A000023 - Arduino - Datasheet.Directory

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Features

•High Performance, Low Power AVR® 8-Bit Microcontroller

•Advanced RISC Architecture

– 135 Powerful Instructions – Most Single Clock Cycle Execution

– 32 x 8 Genera l Pur pos e Wor ki ng Re gisters

– Fully Static Operatio n

– Up to 16 MIPS Throughput at 16MHz

– On-Chip 2-cycle Multi pl ier

•Non-volatile Program and Data Memories

– 16/32KB of In-System Self-Programmable Flash

– 1.25/2.5KB Internal SRAM

– 512Bytes/1KB Internal EEPROM

– Write/Erase Cycles: 10,000 Flash/100,000 EEPROM

– Data retention: 20 years at 85C/ 100 years at 25C(1)

– Optional Boot Code Section with Inde pe nd e nt Lo ck Bits

In-System Programming by On-chip Boot Progra m

True Read-While-Write Op eratio n

Parts using exte rna l XTAL cl oc k are pre -programed with a default USB bootloa de r

– Programming Lock for Software Security

•JTAG (IEEE® std. 1149.1 compliant) Interface

– Boundary-scan Capabilities According to the JTAG Standard

– Extensive On-c hip Debug Support

– Programming of Flash, EEPROM, Fuses, and Lock Bits through the JTAG Interface

•USB 2.0 Full-speed/Low Speed Device Module with Interrupt on Transfer Completion

– Complies fully with Universal Serial Bus Specification Rev 2.0

– Supports data transfer rates up to 12Mbit/s and 1.5Mbit/s

– Endpoint 0 for Control Transfers: up to 64-bytes

– Six Programmab le Endpoints with IN or Out Directions and with Bulk, Interrup t or

Isochronous Transfers

– Configurable Endpoints size up to 256 bytes in double bank mode

– Fully independent 832 bytes USB DPRAM for endpoint memory allocation

– Suspend/Resume Interrupts

– CPU Reset possible on USB Bus Reset detection

– 48MHz from PLL for Full-speed Bus Operation

– USB Bus Connection/Disconnection on Microcontroller Request

– Crystal-less operation for Low Speed mode

•Peripheral Features

– On-chip PLL for USB and High Speed Timer: 32 up to 96MHz operation

– One 8-bit Timer/Counter with Separate Prescaler and Compare Mode

ATmega16U4/ATmega32U4

8-bit Microcontroller with 16/32K bytes of ISP Flash and

USB Controller

DATASHEET

ATmega16U4/32U4 [DATASHEET]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

– Two 16-bit Timer/Counter with Separate Prescaler, Compare- and Capture Mode

– One 10-bit High-Speed Timer/Counter with PLL (64MHz) and Compare Mode

– Four 8-bit PWM Channels

– Four PWM Channels with Programmabl e Resolution from 2 to 16 Bits

– Six PWM Channels for High Speed Operation, with Programmable Resolution from 2 to 11 Bits

– Output Compare Modulator

– 12-channels, 10-bit ADC (features Differential Channels with Programmable Gain)

– Programmable Serial USART w ith Hardware Flow Control

– Master/Slave SPI Serial Interface

– Byte Oriented 2-wire Serial Interface

– Programmable Watchdog Timer with Separate On-chip Oscillator

– On-chip Analog Comparator

– Interrupt a nd Wake-up on Pin Change

– On-chip Temper ature Sensor

•Spec ia l Mic r oc ontroller Feat ure s

– Power-on Reset and Programmable Brown-out Detection

– Internal 8MHz Calibrated Oscillator

– Internal clock prescaler and On-the-fly Clock Switching (Int RC / Ext Osc)

– External and Internal Interrupt Sources

– Six Sleep Modes: Idle, ADC Noise Reduction, Power-save, Power-down, Standby, and Extended Standby

•I/O and Packages

– All I/O combine CMOS outputs and LVTTL inputs

– 26 Programmable I/O Lines

– 44-lead TQFP Package, 10x10mm

– 44-lead QFN Package, 7x7mm

•Operating Voltages

– 2.7 - 5.5V

•Operating temperature

– Industrial (-40°C to +85°C)

•Maximum Frequency

– 8MHz at 2.7V - Indust r ia l ra ng e

– 16MHz at 4.5V - Industrial range

Note: 1. See “Data Retention” on page 8 for details.

ATmega16U4/32U4 [DATASHEET ]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

1. Pin Configurations

Figure 1-1. Pinout

2. Overview

The ATmega16U4/ATmega32U4 is a low-power CMOS 8-bit microcontroller based on the AVR enhanced RISC

architecture. By executing powerful instructions in a single clock cycle, the device achieves throughputs

approaching 1 MIPS per MHz allowing the system designer to optimize power consumption versus processing

speed.

ATmega32U4

ATmega16U4

44-pin QFN/TQFP

UVcc

D-

D+

UGnd

UCap

VBus

(SS/PCINT0) PB0

(INT.6/AIN0) PE6

(PCINT1/SCLK) PB1

(PDI/PCINT2/MOSI) PB2

(

PDO/PCINT3/MISO) PB3

(

PCINT7/OC0A/OC1C/RTS) PB7

RESET

VCC

GND

XTAL2

XTAL1

(OC 0B /SC L/INT0) P D0

(SDA/INT 1) PD1

(RX D1/INT 2) P D2

(T XD 1/IN T3) PD 3

(X CK 1/C T S) PD5

PE2 (HWB)

PC7 (ICP3/CLK0/OC4A)

PC6 (OC3A/OC4A)

PB6 (PCINT6/OC1B/OC4B/AD

PB4 (PCINT4/ADC11)

PD7 (T0/OC4D/ADC10)

PD6 (T1/OC4D/ADC9)

PD4 (ICP1/ADC8)

AVCC

GND

AREF

P F0 (ADC0)

P F1 (ADC1)

P F4 (ADC4/T CK

)

P F5 (ADC5/T M

P F6 (ADC6/T D

P F7 (ADC7/T D

GND

VCC

INDEX CORNER

PB5 (PCINT5/OC1A/OC4B/AD

AVCC

GND

ATmega16U4/32U4 [DATASHEET]

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2.1 Block Diagram

Figure 2-1. Block Diagram

The AVR core combines a rich instruction set with 32 general purpose working registers. All the 32 registers are

directly connected to the Arithmetic Logic Unit (ALU), allowing two independent registers to be accessed in one

single instruction executed in one clock cycle. The resulting architecture is more code efficient while achieving

throughputs up to ten times faster than conventional CISC microcontrollers.

The device provides the following features: 16/32K bytes of In-System Programmable Flash with Read-While-

Write capabilities, 512Bytes/1K bytes EEPROM, 1.25/2.5K bytes SRAM, 26 general purpose I/O lines (CMOS

outputs and LVTTL inputs), 32 general purpose working registers, four flexible Timer/Counters with compare

modes and PWM, one more high-speed Timer/Counter with compare modes and PLL adjustable source, one

USART (including CTS/RTS flow control signals), a byte oriented 2-wire Serial Interface, a 12-channels 10-bit

ADC with optional differential input stage with programmable gain, an on-chip calibrated temperature sensor, a

programmable Watchdog Timer with Internal Oscillator, an SPI serial port, IEEE std. 1149.1 compliant JTAG

test interface, also used for accessing the On-chip Debug system and programming and six software selectable

PROGRAM

COUNTER

STACK

POINTER

PROGRAM

FLASH

MCU CONTROL

GENERAL

PURPOSE

REGISTERS

INSTRUCTION

TIMERS/

COUNTERS

INSTRUCTION

DECODER

DATA DIR.

REG. PORTB

DATA DIR.

REG. PORTE

DATA DIR.

REG. PORTD

DATA REGISTER

PORTB

DATA REGISTER

PORTE

DATA REGISTER

PORTD

INTERRUPT

UNIT

EEPROM

SPI

STATUS

SRAM

USART1

ALU

PORTB DRIVERS

PORTE DRIVERS

PORTF DRIVERS

PORTD DRIVERS

PORTC DRIVERS

PB7 - PB0

PE6

PF7 - PF4

RESET

VCC

GND

XTAL1

XTAL2

CONTROL

LINES

PC7

INTERNAL

OSCILLATOR

WATCHDOG

TIMER

8-BIT DA TA BUS

USB 2.0

TIMING AND

CONTROL

OSCILLATOR

CALIB. OSC

DATA DIR.

REG. PORTC

DATA REGISTER

PORTC

ON-CHIP DEBUG

JTAG TAP

PROGRAMMING

LOGIC

BOUNDARY-

SCAN

DATA DIR.

REG. PORTF

DATA REGISTER

PORTF

POR - BOD

RESET

PD7 - PD0

TWO-WIRE SERIAL

INTERFACE

PLL

HIGH SPEED

TIMER/PWM

PE2

PC6PF1 PF0

ON-CHIP

USB PAD 3V

REGULATOR

UVcc

UCap

1uF

ANALOG

COMPARATOR

VBUS

ADC

AGND

AREF

AVCC

TEMPERATURE

SENSOR

ATmega16U4/32U4 [DATASHEET ]

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power saving modes. The Idle mode stops the CPU while allowing the SRAM, Timer/Counters, SPI port, and

interrupt system to continue functioning. The Power-down mode saves the register contents but freezes the

Oscillator, disabling all other chip functions until the next interrupt or Hardware Reset. The ADC Noise

Reduction mode stops the CPU and all I/O modules except ADC, to minimize switching noise during ADC

conversions. In Standby mode, the Crystal/Resonator Oscillator is running while the rest of the device is

sleeping. This allows very fast start-up combined with low power consumption.

The device is manufactured using the Atmel® high-density nonvolatile memory technology. The On-chip ISP

Flash allows the program memory to be reprogrammed in-system through an SPI serial interface, by a

conventional nonvolatile memory programmer, or by an On-chip Boot program running on the AVR core. The

boot program can use any interface to download the application program in the application Flash memory.

Software in the Boot Flash section will continue to run while the Application Flash section is updated, providing

true Read-While-Write operation. By combining an 8-bit RISC CPU with In-System Self-Programmable Flash on

a monolithic chip, the device is a powerful microcontroller that provides a highly flexible and cost effective

solution to many embedded control applications.

The ATmega16U4/ATmega32U4 AVR is supported with a full suite of program and system development tools

including: C compilers, macro assemblers, program debugger/simulators, in-circuit emulators, and evaluation

kits.

2.2 Pin Descriptions

2.2.1 VCC

Digital supply voltage.

2.2.2 GND

Ground.

2.2.3 Port B (PB7..PB0)

Port B is an 8-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The Port B output

buffers have symmetrical drive characteristics with both high sink and source capability. As inputs, Port B pins

that are externally pulled low will source current if the pull-up resistors are activated. The Port B pins are tri-

stated when a reset condition becomes active, even if the clock is not running.

Port B has better driving capabilities than the other ports.

Port B also serves the functions of various special features of the device as listed on page 74.

2.2.4 Port C (PC7,PC6)

Port C is an 8-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The Port C output

buffers have symmetrical drive characteristics with both high sink and source capability. As inputs, Port C pins

that are externally pulled low will source current if the pull-up resistors are activated. The Port C pins are tri-

stated when a reset condition becomes active, even if the clock is not running.

Only bits 6 and 7 are present on the product pinout.

Port C also serves the functions of special features of the device as listed on page 77.

2.2.5 Port D (PD7..PD0)

Port D is an 8-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The Port D output

buffers have symmetrical drive characteristics with both high sink and source capability. As inputs, Port D pins

that are externally pulled low will source current if the pull-up resistors are activated. The Port D pins are tri-

stated when a reset condition becomes active, even if the clock is not running.

ATmega16U4/32U4 [DATASHEET]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Port D also serves the functions of various special features of the ATmega16U4/ATmega32U4 as listed on

page 78.

2.2.6 Port E (PE6,PE2)

Port E is an 8-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The Port E output

buffers have symmetrical drive characteristics with both high sink and source capability. As inputs, Port E pins

that are externally pulled low will source current if the pull-up resistors are activated. The Port E pins are tri-

stated when a reset condition becomes active, even if the clock is not running.

Only bits 2 and 6 are present on the product pinout.

Port E also serves the functions of various special features of the ATmega16U4/ATmega32U4 as listed on

page 81.

2.2.7 Port F (PF7..PF4, PF1,PF0)

Port F serves as analog inputs to the A/D Converter.

Port F also serves as an 8-bit bi-directional I/O port, if the A/D Converter channels are not used. Port pins can

provide internal pull-up resistors (selected for each bit). The Port F output buffers have symmetrical drive

characteristics with both high sink and source capability. As inputs, Port F pins that are externally pulled low will

source current if the pull-up resistors are activated. The Port F pins are tri-stated when a reset condition

becomes active, even if the clock is not running.

Bits 2 and 3 are not present on the product pinout.

Port F also serves the functions of the JTAG interface. If the JTAG interface is enabled, the pull-up resistors on

pins PF7(TDI), PF5(TMS), and PF4(TCK) will be activated even if a reset occurs.

2.2.8 D-

USB Full speed / Low Speed Negative Data Upstream Port. Should be connected to the USB D- connector pin

with a serial 22 resistor.

2.2.9 D+

USB Full speed / Low Speed Positive Data Upstream Port. Should be connected to the USB D+ connector pin

with a serial 22 resistor.

2.2.10 UGND

USB Pads Ground.

2.2.11 UVCC

USB Pads Internal Regulator Input supply voltage.

2.2.12 UCAP

USB Pads Internal Regulator Output supply voltage. Should be connected to an external capacitor (1µF).

2.2.13 VBUS

USB VBUS monitor input.

ATmega16U4/32U4 [DATASHEET ]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

2.2.14 RESET

Reset input. A low level on this pin for longer than the minimum pulse length will generate a reset, even if the

clock is not running. The minimum pulse length is given in Table 8-2 on page 53. Shorter pulses are not

guaranteed to generate a reset.

2.2.15 XTAL1

Input to the inverting Oscillator amplifier and input to the internal clock operating circuit.

2.2.16 XTAL2

Output from the inverting Oscillator amplifier.

2.2.17 AVCC

AVCC is the supply voltage pin (input) for all the A/D Converter channels. If the ADC is not used, it should be

externally connected to VCC. If the ADC is used, it should be connected to VCC through a low-pass filter.

2.2.18 AREF

This is the analog reference pin (input) for the A/D Converter.

ATmega16U4/32U4 [DATASHEET]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

3. About

3.1 Disclaimer

Typical values contained in this datasheet are based on simulations and characterization of other AVR

microcontrollers manufactured on the same process technology. Min. and Max. values will be available after the

device is characterized.

3.2 Resources

A comprehensive set of development tools, application notes and datasheets are available for download on

http://www.atmel.com/avr.

3.3 Code Examples

This documentation contains simple code examples that briefly show how to use various parts of the device. Be

aware that not all C compiler vendors include bit definitions in the header files and interrupt handling in C is

compiler dependent. Confirm with the C compiler documentation for more details.

These code examples assume that the part specific header file is included before compilation. For I/O registers

located in extended I/O map, "IN", "OUT", "SBIS", "SBIC", "CBI", and "SBI" instructions must be replaced with

instructions that allow access to extended I/O. Typically "LDS" and "STS" combined with "SBRS", "SBRC",

"SBR", and "CBR".

3.4 Data Retention

Reliability Qualification results show that the projected data retention failure rate is much less than 1PPM over

20 years at 85°C or 100 years at 25°C.

ATmega16U4/32U4 [DATASHEET]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

4. AVR CPU Core

4.1 Introduction

This section discusses the AVR core architecture in general. The main function of the CPU core is to ensure

correct program execution. The CPU must therefore be able to access memories, perform calculations, control

peripherals, and handle interrupts.

4.2 Architectural Overview

Figure 4-1. Block Diagram of the AVR Architecture

In order to maximize performance and parallelism, the AVR uses a Harvard architecture – with separate

memories and buses for program and data. Instructions in the program memory are executed with a single level

pipelining. While one instruction is being executed, the next instruction is pre-fetched from the program memory.

This concept enables instructions to be executed in every clock cycle. The program memory is In-System

Reprogrammable Flash memory.

The fast-access Register File contains 32 x 8-bit general purpose working registers with a single clock cycle

access time. This allows single-cycle Arithmetic Logic Unit (ALU) operation. In a typical ALU operation, two

Flash

Program

Memory

Instruction

Decoder

Program

Counter

Control Lines

32 x 8

General

Purpose

Registrers

ALU

Status

and Control

I/O Lines

EEPROM

Data Bus 8-bit

Data

SRAM

Direct Addressing

Indirect Addressing

Interrupt

Unit

SPI

Unit

Watchdog

Timer

Analog

Comparator

I/O Module 2

I/O Module1

I/O Module n

ATmega16U4/32U4 [DATASHEET]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

operands are output from the Register File, the operation is executed, and the result is stored back in the

Six of the 32 registers can be used as three 16-bit indirect address register pointers for Data Space addressing

– enabling efficient address calculations. One of the these address pointers can also be used as an address

pointer for look up tables in Flash program memory. These added function registers are the 16-bit X-, Y-, and Z-

The ALU supports arithmetic and logic operations between registers or between a constant and a register.

Single register operations can also be executed in the ALU. After an arithmetic operation, the Status Register is

updated to reflect information about the result of the operation.

Program flow is provided by conditional and unconditional jump and call instructions, able to directly address the

whole address space. Most AVR instructions have a single 16-bit word format. Every program memory address

contains a 16- or 32-bit instruction.

Program Flash memory space is divided in two sections, the Boot Program section and the Application Program

section. Both sections have dedicated Lock bits for write and read/write protection. The SPM instruction that

writes into the Application Flash memory section must reside in the Boot Program section.

During interrupts and subroutine calls, the return address Program Counter (PC) is stored on the Stack. The

Stack is effectively allocated in the general data SRAM, and consequently the Stack size is only limited by the

total SRAM size and the usage of the SRAM. All user programs must initialize the SP in the Reset routine

(before subroutines or interrupts are executed). The Stack Pointer (SP) is read/write accessible in the I/O space.

The data SRAM can easily be accessed through the five different addressing modes supported in the AVR

architecture.

The memory spaces in the AVR architecture are all linear and regular memory maps.

A flexible interrupt module has its control registers in the I/O space with an additional Global Interrupt Enable bit

in the Status Register. All interrupts have a separate Interrupt Vector in the Interrupt Vector table. The interrupts

have priority in accordance with their Interrupt Vector position. The lower the Interrupt Vector address, the

higher the priority.

The I/O memory space contains 64 addresses for CPU peripheral functions as Control Registers, SPI, and other

I/O functions. The I/O Memory can be accessed directly, or as the Data Space locations following those of the

0x0FF in SRAM where only the ST/STS/STD and LD/LDS/LDD instructions can be used.

4.3 ALU – Arithmetic Logic Unit

The high-performance AVR ALU operates in direct connection with all the 32 general purpose working registers.

Within a single clock cycle, arithmetic operations between general purpose registers or between a register and

an immediate are executed. The ALU operations are divided into three main categories – arithmetic, logical, and

bit-functions. Some implementations of the architecture also provide a powerful multiplier supporting both

signed/unsigned multiplication and fractional format. See “Instruction Set Summary” on page 418 for a detailed

description.

4.4 S tatus Register

The Status Register contains information about the result of the most recently executed arithmetic instruction.

This information can be used for altering program flow in order to perform conditional operations. Note that the

Status Register is updated after all ALU operations, as specified in the Instruction Set Reference. This will in

many cases remove the need for using the dedicated compare instructions, resulting in faster and more

compact code.

The Status Register is not automatically stored when entering an interrupt routine and restored when returning

from an interrupt. This must be handled by software.

ATmega16U4/32U4 [DATASHEET]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

The AVR Status Register – SREG – is defined as:

• Bit 7 – I: Global Interrupt Enable

The Global Interrupt Enable bit must be set for the interrupts to be enabled. The individual interrupt enable

control is then performed in separate control registers. If the Global Interrupt Enable Register is cleared, none of

the interrupts are enabled independent of the individual interrupt enable settings. The I-bit is cleared by

hardware after an interrupt has occurred, and is set by the RETI instruction to enable subsequent interrupts.

The I-bit can also be set and cleared by the application with the SEI and CLI instructions, as described in the

instruction set reference.

• Bit 6 – T: Bit Copy Storage

The Bit Copy instructions BLD (Bit LoaD) and BST (Bit STore) use the T-bit as source or destination for the

operated bit. A bit from a register in the Register File can be copied into T by the BST instruction, and a bit in T

can be copied into a bit in a register in the Register File by the BLD instruction.

• Bit 5 – H: Half Carry Flag

The Half Carry Flag H indicates a Half Carry in some arithmetic operations. Half Carry Is useful in BCD

arithmetic. See “Instruction Set Summary” on page 418 for detailed information.

• Bit 4 – S: Sign Bit, S = N V

The S-bit is always an exclusive or between the Negative Flag N and the Two’s Complement Overflow Flag V.

See “Instruction Set Summary” on page 418 for detailed information.

• Bit 3 – V: Two’s Complement Overflow Flag

The Two’s Complement Overflow Flag V supports two’s arithmetic complements. See “Instruction Set

Summary” on page 418 for detailed information.

• Bit 2 – N: Negative Flag

The Negative Flag N indicates a negative result in an arithmetic or logic operation. See “Instruction Set

Summary” on page 418 for detailed information.

• Bit 1 – Z: Zero Flag

The Zero Flag Z indicates a zero result in an arithmetic or logic operation. See “Instruction Set Summary” on

page 418 for detailed information.

• Bit 0 – C: Carry Flag

The Carry Flag C indicates a carry in an arithmetic or logic operation. See “Instruction Set Summary” on

page 418 for detailed information.

4.5 General Purpose Register File

The Register File is optimized for the AVR Enhanced RISC instruction set. In order to achieve the required

performance and flexibility, the following input/output schemes are supported by the Register File:

One 8-bit output operand and one 8-bit result input

Two 8-bit output operands and one 8-bit result input

Bit 76543210

I T H S V N Z C SREG

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value00000000

ATmega16U4/32U4 [DATASHEET]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Two 8-bit output operands and one 16-bit result input

One 16-bit output operand and one 16-bit result input

Figure 4-2 shows the structure of the 32 general purpose working registers in the CPU.

Figure 4-2. AVR CPU General Purpose Working Registers

Most of the instructions operating on the Register File have direct access to all registers, and most of them are

single cycle instructions.

As shown in Figure 4-2, each register is also assigned a data memory address, mapping them directly into the

first 32 locations of the user Data Space. Although not being physically implemented as SRAM locations, this

memory organization provides great flexibility in access of the registers, as the X-, Y-, and Z-pointer registers

can be set to index any register in the file.

4.5.1 The X-register, Y-register, and Z-register

The registers R26..R31 have some added functions to their general purpose usage. These registers are 16-bit

address pointers for indirect addressing of the data space. The three indirect address registers X, Y, and Z are

defined as described in Figure 4-3.

70Addr.

R0 0x00

R1 0x01

R2 0x02

…

R13 0x0D

General R14 0x0E

Purpose R15 0x0F

Working R16 0x10

Registers R17 0x11

…

R26 0x1A X-register Low Byte

R27 0x1B X-register High Byte

R28 0x1C Y-register Low Byte

R29 0x1D Y-register High Byte

R30 0x1E Z-register Low Byte

R31 0x1F Z-register High Byte

ATmega16U4/32U4 [DATASHEET]

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Figure 4-3. The X-, Y-, and Z-reg isters

In the different addressing modes these address registers have functions as fixed displacement, automatic

increment, and automatic decrement (See “Instruction Set Summary” on page 418 for detailed information).

4.6 Stack Pointer

The Stack is mainly used for storing temporary data, for storing local variables and for storing return addresses

after interrupts and subroutine calls. The Stack Pointer Register always points to the top of the Stack. Note that

the Stack is implemented as growing from higher memory locations to lower memory locations. This implies that

a Stack PUSH command decreases the Stack Pointer.

The Stack Pointer points to the data SRAM Stack area where the Subroutine and Interrupt Stacks are located.

This Stack space in the data SRAM must be defined by the program before any subroutine calls are executed or

interrupts are enabled. The Stack Pointer must be set to point above 0x0100. The initial value of the stack

pointer is the last address of the internal SRAM. The Stack Pointer is decremented by one when data is pushed

onto the Stack with the PUSH instruction, and it is decremented by three when the return address is pushed

onto the Stack with subroutine call or interrupt. The Stack Pointer is incremented by one when data is popped

from the Stack with the POP instruction, and it is incremented by three when data is popped from the Stack with

return from subroutine RET or return from interrupt RETI.

The AVR Stack Pointer is implemented as two 8-bit registers in the I/O space. The number of bits actually used

is implementation dependent. Note that the data space in some implementations of the AVR architecture is so

small that only SPL is needed. In this case, the SPH Register will not be present.

15 XH XL 0

X-register 7 0 7 0

R27 (0x1B) R26 (0x1A)

15 YH YL 0

Y-register 7 0 7 0

R29 (0x1D) R28 (0x1C)

15 ZH ZL 0

Z-register 7 0 7 0

R31 (0x1F) R30 (0x1E)

Bit 1514131211109 8

SP15 SP14 SP13 SP12 SP11 SP10 SP9 SP8 SPH

SP7 SP6 SP5 SP4 SP3 SP2 SP1 SP0 SPL

76543210

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value 0 0 1 0 0 0 0 0

11111111

ATmega16U4/32U4 [DATASHEET]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

4.6.1 Ex tended Z-pointer Register for ELPM/SPM - RAMPZ

For ELPM/SPM instructions, the Z-pointer is a concatenation of RAMPZ, ZH, and ZL, as shown in Figure 4-4.

Note that LPM is not affected by the RAMPZ setting.

Figure 4-4. The Z-pointer used by ELPM and SPM

The actual number of bits is implementation dependent. Unused bits in an implementation will always read as

zero. For compatibility with future devices, be sure to write these bits to zero.

4.7 Instruction Execution Timing

This section describes the general access timing concepts for instruction execution. The AVR CPU is driven by

the CPU clock clkCPU, directly generated from the selected clock source for the chip. No internal clock division is

used.

Figure 4-5 shows the parallel instruction fetches and instruction executions enabled by the Harvard architecture

and the fast-access Register File concept. This is the basic pipelining concept to obtain up to 1 MIPS per MHz

with the corresponding unique results for functions per cost, functions per clocks, and functions per power-unit.

Figure 4-5. The Parallel Instruction Fetches and Instruc tion Executions

Figure 4-6 shows the internal timing concept for the Register File. In a single clock cycle an ALU operation using

two register operands is executed, and the result is stored back to the destination register.

Bit 7654321 0

RAMPZ7 RAMPZ6 RAMPZ5 RAMPZ4 RAMPZ3 RAMPZ2 RAMPZ1 RAMPZ0 RAMPZ

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value0000000 0

Bit (Individually) 7 0 7 0 7 0

RAMPZ ZH ZL

Bit (Z-pointer) 23 16 15 8 7 0

clk

1st Instruction Fetch

1st Instruction Execute

2nd Instruction Fetch

2nd Instruction Execute

3rd Instruction Fetch

3rd Instruction Execute

4th Instruction Fetch

T1 T2 T3 T4

CPU

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Figure 4-6. Single Cycle ALU Operation

4.8 Reset and Interrupt Handling

The AVR provides several different interrupt sources. These interrupts and the separate Reset Vector each

have a separate program vector in the program memory space. All interrupts are assigned individual enable bits

which must be written logic one together with the Global Interrupt Enable bit in the Status Register in order to

enable the interrupt. Depending on the Program Counter value, interrupts may be automatically disabled when

Boot Lock bits BLB02 or BLB12 are programmed. This feature improves software security. See the section

“Memory Programming” on page 353 for details.

The lowest addresses in the program memory space are by default defined as the Reset and Interrupt Vectors.

The complete list of vectors is shown in “Interrupts” on page 63. The list also determines the priority levels of the

different interrupts. The lower the address the higher is the priority level. RESET has the highest priority, and

next is INT0 – the External Interrupt Request 0. The Interrupt Vectors can be moved to the start of the Boot

Flash section by setting the IVSEL bit in the MCU Control Register (MCUCR). Refer to “Interrupts” on page 63

for more information. The Reset Vector can also be moved to the start of the Boot Flash section by

programming the BOOTRST Fuse, see “Memory Programming” on page 353.

When an interrupt occurs, the Global Interrupt Enable I-bit is cleared and all interrupts are disabled. The user

software can write logic one to the I-bit to enable nested interrupts. All enabled interrupts can then interrupt the

current interrupt routine. The I-bit is automatically set when a Return from Interrupt instruction – RETI – is

executed.

There are basically two types of interrupts. The first type is triggered by an event that sets the Interrupt Flag. For

these interrupts, the Program Counter is vectored to the actual Interrupt Vector in order to execute the interrupt

handling routine, and hardware clears the corresponding Interrupt Flag. Interrupt Flags can also be cleared by

writing a logic one to the flag bit position(s) to be cleared. If an interrupt condition occurs while the

corresponding interrupt enable bit is cleared, the Interrupt Flag will be set and remembered until the interrupt is

enabled, or the flag is cleared by software. Similarly, if one or more interrupt conditions occur while the Global

Interrupt Enable bit is cleared, the corresponding Interrupt Flag(s) will be set and remembered until the Global

Interrupt Enable bit is set, and will then be executed by order of priority.

The second type of interrupts will trigger as long as the interrupt condition is present. These interrupts do not

necessarily have Interrupt Flags. If the interrupt condition disappears before the interrupt is enabled, the

interrupt will not be triggered.

When the AVR exits from an interrupt, it will always return to the main program and execute one more

instruction before any pending interrupt is served.

Note that the Status Register is not automatically stored when entering an interrupt routine, nor restored when

returning from an interrupt routine. This must be handled by software.

Total Execution Time

ALU Operation Execute

Result Write Back

T1 T2 T3 T4

clkCPU

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When using the CLI instruction to disable interrupts, the interrupts will be immediately disabled. No interrupt will

be executed after the CLI instruction, even if it occurs simultaneously with the CLI instruction. The following

example shows how this can be used to avoid interrupts during the timed EEPROM write sequence.

When using the SEI instruction to enable interrupts, the instruction following SEI will be executed before any

pending interrupts, as shown in this example.

4.8.1 Interrupt Response Time

The interrupt execution response for all the enabled AVR interrupts is five clock cycles minimum. After five clock

cycles the program vector address for the actual interrupt handling routine is executed. During these five clock

cycle period, the Program Counter is pushed onto the Stack. The vector is normally a jump to the interrupt

routine, and this jump takes three clock cycles. If an interrupt occurs during execution of a multi-cycle

instruction, this instruction is completed before the interrupt is served. If an interrupt occurs when the MCU is in

sleep mode, the interrupt execution response time is increased by five clock cycles. This increase comes in

addition to the start-up time from the selected sleep mode.

Assembly Code Example

in r16, SREG ;

store SREG value

cli ; disable interrupts during

timed sequence

sbi EECR, EEMPE ;

start EEPROM write

sbi EECR, EEPE

out SREG, r16 ;

restore SREG value (I-bit)

C Code Example

char cSREG;

cSREG = SREG; /*

store SREG value */

/* disable interrupts during timed sequence */

__disable_interrupt();

EECR |= (1<<EEMPE); /* start EEPROM write */

EECR |= (1<<EEPE);

SREG = cSREG; /* restore SREG value (I-bit) */

Assembly Code Example

sei ; set Global Interrupt Enable

sleep ; enter sleep, waiting for interrupt

; note: will enter sleep before any pending

; interrupt(s)

C Code Example

__enable_interrupt(); /* set Global Interrupt Enable */

__sleep(); /* enter sleep, waiting for interrupt */

/* note: will enter sleep before any pending

interrupt(s) */

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A return from an interrupt handling routine takes five clock cycles. During these five clock cycles, the Program

Counter (three bytes) is popped back from the Stack, the Stack Pointer is incremented by three, and the I-bit in

SREG is set.

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5. AVR Memories

This section describes the different memories in the device. The AVR architecture has two main memory

spaces, the Data Memory and the Program Memory space. In addition, the device features an EEPROM

Memory for data storage. All three memory spaces are linear and regular.

Notes: 1. Byte address.

2. Word (16-bit) address.

5.1 In-System Reprogrammable Flash Program Memory

The device contains 16/32K bytes On-chip In-System Reprogrammable Flash memory for program storage.

Since all AVR instructions are 16 or 32 bits wide, the Flash is organized as 16K x 16. For software security, the

Flash Program memory space is divided into two sections, Boot Program section and Application Program

section.

The Flash memory has an endurance of at least 100,000 write/erase cycles. The device Program Counter (PC)

is 16 bits wide, thus addressing the 32K program memory locations. The operation of Boot Program section and

associated Boot Lock bits for software protection are described in detail in “Memory Programming” on

page 353. “Memory Programming” on page 353 contains a detailed description on Flash data serial

downloading using the SPI pins or the JTAG interface.

Table 5-1. Memory Mapping

Memory Mnemonic ATmega32U4 ATmega16U4

Flash

Size Flash size 32KB 16KB

Start Address -

0x0000

End Address Flash end 0x7FFF(1)

0x3FFF(2)

0x3FFF(1)

0x1FFF(2)

32 Registers

Size -32 bytes 32 bytes

Start Address -0x0000 0x0000

End Address -0x001F 0x001F

I/O Registers

Size -64 bytes 64 bytes

Start Address -0x0020 0x0020

End Address -0x005F 0x005F

Ext I/O Registers

Size -160 bytes 160 bytes

Start Address -0x0060 0x0060

End Address -0x00FF 0x00FF

Internal SRAM

Size ISRAM size 2.5KB 1.25KB

Start Address ISRAM start 0x100 0x100

End Address ISRAM end 0x0AFF 0x05FF

External Memory Not Present.

EEPROM

Size E2 size 1KB 512 bytes

End Address E2 end 0x03FF 0x01FF

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Constant tables can be allocated within the entire program memory address space (see the LPM – Load

Program Memory instruction description and ELPM - Extended Load Program Memory instruction description).

Timing diagrams for instruction fetch and execution are presented in “Instruction Execution Timing” on page 14.

Figure 5-1. Program Memory Map

5.2 SRAM Data Memory

Figure 5-2 on page 20 shows how the device SRAM Memory is organized.

The device is a complex microcontroller with more peripheral units than can be supported within the 64 location

reserved in the Opcode for the IN and OUT instructions. For the Extended I/O space from $060 - $0FF in

SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.

The first 2,816 Data Memory locations address both the Register File, the I/O Memory, Extended I/O Memory,

and the internal data SRAM. The first 32 locations address the Register file, the next 64 location the standard

I/O Memory, then 160 locations of Extended I/O memory and the next 2,560 locations address the internal data

SRAM.

The five different addressing modes for the data memory cover: Direct, Indirect with Displacement, Indirect,

Indirect with Pre-decrement, and Indirect with Post-increment. In the Register file, registers R26 to R31 feature

the indirect addressing pointer registers.

The direct addressing reaches the entire data space.

The Indirect with Displacement mode reaches 63 address locations from the base address given by the Y- or Z-

When using register indirect addressing modes with automatic pre-decrement and post-increment, the address

registers X, Y, and Z are decremented or incremented.

The 32 general purpose working registers, 64 I/O registers, and the 1.25/2.5Kbytes of internal data SRAM in the

device are all accessible through all these addressing modes. The Register File is described in “General

Purpose Register File” on page 11.

0x00000

Program Memory

Application Flash Section

Boot Flash Section

0x7FFF (32KBytes)

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Figure 5-2. Dat a Memory Map

5.2.1 Data Memory Access Times

This section describes the general access timing concepts for internal memory access. The internal data SRAM

access is performed in two clkCPU cycles as described in Figure 5-3.

Figure 5-3. On-chip Data SRAM Access Cycles

5.3 EEPROM Data Memory

The device contains 512Bytes/1K bytes of data EEPROM memory. It is organized as a separate data space, in

which single bytes can be read and written. The EEPROM has an endurance of at least 100,000 write/erase

cycles. The access between the EEPROM and the CPU is described in the following, specifying the EEPROM

Address Registers, the EEPROM Data Register, and the EEPROM Control Register.

For a detailed description of SPI, JTAG and Parallel data downloading to the EEPROM, see page 367,

page 371, and page 356 respectively.

I/O

$0000

$001

$0020

$005

FFFF

$0060

$00

160

I/O

ISRAM end : $05FF / $0AFF

ISRAM start : $0100

clk

Data

Address Address valid

T1 T2 T3

Compute Address

Read Write

CPU

Memory Access Instruction Next Instruction

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5.3.1 EEPROM Read/Write Access

The EEPROM Access Registers are accessible in the I/O space.

The write access time for the EEPROM is given in Table 5-3 on page 23. A self-timing function, however, lets

the user software detect when the next byte can be written. If the user code contains instructions that write the

EEPROM, some precautions must be taken. In heavily filtered power supplies, VCC is likely to rise or fall slowly

on power-up/down. This causes the device for some period of time to run at a voltage lower than specified as

minimum for the clock frequency used. See “Preventing EEPROM Corruption” on page 25. for details on how to

avoid problems in these situations.

In order to prevent unintentional EEPROM writes, a specific write procedure must be followed. Refer to the

description of the EEPROM Control Register for details on this.

When the EEPROM is read, the CPU is halted for four clock cycles before the next instruction is executed.

When the EEPROM is written, the CPU is halted for two clock cycles before the next instruction is executed.

5.3.2 The EEPROM Address Register – EEARH and EEARL

• Bits 15..12 – Res: Reserved Bits

These bits are reserved bits and will always read as zero.

• Bits 11..0 – EEAR8..0: EEPROM Address

The EEPROM Address Registers – EEARH and EEARL specify the EEPROM address in the 512Bytes/1K

bytes EEPROM space. The EEPROM data bytes are addressed linearly between 0 and E2_END. The initial

value of EEAR is undefined. A proper value must be written before the EEPROM may be accessed.

5.3.3 The EEPROM Data Register – EEDR

• Bits 7..0 – EEDR7.0: EEPROM Data

For the EEPROM write operation, the EEDR Register contains the data to be written to the EEPROM in the

address given by the EEAR Register. For the EEPROM read operation, the EEDR contains the data read out

from the EEPROM at the address given by EEAR.

5.3.4 The EEPROM Control Register – EECR

Bit 1514131211 10 9 8

––––EEAR11EEAR10EEAR9EEAR8EEARH

EEAR7 EEAR6 EEAR5 EEAR4 EEAR3 EEAR2 EEAR1 EEAR0 EEARL

76543 2 10

Read/WriteRRRRR/WR/WR/WR/W

R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value0000X X XX

XXXXX X XX

Bit 76543210

MSB LSB EEDR

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value 0 0 0 0 0 0 0 0

Bit 765432 10

– – EEPM1 EEPM0 EERIE EEMPE EEPE EERE EECR

Read/Write R R R/W R/W R/W R/W R/W R/W

Initial Value 0 0 X X 0 0 X 0

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• Bits 7..6 – Res: Reserved Bits

These bits are reserved and will always read as zero.

• Bits 5, 4 – EEPM1 and EEPM0: EEPROM Programming Mode Bits

The EEPROM Programming mode bit setting defines which programming action that will be triggered when

writing EEPE. It is possible to program data in one atomic operation (erase the old value and program the new

value) or to split the Erase and Write operations in two different operations. The Programming times for the

different modes are shown in the table below. While EEPE is set, any write to EEPMn will be ignored. During

reset, the EEPMn bits will be reset to 0b00 unless the EEPROM is busy programming.

• Bit 3 – EERIE: EEPROM Ready Interrupt Enable

Writing EERIE to one enables the EEPROM Ready Interrupt if the I bit in SREG is set. Writing EERIE to zero

disables the interrupt. The EEPROM Ready interrupt generates a constant interrupt when EEPE is cleared.

• Bit 2 – EEMPE: EEPROM Master Programming Enable

The EEMPE bit determines whether setting EEPE to one causes the EEPROM to be written. When EEMPE is

set, setting EEPE within four clock cycles will write data to the EEPROM at the selected address If EEMPE is

zero, setting EEPE will have no effect. When EEMPE has been written to one by software, hardware clears the

bit to zero after four clock cycles. See the description of the EEPE bit for an EEPROM write procedure.

• Bit 1 – EEPE: EEPROM Programming Enable

The EEPROM Write Enable Signal EEPE is the write strobe to the EEPROM. When address and data are

correctly set up, the EEPE bit must be written to one to write the value into the EEPROM. The EEMPE bit must

be written to one before a logical one is written to EEPE, otherwise no EEPROM write takes place. The

following procedure should be followed when writing the EEPROM (the order of steps 3 and 4 is not essential):

1. Wait until EEPE becomes zero.

2. Wait until SELFPRGEN in SPMCSR becomes zero.

3. Write new EEPROM address to EEAR (optional).

4. Write new EEPROM data to EEDR (optional).

5. Write a logical one to the EEMPE bit while writing a zero to EEPE in EECR.

6. Within four clock cycles after setting EEMPE, write a logical one to EEPE.

The EEPROM can not be programmed during a CPU write to the Flash memory. The software must check that

the Flash programming is completed before initiating a new EEPROM write. Step 2 is only relevant if the

software contains a Boot Loader allowing the CPU to program the Flash. If the Flash is never being updated by

the CPU, step 2 can be omitted. See “Memory Programming” on page 353 for details about Boot programming.

Caution: An interrupt between step 5 and step 6 will make the write cycle fail, since the EEPROM Master Write

Enable will time-out. If an interrupt routine accessing the EEPROM is interrupting another EEPROM access, the

Table 5-2. EEPROM Mode Bits

EEPM1 EEPM0 Programming Time Operation

0 0 3.4ms Erase and Write in one operation (Atomic Operation)

0 1 1.8ms Erase Only

1 0 1.8ms Write Only

1 1 – Reserved for future use

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EEAR or EEDR Register will be modified, causing the interrupted EEPROM access to fail. It is recommended to

have the Global Interrupt Flag cleared during all the steps to avoid these problems.

When the write access time has elapsed, the EEPE bit is cleared by hardware. The user software can poll this

bit and wait for a zero before writing the next byte. When EEPE has been set, the CPU is halted for two cycles

before the next instruction is executed.

• Bit 0 – EERE: EEPROM Read Enable

The EEPROM Read Enable Signal EERE is the read strobe to the EEPROM. When the correct address is set

up in the EEAR Register, the EERE bit must be written to a logic one to trigger the EEPROM read. The

EEPROM read access takes one instruction, and the requested data is available immediately. When the

EEPROM is read, the CPU is halted for four cycles before the next instruction is executed.

The user should poll the EEPE bit before starting the read operation. If a write operation is in progress, it is

neither possible to read the EEPROM, nor to change the EEAR Register.

The calibrated Oscillator is used to time the EEPROM accesses. The following table lists the typical

programming time for EEPROM access from the CPU.

The following code examples show one assembly and one C function for writing to the EEPROM. The examples

assume that interrupts are controlled (e.g. by disabling interrupts globally) so that no interrupts will occur during

execution of these functions. The examples also assume that no Flash Boot Loader is present in the software. If

such code is present, the EEPROM write function must also wait for any ongoing SPM command to finish.

Table 5-3. EEPROM Programming Time

Symbol Number of Calibrated RC Oscillator Cycles Typ Programming Time

EEPROM write

(from CPU) 26,368 3.3ms

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Note: 1. See “Code Examples” on page 8.

The next code examples show assembly and C functions for reading the EEPROM. The examples assume that

interrupts are controlled so that no interrupts will occur during execution of these functions.

Assembly Code Example(1)

EEPROM_write:

; Wait for completion of previous write

sbic EECR,EEPE

rjmp EEPROM_write

; Set up address (r18:r17) in address register

out EEARH, r18

out EEARL, r17

; Write data (r16) to Data Register

out EEDR,r16

; Write logical one to EEMPE

sbi EECR,EEMPE

; Start eeprom write by setting EEPE

sbi EECR,EEPE

ret

C Code Example(1)

void EEPROM_write(unsigned int uiAddress, unsigned char

ucData)

{/* Wait for completion of previous write */

while(EECR & (1<<EEPE))

;

/* Set up address and Data Registers */

EEAR = uiAddress;

EEDR = ucData;

/* Write logical one to EEMPE */

EECR |= (1<<EEMPE);

/* Start eeprom write by setting EEPE */

EECR |= (1<<EEPE);

}

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Note: 1. See “Code Examples” on page 8.

5.3.5 Preventing EEPROM Corruption

During periods of low VCC, the EEPROM data can be corrupted because the supply voltage is too low for the

CPU and the EEPROM to operate properly. These issues are the same as for board level systems using

EEPROM, and the same design solutions should be applied.

An EEPROM data corruption can be caused by two situations when the voltage is too low. First, a regular write

sequence to the EEPROM requires a minimum voltage to operate correctly. Secondly, the CPU itself can

execute instructions incorrectly, if the supply voltage is too low.

EEPROM data corruption can easily be avoided by following this design recommendation:

Keep the AVR RESET active (low) during periods of insufficient power supply voltage. This can be done by

enabling the internal Brown-out Detector (BOD). If the detection level of the internal BOD does not match the

needed detection level, an external low VCC reset Protection circuit can be used. If a reset occurs while a write

operation is in progress, the write operation will be completed provided that the power supply voltage is

sufficient.

5.4 I/O Memory

The I/O space definition of the device is shown in “Register Summary” on page 414.

Assembly Code Example(1)

EEPROM_read:

; Wait for completion of previous write

sbic EECR,EEPE

rjmp EEPROM_read

; Set up address (r18:r17) in address register

out EEARH, r18

out EEARL, r17

; Start eeprom read by writing EERE

sbi EECR,EERE

; Read data from Data Register

in r16,EEDR

ret

C Code Example(1)

unsigned char EEPROM_read(unsigned int uiAddress)

{/* Wait for completion of previous write */

while(EECR & (1<<EEPE))

;

/* Set up address register */

EEAR = uiAddress;

/* Start eeprom read by writing EERE */

EECR |= (1<<EERE);

/* Return data from Data Register */

return EEDR;

}

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All I/Os and peripherals are placed in the I/O space. All I/O locations may be accessed by the LD/LDS/LDD and

ST/STS/STD instructions, transferring data between the 32 general purpose working registers and the I/O

space. I/O Registers within the address range 0x00 - 0x1F are directly bit-accessible using the SBI and CBI

instructions. In these registers, the value of single bits can be checked by using the SBIS and SBIC instructions.

Refer to the instruction set section for more details. When using the I/O specific commands IN and OUT, the I/O

addresses 0x00 - 0x3F must be used. When addressing I/O Registers as data space using LD and ST

instructions, 0x20 must be added to these addresses. The device is a complex microcontroller with more

peripheral units than can be supported within the 64 location reserved in Opcode for the IN and OUT

instructions. For the Extended I/O space from 0x60 - 0xFF in SRAM, only the ST/STS/STD and LD/LDS/LDD

instructions can be used.

For compatibility with future devices, reserved bits should be written to zero if accessed. Reserved I/O memory

addresses should never be written.

Some of the Status Flags are cleared by writing a logical one to them. Note that, unlike most other AVRs, the

CBI and SBI instructions will only operate on the specified bit, and can therefore be used on registers containing

such Status Flags. The CBI and SBI instructions work with registers 0x00 to 0x1F only.

The I/O and peripherals control registers are explained in later sections.

5.4.1 General Purpose I/O Registers

The device contains three General Purpose I/O Registers. These registers can be used for storing any

information, and they are particularly useful for storing global variables and Status Flags. General Purpose I/O

Registers within the address range 0x00 - 0x1F are directly bit-accessible using the SBI, CBI, SBIS, and SBIC

instructions.

5.4.2 General Purpose I/O Register 2 – GPIOR2

5.4.3 General Purpose I/O Register 1 – GPIOR1

5.4.4 General Purpose I/O Register 0 – GPIOR0

Bit 76543210

MSB LSB GPIOR2

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value00000000

Bit 76543210

MSB LSB GPIOR1

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value 0 0 0 0 0 0 0 0

Bit 76543210

MSB LSB GPIOR0

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value 0 0 0 0 0 0 0 0

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6. System Clock and Clock Options

6.1 Clock Systems and their Distribution

Figure 6-1 presents the principal clock systems in the AVR and their distribution. All of the clocks need not be

active at a given time. In order to reduce power consumption, the clocks to modules not being used can be

halted by using different sleep modes, as described in “Power Management and Sleep Modes” on page 43. The

clock systems are detailed below.

Figure 6-1. Clock Distribution

6.1.1 CPU Clock – clkCPU

The CPU clock is routed to parts of the system concerned with operation of the AVR core. Examples of such

modules are the General Purpose Register File, the Status Register and the data memory holding the Stack

Pointer. Halting the CPU clock inhibits the core from performing general operations and calculations.

6.1.2 I/O Clock – clkI/O

The I/O clock is used by the majority of the I/O modules, like Timer/Counters, SPI, and USART. The I/O clock is

also used by the External Interrupt module, but note that some external interrupts are detected by asynchronous

logic, allowing such interrupts to be detected even if the I/O clock is halted. Also, TWI address recognition is

handled in all sleep modes.

6.1.3 Flash Clock – clkFLASH

The Flash clock controls operation of the Flash interface. The Flash clock is usually active simultaneously with

the CPU clock.

General I/O

Modules CPU Core RAM

clkI/O AVR Clock

Control Unit clkCPU

Flash and

EEPROM

clkFLASH

Source clock

Watchdog TimerReset Logic

Clock

Multiplexer

Watchdog

clock

Calibrated RC

Oscillator

Crystal

Oscillator External Clock

ADC

clkADC

System Clock

Prescaler

Watchdog

Oscillator

USB

clkUSB (48MHz)

PLL Clock

Prescaler

PLL

clkPllPresc

High Speed

Timer

clkPLL

PLL Postcaler

(1) (2)

clkTMR

PLL Input

Multiplexer

Clock Switch

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6.1.4 ADC Clock – clkADC

The ADC is provided with a dedicated clock domain. This allows halting the CPU and I/O clocks in order to

reduce noise generated by digital circuitry. This gives more accurate ADC conversion results.

6.1.5 PLL Prescaler Clock – clkPllPresc

The PLL requires a 8MHz input. A prescaler allows user to use either a 8MHz or a 16MHz source (from a crystal

or an external source), using a divider (by 2) if necessary. The output of the prescaler goes into the PLL Input

multiplexer, that allows the user to select either the prescaler output of the System Clock Multiplexer, or the

Internal 8MHz Calibrated Oscillator.

6.1.6 PLL Output Clock – clkPll

When enabled, the PLL outputs one frequency among numerous choices between 32MHz and 96MHz. The

output frequency is determined by the PLL clock register. The frequency is independent of the power supply

voltage. The PLL Output is connected to a postscaler that allows user to generate two different frequencies

(clkUSB and clkTMR) from the common PLL signal, each on them resulting of a selected division ratio (/1, /1.5, /2).

6.1.7 High-Speed Timer Clock– clkTMR

When enabled, the PLL outputs one frequency among numerous choices between 32MHz and 96MHz, that

goes into the PLL Postcaler. The High Speed Timer frequency input is generated from the PLL Postcaler, that

proposes /1, /1.5 and /2 ratios. That can be determined from the PLL clock register. The High Speed Timer

maximum frequency input depends on the power supply voltage and reaches its maximum of 64MHz at 5V.

6.1.8 USB Clock – clkUSB

The USB hardware module needs for a 48MHz clock. This clock is generated from the on-chip PLL. The output

of the PLL passes through the PLL Postcaler where the frequency can be either divided by 2 or directly

connected to the clkUSB signal.

6.2 Clock Sources

The device has the following clock source options, selectable by Flash Fuse bits as shown below. The clock

from the selected source is input to the AVR clock generator, and routed to the appropriate modules.

Note: 1. For all fuses “1” means unprogrammed while “0” means programmed.

Table 6-1. Device Clocking Option s Select(1)

Device Clocking Option CKSEL[3:0] (or EXCKSEL[3:0])

Low Power Crystal Oscillator 1111 - 1000

Reserved 0111 - 0110

Low Frequency Crystal Oscillator 0101 - 0100

Reserved 0011

Calibrated Internal RC Oscillator 0010

External Clock 0000

Reserved 0001

ATmega16U4/32U4 [DATASHEET]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

6.2.1 Default Clock Source ATmega16U4 and ATmega32U4

The device is shipped with Low Power Crystal Oscillator (8.0 - 16MHz) enabled and with the fuse CKDIV8

programmed, resulting in 1.0MHz system clock with an 8MHz crystal. See Table 28-5 on page 355 for an

overview of the default Clock Selection Fuse setting.

6.2.2 Default Clock Source ATmega16U4RC and ATmega32U4RC

The device is shipped with Calibrated Internal RC oscillator (8.0MHz) enabled and with the fuse CKDIV8

programmed, resulting in 1.0MHz system clock. See Table 28-5 on page 355 for an overview of the default

Clock Selection Fuse setting.

6.2.3 Clock Startup Sequence

Any clock source needs a sufficient VCC to start oscillating and a minimum number of oscillating cycles before it

can be considered stable.

To ensure sufficient VCC, the device issues an internal reset with a time-out delay (tTOUT) after the device reset is

released by all other reset sources. “On-chip Debug System” on page 46 describes the start conditions for the

internal reset. The delay (tTOUT) is timed from the Watchdog Oscillator and the number of cycles in the delay is

set by the SUTx and CKSELx fuse bits. The selectable delays are shown in the following table. The frequency of

the Watchdog Oscillator is voltage dependent as shown in this table.

Main purpose of the delay is to keep the AVR in reset until it is supplied with minimum VCC. The delay will not

monitor the actual voltage and it will be required to select a delay longer than the VCC rise time. If this is not

possible, an internal or external Brown-Out Detection circuit should be used. A BOD circuit will ensure sufficient

VCC before it releases the reset, and the time-out delay can be disabled. Disabling the time-out delay without

utilizing a Brown-Out Detection circuit is not recommended.

The oscillator is required to oscillate for a minimum number of cycles before the clock is considered stable. An

internal ripple counter monitors the oscillator output clock, and keeps the internal reset active for a given

number of clock cycles. The reset is then released and the device will start to execute. The recommended

oscillator start-up time is dependent on the clock type, and varies from six cycles for an externally applied clock

to 32K cycles for a low frequency crystal.

The start-up sequence for the clock includes both the time-out delay and the start-up time when the device

starts up from reset. When starting up from Power-save or Power-down mode, VCC is assumed to be at a

sufficient level and only the start-up time is included.

6.3 Low Power Crystal Oscillator

Pins XTAL1 and XTAL2 are input and output, respectively, of an inverting amplifier which can be configured for

use as an On-chip Oscillator, as shown in Figure 6-2. Either a quartz crystal or a ceramic resonator may be

used.

This Crystal Oscillator is a low power oscillator, with reduced voltage swing on the XTAL2 output. It gives the

lowest power consumption, but is not capable of driving other clock inputs.

Table 6-2. Number of Watchdog Oscillator Cycles

Typ Time-out (VCC = 5.0V) Typ Time-out (VCC = 3.0V) Number of Cycles

0ms 0ms 0

4.1ms 4.3ms 512

65ms 69ms 8K (8,192)

ATmega16U4/32U4 [DATASHEET]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

C1 and C2 should always be equal for both crystals and resonators. The optimal value of the capacitors

depends on the crystal or resonator in use, the amount of stray capacitance, and the electromagnetic noise of

the environment. Some initial guidelines for choosing capacitors for use with crystals are given in the below

table. For ceramic resonators, the capacitor values given by the manufacturer should be used.

Figure 6-2. Crystal Oscillator Connections

The Low Power Oscillator can operate in three different modes, each optimized for a specific frequency range.

The operating mode is selected by the fuses CKSEL[3..1] as shown in this table.

Notes: 1. This option should not be used with crystals, only with ceramic resonators.

2. If 8 MHz frequency exceeds the specification of the device (depends on VCC), the CKDIV8 Fuse can be

programmed in order to divide the internal frequency by 8. It must be ensured that the resulting divided clock

meets the frequency specification of the device.

The CKSEL0 Fuse together with the SUT1..0 Fuses select the start-up times as shown in this table.

Table 6-3. Low Power Crystal Oscillator Operating Modes

Frequency Range(1) [MHz] CKSEL3..1 Recommended Range for Capacitors C1 and C2 [pF]

0.4 - 0.9 100(2) –

0.9 - 3.0 101 12 - 22

3.0 - 8.0 110 12 - 22

8.0 - 16.0 111 12 - 22

Table 6-4. Start-up Times for the Low Power Crystal Oscillator Clock Selection

Oscillator Source /

Power Conditions

Start-up Time from

Power-down and

Power-save

Additional Delay from

Reset

(VCC = 5.0V) CKSEL0 SUT1..0

Ceramic resonator,

fast rising power 258CK 14CK + 4.1ms(1) 000

Ceramic resonator,

slowly rising power 258CK 14CK + 65ms(1) 001

Ceramic resonator,

BOD enabled 1K CK 14CK(2) 010

Ceramic resonator,

fast rising power 1K CK 14CK + 4.1ms(2) 011

Ceramic resonator,

slowly rising power 1K CK 14CK + 65ms(2) 100

XTAL2

XTAL1

GND

ATmega16U4/32U4 [DATASHEET]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Notes: 1. These options should only be used when not operating close to the maximum frequency of the device, and

only if frequency stability at start-up is not important for the application. These options are not suitable for

crystals.

2. These options are intended for use with ceramic resonators and will ensure frequency stability at start-up. They

can also be used with crystals when not operating close to the maximum frequency of the device, and if

frequency stability at start-up is not important for the application.

Note: 1. The device is shipped with this option selected.

6.4 Low Frequency Crystal Oscillator

The device can utilize a 32.768kHz watch crystal as clock source by a dedicated Low Frequency Crystal

Oscillator. The crystal should be connected as shown in Figure 6-2 on page 30. When this Oscillator is selected,

start-up times are determined by the SUT Fuses and CKSEL0 as shown in the table below.

Crystal Oscillator,

BOD enabled 16K CK 14CK 101

Crystal Oscillator,

fast rising power 16K CK 14CK + 4.1ms 110

Crystal Oscillator,

slowly rising power 16K CK 14CK + 65ms 111

Table 6-5. Start-up times for the internal calibrated RC Oscillator clock selection

Power Conditions Start-up Time from Power-

down and Power-save Additional Delay from

Reset (VCC = 5.0V) SUT1..0

BOD enabled 6CK 14CK 00

Fast rising power 6CK 14CK + 4.1ms 01

Slowly rising power 6CK 14CK + 65ms(1) 10

Reserved 11

Table 6-4. Start-up Times for the Low Power Crystal Oscillator Clock Selection

Oscillator Source /

Power Conditions

Start-up Time from

Power-down and

Power-save

Additional Delay from

Reset

(VCC = 5.0V) CKSEL0 SUT1..0

Table 6-6. Start-up Times for the Low Frequency Crystal Oscillator Clock Select ion

Power Conditions

Start-up Time from

Power-down and

Power-save

Additional Dela y from

Reset

(VCC = 5.0V) CKSEL0 SUT1..0

BOD enabled 1K CK 14CK(1) 000

Fast rising power 1K CK 14CK + 4.1ms(1) 001

Slowly rising power 1K CK 14CK + 65ms(1) 010

Reserved 011

BOD enabled 32K CK 14CK 100

Fast rising power 32K CK 14CK + 4.1ms 101

Slowly rising power 32K CK 14CK + 65ms 110

ATmega16U4/32U4 [DATASHEET]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Note: 1. These options should only be used if frequency stability at start-up is not important for the application.

6.5 Calibrated Internal RC Oscillator

The calibrated internal RC Oscillator by default provides a 8.0MHz clock. This frequency is nominal value at 3V

and 25C. The device is shipped with the CKDIV8 Fuse programmed. See “System Clock Prescaler” on

page 35 for more details. This clock may be selected as the system clock by programming the CKSEL Fuses as

shown in the table below. If selected, it will operate with no external components. During reset, hardware loads

the calibration byte into the OSCCAL Register and thereby automatically calibrates the RC Oscillator. At 3V and

25C, this calibration gives a frequency of 8MHz ±1%. The oscillator can be calibrated to any frequency in the

range 7.3 - 8.1MHz within ±1% accuracy, by changing the OSCCAL register. When this Oscillator is used as the

chip clock, the Watchdog Oscillator will still be used for the Watchdog Timer and for the Reset Time-out. For

more information on the pre-programmed calibration value, see the section “Calibration Byte” on page 356

Notes: 1. The device is shipped with this option selected.

2. If 8 MHz frequency exceeds the specification of the device (depends on VCC), the CKDIV8 Fuse can be

programmed in order to divide the internal frequency by 8.

When this Oscillator is selected, start-up times are determined by the SUT Fuses as shown in the table.

6.5.1 Oscillator Calibration Register – OSCCAL

• Bits 7..0 – CAL7..0: Oscillator Calibration Value

The Oscillator Calibration Register is used to trim the Calibrated Internal RC Oscillator to remove process

variations from the oscillator frequency. The factory-calibrated value is automatically written to this register

during chip reset, giving an oscillator frequency of 8.0MHz at 25°C. The application software can write this

Reserved 111

Table 6-6. Start-up Times for the Low Frequency Crystal Oscillator Clock Select ion

Power Conditions

Start-up Time from

Power-down and

Power-save

Additional Dela y from

Reset

(VCC = 5.0V) CKSEL0 SUT1..0

Table 6-7. Internal Calibrated RC Oscillator Operating Modes

Frequency Range [MHz] CKSEL[3:0]

7.3 - 8.1 0010

Table 6-8. Start-up times for the internal calibrated RC Oscillator clock selection

Power Conditions Start-up Time from Power-

down and Power-save Additional Delay from

Reset (VCC = 5.0V) SUT1..0

BOD enabled 6CK 14CK 00

Fast rising power 6CK 14CK + 4.1ms 01

Slowly rising power 6CK 14CK + 65ms 10

Reserved 11

Bit 76543210

CAL7 CAL6 CAL5 CAL4 CAL3 CAL2 CAL1 CAL0 OSCCAL

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value Device Specific Calibration Value

ATmega16U4/32U4 [DATASHEET]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

With typical process at 25°C the code should be 127 for 8MHz. Input value of 0x00 gives the lowest frequency,

and 0xFF the highest.

The temperature sensitivity is quite linear but as said previously depends on the process. To determine its

slope, the frequency must be measured at two temperatures. The temperature sensor of the device allows such

an operation, that is detailed on “Sensor Calibration” on page 304. It is then possible to calibrate the oscillator

frequency in function of the temperature measured.

Note that this oscillator is used to time EEPROM and Flash write accesses, and these write times will be

affected accordingly. If the EEPROM or Flash are written, do not calibrate to more than 8.8 MHz. Otherwise, the

EEPROM or Flash write may fail.

6.5.2 Oscillator Control Register – RCCTRL

Bits 7..1 – Reserved

Do not set these bits. Bits should be read as ‘0’.

Bit 0– RCFREQ: RC Oscillator Frequency Select

When this bit is cleared (default value), the RC Oscillator output frequency is set to 8MHz. When the bit is set,

the RC output frequency is 1MHz. Note that the OSCCAL value has the same effect on both 8MHz and 1MHz

output modes (~0.4% / step).

6.6 External Clock

The device can utilize a external clock source as shown in Figure 6-3. To run the device on an external clock,

the CKSEL Fuses must be programmed as shown in Table 6-1 on page 28.

Figure 6-3. External Clock Drive Configuration

When this clock source is selected, start-up times are determined by the SUT Fuses as shown in the table

below.

Bit 76543210

- - - - - - - RCFREQ RCCTRL

Read/WriteRRRRRRRR/W

Initial Value00000000

EXTERNAL

CLOCK

SIGNAL

XTAL2

XTAL1

GND

ATmega16U4/32U4 [DATASHEET]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

When applying an external clock, it is required to avoid sudden changes in the applied clock frequency to

ensure stable operation of the MCU. A variation in frequency of more than 2% from one clock cycle to the next

can lead to unpredictable behavior. If changes of more than 2% is required, ensure that the MCU is kept in

Reset during the changes.

Note that the System Clock Prescaler can be used to implement run-time changes of the internal clock

frequency while still ensuring stable operation. Refer to “System Clock Prescaler” on page 35 for details.

6.7 Clock Switch

The device includes a Clock Switch controller, that allows user to switch from one clock source to another one

by software, in order to control application power and execution time with more accuracy.

6.7.1 Example of use

The modification may be needed when the device enters in USB Suspend mode. It then switches from External

Clock to Calibrated RC Oscillator in order to reduce consumption and wake-up delay. In such a configuration,

the External Clock is disabled. The firmware can then use the watchdog timer to be woken-up from power-down

in order to check if there is an event on the application. If an event occurs on the application or if the USB con-

troller signals a non-idle state on the USB line (Resume for example), the firmware switches the Clock

Multiplexer from the Calibrated RC Oscillator to the External Clock. in order to restart USB operation.

This feature can only be used to switch between Calibrated 8MHz RC Oscillator, External Clock and Low Power

Crystal Oscillator. The Low Frequency Crystal Oscillator must not be used with this feature.

Figure 6-4. Example of Clock Switc hing with Wake-up from USB Host

Table 6-9. Start-up Times for the Externa l Cloc k Selection

Power Conditions Start-up Time from Po wer-

down and Power-save Additional Delay from

Reset (VCC = 5.0V) SUT1..0

BOD enabled 6CK 14CK 00

Fast rising power 6CK 14CK + 4.1ms 01

Slowly rising power 6CK 14CK + 65ms 10

Reserved 11

USB

CPU Clock

External

Oscillator

RC oscillator

Ext RC Ext

non-Idle Idle (Suspend) non-Idle

3ms

resume

1Resume from Host

watchdog wake-up

from power-down

ATmega16U4/32U4 [DATASHEET]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 6-5. Example of Clock Swit ching with Wake-up from Device

6.8 Clock Output Buffer

The device can output the system clock on the CLKO pin. To enable the output, the CKOUT Fuse has to be

programmed. This mode is suitable when the chip clock is used to drive other circuits on the system. The clock

also will be output during reset, and the normal operation of I/O pin will be overridden when the fuse is

programmed. Any clock source, including the internal RC Oscillator, can be selected when the clock is output on

CLKO. If the System Clock Prescaler is used, it is the divided system clock that is output.

6.8.1 System Clock P res ca le r

The AVR USB has a system clock prescaler, and the system clock can be divided by setting the “CLKPR –

Clock Prescaler Register” on page 39. This feature can be used to decrease the system clock frequency and

the power consumption when the requirement for processing power is low. This can be used with all clock

source options, and it will affect the clock frequency of the CPU and all synchronous peripherals. clkI/O, clkADC,

clkCPU, and clkFLASH are divided by a factor as shown in Table 6-10 on page 40.

When switching between prescaler settings, the System Clock Prescaler ensures that no glitches occurs in the

clock system. It also ensures that no intermediate frequency is higher than neither the clock frequency

corresponding to the previous setting, nor the clock frequency corresponding to the new setting.

The ripple counter that implements the prescaler runs at the frequency of the undivided clock, which may be

faster than the CPU's clock frequency. Hence, it is not possible to determine the state of the prescaler - even if

it were readable, and the exact time it takes to switch from one clock division to the other cannot be exactly

predicted. From the time the CLKPS values are written, it takes between T1 + T2 and T1 + 2 * T2 before the

new clock frequency is active. In this interval, two active clock edges are produced. Here, T1 is the previous

clock period, and T2 is the period corresponding to the new prescaler setting.

To avoid unintentional changes of clock frequency, a special write procedure must be followed to change the

CLKPS bits:

1. Write the Clock Prescaler Change Enable (CLKPCE) bit to one and all other bits in CLKPR to zero.

2. Within four cycles, write the desired value to CLKPS while writing a zero to CLKPCE.

Interrupts must be disabled when changing prescaler setting to make sure the write procedure is not interrupted.

USB

CPU Clock

Exte rnal

Oscillator

RC oscillator

Ext RC Ext

non-Idle Idle (Suspend) non-Idle

3ms

upstream-resume

2Upstream Resume from device

w atchdog wake-up

from power-down

ATmega16U4/32U4 [DATASHEET]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

6.9 PLL

The PLL is used to generate internal high frequency (up to 96MHz) clock for USB interface and/or High Speed

Timer module, the PLL input is supplied from an external low-frequency clock (the crystal oscillator or external

clock input pin from XTAL1).

6.9.1 Internal PLL

The internal PLL in the device generates a clock frequency between 32MHz and 96MHz from nominally 8MHz

input.

The source of the 8MHz PLL input clock is the output of the internal PLL clock prescaler that generates the

8MHz from the clock source multiplexer output (See “PLL Control and Status Register – PLLCSR” on page 40.

for PLL interface). The PLL prescaler allows a direct connection (8MHz oscillator) or a divide-by-2 stage for a

16MHz clock input.

The PLL output signal enters the PLL Postcaler stage before being distributed to the USB and High Speed

Timer modules. Each of these modules can choose an independent division ratio.

Figure 6-6. PLL Clocking System

6.10 Clock switch Algorithm

6.10.1 Switch from External Clock to RC Clock

if (Usb_suspend_detected()) // if (UDINT.SUSPI == 1)

{Usb_ack_suspend(); // UDINT.SUSPI = 0;

Usb_freeze_clock(); // USBCON.FRZCLK = 1;

Disable_pll(); // PLLCSR.PLLE = 0;

Enable_RC_clock(); // CLKSEL0.RCE = 1;

while (!RC_clock_ready()); // while (CLKSTA.RCON != 1);

Select_RC_clock(); // CLKSEL0.CLKS = 0;

Disable_external_clock(); // CLKSEL0.EXTE = 0;

}

6.10.2 Switch from RC Clock to External Clock

if (Usb_wake_up_detected()) // if (UDINT.WAKEUPI == 1)

{Usb_ack_wake_up(); // UDINT.WAKEUPI = 0;

Enable_external_clock(); // CKSEL0.EXTE = 1;

while (!External_clock_ready()); // while (CLKSTA.EXTON != 1);

8 MHz

RC OSCILLATOR

XTAL1

XTAL2 XTAL

OSCILLATOR

PLL

PLLE

Lock

Detector clkTMR

To System

Clock Prescaler

clk

8MHz

PLL clock

Prescaler

PINDIV

PDIV3..0

clkUSB

/1.5

PLLTM1:0

PLLUSB

PINMUX

CKSEL3:0 PLOCK

ATmega16U4/32U4 [DATASHEET]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Select_external_clock(); // CLKSEL0.CLKS = 1;

Enable_pll(); // PLLCSR.PLLE = 1;

Disable_RC_clock(); // CLKSEL0.RCE = 0;

while (!Pll_ready()); // while (PLLCSR.PLOCK != 1);

Usb_unfreeze_clock(); // USBCON.FRZCLK = 0;

}

ATmega16U4/32U4 [DATASHEET]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

6.11 Register Description

6.11.1 CLKSEL0 – Clock Selection Register 0

• Bit 7-6 – RCSUT[1:0]: SUT for RC oscillator

These two bits are the SUT value for the RC Oscillator. If the RC oscillator is selected by fuse bits, the SUT fuse

are copied into these bits. A firmware change will not have any effect because this additional start-up time is

only used after a reset and not after a clock switch.

• Bit 5-4 – EXSUT[1:0]: SUT for External Clock/ Low Power Crystal Oscillator

These two bits are the SUT value for the External Clock / Low Power Crystal Oscillator. If the External Clock /

Low Power Crystal Oscillator is selected by fuse bits, the SUT fuses are copied into these bits. The firmware

can modify these bits by writing a new value. This value will be used at the next start of the External Clock / Low

Power Crystal Oscillator.

• Bit 3 – RCE: Enable RC Oscillator

The RCE bit must be written to logic one to enable the RC Oscillator. The RCE bit must be written to logic zero

to disable the RC Oscillator.

• Bit 2 – EXTE: Enable External Clock / Low Power Crystal Oscillator

The OSCE bit must be written to logic one to enable External Clock / Low Power Crystal Oscillator. The OSCE

bit must be written to logic zero to disable the External Clock / Low Power Crystal Oscillator.

• Bit 0 – CLKS: Clock Selector

The CLKS bit must be written to logic one to select the External Clock / Low Power Crystal Oscillator as CPU

clock. The CLKS bit must be written to logic zero to select the RC Oscillator as CPU clock. After a reset, the

CLKS bit is set by hardware if the External Clock / Low Power Crystal Oscillator is selected by the fuse bits con-

figuration.

The firmware has to check if the clock is correctly started before selected it.

6.11.2 CLKSEL1 – Clock Selection Register 1

• Bit 7-4 – RCCKSEL[3:0]: CKSEL for RC oscillator

Clock configuration for the RC Oscillator. After a reset, this part of the register is loaded with the 0010b value

that corresponds to the RC oscillator. Modifying this value by firmware before switching to RC oscillator is pro-

hibited because the RC clock will not start.

• Bit 3-0 – EXCKSEL[3:0]: CKSEL for External Clock / Low Power Crystal Oscillator

Clock configuration for the External Clock / Low Power Crystal Oscillator. After a reset, if the External Clock /

Low Power Crystal Oscillator is selected by fuse bits, this part of the register is loaded with the fuse configura-

tion. Firmware can modify it to change the start-up time after the clock switch.

See Table 6-1 on page 28 for EXCKSEL[3:0] configuration. Only Low Power Crystal Oscillator, Calibrated Inter-

nal RC Oscillator, and External Clock modes are allowed.

Bit 7 6 5 4 3210

RCSUT1 RCSUT0 EXSUT1 EXSUT0 RCE EXTE - CLKS CLKSEL0

Read/Write R/W R/W R/W R/W R/W R/W R R/W

Initial Value 0 0 0 0 See Bit Description

Bit7 6 5 4 3210

RCCKSEL3 RCCKSEL2 RCCKSEL1 RCCKSEL0 EXCKSEL3 EXCKSEL2 EXCKSEL1 EXCKSEL0 CLKSEL1

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value0 0 1 0 0000

ATmega16U4/32U4 [DATASHEET]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

6.11.3 CLKSTA – Clock Status Register

• Bit 7-2 - Reserved bits

These bits are reserved and will always read as zero.

• Bit 1 – RCON: RC Oscillator On

This bit is set by hardware to one if the RC Oscillator is running.

This bit is set by hardware to zero if the RC Oscillator is stopped.

• Bit 0 – EXTON: External Clock / Low Power Crystal Oscillator On

This bit is set by hardware to one if the External Clock / Low Power Crystal Oscillator is running.

This bit is set by hardware to zero if the External Clock / Low Power Crystal Oscillator is stopped.

6.11.4 CLKPR – Clock Prescaler Register

• Bit 7 – CLKPCE: Clock Prescaler Change Enable

The CLKPCE bit must be written to logic one to enable change of the CLKPS bits. The CLKPCE bit is only

updated when the other bits in CLKPR are simultaneously written to zero. CLKPCE is cleared by hardware four

cycles after it is written or when CLKPS bits are written. Rewriting the CLKPCE bit within this time-out period

does neither extend the time-out period, nor clear the CLKPCE bit.

• Bits 3..0 – CLKPS[3..0]: Clock Prescaler Select Bits 3 - 0

These bits define the division factor between the selected clock source and the internal system clock. These bits

can be written run-time to vary the clock frequency to suit the application requirements. As the divider divides

the master clock input to the MCU, the speed of all synchronous peripherals is reduced when a division factor is

used. The division factors are given in the table below.

The CKDIV8 Fuse determines the initial value of the CLKPS bits. If CKDIV8 is unprogrammed, the CLKPS bits

will be reset to “0000”. If CKDIV8 is programmed, CLKPS bits are reset to “0011”, giving a division factor of 8 at

start up. This feature should be used if the selected clock source has a higher frequency than the maximum

frequency of the device at the present operating conditions. Note that any value can be written to the CLKPS

bits regardless of the CKDIV8 Fuse setting. The Application software must ensure that a sufficient division factor

is chosen if the selected clock source has a higher frequency than the maximum frequency of the device at the

present operating conditions. The device is shipped with the CKDIV8 Fuse programmed.

Bit 7 6543210

- - - - - - RCON EXTON CLKSTA

Read/WriteR RRRRRRR

Initial Value 0 0 0 0 0

Bit 7 6543210

CLKPCE–––CLKPS3CLKPS2CLKPS1CLKPS0CLKPR

Read/WriteR/WRRRR/WR/WR/WR/W

Initial Value0 000See Bit Description

ATmega16U4/32U4 [DATASHEET]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

6.11.5 PLL Control and Status Register – PLLCSR

• Bit 7:5 – Res: Reserved Bits

These bits are reserved and always read as zero.

• Bit 4 – PINDIV PLL Input Prescaler (1:1, 1:2)

These bits allow to configure the PLL input prescaler to generate the 8MHz input clock for the PLL from either a

8 or 16MHz input.

When using a 8MHz clock source, this bit must be set to 0 before enabling PLL (1:1).

When using a 16MHz clock source, this bit must be set to 1 before enabling PLL (1:2).

• Bit 3:2 – Res: Reserved Bits

These bits are reserved and always read as zero.

Table 6-10. Clock Prescaler Select

CLKPS3 CLKPS2 CLKPS1 CLKPS0 Clock Division Factor

0 0 0 0 1

0 0 0 1 2

0 0 1 0 4

0 0 1 1 8

0 1 0 0 16

0 1 0 1 32

0 1 1 0 64

0 1 1 1 128

1 0 0 0 256

1 0 0 1 Reserved

1 0 1 0 Reserved

1 0 1 1 Reserved

1 1 0 0 Reserved

1 1 0 1 Reserved

1 1 1 0 Reserved

1 1 1 1 Reserved

Bit 76543210

$29 ($29) PINDIV PLLE PLOCK PLLCSR

Read/Write R R R R/W R R R/W R

Initial Value 0 0 0 0 0 0 0 0

ATmega16U4/32U4 [DATASHEET]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

• Bit 1 – PLLE: PLL Enable

When the PLLE is set, the PLL is started. Note that the Calibrated 8MHz Internal RC oscillator is automatically

enabled when the PLLE bit is set and with PINMUX (see PLLFRQ register) is set. The PLL must be disabled

before entering Power down mode in order to stop Internal RC Oscillator and avoid extra-consumption.

• Bit 0 – PLOCK: PLL Lock Detector

When the PLOCK bit is set, the PLL is locked to the reference clock. After the PLL is enabled, it takes about

several ms for the PLL to lock. To clear PLOCK, clear PLLE.

6.11.6 PLL Frequency Control Register – PLLFRQ

• Bit 7– PINMUX: PLL Input Multiplexer

This bit selects the clock input of the PLL:

PINMUX = 0: the PLL input is connected to the PLL Prescaler, that has the Primary System Clock

as source

PINMUX = 1: the PLL input is directly connected to the Internal Calibrated 8MHz RC Oscillator. This

mode allows to work in USB Low Speed mode with no crystal or using a crystal with a value

different of 8/16MHz.

• Bit 6– PLLUSB: PLL Postcaler for USB Peripheral

This bit select the division factor between the PLL output frequency and the USB module input frequency:

PLLUSB = 0: no division, direct connection (if PLL Output = 48MHz)

PLLUSB = 1: PLL Output frequency is divided by two and sent to USB module

(if PLL Output = 96MHz)

• Bit 5:4 – PLLTM1:0: PLL Postcaler for High S peed Timer

These bits codes for the division factor between the PLL Output Frequency and the High Speed Timer input

frequency.

Note that the division factor 1.5 will introduce some jitter in the clock, but keeping the error null since the aver-

age duty cycle is 50%. See Figures 6-7 for more details.

Bit 76543210

$32 PINMUX PLLUSB PLLTM1 PLLTM0 PDIV3 PDIV2 PDIV1 PDIV0 PLLFRQ

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value00000100

PLLTM1 PLLTM0 PLL Postcaler Factor for High-Speed Timer

0 0 0 (Disconnected)

0 1 1

1 0 1.5

1 1 2

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Figure 6-7. PLL Postcaler operation with division factor = 1.5

• Bit 3:0 – PDIV3:0 PLL Lock Frequency

These bits configure the PLL internal VCO clock reference according to the required output frequency value.

The optimal PLL configuration at 5V is: PLL output frequency = 96MHz, divided by 1.5 to generate the 64MHz

High Speed Timer clock, and divided by 2 to generate the 48MHz USB clock.

PDIV3 PDIV2 PDIV1 PDIV0 PLL Output Frequency

0 0 0 0 Not allowed

0 0 0 1 Not allowed

0 0 1 0 Not allowed

0 0 1 1 40MHz

0 1 0 0 48MHz

0 1 0 1 56MHz

0 1 1 0 Not allowed

0 1 1 1 72MHz

1 0 0 0 80MHz

1 0 0 1 88MHz

1 0 1 0 96MHz

1 0 1 1 Not allowed

1 1 0 0 Not allowed

1 1 0 1 Not allowed

1 1 1 0 Not allowed

1 1 1 1 Not allowed

Fi x ---

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7. Power Management and Sleep Modes

Sleep modes enable the application to shut down unused modules in the MCU, thereby saving power. The AVR

provides various sleep modes allowing the user to tailor the power consumption to the application’s

requirements.

To enter any of the five sleep modes, the SE bit in SMCR must be written to logic one and a SLEEP instruction

must be executed. The SM2, SM1, and SM0 bits in the SMCR Register select which sleep mode (Idle, ADC

Noise Reduction, Power-down, Power-save, or Standby) will be activated by the SLEEP instruction.

See Table 7-1 on page 44 for a summary.

If an enabled interrupt occurs while the MCU is in a sleep mode, the MCU wakes up. The MCU is then halted for

four cycles in addition to the start-up time, executes the interrupt routine, and resumes execution from the

instruction following SLEEP. The contents of the Register File and SRAM are unaltered when the device wakes

up from sleep. If a reset occurs during sleep mode, the MCU wakes up and executes from the Reset Vector.

Figure 6-1 on page 27 presents the different clock systems in the ATmega16U4/ATmega32U4, and their

distribution. The figure is helpful in selecting an appropriate sleep mode.

7.1 Idle Mode

When the SM2..0 bits are written to 000, the SLEEP instruction makes the MCU enter Idle mode, stopping the

CPU but allowing the USB, SPI, USART, Analog Comparator, ADC, 2-wire Serial Interface, Timer/Counters,

Watchdog, and the interrupt system to continue operating. This sleep mode basically halts clkCPU and clkFLASH,

while allowing the other clocks to run.

Idle mode enables the MCU to wake up from external triggered interrupts as well as internal ones like the Timer

Overflow and USART Transmit Complete interrupts. If wake-up from the Analog Comparator interrupt is not

required, the Analog Comparator can be powered down by setting the ACD bit in the Analog Comparator

Control and Status Register – ACSR. This will reduce power consumption in Idle mode. If the ADC is enabled, a

conversion starts automatically when this mode is entered.

7.2 ADC Noise Reduction Mode

When the SM2..0 bits are written to 001, the SLEEP instruction makes the MCU enter ADC Noise Reduction

mode, stopping the CPU but allowing the ADC, the external interrupts, 2-wire Serial Interface address match

and the Watchdog to continue operating (if enabled). This sleep mode basically halts clkI/O, clkCPU, and

clkFLASH, while allowing the other clocks to run (including clkUSB).

This improves the noise environment for the ADC, enabling higher resolution measurements. If the ADC is

enabled, a conversion starts automatically when this mode is entered. Apart form the ADC Conversion

Complete interrupt, only an External Reset, a Watchdog System Reset, a Watchdog interrupt, a Brown-out

Reset, a 2-wire serial interface interrupt, an SPM/EEPROM ready interrupt, an external level interrupt on INT6,

an external interrupt on INT3:0 or a pin change interrupt can wake up the MCU from ADC Noise Reduction

mode.

7.3 Power-down Mode

When the SM2..0 bits are written to 010, the SLEEP instruction makes the MCU enter Power-down mode. In

this mode, the external Oscillator is stopped, while the external interrupts, the 2-wire Serial Interface, and the

Watchdog continue operating (if enabled). Only an External Reset, a Watchdog Reset, a Brown-out Reset, 2-

wire Serial Interface address match, an external level interrupt on INT6, an external interrupt on INT3:0, a pin

change interrupt or an asynchronous USB interrupt sources (VBUSTI, WAKEUPI), can wake up the MCU. This

sleep mode basically halts all generated clocks, allowing operation of asynchronous modules only.

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Note that if a level triggered interrupt is used for wake-up from Power-down mode, the changed level must be

held for some time to wake up the MCU. Refer to “External Interrupts” on page 88 for details.

When waking up from Power-down mode, there is a delay from the wake-up condition occurs until the wake-up

becomes effective. This allows the clock to restart and become stable after having been stopped. The wake-up

period is defined by the same CKSEL Fuses that define the Reset Time-out period, as described in “Clock

Sources” on page 28.

7.4 Power-save Mode

When the SM2..0 bits are written to 011, the SLEEP instruction makes the MCU enter Power-save mode. For

compatibility reasons with AT90USB64/128 this mode is still present but since Timer 2 Asynchronous operation

is not present here, this mode is identical to Power-down.

7.5 Standby Mode

When the SM2..0 bits are 110 and an external crystal/resonator clock option is selected, the SLEEP instruction

makes the MCU enter Standby mode. This mode is identical to Power-down with the exception that the

Oscillator is kept running. From Standby mode, the device wakes up in six clock cycles.

7.6 Extended Standby Mode

When the SM2..0 bits are 111 and an external crystal/resonator clock option is selected, the SLEEP instruction

makes the MCU enter Extended Standby mode. For compatibility reasons with AT90USB64/128 this mode is

still present but since Timer 2 Asynchronous operation is not present here, this mode is identical to Standby-

mode.

Notes: 1. Only recommended with external crystal or resonator selected as clock source.

2. For INT6, only level interrupt.

3. Asynchronous USB interrupts are VBUSTI and WAKEUPI.

Table 7-1. Active Clock Domains and Wake-up Sources in the Differen t Sleep Mo des

Active Clock

Domains Oscillators Wake-up Sources

Sleep Mode

clkCPU

clkFLASH

clkIO

clkADC

Main Clock

Source

Enabled

INT6, INT3:0 and

Pin Change

TWI Address

Match

SPM/

EEPROM Ready

ADC

WDT Interrupt

Other I/O

USB Synchronous

Interrupts

USB Asynchronous

Interrupts(3)

Idle X X X X X X X X X X X

ADCNRM X X X(2) X X X X X X

Power-down X(2) X X X

Power-save X(2) X X X

Standby(1) X X(2) X X X

Extended

Standby X X(2) X X X

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7.7 Power Reduction Register

The Power Reduction Register, PRR, provides a method to stop the clock to individual peripherals to reduce

power consumption. The current state of the peripheral is frozen and the I/O registers can not be read or written.

Resources used by the peripheral when stopping the clock will remain occupied, hence the peripheral should in

most cases be disabled before stopping the clock. Waking up a module, which is done by clearing the bit in

PRR, puts the module in the same state as before shutdown.

Module shutdown can be used in Idle mode and Active mode to significantly reduce the overall power

consumption. In all other sleep modes, the clock is already stopped.

7.8 Minimizing Power Consumption

There are several issues to consider when trying to minimize the power consumption in an AVR controlled

system. In general, sleep modes should be used as much as possible, and the sleep mode should be selected

so that as few as possible of the device’s functions are operating. All functions not needed should be disabled.

In particular, the following modules may need special consideration when trying to achieve the lowest possible

power consumption.

7.8.1 Analog to Digital Converter

If enabled, the ADC will be enabled in all sleep modes. To save power, the ADC should be disabled before

entering any sleep mode. When the ADC is turned off and on again, the next conversion will be an extended

conversion. Refer to “Analog to Digital Converter - ADC” on page 297 for details on ADC operation.

7.8.2 A nal og Co mparator

When entering Idle mode, the Analog Comparator should be disabled if not used. When entering ADC Noise

Reduction mode, the Analog Comparator should be disabled. In other sleep modes, the Analog Comparator is

automatically disabled. However, if the Analog Comparator is set up to use the Internal Voltage Reference as

input, the Analog Comparator should be disabled in all sleep modes. Otherwise, the Internal Voltage Reference

will be enabled, independent of sleep mode. Refer to “Analog Comparator” on page 293 for details on how to

configure the Analog Comparator.

7.8.3 Brown-out Dete ct o r

If the Brown-out Detector is not needed by the application, this module should be turned off. If the Brown-out

Detector is enabled by the BODLEVEL Fuses, it will be enabled in all sleep modes, and hence, always consume

power. In the deeper sleep modes, this will contribute significantly to the total current consumption. Refer to

“Brown-out Detection” on page 52 for details on how to configure the Brown-out Detector.

7.8.4 Internal Voltage Reference

The Internal Voltage Reference will be enabled when needed by the Brown-out Detection, the Analog

Comparator or the ADC. If these modules are disabled as described in the sections above, the internal voltage

reference will be disabled and it will not be consuming power. When turned on again, the user must allow the

reference to start up before the output is used. If the reference is kept on in sleep mode, the output can be used

immediately. Refer to “Internal Voltage Reference” on page 54 for details on the start-up time.

7.8.5 Watchdog Timer

If the Watchdog Timer is not needed in the application, the module should be turned off. If the Watchdog Timer

is enabled, it will be enabled in all sleep modes, and hence, always consume power. In the deeper sleep modes,

this will contribute significantly to the total current consumption. Refer to “Interrupts” on page 63 for details on

how to configure the Watchdog Timer.

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7.8.6 Port Pins

When entering a sleep mode, all port pins should be configured to use minimum power. The most important is

then to ensure that no pins drive resistive loads. In sleep modes where both the I/O clock (clkI/O) and the ADC

clock (clkADC) are stopped, the input buffers of the device will be disabled. This ensures that no power is

consumed by the input logic when not needed. In some cases, the input logic is needed for detecting wake-up

conditions, and it will then be enabled. Refer to the section “Digital Input Enable and Sleep Modes” on page 71

for details on which pins are enabled. If the input buffer is enabled and the input signal is left floating or have an

analog signal level close to VCC/2, the input buffer will use excessive power.

For analog input pins, the digital input buffer should be disabled at all times. An analog signal level close to

VCC/2 on an input pin can cause significant current even in active mode. Digital input buffers can be disabled by

writing to the Digital Input Disable Registers (DIDR1 and DIDR0). Refer to “Digital Input Disable Register 1 –

DIDR1” on page 296 and “Digital Input Disable Register 1 – DIDR1” on page 296 for details.

7.8.7 On-chip Debug System

If the On-chip debug system is enabled by the OCDEN Fuse and the chip enters sleep mode, the main clock

source is enabled, and hence, always consumes power. In the deeper sleep modes, this will contribute

significantly to the total current consumption.

There are three alternative ways to disable the OCD system:

Disable the OCDEN Fuse

Disable the JTAGEN Fuse

Write one to the JTD bit in MCUCR

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7.9 Register Description

7.9.1 Sleep Mode Control Register – SMCR

The Sleep Mode Control Register contains control bits for power management.

• Bits 3, 2, 1 – SM2..0: Sleep Mode Select Bits 2, 1, and 0

These bits select between the six available sleep modes as shown in Table 7-2.

Note: 1. Standby modes are only recommended for use with external crystals or resonators.

• Bit 0 – SE: Sleep Enable

The SE bit must be written to logic one to make the MCU enter the sleep mode when the SLEEP instruction is

executed. To avoid the MCU entering the sleep mode unless it is the programmer’s purpose, it is recommended

to write the Sleep Enable (SE) bit to one just before the execution of the SLEEP instruction and to clear it

immediately after waking up.

7.9.2 Power Reduction Register 0 - PRR0

• Bit 7 - PRTWI: Power Reduction TWI

Writing a logic one to this bit shuts down the TWI by stopping the clock to the module. When waking up the TWI

again, the TWI should be re initialized to ensure proper operation.

• Bit 6 - Res: Reserved bit

This bits is reserved and will always read as zero.

Bit 76543210

––––SM2SM1SM0SESMCR

Read/Write R R R R R/W R/W R/W R/W

Initial Value 0 0 0 0 0 0 0 0

Table 7-2. Sleep Mode Select

SM2 SM1 SM0 Sleep Mode

000Idle

001ADC Noise Reduction

010Power-down

011Power-save

100Reserved

101Reserved

110Standby(1)

111Extended Standby(1)

Bit 7654321 0

PRTWI – PRTIM0 – PRTIM1 PRSPI – PRADC PRR0

Read/Write R/W R R/W R R/W R/W R R/W

Initial Val-

0000000 0

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• Bit 5 - PRTIM0: Power Reduction Timer/Counter0

Writing a logic one to this bit shuts down the Timer/Counter0 module. When the Timer/Counter0 is enabled,

operation will continue like before the shutdown.

• Bit 4 - Res: Reserved bit

This bit is reserved and will always read as zero.

• Bit 3 - PRTIM1: Power Reduction Timer/Counter1

Writing a logic one to this bit shuts down the Timer/Counter1 module. When the Timer/Counter1 is enabled,

operation will continue like before the shutdown.

• Bit 2 - PRSPI: Power Reduction Serial Peripheral Interface

Writing a logic one to this bit shuts down the Serial Peripheral Interface by stopping the clock to the module.

When waking up the SPI again, the SPI should be re initialized to ensure proper operation.

• Bit 1 - Res: Reserved bit

These bits are reserved and will always read as zero.

• Bit 0 - PRADC: Power Reduction ADC

Writing a logic one to this bit shuts down the ADC. The ADC must be disabled before shut down. The analog

comparator cannot use the ADC input MUX when the ADC is shut down.

7.9.3 Power Reduction Register 1 - PRR1

• Bit 7 - PRUSB: Power Reduction USB

Writing a logic one to this bit shuts down the USB by stopping the clock to the module. When waking up the USB

again, the USB should be re initialized to ensure proper operation.

• Bit 6..5 - Res: Reserved bits

These bits are reserved and will always read as zero.

• Bit 4- PRTIM4: Power Reduction Timer/Counter4

Writing a logic one to this bit shuts down the Timer/Counter4 module. When the Timer/Counter4 is enabled,

operation will continue like before the shutdown.

• Bit 3 - PRTIM3: Power Reduction Timer/Counter3

Writing a logic one to this bit shuts down the Timer/Counter3 module. When the Timer/Counter3 is enabled,

operation will continue like before the shutdown.

• Bit 2..1 - Res: Reserved bits

These bits are reserved and will always read as zero.

Bit 7 654 3 210

PRUSB – – PRTIM4 PRTIM3 – – PRUSART1 PRR1

Read/Writ

R/W R R R R/W R R R/W

Initial Val-

0 000 0 000

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• Bit 0 - PRUSART1: Power Reduction USART1

Writing a logic one to this bit shuts down the USART1 by stopping the clock to the module. When waking up the

USART1 again, the USART1 should be re initialized to ensure proper operation.

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8. System Control and Reset

8.1 Resetting the AVR

During reset, all I/O Registers are set to their initial values, and the program starts execution from the Reset

Vector. The instruction placed at the Reset Vector must be a JMP – Absolute Jump – instruction to the reset

handling routine. If the program never enables an interrupt source, the Interrupt Vectors are not used, and

regular program code can be placed at these locations. This is also the case if the Reset Vector is in the

Application section while the Interrupt Vectors are in the Boot section or vice versa. The circuit diagram in Figure

8-1 on page 51 shows the reset logic. Table 29-3 on page 386 defines the electrical parameters of the reset

circuitry.

The I/O ports of the AVR are immediately reset to their initial state when a reset source goes active. This does

not require any clock source to be running.

After all reset sources have gone inactive, a delay counter is invoked, stretching the internal reset. This allows

the power to reach a stable level before normal operation starts. The time-out period of the delay counter is

defined by the user through the SUT and CKSEL Fuses. The different selections for the delay period are

presented in “Clock Sources” on page 28.

8.2 Reset Sources

The ATmega16U4/ATmega32U4 has five sources of reset:

Power-on Reset. The MCU is reset when the supply voltage is below the Power-on Reset threshold

(VPOT).

External Reset. The MCU is reset when a low level is present on the RESET pin for longer than the

minimum pulse length.

Watchdog Reset. The MCU is reset when the Watchdog Timer period expires and the Watchdog is

enabled.

Brown-out Reset. The MCU is reset when the supply voltage VCC is below the Brown-out Reset threshold

(VBOT) and the Brown-out Detector is enabled.

JTAG AVR Reset. The MCU is reset as long as there is a logic one in the Reset Register, one of the scan

chains of the JTAG system. Refer to the section “IEEE 1149.1 (JTAG) Boundary-scan” on page 325 for

details.

USB End of Reset. The MCU is reset (excluding the USB controller that remains enabled and attached)

on the detection of a USB End of Reset condition on the bus, if this feature is enabled by the user.

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Figure 8-1. Reset Logic

8.3 Power-on Reset

A Power-on Reset (POR) pulse is generated by an On-chip detection circuit. The detection level is defined in

Table 8-1 on page 53. The POR is activated whenever VCC is below the detection level. The POR circuit can be

used to trigger the start-up Reset, as well as to detect a failure in supply voltage.

A Power-on Reset (POR) circuit ensures that the device is reset from Power-on. Reaching the Power-on Reset

threshold voltage invokes the delay counter, which determines how long the device is kept in RESET after VCC

rise. The RESET signal is activated again, without any delay, when VCC decreases below the detection level.

Figure 8-2. MCU Start-up, RESET Tied to VCC

MCU Status

Brown-out

Reset Circuit

BODLEVEL [2..0]

Delay Counters

CKSEL[3:0]

CK TIMEOUT

WDRF

BORF

EXTRF

PORF

DATA BUS

Clock

Generator

SPIKE

FILTER

Pull-up Resistor

JTRF

JTAG Reset

Watchdog

Oscillator

SUT[1:0]

Power-on Reset

Circuit

USB Reset

Detection

USBRF

RESET

TIME-OUT

INTERNAL

RESET

TOUT

POT

RST

POR

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Figure 8-3. MCU Start-up, RESET Extended Externally

8.4 External Reset

An External Reset is generated by a low level on the RESET pin. Reset pulses longer than the minimum pulse

width (see Table 29-3 on page 386) will generate a reset, even if the clock is not running. Shorter pulses are not

guaranteed to generate a reset. When the applied signal reaches the Reset Threshold Voltage – VRST – on its

positive edge, the delay counter starts the MCU after the Time-out period – tTOUT – has expired.

Figure 8-4. External Rese t Du r i ng Ope ration

8.5 Brown-out Detection

ATmega16U4/ATmega32U4 has an On-chip Brown-out Detection (BOD) circuit for monitoring the VCC level

during operation by comparing it to a fixed trigger level. The trigger level for the BOD can be selected by the

BODLEVEL Fuses. The trigger level has a hysteresis to ensure spike free Brown-out Detection. The hysteresis

on the detection level should be interpreted as VBOT+ = VBOT + VHYST/2 and VBOT- = VBOT - VHYST/2.

RESET

TIME-OUT

INTERNAL

RESET

TOUT

POT

RST

POR

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When the BOD is enabled, and VCC decreases to a value below the trigger level (VBOT- in Figure 8-5), the

Brown-out Reset is immediately activated. When VCC increases above the trigger level (VBOT+ in Figure 8-5), the

delay counter starts the MCU after the Time-out period tTOUT has expired.

The BOD circuit will only detect a drop in VCC if the voltage stays below the trigger level for longer than tBOD

given in Table 29-3 on page 386.

Figure 8-5. Brown-out Reset During Operation

8.6 Watchdog Reset

When the Watchdog times out, it will generate a short reset pulse of one CK cycle duration. On the falling edge

of this pulse, the delay timer starts counting the Time-out period tTOUT. For details on operation of the Watchdog

Timer, see “Watchdog Timer” on page 55.

Table 8-1. BODLEVEL Fuse Coding

BODLEVEL 2..0 Fuses Min. VBOT Typ. VBOT Max. VBOT Units

111 BOD Disabled

110 1.8 2.0 2.2

101 2.0 2.2 2.4

100 2.2 2.4 2.6

011 2.4 2.6 2.8

010 3.2 3.4 3.6

001 3.3 3.5 3.7

000 4.0 4.3 4.5

Table 8-2. BOD characteristic s

Symbol Parameter Min. Typ. Max. Units

VHYST Brown-out Detector Hysteresis 50 mV

tBOD Min Pulse Width on Brown-out Reset ns

VCC

RESET

TIME-OUT

INTERNAL

RESET

VBOT- VBOT+

tTOUT

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Figure 8-6. Watchdog Reset During Operation

8.7 USB Reset

When the USB controller is enabled and configured with the USB Reset CPU feature enabled and if a valid USB

Reset signalling is detected on the bus, the CPU core is reset but the USB controller remains enabled and

attached. This feature may be used to enhance device reliability.

Figure 8-7. USB Reset During Opera tion

8.8 Internal Voltage Reference

ATmega16U4/ATmega32U4 features an internal bandgap reference. This reference is used for Brown-out

Detection, and it can be used as an input to the Analog Comparator or the ADC.

8.8.1 Voltage Reference Enable Signals and Start-up Time

The voltage reference has a start-up time that may influence the way it should be used. The start-up time is

given in Table 8-3 on page 55. To save power, the reference is not always turned on. The reference is on during

the following situations:

1. When the BOD is enabled (by programming the BODLEVEL [2..0] Fuse).

2. When the bandgap reference is connected to the Analog Comparator (by setting the ACBG bit in ACSR).

3. When the ADC is enabled.

Thus, when the BOD is not enabled, after setting the ACBG bit or enabling the ADC, the user must always allow

the reference to start up before the output from the Analog Comparator or ADC is used. To reduce power

USB Traffic USB Traffic

(USB Lines)

USBRSTMIN

End of Reset

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consumption in Power-down mode, the user can avoid the three conditions above to ensure that the reference

is turned off before entering Power-down mode.

8.9 Watchdog Timer

ATmega16U4/ATmega32U4 has an Enhanced Watchdog Timer (WDT). The main features are:

•Clocked from separate On-chip Oscillator

•Three Oper ating modes

– Interrupt

– System Reset

– Interrupt and System Reset

•Selectable Time-out period from 16ms to 8s

•Possible Hardware fuse Watchdog always on (WDTON) for fail-safe mode

Figure 8-8. Watchdog Timer

The Watchdog Timer (WDT) is a timer counting cycles of a separate on-chip 128kHz oscillator. The WDT gives

an interrupt or a system reset when the counter reaches a given time-out value. In normal operation mode, it is

required that the system uses the WDR - Watchdog Timer Reset - instruction to restart the counter before the

time-out value is reached. If the system doesn't restart the counter, an interrupt or system reset will be issued.

In Interrupt mode, the WDT gives an interrupt when the timer expires. This interrupt can be used to wake the

device from sleep-modes, and also as a general system timer. One example is to limit the maximum time

allowed for certain operations, giving an interrupt when the operation has run longer than expected. In System

Reset mode, the WDT gives a reset when the timer expires. This is typically used to prevent system hang-up in

Table 8-3. Internal Voltage Reference Characteristics

Symbol Parameter Condition Min. Typ. Max. Units

VBG Bandgap reference voltage VCC=2.7

TA=25°C 1.0 1.1 1.2 V

tBG Bandgap reference start-up time VCC=2.7

TA=25°C 40 70 µs

IBG Bandgap reference current consumption VCC=2.7

TA=25°C 10 µA

128kHz

OSCILLATOR

OSC/2K

OSC/4K

OSC/8K

OSC/16K

OSC/32K

OSC/64K

OSC/128K

OSC/256K

OSC/512K

OSC/1024K

WDP0

WDP1

WDP2

WDP3

WATCHDOG

RESET

WDE

WDIF

WDIE

MCU RESET

INTERRUPT

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case of runaway code. The third mode, Interrupt and System Reset mode, combines the other two modes by

first giving an interrupt and then switch to System Reset mode. This mode will for instance allow a safe

shutdown by saving critical parameters before a system reset.

The Watchdog always on (WDTON) fuse, if programmed, will force the Watchdog Timer to System Reset mode.

With the fuse programmed the System Reset mode bit (WDE) and Interrupt mode bit (WDIE) are locked to 1

and 0 respectively. To further ensure program security, alterations to the Watchdog set-up must follow timed

sequences. The sequence for clearing WDE and changing time-out configuration is as follows:

1. In the same operation, write a logic one to the Watchdog change enable bit (WDCE) and WDE. A logic

one must be written to WDE regardless of the previous value of the WDE bit.

2. Within the next four clock cycles, write the WDE and Watchdog prescaler bits (WDP) as desired, but with

the WDCE bit cleared. This must be done in one operation.

The following code example shows one assembly and one C function for turning off the Watchdog Timer. The

example assumes that interrupts are controlled (e.g. by disabling interrupts globally) so that no interrupts will

occur during the execution of these functions.

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Note: 1. The example code assumes that the part specific header file is included.

Note: If the Watchdog is accidentally enabled, for example by a runaway pointer or brown-out condition, the

device will be reset and the Watchdog Timer will stay enabled. If the code is not set up to handle the Watchdog,

this might lead to an eternal loop of time-out resets. To avoid this situation, the application software should

always clear the Watchdog System Reset Flag (WDRF) and the WDE control bit in the initialization routine,

even if the Watchdog is not in use.

The following code example shows one assembly and one C function for changing the time-out value of the

Watchdog Timer.

Assembly Code Example(1)

WDT_off:

; Turn off global interrupt

cli

; Reset Watchdog Timer

wdr

; Clear WDRF in MCUSR

in r16, MCUSR

andi r16, (0xff & (0<<WDRF))

out MCUSR, r16

; Write logical one to WDCE and WDE

; Keep old prescaler setting to prevent

unintentional time-out

in r16, WDTCSR

ori r16, (1<<WDCE) | (1<<WDE)

out WDTCSR, r16

; Turn off WDT

ldi r16, (0<<WDE)

out WDTCSR, r16

; Turn on global interrupt

sei

ret

C Code Example(1)

void WDT_off(void)

{__disable_interrupt();

__watchdog_reset();

/* Clear WDRF in MCUSR */

MCUSR &= ~(1<<WDRF);

/* Write logical one to WDCE and WDE */

/* Keep old prescaler setting to prevent

unintentional time-out */

WDTCSR |= (1<<WDCE) | (1<<WDE);

/* Turn off WDT */

WDTCSR = 0x00;

__enable_interrupt();

}

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Note: 1. The example code assumes that the part specific header file is included.

Note: The Watchdog Timer should be reset before any change of the WDP bits, since a change in the WDP bits

can result in a time-out when switching to a shorter time-out period.

Assembly Code Example(1)

WDT_Prescaler_Change:

; Turn off global interrupt

cli

; Reset Watchdog Timer

wdr

; Start timed sequence

in r16, WDTCSR

ori r16, (1<<WDCE) | (1<<WDE)

out WDTCSR, r16

; -- Got four cycles to set the new values from

here - ; Set new prescaler(time-out) value = 64K cycles

(~0.5 s)

ldi r16, (1<<WDE) | (1<<WDP2) | (1<<WDP0)

out WDTCSR, r16

; -- Finished setting new values, used 2 cycles -

; Turn on global interrupt

sei

ret

C Code Example(1)

void WDT_Prescaler_Change(void)

{__disable_interrupt();

__watchdog_reset();

/* Start timed sequence */

WDTCSR |= (1<<WDCE) | (1<<WDE);

/* Set new prescaler(time-out) value = 64K cycles

(~0.5 s) */

WDTCSR = (1<<WDE) | (1<<WDP2) | (1<<WDP0);

__enable_interrupt();

}

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8.10 Register Description

8.11 MCU Status Register – MCUSR

The MCU Status Register provides information on which reset source caused an MCU reset.

• Bit 7..6 - Reserved

These bits are reserved and should be read as 0. Do not set these bits.

• Bit 5– USBRF: USB Reset Flag

This bit is set if a reset is being caused by a logic one in the JTAG Reset Register selected by the JTAG

instruction AVR_RESET. This bit is reset by a Power-on Reset, or by writing a logic zero to the flag.

• Bit 4 – JTRF: JTAG Reset Flag

This bit is set if a reset is being caused by a logic one in the JTAG Reset Register selected by the JTAG

instruction AVR_RESET. This bit is reset by a Power-on Reset, or by writing a logic zero to the flag.

• Bit 3 – WDRF: Watchdog Reset Flag

This bit is set if a Watchdog Reset occurs. The bit is reset by a Power-on Reset, or by writing a logic zero to the

flag.

• Bit 2 – BORF: Brown-out Reset Flag

This bit is set if a Brown-out Reset occurs. The bit is reset by a Power-on Reset, or by writing a logic zero to the

flag.

• Bit 1 – EXTRF: External Reset Flag

This bit is set if an External Reset occurs. The bit is reset by a Power-on Reset, or by writing a logic zero to the

flag.

• Bit 0 – PORF: Power-on Reset Flag

This bit is set if a Power-on Reset occurs. The bit is reset only by writing a logic zero to the flag.

To make use of the Reset Flags to identify a reset condition, the user should read and then Reset the MCUSR

as early as possible in the program. If the register is cleared before another reset occurs, the source of the reset

can be found by examining the Reset Flags.

Bit 76543210

–– USBRF JTRF WDRF BORF EXTRF PORF MCUSR

Read/Write R R R R/W R/W R/W R/W R/W

Initial Value 0 0 0 See Bit Description

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8.11.1 Watchdog Timer Control Register - WDTCSR

• Bit 7 - WDIF: Watchdog Interrupt Flag

This bit is set when a time-out occurs in the Watchdog Timer and the Watchdog Timer is configured for interrupt.

WDIF is cleared by hardware when executing the corresponding interrupt handling vector. Alternatively, WDIF is

cleared by writing a logic one to the flag. When the I-bit in SREG and WDIE are set, the Watchdog Time-out

Interrupt is executed.

• Bit 6 - WDIE: Watchdog Interrupt Enable

When this bit is written to one and the I-bit in the Status Register is set, the Watchdog Interrupt is enabled. If

WDE is cleared in combination with this setting, the Watchdog Timer is in Interrupt Mode, and the

corresponding interrupt is executed if time-out in the Watchdog Timer occurs.

If WDE is set, the Watchdog Timer is in Interrupt and System Reset Mode. The first time-out in the Watchdog

Timer will set WDIF. Executing the corresponding interrupt vector will clear WDIE and WDIF automatically by

hardware (the Watchdog goes to System Reset Mode). This is useful for keeping the Watchdog Timer security

while using the interrupt. To stay in Interrupt and System Reset Mode, WDIE must be set after each interrupt.

This should however not be done within the interrupt service routine itself, as this might compromise the safety-

function of the Watchdog System Reset mode. If the interrupt is not executed before the next time-out, a System

Reset will be applied.

• Bit 4 - WDCE: Watchdog Change Enable

This bit is used in timed sequences for changing WDE and prescaler bits. To clear the WDE bit, and/or change

the prescaler bits, WDCE must be set.

Once written to one, hardware will clear WDCE after four clock cycles.

• Bit 3 - WDE: Watchdog System Reset Enable

WDE is overridden by WDRF in MCUSR. This means that WDE is always set when WDRF is set. To clear

WDE, WDRF must be cleared first. This feature ensures multiple resets during conditions causing failure, and a

safe start-up after the failure.

Bit 76543210

WDIF WDIE WDP3 WDCE WDE WDP2 WDP1 WDP0 WDTCSR

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value 0 0 0 0 X 0 0 0

Table 8-4. Watchdog Timer Configuratio n

WDTON WDE WDIE Mode Action on Time-out

0 0 0 Stopped None

0 0 1 Interrupt Mode Interrupt

0 1 0 System Reset Mode Reset

0 1 1 Interrupt and System Reset

Mode

Interrupt, then go to System

Reset Mode

1 x x System Reset Mode Reset

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• Bit 5, 2..0 - WDP3..0: Watchdog Timer Presc aler 3, 2, 1, and 0

The WDP3..0 bits determine the Watchdog Timer prescaling when the Watchdog Timer is running. The different

prescaling values and their corresponding time-out periods are shown in the following table.

Table 8-5. Watchdog Timer Prescale Select

WDP3 WDP2 WDP1 WDP0 Number of WDT Oscillator

Cycles Typical Time-out at

VCC = 5.0V

0 0 0 0 2K (2048) cycles 16ms

0 0 0 1 4K (4096) cycles 32ms

0 0 1 0 8K (8192) cycles 64ms

0 0 1 1 16K (16384) cycles 0.125s

0 1 0 0 32K (32768) cycles 0.25s

0 1 0 1 64K (65536) cycles 0.5s

0 1 1 0 128K (131072) cycles 1.0s

0 1 1 1 256K (262144) cycles 2.0s

1 0 0 0 512K (524288) cycles 4.0s

1 0 0 1 1024K (1048576) cycles 8.0s

1 0 1 0

Reserved

1 0 1 1

1 1 0 0

1 1 0 1

1 1 1 0

1 1 1 1

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ATmega16U4/32U4 [DATASHEET]

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9. Interrupts

This chapter describes the specifics of the interrupt handling as performed in ATmega16U4/ATmega32U4. For

a general explanation of the AVR interrupt handling, refer to “Reset and Interrupt Handling” on page 15.

9.1 Interrupt Vectors in ATmega16U4/ATmega32U4

Table 9-1. Reset and Interrupt Vectors(cont’d)

Vector

No. Program

Address(2) Source Interrupt Definition

1$0000(1) RESET External Pin, Power-on Reset, Brown-out Reset, Watchdog

Reset, and JTAG AVR Reset

2$0002 INT0 External Interrupt Request 0

3$0004 INT1 External Interrupt Request 1

4$0006 INT2 External Interrupt Request 2

5$0008 INT3 External Interrupt Request 3

6$000A Reserved Reserved

7$000C Reserved Reserved

8$000E INT6 External Interrupt Request 6

9$0010 Reserved Reserved

10 $0012 PCINT0 Pin Change Interrupt Request 0

11 $0014 USB General USB General Interrupt request

12 $0016 USB Endpoint USB Endpoint Interrupt request

13 $0018 WDT Watchdog Time-out Interrupt

14 $001A Reserved Reserved

15 $001C Reserved Reserved

16 $001E Reserved Reserved

17 $0020 TIMER1 CAPT Timer/Counter1 Capture Event

18 $0022 TIMER1 COMPA Timer/Counter1 Compare Match A

19 $0024 TIMER1 COMPB Timer/Counter1 Compare Match B

20 $0026 TIMER1 COMPC Timer/Counter1 Compare Match C

21 $0028 TIMER1 OVF Timer/Counter1 Overflow

22 $002A TIMER0 COMPA Timer/Counter0 Compare Match A

23 $002C TIMER0 COMPB Timer/Counter0 Compare match B

24 $002E TIMER0 OVF Timer/Counter0 Overflow

25 $0030 SPI (STC) SPI Serial Transfer Complete

26 $0032 USART1 RX USART1 Rx Complete

27 $0034 USART1 UDRE USART1 Data Register Empty

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Notes: 1. When the BOOTRST Fuse is programmed, the device will jump to the Boot Loader address at reset, see

“Memory Programming” on page 353.

2. When the IVSEL bit in MCUCR is set, Interrupt Vectors will be moved to the start of the Boot Flash Section.

The address of each Interrupt Vector will then be the address in this table added to the start address of the

Boot Flash Section.

The table shows reset and Interrupt Vectors placement for the various combinations of BOOTRST and IVSEL

settings. If the program never enables an interrupt source, the Interrupt Vectors are not used, and regular

program code can be placed at these locations. This is also the case if the Reset Vector is in the Application

section while the Interrupt Vectors are in the Boot section or vice versa.

Note: 1. The Boot Reset Address is shown in Table 27-4 on page 340. For the BOOTRST Fuse “1” means

unprogrammed while “0” means programmed.

9.1.1 Moving Interrupts Between Application and Boot Space

The General Interrupt Control Register controls the placement of the Interrupt Vector table.

28 $0036 USART1TX USART1 Tx Complete

29 $0038 ANALOG COMP Analog Comparator

30 $003A ADC ADC Conversion Complete

31 $003C EE READY EEPROM Ready

32 $003E TIMER3 CAPT Timer/Counter3 Capture Event

33 $0040 TIMER3 COMPA Timer/Counter3 Compare Match A

34 $0042 TIMER3 COMPB Timer/Counter3 Compare Match B

35 $0044 TIMER3 COMPC Timer/Counter3 Compare Match C

36 $0046 TIMER3 OVF Timer/Counter3 Overflow

37 $0048 TWI 2-wire Serial Interface

38 $004A SPM READY Store Program Memory Ready

39 $004C TIMER4 COMPA Timer/Counter4 Compare Match A

40 $004E TIMER4 COMPB Timer/Counter4 Compare Match B

41 $0050 TIMER4 COMPD Timer/Counter4 Compare Match D

42 $0052 TIMER4 OVF Timer/Counter4 Overflow

43 $0054 TIMER4 FPF Timer/Counter4 Fault Protection Interrupt

Table 9-2. Reset and Interrupt Vectors Placement

BOOTRST IVSEL Reset Address Interrupt Vectors Start Address

1 0 0x0000 0x0002

1 1 0x0000 Boot Reset Address + 0x0002

0 0 Boot Reset Address 0x0002

0 1 Boot Reset Address Boot Reset Address + 0x0002

Table 9-1. Reset and Interrupt Vectors(cont’d)

Vector

No. Program

Address(2) Source Interrupt Definition

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9.2 Register Description

9.2.1 MCU Control Register – MCUCR

• Bit 1 – IVSEL: Interrupt Vector Select

When the IVSEL bit is cleared (zero), the Interrupt Vectors are placed at the start of the Flash memory. When

this bit is set (one), the Interrupt Vectors are moved to the beginning of the Boot Loader section of the Flash.

The actual address of the start of the Boot Flash Section is determined by the BOOTSZ Fuses. Refer to the

section “Memory Programming” on page 353 for details. To avoid unintentional changes of Interrupt Vector

tables, a special write procedure must be followed to change the IVSEL bit:

1. Write the Interrupt Vector Change Enable (IVCE) bit to one.

2. Within four cycles, write the desired value to IVSEL while writing a zero to IVCE.

Interrupts will automatically be disabled while this sequence is executed. Interrupts are disabled in the cycle

IVCE is set, and they remain disabled until after the instruction following the write to IVSEL. If IVSEL is not

written, interrupts remain disabled for four cycles. The I-bit in the Status Register is unaffected by the automatic

disabling.

Note: If Interrupt Vectors are placed in the Boot Loader section and Boot Lock bit BLB02 is programmed, interrupts are

disabled while executing from the Application section. If Interrupt Vectors are placed in the Application section and

Boot Lock bit BLB12 is programed, interrupts are disabled while executing from the Boot Loader section. Refer to

the section “Memory Programming” on page 353 for details on Boot Lock bits.

• Bit 0 – IVCE: Interrupt Vector Change Enable

The IVCE bit must be written to logic one to enable change of the IVSEL bit. IVCE is cleared by hardware four

cycles after it is written or when IVSEL is written. Setting the IVCE bit will disable interrupts, as explained in the

IVSEL description above. See Code Example below.

Bit 76543210

JTD – – PUD –– IVSEL IVCE MCUCR

Read/Write R/W R R R/W R R R/W R/W

Initial Value 0 0 0 0 0 0 0 0

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Assembly Code Example

Move_interrupts:

; Enable change of Interrupt Vectors

ldi r16, (1<<IVCE)

out MCUCR, r16

; Move interrupts to Boot Flash section

ldi r16, (1<<IVSEL)

out MCUCR, r16

ret

C Code Example

void Move_interrupts(void)

{/* Enable change of Interrupt Vectors */

MCUCR = (1<<IVCE);

/* Move interrupts to Boot Flash section */

MCUCR = (1<<IVSEL);

}

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10. I/O-Ports

10.1 Introduction

All AVR ports have true Read-Modify-Write functionality when used as general digital I/O ports. This means that

the direction of one port pin can be changed without unintentionally changing the direction of any other pin with

the SBI and CBI instructions. The same applies when changing drive value (if configured as output) or

enabling/disabling of pull-up resistors (if configured as input). Each output buffer has symmetrical drive

characteristics with both high sink and source capability. The pin driver is strong enough to drive LED displays

directly. All port pins have individually selectable pull-up resistors with a supply-voltage invariant resistance. All

I/O pins have protection diodes to both VCC and Ground as indicated in Figure 10-1. Refer to “Electrical

Characteristics” on page 383 for a complete list of parameters.

Figure 10-1. I/O Pin Equivalent Schematic

All registers and bit references in this section are written in general form. A lower case “x” represents the

numbering letter for the port, and a lower case “n” represents the bit number. However, when using the register

or bit defines in a program, the precise form must be used. For example, PORTB3 for bit no. 3 in Port B, here

documented generally as PORTxn. The physical I/O Registers and bit locations are listed in “” on page 83.

Three I/O memory address locations are allocated for each port, one each for the Data Register – PORTx, Data

Direction Register – DDRx, and the Port Input Pins – PINx. The Port Input Pins I/O location is read only, while

the Data Register and the Data Direction Register are read/write. However, writing a logic one to a bit in the

PINx Register, will result in a toggle in the corresponding bit in the Data Register. In addition, the Pull-up Disable

– PUD bit in MCUCR disables the pull-up function for all pins in all ports when set.

Using the I/O port as General Digital I/O is described in “Ports as General Digital I/O” on page 67. Most port pins

are multiplexed with alternate functions for the peripheral features on the device. How each alternate function

interferes with the port pin is described in “Alternate Port Functions” on page 72. Refer to the individual module

sections for a full description of the alternate functions.

Note that enabling the alternate function of some of the port pins does not affect the use of the other pins in the

port as general digital I/O.

10.2 Ports as General Digital I/O

The ports are bi-directional I/O ports with optional internal pull-ups. Figure 10-2 shows a functional description of

one I/O-port pin, here generically called Pxn.

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Figure 10-2. General Digital I/O(1)

Note: 1. WRx, WPx, WDx, RRx, RPx, and RDx are common to all pins within the same port. clkI/O, SLEEP, and PUD

are common to all ports.

10.2.1 Configuring the Pin

Each port pin consists of three register bits: DDxn, PORTxn, and PINxn. As shown in “” on page 83, the DDxn

bits are accessed at the DDRx I/O address, the PORTxn bits at the PORTx I/O address, and the PINxn bits at

the PINx I/O address.

The DDxn bit in the DDRx Register selects the direction of this pin. If DDxn is written logic one, Pxn is

configured as an output pin. If DDxn is written logic zero, Pxn is configured as an input pin.

If PORTxn is written logic one when the pin is configured as an input pin, the pull-up resistor is activated. To

switch the pull-up resistor off, PORTxn has to be written logic zero or the pin has to be configured as an output

pin. The port pins are tri-stated when reset condition becomes active, even if no clocks are running.

If PORTxn is written logic one when the pin is configured as an output pin, the port pin is driven high (one). If

PORTxn is written logic zero when the pin is configured as an output pin, the port pin is driven low (zero).

10.2.2 Toggling the Pin

Writing a logic one to PINxn toggles the value of PORTxn, independent on the value of DDRxn. Note that the

SBI instruction can be used to toggle one single bit in a port.

10.2.3 Switching Between Input and Output

When switching between tri-state ({DDxn, PORTxn} = 0b00) and output high ({DDxn, PORTxn} = 0b11), an

intermediate state with either pull-up enabled {DDxn, PORTxn} = 0b01) or output low ({DDxn, PORTxn} = 0b10)

occurs. Normally, the pull-up enabled state is fully acceptable, as a high-impedance environment will not notice

clk

RPx

RRx

RDx

WDx

PUD

SYNCHRONIZER

WDx: WRITE DDRx

WRx: WRITE PORTx

RRx: READ PORTx REGISTER

RPx: READ PORTx PIN

PUD: PULLUP DISABLE

clkI/O: I/O CLOCK

RDx: READ DDRx

RESET

CLR

PORTxn

CLR

DDxn

PINxn

DATA BUS

SLEEP

SLEEP: SLEEP CONTROL

Pxn

I/O

WPx

WRx

WPx: WRITE PINx REGISTER

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the difference between a strong high driver and a pull-up. If this is not the case, the PUD bit in the MCUCR

Switching between input with pull-up and output low generates the same problem. The user must use either the

tri-state ({DDxn, PORTxn} = 0b00) or the output high state ({DDxn, PORTxn} = 0b11) as an intermediate step.

Table 10-1 summarizes the control signals for the pin value

10.2.4 Reading the Pin Value

Independent of the setting of Data Direction bit DDxn, the port pin can be read through the PINxn Register bit.

As shown in Figure 10-2 on page 68, the PINxn Register bit and the preceding latch constitute a synchronizer.

This is needed to avoid metastability if the physical pin changes value near the edge of the internal clock, but it

also introduces a delay. Figure 10-3 on page 69 shows a timing diagram of the synchronization when reading an

externally applied pin value. The maximum and minimum propagation delays are denoted tpd,max and tpd,min

respectively.

Figure 10-3. Synchronization when Reading an Externally Applied Pin Value

Consider the clock period starting shortly after the first falling edge of the system clock. The latch is closed when

the clock is low, and goes transparent when the clock is high, as indicated by the shaded region of the “SYNC

LATCH” signal. The signal value is latched when the system clock goes low. It is clocked into the PINxn

signal transition on the pin will be delayed between ½ and 1½ system clock period depending upon the time of

assertion.

Table 10-1. Port Pin Configurations

DDxn PORTxn PUD

(in MCUCR) I/O Pull-up Comment

0 0 X Input No Tri-state (Hi-Z)

0 1 0 Input Yes Pxn will source current if ext. pulled low

0 1 1 Input No Tri-state (Hi-Z)

1 0 X Output No Output Low (Sink)

1 1 X Output No Output High (Source)

XXX in r17, PINx

0x00 0xFF

INSTRUCTIONS

SYNC LATCH

PINxn

r17

XXX

SYSTEM CLK

pd, max

pd, min

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When reading back a software assigned pin value, a nop instruction must be inserted as indicated in Figure 10-

4. The out instruction sets the “SYNC LATCH” signal at the positive edge of the clock. In this case, the delay tpd

through the synchronizer is one system clock period.

Figure 10-4. Synchronization when Reading a Software Assigned Pin Value

The following code example shows how to set port B pins 0 and 1 high, 2 and 3 low, and define the port pins

from 4 to 7 as input with pull-ups assigned to port pins 6 and 7. The resulting pin values are read back again, but

as previously discussed, a nop instruction is included to be able to read back the value recently assigned to

some of the pins.

out PORTx, r16 nop in r17, PINx

0xFF

0x00 0xFF

SYSTEM CLK

r16

INSTRUCTIONS

SYNC LATCH

PINxn

r17

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Note: 1. For the assembly program, two temporary registers are used to minimize the time from pull-ups are set on pins

0, 1, 6, and 7, until the direction bits are correctly set, defining bit 2 and 3 as low and redefining bits 0 and 1 as

strong high drivers.

10.2.5 Digital Input Enable and Sleep Modes

As shown in Figure 10-2, the digital input signal can be clamped to ground at the input of the Schmidt-trigger.

The signal denoted SLEEP in the figure, is set by the MCU Sleep Controller in Power-down mode, Power-save

mode, and Standby mode to avoid high power consumption if some input signals are left floating, or have an

analog signal level close to VCC/2.

SLEEP is overridden for port pins enabled as external interrupt pins. If the external interrupt request is not

enabled, SLEEP is active also for these pins. SLEEP is also overridden by various other alternate functions as

described in “Alternate Port Functions” on page 72.

If a logic high level (“one”) is present on an asynchronous external interrupt pin configured as “Interrupt on

Rising Edge, Falling Edge, or Any Logic Change on Pin” while the external interrupt is not enabled, the

corresponding External Interrupt Flag will be set when resuming from the above mentioned Sleep mode, as the

clamping in these sleep mode produces the requested logic change.

10.2.6 Unconnected Pins

If some pins are unused, it is recommended to ensure that these pins have a defined level. Even though most of

the digital inputs are disabled in the deep sleep modes as described above, floating inputs should be avoided to

Assembly Code Example(1)

...

; Define pull-ups and set outputs high

; Define directions for port pins

ldi

r16,(1<<PB7)|(1<<PB6)|(1<<PB1)|(1<<PB0)

ldi

r17,(1<<DDB3)|(1<<DDB2)|(1<<DDB1)|(1<<DDB0)

out PORTB,r16

out DDRB,r17

; Insert nop for synchronization

nop

; Read port pins

in r16,PINB

...

C Code Example

unsigned char i;

...

/* Define pull-ups and set outputs high */

/* Define directions for port pins */

PORTB = (1<<PB7)|(1<<PB6)|(1<<PB1)|(1<<PB0);

DDRB = (1<<DDB3)|(1<<DDB2)|(1<<DDB1)|(1<<DDB0);

/* Insert nop for synchronization*/

__no_operation();

/* Read port pins */

i = PINB;

...

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reduce current consumption in all other modes where the digital inputs are enabled (Reset, Active mode, and

Idle mode).

The simplest method to ensure a defined level of an unused pin, is to enable the internal pull-up. In this case,

the pull-up will be disabled during reset. If low power consumption during reset is important, it is recommended

to use an external pull-up or pull-down. Connecting unused pins directly to VCC or GND is not recommended,

since this may cause excessive currents if the pin is accidentally configured as an output.

10.3 Alternate Port Functions

Most port pins have alternate functions in addition to being general digital I/Os. Figure 10-5 shows how the port

pin control signals from the simplified Figure 10-2 on page 68 can be overridden by alternate functions. The

overriding signals may not be present in all port pins, but the figure serves as a generic description applicable to

all port pins in the AVR microcontroller family.

Figure 10-5. Alternate Port Functions(1)

Note: 1. WRx, WPx, WDx, RRx, RPx, and RDx are common to all pins within the same port. clkI/O, SLEEP, and PUD

are common to all ports. All other signals are unique for each pin.

The table summarizes the function of the overriding signals. The pin and port indexes from Figure 10-5 on page

72 are not shown in the succeeding tables. The overriding signals are generated internally in the modules

having the alternate function.

clk

RPx

RRx

WRx

RDx

WDx

PUD

SYNCHRONIZER

WDx: WRITE DDRx

WRx: WRITE PORTx

RRx: READ PORTx REGISTER

RPx: READ PORTx PIN

PUD: PULLUP DISABLE

clk

I/O

: I/O CLOCK

RDx: READ DDRx

SET

CLR

DIxn

AIOxn

DIEOExn

PVOVxn

PVOExn

DDOVxn

DDOExn

PUOExn

PUOVxn

PUOExn: Pxn PULL-UP OVERRIDE ENABLE

PUOVxn: Pxn PULL-UP OVERRIDE VALUE

DDOExn: Pxn DATA DIRECTION OVERRIDE ENABLE

DDOVxn: Pxn DATA DIRECTION OVERRIDE VALUE

PVOExn: Pxn PORT VALUE OVERRIDE ENABLE

PVOVxn: Pxn PORT VALUE OVERRIDE VALUE

DIxn: DIGITAL INPUT PIN n ON PORTx

AIOxn: ANALOG INPUT/OUTPUT PIN n ON PORTx

RESET

CLR

PINxn

PORTxn

DDxn

DATA BUS

DIEOVxn

SLEEP

DIEOExn: Pxn DIGITAL INPUT-ENABLE OVERRIDE ENABLE

DIEOVxn: Pxn DIGITAL INPUT-ENABLE OVERRIDE VALUE

SLEEP: SLEEP CONTROL

Pxn

I/O

PTOExn

PTOExn: Pxn, PORT TOGGLE OVERRIDE ENABLE

WPx: WRITE PINx

WPx

ATmega16U4/32U4 [DATASHEET]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

The following subsections shortly describe the alternate functions for each port, and relate the overriding signals

to the alternate function. Refer to the alternate function description for further details.

Table 10-2. Generic Description of Overriding Signals for Alternate Functio ns

Signal Name Full Name Description

PUOE Pull-up Override

Enable

If this signal is set, the pull-up enable is controlled by the PUOV

signal. If this signal is cleared, the pull-up is enabled when {DDxn,

PORTxn, PUD} = 0b010.

PUOV Pull-up Override

Value

If PUOE is set, the pull-up is enabled/disabled when PUOV is

set/cleared, regardless of the setting of the DDxn, PORTxn, and

PUD Register bits.

DDOE Data Direction

Override Enable

If this signal is set, the Output Driver Enable is controlled by the

DDOV signal. If this signal is cleared, the Output driver is enabled

by the DDxn Register bit.

DDOV Data Direction

Override Value

If DDOE is set, the Output Driver is enabled/disabled when DDOV

is set/cleared, regardless of the setting of the DDxn Register bit.

PVOE Port Value

Override Enable

If this signal is set and the Output Driver is enabled, the port value

is controlled by the PVOV signal. If PVOE is cleared, and the

Output Driver is enabled, the port Value is controlled by the

PORTxn Register bit.

PVOV Port Value

Override Value

If PVOE is set, the port value is set to PVOV, regardless of the

setting of the PORTxn Register bit.

PTOE Port Toggle

Override Enable If PTOE is set, the PORTxn Register bit is inverted.

DIEOE

Digital Input

Enable Override

Enable

If this bit is set, the Digital Input Enable is controlled by the DIEOV

signal. If this signal is cleared, the Digital Input Enable is

determined by MCU state (Normal mode, sleep mode).

DIEOV

Digital Input

Enable Override

Value

If DIEOE is set, the Digital Input is enabled/disabled when DIEOV

is set/cleared, regardless of the MCU state (Normal mode, sleep

mode).

DI Digital Input

This is the Digital Input to alternate functions. In the figure, the

signal is connected to the output of the schmitt trigger but before

the synchronizer. Unless the Digital Input is used as a clock

source, the module with the alternate function will use its own

synchronizer.

AIO Analog

Input/Output

This is the Analog Input/output to/from alternate functions. The

signal is connected directly to the pad, and can be used bi-

directionally.

ATmega16U4/32U4 [DATASHEET]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

10.3.1 Alternat e Functions of Port B

The Port B pins with alternate functions are shown below.

The alternate pin configuration is as follows:

• OC0A/OC1C/PCINT7/RTS, Bit 7

OC0A, Output Compare Match A output: The PB7 pin can serve as an external output for the Timer/Counter0

Output Compare. The pin has to be configured as an output (DDB7 set “one”) to serve this function. The OC0A

pin is also the output pin for the PWM mode timer function.

OC1C, Output Compare Match C output: The PB7 pin can serve as an external output for the Timer/Counter1

Output Compare C. The pin has to be configured as an output (DDB7 set “one”) to serve this function. The

OC1C pin is also the output pin for the PWM mode timer function.

PCINT7, Pin Change Interrupt source 7: The PB7 pin can serve as an external interrupt source.

RTS: RTS flow control signal used by enhanced UART.

• OC1B/PCINT6/OC.4B/ADC12, Bit 6

OC1B, Output Compare Match B output: The PB6 pin can serve as an external output for the Timer/Counter1

Output Compare B. The pin has to be configured as an output (DDB6 set “one”) to serve this function. The

OC1B pin is also the output pin for the PWM mode timer function.

PCINT6, Pin Change Interrupt source 6: The PB7 pin can serve as an external interrupt source.

OC.4B: Timer 4 Output Compare B. This pin can be used to generate a high-speed PWM signal from Timer 4

module. The pin has to be configured as an output (DDB6 set “one”) to serve this function.

ADC13: Analog to Digital Converter, channel 13.

Table 10-3. Port B Pins Alternate Functions

Port Pin Alternate Functions

PB7

OC0A/OC1C/PCINT7/RTS (Output Compare and PWM Output A for Timer/Counter0,

Output Compare and PWM Output C for Timer/Counter1 or Pin Change Interrupt 7 or UART

flow control RTS signal)

PB6

OC1B/PCINT6/OC.4B/ADC13 (Output Compare and PWM Output B for Timer/Counter1 or

Pin Change Interrupt 6 or Timer 4 Output Compare B / PWM output or Analog to Digital

Converter channel 13)

PB5

OC1A/PCINT5/OC.4B/ADC12 (Output Compare and PWM Output A for Timer/Counter1 or

Pin Change Interrupt 5 or Timer 4 Complementary Output Compare B / PWM output or

Analog to Digital Converter channel 12)

PB4 PCINT4/ADC11 (Pin Change Interrupt 4 or Analog to Digital Converter channel 11)

PB3 PDO/MISO/PCINT3 (Programming Data Output or SPI Bus Master Input/Slave Output or

Pin Change Interrupt 3)

PB2 PDI/MOSI/PCINT2 (Programming Data Input or SPI Bus Master Output/Slave Input or Pin

Change Interrupt 2)

PB1 SCK/PCINT1 (SPI Bus Serial Clock or Pin Change Interrupt 1)

PB0 SS/PCINT0 (SPI Slave Select input or Pin Change Interrupt 0)

ATmega16U4/32U4 [DATASHEET]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

• OC1A/PCINT5/OC.4B/ADC12, Bit 5

OC1A, Output Compare Match A output: The PB5 pin can serve as an external output for the Timer/Counter1

Output Compare A. The pin has to be configured as an output (DDB5 set (one)) to serve this function. The

OC1A pin is also the output pin for the PWM mode timer function.

PCINT5, Pin Change Interrupt source 5: The PB7 pin can serve as an external interrupt source.

OC.4B: Timer 4 Output Compare B. This pin can be used to generate a high-speed PWM signal from Timer 4

module, complementary to OC.4B (PB5) signal. The pin has to be configured as an output (DDB5 set (one)) to

serve this function.

ADC12: Analog to Digital Converter, channel 12.

• PCINT4/ADC11, Bit 4

PCINT4, Pin Change Interrupt source 4: The PB7 pin can serve as an external interrupt source.

ADC11, Analog to Digital Converter channel 11.

• PDO/MISO/PCINT3 – Port B, Bit 3

PDO, SPI Serial Programming Data Output. During Serial Program Downloading, this pin is used as data output

line for the ATmega16U4/ATmega32U4.

MISO: Master Data input, Slave Data output pin for SPI channel. When the SPI is enabled as a master, this pin

is configured as an input regardless of the setting of DDB3. When the SPI is enabled as a slave, the data

direction of this pin is controlled by DDB3. When the pin is forced to be an input, the pull-up can still be

controlled by the PORTB3 bit.

PCINT3, Pin Change Interrupt source 3: The PB7 pin can serve as an external interrupt source.

• PDI/MOSI/PCINT2 – Port B, Bit 2

PDI, SPI Serial Programming Data Input. During Serial Program Downloading, this pin is used as data input line

for the ATmega16U4/ATmega32U4.

MOSI: SPI Master Data output, Slave Data input for SPI channel. When the SPI is enabled as a slave, this pin is

configured as an input regardless of the setting of DDB2. When the SPI is enabled as a master, the data

direction of this pin is controlled by DDB2. When the pin is forced to be an input, the pull-up can still be

controlled by the PORTB2 bit.

PCINT2, Pin Change Interrupt source 2: The PB7 pin can serve as an external interrupt source.

• SCK/PCINT1 – Port B, Bit 1

SCK: Master Clock output, Slave Clock input pin for SPI channel. When the SPI is enabled as a slave, this pin is

configured as an input regardless of the setting of DDB1. When the SPI0 is enabled as a master, the data

direction of this pin is controlled by DDB1. When the pin is forced to be an input, the pull-up can still be

controlled by the PORTB1 bit.

PCINT1, Pin Change Interrupt source 1: The PB7 pin can serve as an external interrupt source.

•SS/PCINT0 – Port B, Bit 0

SS: Slave Port Select input. When the SPI is enabled as a slave, this pin is configured as an input regardless of

the setting of DDB0. As a slave, the SPI is activated when this pin is driven low. When the SPI is enabled as a

master, the data direction of this pin is controlled by DDB0. When the pin is forced to be an input, the pull-up can

still be controlled by the PORTB0 bit.

Table 10-4 and Table 10-5 on page 76 relate the alternate functions of Port B to the overriding signals shown in

Figure 10-5 on page 72. SPI MSTR INPUT and SPI SLAVE OUTPUT constitute the MISO signal, while MOSI is

divided into SPI MSTR OUTPUT and SPI SLAVE INPUT.

ATmega16U4/32U4 [DATASHEET]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

PCINT0, Pin Change Interrupt source 0: The PB7 pin can serve as an external interrupt source

Table 10-4. Overriding Signals for Alternate Functions in PB7..PB4

Signal

Name PB7/PCINT7/OC0A/O

C1C/RTS PB6/PCINT6/OC1B/

OC.4B/ADC13 PB5/PCINT5/OC1A/

OC.4B/ADC12 PB4/PCINT4/ADC

PUOE 0 0 0 0

PUOV 0 0 0 0

DDOE 0 0 0 0

DDOV 0 0 0 0

PVOE OC0/OC1C ENABLE OC1B ENABLE OC1A ENABLE 0

PVOV OC0/OC1C OC1B OC1A 0

DIEOE PCINT7 • PCIE0 PCINT6 • PCIE0 PCINT5 • PCIE0 PCINT4 • PCIE0

DIEOV 1 1 1 1

DI PCINT7 INPUT PCINT6 INPUT PCINT5 INPUT PCINT4 INPUT

AIO – – – –

Table 10-5. Overriding Signals for Alternate Functions in PB3..PB0

Signal

Name PB3/PD0/PCINT3/

MISO PB2/PDI/PCINT2/

MOSI PB1/PCINT1/

SCK PB0/PCINT0/

PUOE SPE • MSTR SPE • MSTR SPE • MSTR SPE • MSTR

PUOV PORTB3 • PUD PORTB2 • PUD PORTB1 • PUD PORTB0 • PUD

DDOE SPE • MSTR SPE • MSTR SPE • MSTR SPE • MSTR

DDOV 0 0 0 0

PVOE SPE • MSTR SPE • MSTR SPE • MSTR 0

PVOV SPI SLAVE OUTPUT SPI MSTR OUTPUT SCK OUTPUT 0

DIEOE PCINT3 • PCIE0 PCINT2 • PCIE0 PCINT1 • PCIE0 PCINT0 • PCIE0

DIEOV 1 1 1 1

DI SPI MSTR INPUT

PCINT3 INPUT

SPI SLAVE INPUT

PCINT2 INPUT

SCK INPUT

PCINT1 INPUT

SPI SS

PCINT0 INPUT

AIO – – – –

ATmega16U4/32U4 [DATASHEET]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

10.3.2 Alternat e Functions of Port C

The Port C pins with alternate functions are shown below.

• ICP3/CLKO/OC.4A – Port C, Bit 7

ICP3: If Timer 3 is correctly configured, this pin can serve as Input Capture feature.

CLKO: When the corresponding fuse is enabled, this pin outputs the internal microcontroller working frequency.

If the clock prescaler is used, this will affect this output frequency.

OC.4A: Timer 4 Output Compare A. This pin can be used to generate a high-speed PWM signal from Timer 4

module. The pin has to be configured as an output (DDC7 set “one”) to serve this function.

• OC.3A/OC.4A – Port C, Bit 6

OC.3A: Timer 3 Output Compare A. This pin can be used to generate a PWM signal from Timer 3 module.

OC.4A: Timer 4 Output Compare A. This pin can be used to generate a high-speed PWM signal from Timer 4

module, complementary to OC.4A (PC7) signal. The pin has to be configured as an output (DDC6 set “one”) to

serve this function.

Table 10-6. Port C Pins Alternate Functions

Port Pin Alternate Fu nction

PC7 ICP3/CLKO/OC4A(Input Capture Timer 3 or CLK0 (Divided System

Clock) or Output Compare and direct PWM output A for Timer 4)

PC6 OC.3A/OC4A (Output Compare and PWM output A for Timer/Counter3

or Output Compare and complementary PWM output A for Timer 4)

PC5

Not present on pin-out.

PC4

PC3

PC2

PC1

PC0

ATmega16U4/32U4 [DATASHEET]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

The following table shows the alternate functions of Port C to the overriding signals shown in Figure 10-5 on

page 72.

10.3.3 Alternat e Functions of Port D

The Port D pins with alternate functions are shown below.

The alternate pin configuration is as follows:

• T0/OC.4D/ADC10 – Port D, Bit 7

T0, Timer/Counter0 counter source.

OC.4D: Timer 4 Output Compare D. This pin can be used to generate a high-speed PWM signal from Timer 4

module. The pin has to be configured as an output (DDD7 set “one”) to serve this function.

ADC10: Analog to Digital Converter, Channel 10.

Table 10-7. Overriding Signals for Alternate Functions in PC7..PC6

Signal Name PC7/ICP3/CLKO/OC.4A PC6/OC.3A/OC.4A

PUOE SRE • (XMM<1) SRE • (XMM<2)|OC3A enable

PUOV 0 0

DDOE SRE • (XMM<1) SRE • (XMM<2)

DDOV 1 1

PVOE SRE • (XMM<1) SRE • (XMM<2)

PVOV A15 if (SRE.XMM<2) then A14 else OC3A

DIEOE 0 0

DIEOV 0 0

DI ICP3 input –

AIO – –

Table 10-8. Port D Pins Alternate Functions

Port Pin Alternate Functio n

PD7 T0/OC.4D/ADC10 (Timer/Counter0 Clock Input or Timer 4 Output Compare D / PWM

output or Analog to Digital Converter channel 10)

PD6 T1/OC.4D/ADC9 (Timer/Counter1 Clock Input or Timer 4 Output Complementary

Compare D / PWM output or Analog to Digital Converter channel 9)

PD5 XCK1/CTS (USART1 External Clock Input/Output or UART flow control CTS signal)

PD4 ICP1/ADC8 (Timer/Counter1 Input Capture Trigger or Analog to Digital Converter

channel 8)

PD3 INT3/TXD1 (External Interrupt3 Input or USART1 Transmit Pin)

PD2 INT2/RXD1 (External Interrupt2 Input or USART1 Receive Pin)

PD1 INT1/SDA (External Interrupt1 Input or TWI Serial DAta)

PD0 INT0/SCL/OC0B (External Interrupt0 Input or TWI Serial CLock or Output Compare for

Timer/Counter0)

ATmega16U4/32U4 [DATASHEET]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

• T1/OC.4D/ADC9 – Port D, Bit 6

T1, Timer/Counter1 counter source.

OC.4D: Timer 4 Output Compare D. This pin can be used to generate a high-speed PWM signal from Timer 4

module, complementary to OC.4D (PD7) signal. The pin has to be configured as an output (DDD6 set “one”) to

serve this function.

ADC9: Analog to Digital Converter, Channel 9.

• XCK1/CTS – Port D, Bit 5

XCK1, USART1 External clock. The Data Direction Register (DDD5) controls whether the clock is output (DDD5

set) or input (DDD5 cleared). The XCK1 pin is active only when the USART1 operates in Synchronous mode.

CTS: Clear-To-Send flow control signal used by enhanced UART module.

• ICP1/ADC8 – Port D, Bit 4

ICP1 – Input Capture Pin 1: The PD4 pin can act as an input capture pin for Timer/Counter1.

ADC8: Analog to Digital Converter, Channel 8.

•INT3/TXD1 – Port D, Bit 3

INT3, External Interrupt source 3: The PD3 pin can serve as an external interrupt source to the MCU.

TXD1, Transmit Data (Data output pin for the USART1). When the USART1 Transmitter is enabled, this pin is

configured as an output regardless of the value of DDD3.

•INT2/RXD1 – Port D, Bit 2

INT2, External Interrupt source 2. The PD2 pin can serve as an External Interrupt source to the MCU.

RXD1, Receive Data (Data input pin for the USART1). When the USART1 receiver is enabled this pin is

configured as an input regardless of the value of DDD2. When the USART forces this pin to be an input, the

pull-up can still be controlled by the PORTD2 bit.

•INT1/SDA – Port D, Bit 1

INT1, External Interrupt source 1. The PD1 pin can serve as an external interrupt source to the MCU.

SDA, 2-wire Serial Interface Data: When the TWEN bit in TWCR is set (one) to enable the 2-wire Serial

Interface, pin PD1 is disconnected from the port and becomes the Serial Data I/O pin for the 2-wire Serial

Interface. In this mode, there is a spike filter on the pin to suppress spikes shorter than 50ns on the input signal,

and the pin is driven by an open drain driver with slew-rate limitation.

•INT0/SCL/OC0B – Port D, Bit 0

INT0, External Interrupt source 0. The PD0 pin can serve as an external interrupt source to the MCU.

SCL, 2-wire Serial Interface Clock: When the TWEN bit in TWCR is set (one) to enable the 2-wire Serial

Interface, pin PD0 is disconnected from the port and becomes the Serial Clock I/O pin for the 2-wire Serial

Interface. In this mode, there is a spike filter on the pin to suppress spikes shorter than 50ns on the input signal,

and the pin is driven by an open drain driver with slew-rate limitation.

OC.0B: Timer 0 Output Compare B. This pin can be used to generate a PWM signal from the Timer 0 module.

The two following tables relate the alternate functions of Port D to the overriding signals shown in Figure 10-5 on

page 72.

ATmega16U4/32U4 [DATASHEET]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Note: 1. When enabled, the 2-wire Serial Interface enables Slew-Rate controls on the output pins PD0 and PD1. This is not

shown in this table. In addition, spike filters are connected between the AIO outputs shown in the port figure and the

digital logic of the TWI module.

Table 10-9. Overriding Signals for Alternate Functions PD7..PD4

Signal Name PD7/T0/OC4D/ADC10 PD6/T1/OC4D/ADC9 PD5/XCK1/CTS PD4/ICP1/ADC8

PUOE 0 0 0 0

PUOV 0 0 0 0

DDOE 0 0 XCK1 OUTPUT ENABLE 0

DDOV 0 0 1 0

PVOE 0 0 XCK1 OUTPUT ENABLE 0

PVOV 0 0 XCK1 OUTPUT 0

DIEOE 0 0 0 0

DIEOV 0 0 0 0

DI T0 INPUT T1 INPUT XCK1 INPUT ICP1 INPUT

AIO – – – –

Table 10-10. Overrid ing Signals for Alternate Functions in PD3..PD0(1)

Signal Name PD3/INT3/TXD1 PD2/INT2/RXD1 PD1/INT1/SDA PD0/INT0/SCL/OC0B

PUOE TXEN1 RXEN1 TWEN TWEN

PUOV 0PORTD2 • PUD PORTD1 • PUD PORTD0 • PUD

DDOE TXEN1 RXEN1 TWEN TWEN

DDOV 1 0 SDA_OUT SCL_OUT

PVOE TXEN1 0TWEN ENABLE TWEN | OC0B ENABLE

PVOV TXD1 0 0 OC0B

DIEOE INT3 ENABLE INT2 ENABLE INT1 ENABLE INT0 ENABLE

DIEOV 1 1 1 1

DI INT3 INPUT INT2 INPUT/RXD1 INT1 INPUT INT0 INPUT

AIO – – SDA INPUT SCL INPUT

ATmega16U4/32U4 [DATASHEET]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

10.3.4 Alternat e Functions of Port E

The Port E pins with alternate functions are shown below.

• INT6/AIN0 – Port E, Bit 6

INT6, External Interrupt source 6: The PE6 pin can serve as an external interrupt source.

AIN0 – Analog Comparator Negative input. This pin is directly connected to the negative input of the Analog

Comparator.

•HWB – Port E, Bit 2

HWB allows to execute the bootloader section after reset when tied to ground during external reset pulse. The

HWB mode of this pin is active only when the HWBE fuse is enable. During normal operation (excluded Reset),

this pin acts as a general purpose I/O.

10.3.5 Alternat e Functions of Port F

The Port F has an alternate function as analog input for the ADC as shown in Table 10-13 on page 82. If some

Port F pins are configured as outputs, it is essential that these do not switch when a conversion is in progress.

Table 10-11. Port E Pins Alternate Functions

Port Pin Alternate Function

PE7 Not present on pin-out.

PE6 INT6/AIN0 (External Interrupt 6 Input or Analog Comparator Positive Input)

PE5

Not present on pin-out.PE4

PE3

PE2 HWB (Hardware bootloader activation)

PE1

Not present on pin-out.

PE0

Table 10-12. Over riding Signals for Alternate Functions PE6, PE2

Signal Name PE6/INT6/AIN0 PE2/HWB

PUOE 0 0

PUOV 0 0

DDOE 0 0

DDOV 0 1

PVOE 0 0

PVOV 0 0

DIEOE INT6 ENABLE 0

DIEOV 1 0

DI INT6 INPUT HWB

AIO AIN0 INPUT -

ATmega16U4/32U4 [DATASHEET]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

This might corrupt the result of the conversion. If the JTAG interface is enabled, the pull-up resistors on pins

PF7(TDI), PF5(TMS), and PF4(TCK) will be activated even if a Reset occurs.

• TDI, ADC7 – Port F, Bit 7

ADC7, Analog to Digital Converter, Channel 7.

TDI, JTAG Test Data In: Serial input data to be shifted in to the Instruction Register or Data Register (scan

chains). When the JTAG interface is enabled, this pin can not be used as an I/O pin.

• TDO, ADC6 – Port F, Bit 6

ADC6, Analog to Digital Converter, Channel 6.

TDO, JTAG Test Data Out: Serial output data from Instruction Register or Data Register. When the JTAG

interface is enabled, this pin can not be used as an I/O pin.

The TDO pin is tri-stated unless TAP states that shift out data are entered.

• TMS, ADC5 – Port F, Bit 5

ADC5, Analog to Digital Converter, Channel 5.

TMS, JTAG Test Mode Select: This pin is used for navigating through the TAP-controller state machine. When

the JTAG interface is enabled, this pin can not be used as an I/O pin.

• TCK, ADC4 – Port F, Bit 4

ADC4, Analog to Digital Converter, Channel 4.

TCK, JTAG Test Clock: JTAG operation is synchronous to TCK. When the JTAG interface is enabled, this pin

can not be used as an I/O pin.

• ADC3 – ADC0 – Port F, Bit 1..0

Analog to Digital Converter, Channel 1.0

Table 10-13. Port F Pins Alternate Functions

Port Pin Alternate Function

PF7 ADC7/TDI (ADC input channel 7 or JTAG Test Data Input)

PF6 ADC6/TDO (ADC input channel 6 or JTAG Test Data Output)

PF5 ADC5/TMS (ADC input channel 5 or JTAG Test Mode Select)

PF4 ADC4/TCK (ADC input channel 4 or JTAG Test ClocK)

PF3

Not present on pin-out

PF2

PF1 ADC1 (ADC input channel 1)

PF0 ADC0 (ADC input channel 0)

ATmega16U4/32U4 [DATASHEET]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Table 10-14. Overriding Signals for Alternate Functions in PF7..PF4

Signal Name PF7/ADC7/TDI PF6/ADC6/TDO PF5/ADC5/TMS PF4/ADC4/TCK

PUOE JTAGEN JTAGEN JTAGEN JTAGEN

PUOV 1 0 1 1

DDOE JTAGEN JTAGEN JTAGEN JTAGEN

DDOV 0SHIFT_IR +

SHIFT_DR 0 0

PVOE 0JTAGEN 0 0

PVOV 0TDO 0 0

DIEOE JTAGEN JTAGEN JTAGEN JTAGEN

DIEOV 0 0 0 0

DI – – – –

AIO TDI/ADC7 INPUT ADC6 INPUT TMS/ADC5 INPUT TCK/ADC4 INPUT

Table 10-15. Overriding Signals for Alternate Functions in PF1..PF0

Signal Name PF1/ADC1 PF0/ADC0

PUOE 0 0

PUOV 0 0

DDOE 0 0

DDOV 0 0

PVOE 0 0

PVOV 0 0

DIEOE 0 0

DIEOV 0 0

DI – –

AIO ADC1 INPUT ADC0 INPUT

ATmega16U4/32U4 [DATASHEET]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

10.4 Register Description for I/O-Ports

10.4.1 MCU Control Register – MCUCR

• Bit 4 – PUD: Pull-up Disable

When this bit is written to one, the pull-ups in the I/O ports are disabled even if the DDxn and PORTxn Registers

are configured to enable the pull-ups ({DDxn, PORTxn} = 0b01). See “Configuring the Pin” on page 68 for more

details about this feature.

10.4.2 Port B Data Register – PORTB

10.4.3 Port B Data Direction Register – DDRB

10.4.4 Port B Input Pins Address – PINB

10.4.5 Port C Data Register – PORTC

10.4.6 Port C Data Direction Register – DDRC

Bit7654 3210

JTD ––PUD– – IVSEL IVCE MCUCR

Read/Writ

R/W R R R/W R R R/W R/W

Initial Val-

0000 0000

Bit 76543210

PORTB7 PORTB6 PORTB5 PORTB4 PORTB3 PORTB2 PORTB1 PORTB0 PORTB

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value 0 0 0 0 0 0 0 0

Bit 76543210

DDB7 DDB6 DDB5 DDB4 DDB3 DDB2 DDB1 DDB0 DDRB

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value 0 0 0 0 0 0 0 0

Bit 76543210

PINB7 PINB6 PINB5 PINB4 PINB3 PINB2 PINB1 PINB0 PINB

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value N/A N/A N/A N/A N/A N/A N/A N/A

Bit 76543210

PORTC7PORTC6------PORTC

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value00000000

Bit 76543210

DDC7 DDC6 - - ----DDRC

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value00000000

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10.4.7 Port C Input Pins Address – PINC

10.4.8 Port D Data Register – PORTD

10.4.9 Port D Data Direction Register – DDRD

10.4.10 Port D Input Pins Address – PIND

10.4.11 Port E Data Register – PORTE

10.4.12 Port E Data Direction Register – DDRE

10.4.13 Port E Input Pins Address – PINE

Bit 76543210

PINC7PINC6------PINC

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value N/A N/A N/A N/A N/A N/A N/A N/A

Bit 76543210

PORTD7 PORTD6 PORTD5 PORTD4 PORTD3 PORTD2 PORTD1 PORTD0 PORTD

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value00000000

Bit 76543210

DDD7 DDD6 DDD5 DDD4 DDD3 DDD2 DDD1 DDD0 DDRD

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value00000000

Bit 76543210

PIND7 PIND6 PIND5 PIND4 PIND3 PIND2 PIND1 PIND0 PIND

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value N/A N/A N/A N/A N/A N/A N/A N/A

Bit 76543210

-PORTE6---PORTE2--PORTE

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value00000000

Bit 76543210

- DDE6 - - - DDE2 - - DDRE

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value00000000

Bit 76543210

-PINE6---PINE2--PINE

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value N/A N/A N/A N/A N/A N/A N/A N/A

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10.4.14 Port F Data Register – PORTF

10.4.15 Port F Data Direction Register – DDRF

10.4.16 Port F Input Pins Address – PINF

Bit 76543210

PORTF7 PORTF6 PORTF5 PORTF4 - - PORTF1 PORTF0 PORTF

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value00000000

Bit 76543210

DDF7 DDF6 DDF5 DDF4 - - DDF1 DDF0 DDRF

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value00000000

Bit 76543210

PINF7 PINF6 PINF5 PINF4 - - PINF1 PINF0 PINF

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value N/A N/A N/A N/A N/A N/A N/A N/A

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ATmega16U4/32U4 [DATASHEET]

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11. External Interrupts

The External Interrupts are triggered by the INT6, INT3:0 pin or any of the PCINT7..0 pins. Observe that, if

enabled, the interrupts will trigger even if the INT[6;3:0] or PCINT7..0 pins are configured as outputs. This

feature provides a way of generating a software interrupt.

The Pin change interrupt PCI0 will trigger if any enabled PCINT7:0 pin toggles. PCMSK0 Register control which

pins contribute to the pin change interrupts. Pin change interrupts on PCINT7 ..0 are detected asynchronously.

This implies that these interrupts can be used for waking the part also from sleep modes other than Idle mode.

The External Interrupts can be triggered by a falling or rising edge or a low level. This is set up as indicated in

the specification for the External Interrupt Control Registers – EICRA (INT3:0) and EICRB (INT6). When the

external interrupt is enabled and is configured as level triggered, the interrupt will trigger as long as the pin is

held low. Note that recognition of falling or rising edge interrupts on INT6 requires the presence of an I/O clock,

described in “System Clock and Clock Options” on page 27. Low level interrupts and the edge interrupt on

INT3:0 are detected asynchronously. This implies that these interrupts can be used for waking the part also

from sleep modes other than Idle mode. The I/O clock is halted in all sleep modes except Idle mode.

Note that if a level triggered interrupt is used for wake-up from Power-down, the required level must be held long

enough for the MCU to complete the wake-up to trigger the level interrupt. If the level disappears before the end

of the Start-up Time, the MCU will still wake up, but no interrupt will be generated. The start-up time is defined

by the SUT and CKSEL Fuses as described in “System Clock and Clock Options” on page 27.

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11.1 Register Description

11.1.1 External Interrupt Control Register A – EICRA

The External Interrupt Control Register A contains control bits for interrupt sense control.

• Bits 7..0 – ISC31, ISC30 – ISC00, ISC00: External Interrupt 3 - 0 Sense Control Bits

The External Interrupts 3 - 0 are activated by the external pins INT3:0 if the SREG I-flag and the corresponding

interrupt mask in the EIMSK is set. The level and edges on the external pins that activate the interrupts are

defined in the below table. Edges on INT3..INT0 are registered asynchronously. Pulses on INT3:0 pins wider

than the minimum pulse width given in the below table will generate an interrupt. Shorter pulses are not

guaranteed to generate an interrupt. If low level interrupt is selected, the low level must be held until the

completion of the currently executing instruction to generate an interrupt. If enabled, a level triggered interrupt

will generate an interrupt request as long as the pin is held low. When changing the ISCn bit, an interrupt can

occur. Therefore, it is recommended to first disable INTn by clearing its Interrupt Enable bit in the EIMSK

logical one to its Interrupt Flag bit (INTFn) in the EIFR Register before the interrupt is re-enabled.

Note: 1. n = 3, 2, 1, or 0.

When changing the ISCn1/ISCn0 bits, the interrupt must be disabled by clearing its Interrupt Enable bit in the

EIMSK Register. Otherwise an interrupt can occur when the bits are changed.

11.1.2 External Interrupt Control Register B – EICRB

• Bit 7..6 – Reserved Bits

These bits are reserved bits in the ATmega16U4/ATmega32U4 and always read as zero.

• Bits 5, 4 – ISC61, ISC60: External Interrupt 6 Sense Control Bits

The External Interrupt 6 is activated by the external pin INT6 if the SREG I-flag and the corresponding interrupt

mask in the EIMSK is set. The level and edges on the external pin that activate the interrupt are defined in the

following table. The value on the INT6 pin are sampled before detecting edges. If edge or toggle interrupt is

Bit 76543210

ISC31 ISC30 ISC21 ISC20 ISC11 ISC10 ISC01 ISC00 EICRA

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value 0 0 0 0 0 0 0 0

ISCn1 ISCn0 Description

0 0 The low level of INTn generates an interrupt request.

0 1 Any edge of INTn generates asynchronously an interrupt request.

1 0 The falling edge of INTn generates asynchronously an interrupt request.

1 1 The rising edge of INTn generates asynchronously an interrupt request.

Symbol Parameter Condition Min. Typ. Max. Units

tINT

Minimum pulse width for asynchronous

external interrupt 50 ns

Bit 76543210

--ISC61ISC60----EICRB

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value00000000

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selected, pulses that last longer than one clock period will generate an interrupt. Shorter pulses are not

guaranteed to generate an interrupt. Observe that CPU clock frequency can be lower than the XTAL frequency

if the XTAL divider is enabled. If low level interrupt is selected, the low level must be held until the completion of

the currently executing instruction to generate an interrupt. If enabled, a level triggered interrupt will generate an

interrupt request as long as the pin is held low.

Note: 1. When changing the ISC61/ISC60 bits, the interrupt must be disabled by clearing its Interrupt Enable bit in the

EIMSK Register. Otherwise an interrupt can occur when the bits are changed.

• Bit 3..0 – Reserved Bits

These bits are reserved bits and always read as zero.

11.1.3 External Interrupt Mask Register – EIMSK

• Bits 7..0 – INT6, INT3 – INT0: External Interrupt Request 6, 3 - 0 Enable

When an INT[6;3:0] bit is written to one and the I-bit in the Status Register (SREG) is set (one), the

corresponding external pin interrupt is enabled. The Interrupt Sense Control bits in the External Interrupt Control

Registers – EICRA and EICRB – defines whether the external interrupt is activated on rising or falling edge or

level sensed. Activity on any of these pins will trigger an interrupt request even if the pin is enabled as an output.

This provides a way of generating a software interrupt.

11.1.4 External Interrupt Flag Register – EIFR

• Bits 7..0 – INTF6, INTF3 - INTF0: External Interr upt Flags 6, 3 - 0

When an edge or logic change on the INT[6;3:0] pin triggers an interrupt request, INTF7:0 becomes set (one). If

the I-bit in SREG and the corresponding interrupt enable bit, INT[6;3:0] in EIMSK, are set (one), the MCU will

jump to the interrupt vector. The flag is cleared when the interrupt routine is executed. Alternatively, the flag can

be cleared by writing a logical one to it. These flags are always cleared when INT[6;3:0] are configured as level

interrupt. Note that when entering sleep mode with the INT3:0 interrupts disabled, the input buffers on these

pins will be disabled. This may cause a logic change in internal signals which will set the INTF3:0 flags. See

“Digital Input Enable and Sleep Modes” on page 71 for more information.

ISC61 ISC60 Description

0 0 The low level of INT6 generates an interrupt request.

0 1 Any logical change on INT6 generates an interrupt request

1 0 The falling edge between two samples of INT6 generates an interrupt request.

1 1 The rising edge between two samples of INT6 generates an interrupt request.

Bit 76543210

- INT6 - - INT3 INT2 INT1 IINT0 EIMSK

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value 0 0 0 0 0 0 0 0

Bit 76543210

- INTF6 - - INTF3 INTF2 INTF1 IINTF0 EIFR

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value 0 0 0 0 0 0 0 0

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11.1.5 Pin Change Interrupt Control Register - PCICR

• Bit 0 – PCIE0: Pin Change Interrupt Enable 0

When the PCIE0 bit is set (one) and the I-bit in the Status Register (SREG) is set (one), pin change interrupt 0 is

enabled. Any change on any enabled PCINT7..0 pin will cause an interrupt. The corresponding interrupt of Pin

Change Interrupt Request is executed from the PCI0 Interrupt Vector. PCINT7..0 pins are enabled individually

by the PCMSK0 Register.

11.1.6 Pin Change Interrupt Flag Register – PCIFR

• Bit 0 – PCIF0: Pin Change Interr upt Flag 0

When a logic change on any PCINT7..0 pin triggers an interrupt request, PCIF0 becomes set (one). If the I-bit in

SREG and the PCIE0 bit in EIMSK are set (one), the MCU will jump to the corresponding Interrupt Vector. The

flag is cleared when the interrupt routine is executed. Alternatively, the flag can be cleared by writing a logical

one to it.

11.1.7 Pin Change Mask Register 0 – PCMSK0

• Bit 7..0 – PCINT7..0: Pin Change Enable Mask 7..0

Each PCINT7..0 bit selects whether pin change interrupt is enabled on the corresponding I/O pin. If PCINT7..0

is set and the PCIE0 bit in PCICR is set, pin change interrupt is enabled on the corresponding I/O pin. If

PCINT7..0 is cleared, pin change interrupt on the corresponding I/O pin is disabled.

Bit 76543210

– – – – –PCIE0PCICR

Read/WriteRRRRRRRR/W

Initial Value 0 0 0 0 0 0 0 0

Bit 76543210

–––––PCIF0PCIFR

Read/WriteRRRRRRRR/W

Initial Value 0 0 0 0 0 0 0 0

Bit 76543210

PCINT7 PCINT6 PCINT5 PCINT4 PCINT3 PCINT2 PCINT1 PCINT0 PCMSK0

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Val-

00000000

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12. Timer/Counter0, Timer/Counter1, and Timer/Counter3 Prescalers

Timer/Counter0, 1, and 3 share the same prescaler module, but the Timer/Counters can have different

prescaler settings. The description below applies to all Timer/Counters. Tn is used as a general name, n = 0, 1,

or 3.

12.1 Internal Clock Source

The Timer/Counter can be clocked directly by the system clock (by setting the CSn2:0 = 1). This provides the

fastest operation, with a maximum Timer/Counter clock frequency equal to system clock frequency (fCLK_I/O).

Alternatively, one of four taps from the prescaler can be used as a clock source. The prescaled clock has a fre-

quency of either fCLK_I/O/8, fCLK_I/O/64, fCLK_I/O/256, or fCLK_I/O/1024.

12.2 Prescaler Reset

The prescaler is free running, i.e., operates independently of the Clock Select logic of the Timer/Counter, and it

is shared by the Timer/Counter Tn. Since the prescaler is not affected by the Timer/Counter’s clock select, the

state of the prescaler will have implications for situations where a prescaled clock is used. One example of pres-

caling artifacts occurs when the timer is enabled and clocked by the prescaler (6 > CSn2:0 > 1). The number of

system clock cycles from when the timer is enabled to the first count occurs can be from 1 to N+1 system clock

cycles, where N equals the prescaler divisor (8, 64, 256, or 1024).

It is possible to use the prescaler reset for synchronizing the Timer/Counter to program execution. However,

care must be taken if the other Timer/Counter that shares the same prescaler also uses prescaling. A prescaler

reset will affect the prescaler period for all Timer/Counters it is connected to.

12.3 External Clock Source

An external clock source applied to the Tn pin can be used as Timer/Counter clock (clkTn). The Tn pin is sam-

pled once every system clock cycle by the pin synchronization logic. The synchronized (sampled) signal is then

passed through the edge detector. Figure 12-1 shows a functional equivalent block diagram of the Tn synchro-

nization and edge detector logic. The registers are clocked at the positive edge of the internal system clock

(clkI/O). The latch is transparent in the high period of the internal system clock.

The edge detector generates one clkTn pulse for each positive (CSn2:0 = 7) or negative (CSn2:0 = 6) edge it

detects.

Figure 12-1. Tn/T0 Pin Sampling

The synchronization and edge detector logic introduces a delay of 2.5 to 3.5 system clock cycles from an edge

has been applied to the Tn pin to the counter is updated.

Enabling and disabling of the clock input must be done when Tn has been stable for at least one system clock

cycle, otherwise it is a risk that a false Timer/Counter clock pulse is generated.

Each half period of the external clock applied must be longer than one system clock cycle to ensure correct

sampling. The external clock must be guaranteed to have less than half the system clock frequency (fExtClk <

fclk_I/O/2) given a 50/50% duty cycle. Since the edge detector uses sampling, the maximum frequency of an

external clock it can detect is half the sampling frequency (Nyquist sampling theorem). However, due to varia-

tion of the system clock frequency and duty cycle caused by Oscillator source (crystal, resonator, and

Tn_sync

(To Clock

Select Logic)

Edge DetectorSynchronization

DQDQ

clkI/O

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capacitors) tolerances, it is recommended that maximum frequency of an external clock source is less than

fclk_I/O/2.5.

An external clock source can not be prescaled.

Figure 12-2. Prescaler for Synchronous Timer/Counters

Note: T3 input is not available on the ATmega16U4/ATmega32U4 products. “Tn” only refers to either T0 or T1

inputs.

12.4 Register Description

12.4.1 General Timer/Counter Control Register – GTCCR

• Bit 7 – TSM: Timer/Counter Synchronization Mode

Writing the TSM bit to one activates the Timer/Counter Synchronization mode. In this mode, the value that is

written to the PSRASY and PSRSYNC bits is kept, hence keeping the corresponding prescaler reset signals

asserted. This ensures that the corresponding Timer/Counters are halted and can be configured to the same

value without the risk of one of them advancing during configuration. When the TSM bit is written to zero, the

PSRASY and PSRSYNC bits are cleared by hardware, and the Timer/Counters start counting simultaneously.

• Bit 0 – PSRSYNC: Prescaler Reset for Synchronous Timer/Count ers

When this bit is one, Timer/Counter0 and Timer/Counter1 and Timer/Counter3 prescaler will be Reset. This bit

is normally cleared immediately by hardware, except if the TSM bit is set. Note that Timer/Counter0,

Timer/Counter1 and Timer/Counter3 share the same prescaler and a reset of this prescaler will affect all timers.

PSR10

Clear

clkI/O

Synchronization

TIMER/COUNTERn CLOCK SOURCE

clkTn

TIMER/COUNTERn CLOCK SOURCE

clkTn

CSn0

CSn1

CSn2

CSn0

CSn1

CSn2

Bit 7 6 5 4 3 2 1 0

TSM – – – – – PSRASY PSRSYNC GTCCR

Read/Write R/W R R R R R R/W R/W

Initial Value 0 0 0 0 0 0 0 0

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13. 8-bit Timer/Counter0 with PWM

Timer/Counter0 is a general purpose 8-bit Timer/Counter module, with two independent Output Compare Units,

and with PWM support. It allows accurate program execution timing (event management) and wave generation.

The main features are:

•Two Independ ent Output Compare Units

•Double Buffered Output Compare Registers

•Clear Timer on Compare Match (Auto Reload)

•Glitch Free, Phase Correct Pulse Width Modulator (PWM)

•Variable PWM Period

•Frequency Generator

•Three Independe nt Interrupt Sources (TOV0 , OCF0 A, an d OCF0 B)

13.1 Overview

A simplified block diagram of the 8-bit Timer/Counter is shown in Figure 13-1. For the actual placement of I/O

pins, refer to “Pinout” on page 3. CPU accessible I/O Registers, including I/O bits and I/O pins, are shown in

bold. The device-specific I/O Register and bit locations are listed in the “8-bit Timer/Counter Register

Description” on page 104.

Figure 13-1. 8-bit Timer/Counter Block Diagram

13.1.1 Registers

The Timer/Counter (TCNT0) and Output Compare Registers (OCR0A and OCR0B) are 8-bit registers. Interrupt

request (abbreviated to Int.Req. in the figure) signals are all visible in the Timer Interrupt Flag Register (TIFR0).

All interrupts are individually masked with the Timer Interrupt Mask Register (TIMSK0). TIFR0 and TIMSK0 are

not shown in the figure.

The Timer/Counter can be clocked internally, via the prescaler, or by an external clock source on the T0 pin.

The Clock Select logic block controls which clock source and edge the Timer/Counter uses to increment (or

decrement) its value. The Timer/Counter is inactive when no clock source is selected. The output from the Clock

Select logic is referred to as the timer clock (clkT0).

Clock Select

Timer/Counter

DATA BUS

OCRnA

OCRnB

TCNTn

Waveform

Generation

Waveform

Generation

OCnA

OCnB

Fixed

TOP

Value

Control Logic

TOP BOTTOM

Count

Clear

Direction

TOVn

(Int.Req.)

OCnA

(Int.Req.)

OCnB

(Int.Req.)

TCCRnA TCCRnB

Edge

Detector

( From Prescaler )

clk

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The double buffered Output Compare Registers (OCR0A and OCR0B) are compared with the Timer/Counter

value at all times. The result of the compare can be used by the Waveform Generator to generate a PWM or

variable frequency output on the Output Compare pins (OC0A and OC0B). See “Output Compare Unit” on

page 96. for details. The Compare Match event will also set the Compare Flag (OCF0A or OCF0B) which can

be used to generate an Output Compare interrupt request.

13.1.2 Definitions

Many register and bit references in this section are written in general form. A lower case “n” replaces the

Timer/Counter number, in this case 0. A lower case “x” replaces the Output Compare Unit, in this case Compare

Unit A or Compare Unit B. However, when using the register or bit defined in a program, the precise form must

be used, i.e., TCNT0 for accessing Timer/Counter0 counter value and so on.

The definitions in the table are also used extensively throughout the document.

13.2 Timer/Counter Clock Sources

The Timer/Counter can be clocked by an internal or an external clock source. The clock source is selected by

the Clock Select logic which is controlled by the Clock Select (CS02:0) bits located in the Timer/Counter Control

Timer/Counter3 Prescalers” on page 92.

13.3 Counter Unit

The main part of the 8-bit Timer/Counter is the programmable bi-directional counter unit. Figure 13-2 shows a

block diagram of the counter and its surroundings.

Figure 13-2. Counter Un it Block Diagram

Signal description (internal signals):

count: Increment or decrement TCNT0 by 1.

direction: Select between increment and decrement.

clear: Clear TCNT0 (set all bits to zero).

clkTn: Timer/Counter clock, referred to as clkT0 in the following.

top: Signalize that TCNT0 has reached maximum value.

BOTTOM The counter reaches the BOTTOM when it becomes 0x00.

MAX The counter reaches its MAXimum when it becomes 0xFF (decimal 255).

TOP

The counter reaches the TOP when it becomes equal to the highest value in the count

sequence. The TOP value can be assigned to be the fixed value 0xFF (MAX) or the value

stored in the OCR0A Register. The assignment is dependent on the mode of operation.

DATA BUS

TCNTn Control Logic

count

TOVn

(Int.Req.)

Clock Select

top

Edge

Detector

( From Prescaler )

clk

bottom

direction

clear

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bottom: Signalize that TCNT0 has reached minimum value (zero).

Depending of the mode of operation used, the counter is cleared, incremented, or decremented at each timer

clock (clkT0). clkT0 can be generated from an external or internal clock source, selected by the Clock Select bits

(CS02:0). When no clock source is selected (CS02:0 = 0) the timer is stopped. However, the TCNT0 value can

be accessed by the CPU, regardless of whether clkT0 is present or not. A CPU write overrides (has priority over)

all counter clear or count operations.

The counting sequence is determined by the setting of the WGM01 and WGM00 bits located in the

Timer/Counter Control Register (TCCR0A) and the WGM02 bit located in the Timer/Counter Control Register B

(TCCR0B). There are close connections between how the counter behaves (counts) and how waveforms are

generated on the Output Compare outputs OC0A and OC0B. For more details about advanced counting

sequences and waveform generation, see “Modes of Operation” on page 98.

The Timer/Counter Overflow Flag (TOV0) is set according to the mode of operation selected by the WGM02:0

bits. TOV0 can be used for generating a CPU interrupt.

13.4 Output Compare Unit

The 8-bit comparator continuously compares TCNT0 with the Output Compare Registers (OCR0A and

OCR0B). Whenever TCNT0 equals OCR0A or OCR0B, the comparator signals a match. A match will set the

Output Compare Flag (OCF0A or OCF0B) at the next timer clock cycle. If the corresponding interrupt is

enabled, the Output Compare Flag generates an Output Compare interrupt. The Output Compare Flag is

automatically cleared when the interrupt is executed. Alternatively, the flag can be cleared by software by writing

a logical one to its I/O bit location. The Waveform Generator uses the match signal to generate an output

according to operating mode set by the WGM02:0 bits and Compare Output mode (COM0x1:0) bits. The max.

and bottom signals are used by the Waveform Generator for handling the special cases of the extreme values in

some modes of operation (“Modes of Operation” on page 98).

Figure 13-3 on page 96 shows a block diagram of the Output Compare unit.

Figure 13-3. Output Com pare Unit, Block Diagram

The OCR0x Registers are double buffered when using any of the Pulse Width Modulation (PWM) modes. For

the normal and Clear Timer on Compare (CTC) modes of operation, the double buffering is disabled. The

double buffering synchronizes the update of the OCR0x Compare Registers to either top or bottom of the

OCFnx (Int.Req.)

= (8-bit Comparator )

OCRnx

OCnx

DATA BUS

TCNTn

WGMn1:0

Waveform Generator

top

FOCn

COMnX1:0

bottom

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counting sequence. The synchronization prevents the occurrence of odd-length, non-symmetrical PWM pulses,

thereby making the output glitch-free.

The OCR0x Register access may seem complex, but this is not case. When the double buffering is enabled, the

CPU has access to the OCR0x Buffer Register, and if double buffering is disabled the CPU will access the

OCR0x directly.

13.4.1 Force Output Compare

In non-PWM waveform generation modes, the match output of the comparator can be forced by writing a one to

the Force Output Compare (FOC0x) bit. Forcing Compare Match will not set the OCF0x Flag or reload/clear the

timer, but the OC0x pin will be updated as if a real Compare Match had occurred (the COM0x1:0 bits settings

define whether the OC0x pin is set, cleared or toggled).

13.4.2 Compare Match Blocking by TCNT0 Write

All CPU write operations to the TCNT0 Register will block any Compare Match that occur in the next timer clock

cycle, even when the timer is stopped. This feature allows OCR0x to be initialized to the same value as TCNT0

without triggering an interrupt when the Timer/Counter clock is enabled.

13.4.3 Using the Output Compare Unit

Since writing TCNT0 in any mode of operation will block all Compare Matches for one timer clock cycle, there

are risks involved when changing TCNT0 when using the Output Compare Unit, independently of whether the

Timer/Counter is running or not. If the value written to TCNT0 equals the OCR0x value, the Compare Match will

be missed, resulting in incorrect waveform generation. Similarly, do not write the TCNT0 value equal to

BOTTOM when the counter is down-counting.

The setup of the OC0x should be performed before setting the Data Direction Register for the port pin to output.

The easiest way of setting the OC0x value is to use the Force Output Compare (FOC0x) strobe bits in Normal

mode. The OC0x Registers keep their values even when changing between Waveform Generation modes.

Be aware that the COM0x1:0 bits are not double buffered together with the compare value. Changing the

COM0x1:0 bits will take effect immediately.

13.5 Compa re Match Output Unit

The Compare Output mode (COM0x1:0) bits have two functions. The Waveform Generator uses the COM0x1:0

bits for defining the Output Compare (OC0x) state at the next Compare Match. Also, the COM0x1:0 bits control

the OC0x pin output source. Figure 13-4 shows a simplified schematic of the logic affected by the COM0x1:0 bit

setting. The I/O Registers, I/O bits, and I/O pins in the figure are shown in bold. Only the parts of the general I/O

Port Control Registers (DDR and PORT) that are affected by the COM0x1:0 bits are shown. When referring to

the OC0x state, the reference is for the internal OC0x Register, not the OC0x pin. If a system reset occur, the

OC0x Register is reset to “0”.

ATmega16U4/32U4 [DATASHEET]

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Figure 13-4. Compare Match Output Unit, Schematic

The general I/O port function is overridden by the Output Compare (OC0x) from the Waveform Generator if

either of the COM0x1:0 bits are set. However, the OC0x pin direction (input or output) is still controlled by the

Data Direction Register (DDR) for the port pin. The Data Direction Register bit for the OC0x pin (DDR_OC0x)

must be set as output before the OC0x value is visible on the pin. The port override function is independent of

the Waveform Generation mode.

The design of the Output Compare pin logic allows initialization of the OC0x state before the output is enabled.

Note that some COM0x1:0 bit settings are reserved for certain modes of operation. See “8-bit Timer/Counter

13.5.1 Compare Output Mode and Waveform Generation

The Waveform Generator uses the COM0x1:0 bits differently in Normal, CTC, and PWM modes. For all modes,

setting the COM0x1:0 = 0 tells the Waveform Generator that no action on the OC0x Register is to be performed

on the next Compare Match. For compare output actions in the non-PWM modes refer to Table 13-1 on

page 104. For fast PWM mode, refer to Table 13-2 on page 104, and for phase correct PWM refer to Table 13-

3 on page 105.

A change of the COM0x1:0 bits state will have effect at the first Compare Match after the bits are written. For

non-PWM modes, the action can be forced to have immediate effect by using the FOC0x strobe bits.

13.6 Modes of Operation

The mode of operation, i.e., the behavior of the Timer/Counter and the Output Compare pins, is defined by the

combination of the Waveform Generation mode (WGM02:0) and Compare Output mode (COM0x1:0) bits. The

Compare Output mode bits do not affect the counting sequence, while the Waveform Generation mode bits do.

The COM0x1:0 bits control whether the PWM output generated should be inverted or not (inverted or non-

inverted PWM). For non-PWM modes the COM0x1:0 bits control whether the output should be set, cleared, or

toggled at a Compare Match (See “Compare Match Output Unit” on page 97.).

For detailed timing information see “Timer/Counter Timing Diagrams” on page 102.

13.6.1 Normal Mode

The simplest mode of operation is the Normal mode (WGM02:0 = 0). In this mode the counting direction is

always up (incrementing), and no counter clear is performed. The counter simply overruns when it passes its

PORT

DDR

OCnx

Waveform

Generator

COMnx1

COMnx0

DATA BU S

FOCn

clk

I/O

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maximum 8-bit value (TOP = 0xFF) and then restarts from the bottom (0x00). In normal operation the

Timer/Counter Overflow Flag (TOV0) will be set in the same timer clock cycle as the TCNT0 becomes zero. The

TOV0 Flag in this case behaves like a ninth bit, except that it is only set, not cleared. However, combined with

the timer overflow interrupt that automatically clears the TOV0 Flag, the timer resolution can be increased by

software. There are no special cases to consider in the Normal mode, a new counter value can be written

anytime.

The Output Compare Unit can be used to generate interrupts at some given time. Using the Output Compare to

generate waveforms in Normal mode is not recommended, since this will occupy too much of the CPU time.

13.6.2 Clear Timer on Compare Match (CTC) Mode

In Clear Timer on Compare or CTC mode (WGM02:0 = 2), the OCR0A Register is used to manipulate the

counter resolution. In CTC mode the counter is cleared to zero when the counter value (TCNT0) matches the

OCR0A. The OCR0A defines the top value for the counter, hence also its resolution. This mode allows greater

control of the Compare Match output frequency. It also simplifies the operation of counting external events.

The timing diagram for the CTC mode is shown in Figure 13-5 on page 99. The counter value (TCNT0)

increases until a Compare Match occurs between TCNT0 and OCR0A, and then counter (TCNT0) is cleared.

Figure 13-5. CTC Mode, Timing Dia gram

An interrupt can be generated each time the counter value reaches the TOP value by using the OCF0A Flag. If

the interrupt is enabled, the interrupt handler routine can be used for updating the TOP value. However,

changing TOP to a value close to BOTTOM when the counter is running with none or a low prescaler value must

be done with care since the CTC mode does not have the double buffering feature. If the new value written to

OCR0A is lower than the current value of TCNT0, the counter will miss the Compare Match. The counter will

then have to count to its maximum value (0xFF) and wrap around starting at 0x00 before the Compare Match

can occur.

For generating a waveform output in CTC mode, the OC0A output can be set to toggle its logical level on each

Compare Match by setting the Compare Output mode bits to toggle mode (COM0A1:0 = 1). The OC0A value

will not be visible on the port pin unless the data direction for the pin is set to output. The waveform generated

will have a maximum frequency of fOC0 = fclk_I/O/2 when OCR0A is set to zero (0x00). The waveform frequency is

defined by the following equation:

The N variable represents the prescaler factor (1, 8, 64, 256, or 1024).

As for the Normal mode of operation, the TOV0 Flag is set in the same timer clock cycle that the counter counts

from MAX to 0x00.

TCNTn

OCn

(Toggle)

OCnx Interrupt Flag Set

1 4

Period

2 3

(COMnx1:0 = 1)

fOCnx fclk_I/O

2N1OCRnx+

--------------------------------------------------=

100

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13.6.3 Fast PWM Mode

The fast Pulse Width Modulation or fast PWM mode (WGM02:0 = 3 or 7) provides a high frequency PWM

waveform generation option. The fast PWM differs from the other PWM option by its single-slope operation. The

counter counts from BOTTOM to TOP then restarts from BOTTOM. TOP is defined as 0xFF when WGM2:0 = 3,

and OCR0A when WGM2:0 = 7. In non-inverting Compare Output mode, the Output Compare (OC0x) is cleared

on the Compare Match between TCNT0 and OCR0x, and set at BOTTOM. In inverting Compare Output mode,

the output is set on Compare Match and cleared at BOTTOM. Due to the single-slope operation, the operating

frequency of the fast PWM mode can be twice as high as the phase correct PWM mode that use dual-slope

operation. This high frequency makes the fast PWM mode well suited for power regulation, rectification, and

DAC applications. High frequency allows physically small sized external components (coils, capacitors), and

therefore reduces total system cost.

In fast PWM mode, the counter is incremented until the counter value matches the TOP value. The counter is

then cleared at the following timer clock cycle. The timing diagram for the fast PWM mode is shown in Figure

13-6. The TCNT0 value is in the timing diagram shown as a histogram for illustrating the single-slope operation.

The diagram includes non-inverted and inverted PWM outputs. The small horizontal line marks on the TCNT0

slopes represent Compare Matches between OCR0x and TCNT0.

Figure 13-6. Fast PWM Mode, Timing Diagram

The Timer/Counter Overflow Flag (TOV0) is set each time the counter reaches TOP. If the interrupt is enabled,

the interrupt handler routine can be used for updating the compare value.

In fast PWM mode, the compare unit allows generation of PWM waveforms on the OC0x pins. Setting the

COM0x1:0 bits to two will produce a non-inverted PWM and an inverted PWM output can be generated by

setting the COM0x1:0 to three: Setting the COM0A1:0 bits to one allows the OC0A pin to toggle on Compare

Matches if the WGM02 bit is set. This option is not available for the OC0B pin (See Table 13-2 on page 104).

The actual OC0x value will only be visible on the port pin if the data direction for the port pin is set as output. The

PWM waveform is generated by setting (or clearing) the OC0x Register at the Compare Match between OCR0x

and TCNT0, and clearing (or setting) the OC0x Register at the timer clock cycle the counter is cleared (changes

from TOP to BOTTOM).

The PWM frequency for the output can be calculated by the following equation:

The N variable represents the prescaler factor (1, 8, 64, 256, or 1024).

TCNTn

OCRnx Update and

TOVn Interrupt Flag Set

Period 2 3

OCnx

(COMnx1:0 = 2)

(COMnx1:0 = 3)

OCRnx Interrupt Flag Set

4 5 6 7

fOCnxPWM fclk_I/O

N256

------------------=

101

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The extreme values for the OCR0A Register represents special cases when generating a PWM waveform

output in the fast PWM mode. If the OCR0A is set equal to BOTTOM, the output will be a narrow spike for each

MAX+1 timer clock cycle. Setting the OCR0A equal to MAX will result in a constantly high or low output

(depending on the polarity of the output set by the COM0A1:0 bits.)

A frequency (with 50% duty cycle) waveform output in fast PWM mode can be achieved by setting OC0x to

toggle its logical level on each Compare Match (COM0x1:0 = 1). The waveform generated will have a maximum

frequency of fOC0 = fclk_I/O/2 when OCR0A is set to zero. This feature is similar to the OC0A toggle in CTC mode,

except the double buffer feature of the Output Compare unit is enabled in the fast PWM mode.

13.6.4 Phase Correct PWM Mode

The phase correct PWM mode (WGM02:0 = 1 or 5) provides a high resolution phase correct PWM waveform

generation option. The phase correct PWM mode is based on a dual-slope operation. The counter counts

repeatedly from BOTTOM to TOP and then from TOP to BOTTOM. TOP is defined as 0xFF when WGM2:0 = 1,

and OCR0A when WGM2:0 = 5. In non-inverting Compare Output mode, the Output Compare (OC0x) is cleared

on the Compare Match between TCNT0 and OCR0x while up counting, and set on the Compare Match while

down-counting. In inverting Output Compare mode, the operation is inverted. The dual-slope operation has

lower maximum operation frequency than single slope operation. However, due to the symmetric feature of the

dual-slope PWM modes, these modes are preferred for motor control applications.

In phase correct PWM mode the counter is incremented until the counter value matches TOP. When the

counter reaches TOP, it changes the count direction. The TCNT0 value will be equal to TOP for one timer clock

cycle. The timing diagram for the phase correct PWM mode is shown on Figure 13-7. The TCNT0 value is in the

timing diagram shown as a histogram for illustrating the dual-slope operation. The diagram includes non-

inverted and inverted PWM outputs. The small horizontal line marks on the TCNT0 slopes represent Compare

Matches between OCR0x and TCNT0.

Figure 13-7. Phase Correct PWM Mode, Timing Diagram

The Timer/Counter Overflow Flag (TOV0) is set each time the counter reaches BOTTOM. The Interrupt Flag

can be used to generate an interrupt each time the counter reaches the BOTTOM value.

In phase correct PWM mode, the compare unit allows generation of PWM waveforms on the OC0x pins. Setting

the COM0x1:0 bits to two will produce a non-inverted PWM. An inverted PWM output can be generated by

TOVn Interrupt Flag Set

OCnx Interrupt Flag Set

1 2 3

TCNTn

Period

OCnx

(COMnx1:0 = 2)

(COMnx1:0 = 3)

OCRnx Update

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setting the COM0x1:0 to three: Setting the COM0A0 bits to one allows the OC0A pin to toggle on Compare

Matches if the WGM02 bit is set. This option is not available for the OC0B pin. The actual OC0x value will only

be visible on the port pin if the data direction for the port pin is set as output. The PWM waveform is generated

by clearing (or setting) the OC0x Register at the Compare Match between OCR0x and TCNT0 when the counter

increments, and setting (or clearing) the OC0x Register at Compare Match between OCR0x and TCNT0 when

the counter decrements. The PWM frequency for the output when using phase correct PWM can be calculated

by the following equation:

The N variable represents the prescaler factor (1, 8, 64, 256, or 1024).

The extreme values for the OCR0A Register represent special cases when generating a PWM waveform output

in the phase correct PWM mode. If the OCR0A is set equal to BOTTOM, the output will be continuously low and

if set equal to MAX the output will be continuously high for non-inverted PWM mode. For inverted PWM the

output will have the opposite logic values.

At the very start of period 2 in Figure 13-7 on page 101 OCnx has a transition from high to low even though

there is no Compare Match. The point of this transition is to guarantee symmetry around BOTTOM. There are

two cases that give a transition without Compare Match.

OCR0A changes its value from MAX, like in Figure 13-7 on page 101. When the OCR0A value is MAX the

OCn pin value is the same as the result of a down-counting Compare Match. To ensure symmetry around

BOTTOM the OCn value at MAX must correspond to the result of an up-counting Compare Match.

The timer starts counting from a value higher than the one in OCR0A, and for that reason misses the

Compare Match and hence the OCn change that would have happened on the way up.

13.7 Timer/Counter Ti ming Diagrams

The Timer/Counter is a synchronous design and the timer clock (clkT0) is therefore shown as a clock enable

signal in the following figures. The figures include information on when Interrupt Flags are set. Figure 13-8

contains timing data for basic Timer/Counter operation. The figure shows the count sequence close to the MAX

value in all modes other than phase correct PWM mode.

Figure 13-8. Timer/Counter Timing Diagram, no Prescaling

Figure 13-9 on page 103 shows the same timing data, but with the prescaler enabled.

fOCnxPCPWM fclk_I/O

N510

------------------=

clk

(clk

I/O

/1)

TOVn

clk

I/O

TCNTn MAX - 1 MAX BOTTOM BOTTOM + 1

103

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Figure 13-9. Timer/Counter Timing Diagram, with Prescaler (fclk_I/O/8)

Figure 13-10 shows the setting of OCF0B in all modes and OCF0A in all modes except CTC mode and PWM

mode, where OCR0A is TOP.

Figure 13-10. Timer/Counter Timing Diagram, Setting of OCF0x, with Prescaler (fclk_I/O/8)

Figure 13-11 shows the setting of OCF0A and the clearing of TCNT0 in CTC mode and fast PWM mode where

OCR0A is TOP.

Figure 13-11. Timer/Counter Timing Diagram, Clear Timer on Compare Match mode, with Prescaler (fclk_I/O/8)

TOVn

TCNTn MAX - 1 MAX BOTTOM BOTTOM + 1

clk

I/O

clk

(clk

I/O

/8)

OCFnx

OCRnx

TCNTn

OCRnx Value

OCRnx - 1 OCRnx OCRnx + 1 OCRnx + 2

clkI/O

clkTn

(clkI/O/8)

OCFnx

OCRnx

TCNTn

(CTC)

TOP

TOP - 1 TOP BOTTOM BOTTOM + 1

clkI/O

clkTn

(clkI/O/8)

104

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13.8 8-bit Timer/Counter Register Description

13.8.1 Timer/Counter Control Register A – TCCR0A

• Bits 7:6 – COM01A:0: Compare Match Output A Mode

These bits control the Output Compare pin (OC0A) behavior. If one or both of the COM0A1:0 bits are set, the

OC0A output overrides the normal port functionality of the I/O pin it is connected to. However, note that the Data

Direction Register (DDR) bit corresponding to the OC0A pin must be set in order to enable the output driver.

When OC0A is connected to the pin, the function of the COM0A1:0 bits depends on the WGM02:0 bit setting.

The table shows the COM0A1:0 bit functionality when the WGM02:0 bits are set to a normal or CTC mode (non-

PWM).

The table shows the COM0A1:0 bit functionality when the WGM01:0 bits are set to fast PWM mode

Note: 1. A special case occurs when OCR0A equals TOP and COM0A1 is set. In this case, the Compare Match is

ignored, but the set or clear is done at TOP. See “Fast PWM Mode” on page 100 for more details.

The table shows the COM0A1:0 bit functionality when the WGM02:0 bits are set to phase correct PWM mode.

Bit 7 6 5 4 3 2 1 0

COM0A1COM0A0COM0B1COM0B0––WGM01 WGM00 TCCR0A

Read/Write R/W R/W R/W R/W R R R/W R/W

Initial Value 0 0 0 0 0 0 0 0

Table 13-1. Compare Outpu t Mode, non-PWM Mode

COM0A1 COM0A0 Description

0 0 Normal port operation, OC0A disconnected

0 1 Toggle OC0A on Compare Match

1 0 Clear OC0A on Compare Match

1 1 Set OC0A on Compare Match

Table 13-2. Compare Output Mode, Fast PWM Mode(1)

COM0A1 COM0A0 Description

0 0 Normal port operation, OC0A disconnected

0 1 WGM02 = 0: Normal Port Operation, OC0A Disconnected

WGM02 = 1: Toggle OC0A on Compare Match

1 0 Clear OC0A on Compare Match, set OC0A at TOP

1 1 Set OC0A on Compare Match, clear OC0A at TOP

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Note: 1. A special case occurs when OCR0A equals TOP and COM0A1 is set. In this case, the Compare Match is

ignored, but the set or clear is done at TOP. See “Phase Correct PWM Mode” on page 101 for more details.

• Bits 5:4 – COM0B1 :0: Compare Match Output B Mode

These bits control the Output Compare pin (OC0B) behavior. If one or both of the COM0B1:0 bits are set, the

OC0B output overrides the normal port functionality of the I/O pin it is connected to. However, note that the Data

Direction Register (DDR) bit corresponding to the OC0B pin must be set in order to enable the output driver.

When OC0B is connected to the pin, the function of the COM0B1:0 bits depends on the WGM02:0 bit setting.

The table shows the COM0A1:0 bit functionality when the WGM02:0 bits are set to a normal or CTC mode (non-

PWM).

The table shows the COM0B1:0 bit functionality when the WGM02:0 bits are set to fast PWM mode

Note: 1. A special case occurs when OCR0B equals TOP and COM0B1 is set. In this case, the Compare Match is

ignored, but the set or clear is done at TOP. See “Fast PWM Mode” on page 100 for more details.

Table 13-3. Compare Output Mode, Phase Correct PWM Mode(1)

COM0A1 COM0A0 Description

0 0 Normal port operation, OC0A disconnected

0 1 WGM02 = 0: Normal Port Operation, OC0A Disconnected

WGM02 = 1: Toggle OC0A on Compare Match

1 0 Clear OC0A on Compare Match when up-counting. Set OC0A on Compare

Match when down-counting.

1 1 Set OC0A on Compare Match when up-counting. Clear OC0A on Compare

Match when down-counting.

Table 13-4. Compare Outpu t Mode, non-PWM Mode

COM01 COM00 Description

0 0 Normal port operation, OC0B disconnected

0 1 Toggle OC0B on Compare Match

1 0 Clear OC0B on Compare Match

1 1 Set OC0B on Compare Match

Table 13-5. Compare Output Mode, Fast PWM Mode(1)

COM01 COM00 Description

0 0 Normal port operation, OC0B disconnected

0 1 Reserved

1 0 Clear OC0B on Compare Match, set OC0B at TOP

1 1 Set OC0B on Compare Match, clear OC0B at TOP

106

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The table shows the COM0B1:0 bit functionality when the WGM02:0 bits are set to phase correct PWM mode.

Note: 1. A special case occurs when OCR0B equals TOP and COM0B1 is set. In this case, the Compare Match is

ignored, but the set or clear is done at TOP. See “Phase Correct PWM Mode” on page 101 for more details.

• Bits 3, 2 – Res: Reserved Bit s

These bits are reserved bits in the ATmega16U4/ATmega32U4 and will always read as zero.

• Bits 1:0 – WGM01:0: Waveform Ge neration Mode

Combined with the WGM02 bit found in the TCCR0B Register, these bits control the counting sequence of the

counter, the source for maximum (TOP) counter value, and what type of waveform generation to be used as

shown in the table. Modes of operation supported by the Timer/Counter unit are: Normal mode (counter), Clear

Timer on Compare Match (CTC) mode, and two types of Pulse Width Modulation (PWM) modes (see “Modes of

Operation” on page 98).

Notes: 1. MAX = 0xFF

2. BOTTOM = 0x00

Table 13-6. Compare Output Mode, Phase Correct PWM Mode(1)

COM0A1 COM0A0 Description

0 0 Normal port operation, OC0B disconnected

0 1 Reserved

1 0 Clear OC0B on Compare Match when up-counting. Set OC0B on Compare

Match when down-counting.

1 1 Set OC0B on Compare Match when up-counting. Clear OC0B on Compare

Match when down-counting.

Table 13-7. Waveform Generation Mode Bit Description

Mode WGM2 WGM1 WGM0 Timer/Counter Mode of

Operation TOP Update of

OCRx at TOV Flag

Set on(1)(2)

0 0 0 0 Normal 0xFF Immediate MAX

1 0 0 1 PWM, Phase Correct 0xFF TOP BOTTOM

2 0 1 0 CTC OCRA Immediate MAX

3 0 1 1 Fast PWM 0xFF TOP MAX

4 1 0 0 Reserved – – –

5 1 0 1 PWM, Phase Correct OCRA TOP BOTTOM

6 1 1 0 Reserved – – –

7 1 1 1 Fast PWM OCRA TOP TOP

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13.8.2 Timer/Counter Control Register B – TCCR0B

• Bit 7 – FOC0A: Force Output Compare A

The FOC0A bit is only active when the WGM bits specify a non-PWM mode.

However, for ensuring compatibility with future devices, this bit must be set to zero when TCCR0B is written

when operating in PWM mode. When writing a logical one to the FOC0A bit, an immediate Compare Match is

forced on the Waveform Generation unit. The OC0A output is changed according to its COM0A1:0 bits setting.

Note that the FOC0A bit is implemented as a strobe. Therefore it is the value present in the COM0A1:0 bits that

determines the effect of the forced compare.

A FOC0A strobe will not generate any interrupt, nor will it clear the timer in CTC mode using OCR0A as TOP.

The FOC0A bit is always read as zero.

• Bit 6 – FOC0B: Force Output Compare B

The FOC0B bit is only active when the WGM bits specify a non-PWM mode.

However, for ensuring compatibility with future devices, this bit must be set to zero when TCCR0B is written

when operating in PWM mode. When writing a logical one to the FOC0B bit, an immediate Compare Match is

forced on the Waveform Generation unit. The OC0B output is changed according to its COM0B1:0 bits setting.

Note that the FOC0B bit is implemented as a strobe. Therefore it is the value present in the COM0B1:0 bits that

determines the effect of the forced compare.

A FOC0B strobe will not generate any interrupt, nor will it clear the timer in CTC mode using OCR0B as TOP.

The FOC0B bit is always read as zero.

• Bits 5:4 – Res: Reserved Bits

These bits are reserved bits and will always read as zero.

• Bit 3 – WGM02: Waveform Generation Mode

See the description in the “Timer/Counter Control Register A – TCCR0A” on page 104.

Bit 7 6 5 4 3 2 1 0

FOC0A FOC0B – – WGM02 CS02 CS01 CS00 TCCR0B

Read/Write W W R R R/W R/W R/W R/W

Initial Value 0 0 0 0 0 0 0 0

108

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• Bits 2:0 – CS02:0: Clock Select

The three Clock Select bits select the clock source to be used by the Timer/Counter.

If external pin modes are used for the Timer/Counter0, transitions on the T0 pin will clock the counter even if the

pin is configured as an output. This feature allows software control of the counting.

13.8.3 Timer/Counter Register – TCNT0

The Timer/Counter Register gives direct access, both for read and write operations, to the Timer/Counter unit

8 -bit counter. Writing to the TCNT0 Register blocks (removes) the Compare Match on the following timer clock.

Modifying the counter (TCNT0) while the counter is running, introduces a risk of missing a Compare Match

between TCNT0 and the OCR0x Registers.

13.8.4 Output Compare Register A – OCR0A

The Output Compare Register A contains an 8-bit value that is continuously compared with the counter value

(TCNT0). A match can be used to generate an Output Compare interrupt, or to generate a waveform output on

the OC0A pin.

13.8.5 Output Compare Register B – OCR0B

Table 13-8. Clock Select Bit Description

CS02 CS01 CS00 Description

000No clock source (Timer/Counter stopped)

001clkI/O/(No prescaling)

010clkI/O/8 (From prescaler)

011clkI/O/64 (From prescaler)

100clkI/O/256 (From prescaler)

101clkI/O/1024 (From prescaler)

110External clock source on T0 pin. Clock on falling edge.

111External clock source on T0 pin. Clock on rising edge.

Bit 76543210

TCNT0[7:0] TCNT0

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value00000000

Bit 76543210

OCR0A[7:0] OCR0A

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value 0 0 0 0 0 0 0 0

Bit 76543210

OCR0B[7:0] OCR0B

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value 0 0 0 0 0 0 0 0

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The Output Compare Register B contains an 8-bit value that is continuously compared with the counter value

(TCNT0). A match can be used to generate an Output Compare interrupt, or to generate a waveform output on

the OC0B pin.

13.8.6 Timer/Counter Interrupt Mask Register – TIMSK0

• Bits 7..3, 0 – Res: Reserved Bits

These bits are reserved bits and will always read as zero.

• Bit 2 – OCIE0B: Timer/Counter Output Compare Match B Interrupt Enable

When the OCIE0B bit is written to one, and the I-bit in the Status Register is set, the Timer/Counter Compare

Match B interrupt is enabled. The corresponding interrupt is executed if a Compare Match in Timer/Counter

occurs, i.e., when the OCF0B bit is set in the Timer/Counter Interrupt Flag Register – TIFR0.

• Bit 1 – OCIE0A: Timer/Counter0 Output Compare Match A Interrupt Enable

When the OCIE0A bit is written to one, and the I-bit in the Status Register is set, the Timer/Counter0 Compare

Match A interrupt is enabled. The corresponding interrupt is executed if a Compare Match in Timer/Counter0

occurs, i.e., when the OCF0A bit is set in the Timer/Counter 0 Interrupt Flag Register – TIFR0.

• Bit 0 – TOIE0: Timer/Counter0 Overflow Interrupt Enable

When the TOIE0 bit is written to one, and the I-bit in the Status Register is set, the Timer/Counter0 Overflow

interrupt is enabled. The corresponding interrupt is executed if an overflow in Timer/Counter0 occurs, i.e., when

the TOV0 bit is set in the Timer/Counter 0 Interrupt Flag Register – TIFR0.

13.8.7 Timer/Counter 0 Interrupt Flag Register – TIFR0

• Bits 7..3, 0 – Res: Reserved Bits

These bits are reserved bits in the ATmega16U4/ATmega32U4 and will always read as zero.

• Bit 2 – OCF0B: Timer/Counter 0 Output Compare B Match Flag

The OCF0B bit is set when a Compare Match occurs between the Timer/Counter and the data in OCR0B –

Output Compare Register0 B. OCF0B is cleared by hardware when executing the corresponding interrupt

handling vector. Alternatively, OCF0B is cleared by writing a logic one to the flag. When the I-bit in SREG,

OCIE0B (Timer/Counter Compare B Match Interrupt Enable), and OCF0B are set, the Timer/Counter Compare

Match Interrupt is executed.

• Bit 1 – OCF0A: Timer/Counter 0 Output Compare A Match Flag

The OCF0A bit is set when a Compare Match occurs between the Timer/Counter0 and the data in OCR0A –

Output Compare Register0. OCF0A is cleared by hardware when executing the corresponding interrupt

handling vector. Alternatively, OCF0A is cleared by writing a logic one to the flag. When the I-bit in SREG,

Bit 76543 2 1 0

– – – – – OCIE0B OCIE0A TOIE0 TIMSK0

Read/Write R R R R R R/W R/W R/W

Initial Value 0 0 0 0 0 0 0 0

Bit 76543210

–––––OCF0BOCF0ATOV0 TIFR0

Read/Write R R R R R R/W R/W R/W

Initial Value 0 0 0 0 0 0 0 0

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OCIE0A (Timer/Counter0 Compare Match Interrupt Enable), and OCF0A are set, the Timer/Counter0 Compare

Match Interrupt is executed.

• Bit 0 – TOV0: Timer/Counter0 Overflow Flag

The bit TOV0 is set when an overflow occurs in Timer/Counter0. TOV0 is cleared by hardware when executing

the corresponding interrupt handling vector. Alternatively, TOV0 is cleared by writing a logic one to the flag.

When the SREG I-bit, TOIE0 (Timer/Counter0 Overflow Interrupt Enable), and TOV0 are set, the

Timer/Counter0 Overflow interrupt is executed.

The setting of this flag is dependent of the WGM02:0 bit setting.

Refer to “Waveform Generation Mode Bit Description” on page 106.

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14. 16-bit Timers/Counters (Timer/Counter1 and Timer/Counter3)

The 16-bit Timer/Counter unit allows accurate program execution timing (event management), wave generation,

and signal timing measurement. The main features are:

•True 16-bit Design (i.e., Allows 16-bit PWM)

•Three independent Output Compare Units

•Double Buffered Output Compare Registers

•One Input Capture Un it

•Input Capture Noise Canceler

•Clear Timer on Compare Match (Auto Reload)

•Glitch-free, Phase Correct Pulse Width Modulator (PWM)

•Variable PWM Period

•Frequency Generator

•External Event Counter

•Ten independe nt interrupt sources (TOV1, OCF1A, OCF1B, OCF1C, ICF1, TOV3, OCF3A, OCF3B, OCF3C, and

ICF3)

14.1 Overview

Most register and bit references in this section are written in general form. A lower case “n” replaces the

Timer/Counter number, and a) lower case “x” replaces the Output Compare unit channel. However, when using

the register or bit defines in a program, the precise form must be used, i.e., TCNT1 for accessing

Timer/Counter1 counter value and so on.

A simplified block diagram of the 16-bit Timer/Counter is shown in Figure 14-1 on page 112. For the actual

placement of I/O pins, see “Pinout” on page 3. CPU accessible I/O Registers, including I/O bits and I/O pins, are

shown in bold. The device-specific I/O Register and bit locations are listed in the “16-bit Timers/Counters

(Timer/Counter1 and Timer/Counter3)” on page 111.

The Power Reduction Timer/Counter1 bit, PRTIM1, in “Power Reduction Register 0 - PRR0” on page 47 must

be written to zero to enable Timer/Counter1 module.

The Power Reduction Timer/Counter3 bit, PRTIM3, in “Power Reduction Register 1 - PRR1” on page 48 must

be written to zero to enable Timer/Counter3 module.

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Figure 14-1. 16-bit Timer/Counter Block Diagram(1)

Note: 1. Refer to “Pinout” on page 3, Table 10-3 on page 74, and Table 10-6 on page 77 for Timer/Counter1 and 3 and

3 pin placement and description.

2. Tn only refers to T1 since T3 input is not available on the product.

14.1.1 Registers

The Timer/Counter (TCNTn), Output Compare Registers (OCRnA/B/C), and Input Capture Register (ICRn) are

all 16-bit registers. Special procedures must be followed when accessing the 16-bit registers. These procedures

are described in the section “Accessing 16-bit Registers” on page 113. The Timer/Counter Control Registers

(TCCRnA/B/C) are 8-bit registers and have no CPU access restrictions. Interrupt requests (shorten as Int.Req.)

signals are all visible in the Timer Interrupt Flag Register (TIFRn). All interrupts are individually masked with the

Timer Interrupt Mask Register (TIMSKn). TIFRn and TIMSKn are not shown in the figure since these registers

are shared by other timer units.

The Timer/Counter can be clocked internally, via the prescaler, or by an external clock source on the Tn pin.

The Clock Select logic block controls which clock source and edge the Timer/Counter uses to increment (or

decrement) its value. The Timer/Counter is inactive when no clock source is selected. The output from the clock

select logic is referred to as the timer clock (clkTn).

The double buffered Output Compare Registers (OCRnA/B/C) are compared with the Timer/Counter value at all

time. The result of the compare can be used by the Waveform Generator to generate a PWM or variable

frequency output on the Output Compare pin (OCnA/B/C). See “Output Compare Units” on page 119. The

compare match event will also set the Compare Match Flag (OCFnA/B/C) which can be used to generate an

Output Compare interrupt request.

ICFn (Int.Req.)

TOVn

(Int.Req.)

Clock Select

Timer/Counter

DATABUS

ICRn

TCNTn

Waveform

Generation

Waveform

Generation

Waveform

Generation

OCnA

OCnB

OCnC

Noise

Canceler

ICPn

Fixed

TOP

Values

Edge

Detector

Control Logic

= 0

TOP BOTTOM

Count

Clear

Direction

OCFnA

(Int.Req.)

OCFnB

(Int.Req.)

OCFnC

(Int.Req.)

TCCRnA TCCRnB TCCRnC

( From Analog

Comparator Ouput )

Edge

Detector

( From Prescaler )

TCLK

OCRnC

OCRnB

OCRnA

(2)

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The Input Capture Register can capture the Timer/Counter value at a given external (edge triggered) event on

either the Input Capture pin (ICPn) or on the Analog Comparator pins (See “Analog Comparator” on page 293.)

The Input Capture unit includes a digital filtering unit (Noise Canceler) for reducing the chance of capturing

noise spikes.

The TOP value, or maximum Timer/Counter value, can in some modes of operation be defined by either the

OCRnA Register, the ICRn Register, or by a set of fixed values. When using OCRnA as TOP value in a PWM

mode, the OCRnA Register can not be used for generating a PWM output. However, the TOP value will in this

case be double buffered allowing the TOP value to be changed in run time. If a fixed TOP value is required, the

ICRn Register can be used as an alternative, freeing the OCRnA to be used as PWM output.

14.1.2 Definitions

The following definitions are used extensively throughout the document:

14.2 Accessing 16-bit Registers

The TCNTn, OCRnA/B/C, and ICRn are 16-bit registers that can be accessed by the AVR CPU via the 8-bit

data bus. The 16-bit register must be byte accessed using two read or write operations. Each 16-bit timer has a

single 8-bit register for temporary storing of the high byte of the 16-bit access. The same Temporary Register is

shared between all 16-bit registers within each 16-bit timer. Accessing the low byte triggers the 16-bit read or

write operation. When the low byte of a 16-bit register is written by the CPU, the high byte stored in the

Temporary Register, and the low byte written are both copied into the 16-bit register in the same clock cycle.

When the low byte of a 16-bit register is read by the CPU, the high byte of the 16-bit register is copied into the

Temporary Register in the same clock cycle as the low byte is read.

Not all 16-bit accesses uses the Temporary Register for the high byte. Reading the OCRnA/B/C 16-bit registers

does not involve using the Temporary Register.

To do a 16-bit write, the high byte must be written before the low byte. For a 16-bit read, the low byte must be

read before the high byte.

The following code examples show how to access the 16-bit timer registers assuming that no interrupts updates

the temporary register. The same principle can be used directly for accessing the OCRnA/B/C and ICRn

Registers. Note that when using “C”, the compiler handles the 16-bit access.

BOTTOM The counter reaches the BOTTOM when it becomes 0x0000.

MAX The counter reaches its MAXimum when it becomes 0xFFFF (decimal 65535).

TOP

The counter reaches the TOP when it becomes equal to the highest value in the count

sequence. The TOP value can be assigned to be one of the fixed values: 0x00FF,

0x01FF, or 0x03FF, or to the value stored in the OCRnA or ICRn Register. The

assignment is dependent of the mode of operation.

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Note: 1. See “Code Examples” on page 8.

The assembly code example returns the TCNTn value in the r17:r16 register pair.

It is important to notice that accessing 16-bit registers are atomic operations. If an interrupt occurs between the

two instructions accessing the 16-bit register, and the interrupt code updates the temporary register by

accessing the same or any other of the 16-bit Timer Registers, then the result of the access outside the interrupt

will be corrupted. Therefore, when both the main code and the interrupt code update the temporary register, the

main code must disable the interrupts during the 16-bit access.

The following code examples show how to do an atomic read of the TCNTn Register contents. Reading any of

the OCRnA/B/C or ICRn Registers can be done by using the same principle.

Assembly Code Examples(1)

...

; Set TCNTn to 0x01FF

ldi r17,0x01

ldi r16,0xFF

out TCNTnH,r17

out TCNTnL,r16

; Read TCNTn into r17:r16

in r16,TCNTnL

in r17,TCNTnH

...

C Code Examples(1)

unsigned int i;

...

/* Set TCNTn to 0x01FF */

TCNTn = 0x1FF;

/* Read TCNTn into i */

i = TCNTn;

...

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Note: 1. See “Code Examples” on page 8.

The assembly code example returns the TCNTn value in the r17:r16 register pair.

The following code examples show how to do an atomic write of the TCNTn Register contents. Writing any of

the OCRnA/B/C or ICRn Registers can be done by using the same principle.

Assembly Code Example(1)

TIM16_ReadTCNTn:

; Save global interrupt flag

in r18,SREG

; Disable interrupts

cli

; Read TCNTn into r17:r16

in r16,TCNTnL

in r17,TCNTnH

; Restore global interrupt flag

out SREG,r18

ret

C Code Example(1)

unsigned int TIM16_ReadTCNTn( void )

{unsigned char sreg;

unsigned int i;

/* Save global interrupt flag */

sreg = SREG;

/* Disable interrupts */

__disable_interrupt();

/* Read TCNTn into i */

i = TCNTn;

/* Restore global interrupt flag */

SREG = sreg;

return i;

}

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Note: 1. See “Code Examples” on page 8.

The assembly code example requires that the r17:r16 register pair contains the value to be written to TCNTn.

14.2.1 Reusing the Temporary High Byte Register

If writing to more than one 16-bit register where the high byte is the same for all registers written, then the high

byte only needs to be written once. However, note that the same rule of atomic operation described previously

also applies in this case.

14.3 Timer/Counter Clock Sources

The Timer/Counter can be clocked by an internal or an external clock source. The clock source is selected by

the Clock Select logic which is controlled by the Clock Sel ect (CSn2:0) bits located in the Timer/Counter co ntrol

Timer/Counter3 Prescalers” on page 92.

14.4 Counter Unit

The main part of the 16-bit Timer/Counter is the programmable 16-bit bi-directional counter unit. Figure 14-2

shows a block diagram of the counter and its surroundings.

Assembly Code Example(1)

TIM16_WriteTCNTn:

; Save global interrupt flag

in r18,SREG

; Disable interrupts

cli

; Set TCNTn to r17:r16

out TCNTnH,r17

out TCNTnL,r16

; Restore global interrupt flag

out SREG,r18

ret

C Code Example(1)

void TIM16_WriteTCNTn( unsigned int i )

{unsigned char sreg;

unsigned int i;

/* Save global interrupt flag */

sreg = SREG;

/* Disable interrupts */

__disable_interrupt();

/* Set TCNTn to i */

TCNTn = i;

/* Restore global interrupt flag */

SREG = sreg;

}

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Figure 14-2. Counter Un it Block Diagram

Signal description (internal signals):

Count: Increment or decrement TCNTn by 1.

Direction: Select between increment and decrement.

Clear: Clear TCNTn (set all bits to zero).

clkTn: Timer/Counter clock.

TOP: Signalize that TCNTn has reached maximum value.

BOTTOM: Signalize that TCNTn has reached minimum value (zero).

The 16-bit counter is mapped into two 8-bit I/O memory locations: Counter High (TCNTnH) containing the upper

eight bits of the counter, and Cou nter Low (TCNTnL) containing the lower eight bits. The TCNTnH Register can

only be indirectly accessed by the CPU. When the CPU does an access to the TCNTnH I/O location, the CPU

accesses the high byte temporary register (TEMP). The temporary register is updated with the TCNTnH value

when the TCNTnL is read, and TCNTnH is updated with the temporary register value when TCNTnL is written.

This allows the CPU to read or write the entire 16-bit counter value within one clock cycle via the 8-bit data bus.

It is important to notice that there are special cases of writing to the TCNTn Register when the counter is

counting that will give unpredictable results. The special cases are described in the sections where they are of

importance.

Depending on the mode of operation used, the counter is cleared, incremented, or decremented at each timer

clock (clkTn). The clkTn can be generated from an external or internal clock source, selected by the Clock Select

bits (CSn2:0). When no clock source is selected (CSn2:0 = 0) the timer is stopped. However, the TCNTn value

can be accessed by the CPU, independent of whether clkTn is present or not. A CPU write overrides (has priority

over) all counter clear or count operations.

The counting sequence is determined by the setting of the Wave form Generation mode bits (WGMn3:0) located

in the Timer/Counter Control Registers A and B (TCCRnA and TCCRnB). There are close connections between

how the counter behaves (counts) and how waveforms are generated on the Output Compare outputs OCnx.

For more details about advanced counting sequences and waveform generation, see “Modes of Operation” on

page 122.

The Timer/Counter Overflow Flag (TOVn) is set according to the mode of operation selected by the WGMn3:0

bits. TOVn can be used for generating a CPU interrupt.

14.5 Input Capture Unit

The Timer/Counter incorporates an input capture unit that can capture external events and give them a time-

stamp indicating time of occurrence. The external signal indicating an event, or multiple events, can be applied

via the ICPn pin or alternatively, for the Timer/Counter1 only, via the Analog Comparator unit. The time-stamps

can then be used to calculate frequency, duty-cycle, and other features of the signal applied. Alternatively the

time-stamps can be used for creating a log of the events.

TEMP (8-bit)

DATA BUS

(8-bit)

TCNTn (16-bit Counter)

TCNTnH (8-bit) TCNTnL (8-bit) Control Logic

Count

Clear

Direction

TOVn

(Int.Req.)

Clock Select

TOP BOTTOM

Edge

Detector

( From Prescaler )

clkTn

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The Input Capture unit is illustrated by the block diagram shown in Figure 14-3. The elements of the block

diagram that are not directly a part of the input capture unit are gray shaded. The small “n” in register and bit

names indicates the Timer/Counter number.

Figure 14-3. Input Capture Unit Block Diagram

Note: The Analog Comparator Output (ACO) can only trigger the Timer/Counter1 ICP – not Timer/Counter3, 4, or 5.

When a change of the logic level (an event) occurs on the Input Capture Pin (ICPn), alternatively on the analog

Comparator output (ACO), and this change confirms to the setting of the edge detector, a capture will be

triggered. When a capture is triggered, the 16-bit value of the counter (TCNTn) is written to the Input Capture

into ICRn Register. If enabled (TICIEn = 1), the input capture flag generates an input capture interrupt. The ICFn

flag is automatically cleared when the interrupt is executed. Alternatively the ICFn flag can be cleared by

software by writing a logical one to its I/O bit location.

Reading the 16-bit value in the Input Capture Register (ICRn) is done by first reading the low byte (ICRnL) and

then the high byte (ICRnH). When the low byte is read the high byte is copied into the high byte Temporary

The ICRn Register can only be written when using a Waveform Generation mode that utilizes the ICRn Register

for defining the counter’s TOP value. In these cases the Waveform Generation mode (WGMn3:0) bits must be

set before the TOP value can be written to the ICRn Register. When writing the ICRn Register the high byte

must be written to the ICRnH I/O location before the low byte is written to ICRnL.

For more information on how to access the 16-bit registers refer to “Accessing 16-bit Registers” on page 113.

14.5.1 Input Capture Trigger Source

The main trigger source for the input capture unit is the Input Capture Pin (ICPn). Timer/Counter1 can

alternatively use the analog comparator output as trigger source for the input capture unit. The Analog

Comparator is selected as trigger source by setting the analog Comparator Input Capture (ACIC) bit in the

Analog Comparator Control and Status Register (ACSR). Be aware that changing trigger source can trigger a

capture. The input capture flag must therefore be cleared after the change.

ICFn (Int.Req.)

Analog

Comparator

WRITE ICRn (16-bit Register)

ICRnH (8-bit)

Noise

Canceler

ICPn

Edge

Detector

TEMP (8-bit)

DATA BUS (8-bit)

ICRnL (8-bit)

TCNTn (16-bit Counter)

TCNTnH (8-bit) TCNTnL (8-bit)

ACIC* ICNC ICES

ACO*

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Both the Input Capture Pin (ICPn) and the An alog Comparator outp ut (ACO) inputs are sampled using the same

technique as for the Tn pin (Figure 12-1 on page 92). The edge detector is also identical. However, when the

noise canceler is enabled, additional logic is inserted before the edge detector, which increases the delay by

four system clock cycles. Note that the input of the noise canceler and edge detector is always enabled unless

the Timer/Counter is set in a Waveform Generation mode that uses ICRn to define TOP.

An input capture can be triggered by software by controlling the port of the ICPn pin.

14.5.2 Noise Canceler

The noise canceler improves noise immunity by using a simple digital filtering scheme. The noise canceler input

is monitored over four samples, and all four must be equal for changing the output that in turn is used by the

edge detector.

The noise canceler is enabled by setting the Input Capture Noise Canceler (ICNCn) bit in Timer/Counter Control

from a change applied to the input, to the update of the ICRn Register. The noise canceler uses the system

clock and is therefore not affected by the prescaler.

14.5.3 Using the Input Ca pture Unit

The main challenge when using the Input Capture unit is to assign enough processor capacity for handling the

incoming events. The time between two events is critical. If the processor has not read the captured value in the

ICRn Register before the next event occurs, the ICRn will be overwritten with a new value. In this case the result

of the capture will be incorrect.

When using the Input Capture interrupt, the ICRn Register should be read as early in the interrupt handler

routine as possible. Even though the Input Capture interrupt has relatively high priority, the maximum interrupt

response time is dependent on the maximum number of clock cycles it takes to handle any of the other interrupt

requests.

Using the Input Capture unit in any mode of operation when the TOP value (resolution) is actively changed

during operation, is not recommended.

Measurement of an external signal’s duty cycle requires that the trigger edge is changed after each capture.

Changing the edge sensing must be done as early as possible after the ICRn Register has been read. After a

change of the edge, the Input Capture Flag (ICFn) must be cleared by software (writing a logical one to the I/O

bit location). For measuring frequency only, the clearing of the ICFn Flag is not required (if an interrupt handler

is used).

14.6 Output Compare Units

The 16-bit comparator continuously compares TCNTn with the Output Compare Register (OCRnx). If TCNT

equals OCRnx the comparator signals a match. A match will set the Output Compare Flag (OCFnx) at the next

timer clock cycle. If enabled (OCIEnx = 1), the Output Compare Flag generates an Output Compare interrupt.

The OCFnx Flag is automatically cleared when the interrupt is executed. Alternatively the OCFnx Flag can be

cleared by software by writing a logical one to its I/O bit location. The Waveform Generator uses the match

signal to generate an output according to operating mode set by the Waveform Generation mode (WGMn3:0)

bits and Compare Outp ut mod e (COMnx1:0) bits. The TOP and BOTTOM signals are used by the Waveform

Generator for handling the special cases of the extreme values in some modes of operation (See “Modes of

Operation” on page 122.)

A special feature of Output Compare unit A allows it to define the Timer/Counter TOP value (i.e., counter

resolution). In addition to the counter resolution, the TOP value defines the period time for waveforms generated

by the Waveform Generator.

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Figure 14-4 shows a block diagram of the Output Compare unit. The small “n” in the register and bit names

indicates the device number (n = n for Timer/Counter n), and the “x” indicates Output Compare unit (A/B/C). The

elements of the block diagram that are not directly a part of the Output Compare unit are gray shaded.

Figure 14-4. Output Com pare Unit, Block Diagram

The OCRnx Register is double buffered when using any of the twelve Pulse Width Modula tio n (PWM) modes.

For the Normal and Clear Timer on Compare (CTC) modes of operation, the double buffering is disabled. The

double buffering synchronizes the update of the OCRnx Compare Register to either TOP or BOTTOM of the

counting sequence. The synchronization prevents the occurrence of odd-length, non-symmetrical PWM pulses,

thereby making the output glitch-free.

The OCRnx Register access may seem complex, but this is not case. When the double buffering is enabled, the

CPU has access to the OCRnx Buffer Register, and if double buffering is disabled the CPU will access the

OCRnx directly. The content of the OCR1x (Buffer or Compare) Register is only changed by a write operation

(the Timer/Counter does not update this register automatically as the TCNT1 and ICR1 Register). Therefore

OCR1x is not read via the high byte temporary register (TEMP). However, it is a good practice to read the low

byte first as when accessing other 16-bit registers. Writing the OCRnx Registers must be done via the TEMP

When the high byte I/O location is written by the CPU, the TEMP Register will be updated by the value written.

Then when the low byte (OCRnxL) is written to the lower eight bits, the high byte will be copied into the upper 8-

bits of either the OCRnx buffer or OCRnx Compare Register in the same system clock cycle.

For more information of how to access the 16-bit registers refer to “Accessing 16-bit Registers” on page 113.

14.6.1 Force Output Compare

In non-PWM Waveform Generation modes, the match output of the comparator can be forced by writing a one

to the Force Output Compare (FOCnx) bit. Forcing compare match will not set the OCFnx Flag or reload/clear

the timer, but the OCnx pin will be updated as if a real compare match had occurred (the COMn1:0 bits settings

define whether the OCnx pin is set, cleared or toggled).

OCFnx (Int.Req.)

(16-bit Comparator )

OCRnx Buffer (16-bit Register)

OCRnxH Buf. (8-bit)

OCnx

TEMP (8-bit)

DATA BUS

(8-bit)

OCRnxL Buf. (8-bit)

TCNTn (16-bit Counter)

TCNTnH (8-bit) TCNTnL (8-bit)

COMnx1:0WGMn3:0

OCRnx (16-bit Register)

OCRnxH (8-bit) OCRnxL (8-bit)

Waveform Generator

TOP

BOTTOM

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14.6.2 Compare Match Blocking by TCNTn Write

All CPU writes to the TCNTn Register will block any compare match that occurs in the next timer clock cycle,

even when the timer is stopped. This feature allows OCRnx to be initialized to the same value as TCNTn without

triggering an interrupt when the Timer/Counter clock is enabled.

14.6.3 Using the Output Compare Unit

Since writing TCNTn in any mode of operation will block all compare matches for one timer clock cycle, there

are risks involved when changing TCNTn when using any of the Output Compare channels, independent of

whether the Timer/Counter is running or not. If the value written to TCNTn equals the OCRnx value, the

compare match will be missed, resulting in incorrect waveform generation. Do not write the TCNTn equal to

TOP in PWM modes with variable TOP values. The compare match for the TOP will be ignored and the counter

will continue to 0xFFFF. Similarly, do not write the TCNTn value equal to BOTTOM when the counter is down-

counting.

The setup of the OCnx should be performed before setting the Data Direction Register for the port pin to output.

The easiest way of setting the OCnx value is to use the Force Output Compare (FOCnx) strobe bits in Normal

mode. The OCnx Register keeps its value even when changing between Waveform Generation modes.

Be aware that the COMnx1:0 bits are not double buffered together with the compare value. Changing the

COMnx1:0 bits will take effect immediately.

14.7 Compa re Match Output Unit

The Compare Output mode (COMnx1:0) bits have two functions. The Waveform Generator uses the COMnx1:0

bits for defining the Output Compare (OCnx) state at the next compare match. Secondly the COMnx1:0 bits

control the OCnx pin output source. Figure 14-5 shows a simplified schematic of the logic affected by the

COMnx1:0 bit setting. The I/O Registers, I/O bits, and I/O pins in the figure are shown in bold. Only the parts of

the general I/O Port Control Registers (DDR and PORT) that are affected by the COMnx1:0 bits are shown.

When referring to the OCnx state, the reference is for the internal OCnx Register, not the OCnx pin. If a system

reset occur, the OCnx Register is reset to “0”.

Figure 14-5. Compare Match Output Unit, Schematic

PORT

DDR

OCnx

Waveform

Generator

COMnx1

COMnx0

DATA B US

FOCnx

clk

I/O

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The general I/O port function is overridden by the Output Compare (OCnx) from the Waveform Generator if

either of the COMnx1:0 bits are set. However, the OCnx pin direction (input or output) is still controlled by the

Data Direction Register (DDR) for the port pin. The Data Direction Register bit for the OCnx pin (DDR_OCnx)

must be set as output before the OCnx value is visible on the pin. The port override function is generally

independent of the Waveform Generation mode, but there are some exceptions. Refer to Table 14-1 on

page 131, Table 14-2 on page 132, and Table 14-3 on page 132 for details.

The design of the Output Compare pin logic allows initialization of the OCnx state before the output is enabled.

Note that some COMnx1:0 bit settings are reserved for certain modes of operation. See “16-bit Timers/Counters

(Timer/Counter1 and Timer/Counter3)” on page 111.

The COMnx1:0 bits have no effect on the Input Capture unit.

14.7.1 Compare Output Mode and Waveform Generation

The Waveform Generator uses the COMnx1:0 bits differently in normal, CTC, and PWM modes. For all modes,

setting the COMnx1:0 = 0 tells the Waveform Generator that no action on the OCnx Register is to be performed

on the next compare match. For compare output actions in the non-PWM modes refer to Table 14-1 on

page 131. For fast PWM mode refer to Table 14-2 on page 132, and for phase correct and phase and frequency

correct PWM refer to Table 14-3 on page 132.

A change of the COMnx1:0 bits state will have effect at the first compare match after the bits are written. For

non-PWM modes, the action can be forced to have immediate effect by using the FOCnx strobe bits.

14.8 Modes of Operation

The mode of operation, i.e., the behavior of the Timer/Counter and the Output Compare pins, is defined by the

combination of the Waveform Generation mode (WGMn3:0) and Compare Output mode (COMnx1:0) bits. The

Compare Output mode bits do not affect the counting sequence, while the Waveform Generation mode bits do.

The COMnx1:0 bits control whether the PWM output generated should be inverted or not (inverted or non-

inverted PWM). For non-PWM modes the COMnx1:0 bits control whether the output should be set, cleared or

toggle at a compare match (See “Compare Match Output Unit” on page 121.)

For detailed timing information refer to “Timer/Counter Timing Diagrams” on page 129.

14.8.1 Normal Mode

The simplest mode of operation is the Normal mode (WGMn3:0 = 0). In this mode the counting direction is

always up (incrementing), and no counter clear is performed. The counter simply overruns when it passes its

maximum 16-bit value (MAX = 0xFFFF) and then restarts from the BOTTOM (0x0000). In normal operation the

Timer/Counter Overflow Flag (TOVn) will be set in the same timer clock cycle as the TCNTn becomes zero. The

TOVn Flag in this case behaves like a 17th bit, except that it is only set, not cleared. However, combined with

the timer overflow interrupt that automatically clears the TOVn Flag, the timer resolution can be increased by

software. There are no special cases to consider in the Normal mode, a new counter value can be written

anytime.

The Input Capture unit is easy to use in Normal mode. However, observe that the maximum interval between

the external events must not exceed the resolution of the counter. If the interval between events are too long,

the timer overflow interrupt or the prescaler must be used to extend the resolution for the capture unit.

The Output Compare units can be used to generate interrupts at some given time. Using the Output Compare to

generate waveforms in Normal mode is not recommended, since this will occupy too much of the CPU time.

14.8.2 Clear Timer on Compare Match (CTC) Mode

In Clear Timer on Compare or CTC mode (WGMn3:0 = 4 or 12), the OCRnA or ICRn Register are used to

manipulate the counter resolution. In CTC mode the counter is cleared to zero when the counter value (TCNTn)

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matches either the OCRnA (WGMn3:0 = 4) or the ICRn (WGMn3:0 = 12). The OCRnA or ICRn define the top

value for the counter, hence also its resolution. This mode allows greater control of the compare match output

frequency. It also simplifies the operation of counting external events.

The timing diagram for the CTC mode is shown in Figure 14-6. The counter value (TCNTn) increases until a

compare match occurs with either OCRnA or ICRn, and then counter (TCNTn) is cleared.

Figure 14-6. CTC Mode, Timing Dia gram

An interrupt can be generated at each time the counter value reaches the TOP value by either using the OCFnA

or ICFn Flag according to the register used to define the TOP value. If the interrupt is enabled, the interrupt

handler routine can be used for updating the TOP value. However, changing the TOP to a value close to

BOTTOM when the counter is running with none or a low prescaler value must be done with care since the CTC

mode does not have the double buffering feature. If the new value written to OCRnA or ICRn is lower than the

current value of TCNTn, the counter will miss the compare match. The counter will then have to count to its

maximum value (0xFFFF) and wrap around starting at 0x0000 before the compare match can occur. In many

cases this feature is not desirable. An alternative will then be to use the fast PWM mode using OCRnA for

defining TOP (WGMn3:0 = 15) since the OCRnA then will be double buffered.

For generating a waveform output in CTC mode, the OCnA output can be set to toggle its logical level on each

compare match by setting the Compare Output mode bits to toggle mode (COMnA1:0 = 1). The OCnA value will

not be visible on the port pin unless the data direction for the pin is set to output (DDR_OCnA = 1). The

waveform generated will have a maximum frequency of fOCnA = fclk_I/O/2 when OCRnA is set to zero (0x0000).

The waveform frequency is defined by the following equation:

The N variable represents the prescaler factor (1, 8, 64, 256, or 1024).

As for the Normal mode of operation, the TOVn Flag is set in the same timer clock cycle that the counter counts

from MAX to 0x0000.

14.8.3 Fast PWM Mode

The fast Pulse Width Modulation or fast PWM mode (WGMn3:0 = 5, 6, 7, 14, or 15) provides a high frequency

PWM waveform generation option. The fast PWM differs from the other PWM options by its single-slope

operation. The counter counts from BOTTOM to TOP then restarts from BOTTOM. In non-inverting Compare

Output mode, the Output Compare (OCnx) is set on the compare match between TCNTn and OCRnx, and

cleared at TOP. In inverting Compare Output mode output is cleared on compare match and set at TOP. Due to

the single-slope operation, the operating frequency of the fast PWM mode can be twice as high as the phase

correct and phase and frequency correct PWM modes that use dual-slope operation. This high frequency

makes the fast PWM mode well suited for power regulation, rectification, and DAC applications. High frequency

allows physically small sized external components (coils, capacitors), hence reduces total system cost.

TCNTn

OCnA

(Toggle)

OCnA Interrupt Flag Set

or ICFn Interrupt Flag Set

(Interrupt on TOP)

1 4

Period

2 3

(COMnA1:0 = 1)

fOCnA fclk_I/O

2N1OCRnA+

---------------------------------------------------=

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The PWM resolution for fast PWM can be fixed to 8-, 9-, or 10-bit, or defined by either ICRn or OCRnA. The

minimum resolution allowed is 2-bit (ICRn or OCRnA set to 0x0003), and the maximum resolution is 16-bit

(ICRn or OCRnA set to MAX). The PWM resolution in bits can be calculated by using the following equation:

In fast PWM mode the counter is incremented until the counter value matches either one of the fixed values

0x00FF, 0x01FF, or 0x03FF (WGMn3:0 = 5, 6, or 7), the value in ICRn (WGMn3:0 = 14), or the value in OCRnA

(WGMn3:0 = 15). The counter is then cleared at the following timer clock cycle. The timing diagram for the fast

PWM mode is shown in Figure 14-7. The figure shows fast PWM mode when OCRnA or ICRn is used to define

TOP. The TCNTn value is in the timing diagram shown as a histogram for illustrating the single-slope operation.

The diagram includes non-inverted and inverted PWM outputs. The small horizontal line marks on the TCNTn

slopes represent compare matches between OCRnx and TCNTn. The OCnx Interrupt Flag will be set when a

compare match occurs.

Figure 14-7. Fast PWM Mode, Timing Diagram

The Timer/Counter Overflow Flag (TOVn) is set each time the counter reaches TOP. In addition the OCnA or

ICFn Flag is set at the same timer clock cycle as TOVn is set when either OCRnA or ICRn is used for defining

the TOP value. If one of the interrupts are enabled, the interrupt handler routine can be used for updating the

TOP and compare values.

When changing the TOP value the program must ensure that the new TOP value is higher or equal to the value

of all of the Compare Registers. If the TOP value is lower than any of the Compare Registers, a compare match

will never occur between the TCNTn and the OCRnx. Note that when using fixed TOP values the unused bits

are masked to zero when any of the OCRnx Registers are written.

The procedure for updating ICRn differs from updating OCRnA when used for defining the TOP value. The ICRn

with none or a low prescaler value, there is a risk that the new ICRn value written is lower than the current value

of TCNTn. The result will then be that the counter will miss the compare match at the TOP value. The counter

will then have to count to the MAX value (0xFFFF) and wrap around starting at 0x0000 before the compare

match can occur. The OCRnA Register however, is double buffered. This feature allows the OCRnA I/O location

to be written anytime. When the OCRnA I/O location is written the value written will be put into the OCRnA

Buffer Register. The OCRnA Compare Register will then be updated with the value in the Buffer Register at the

next timer clock cycle the TCNTn matches TOP. The update is done at the same timer clock cycle as the

TCNTn is cleared and the TOVn Flag is set.

Using the ICRn Register for defining TOP works well when using fixed TOP values. By using ICRn, the OCRnA

RFPWM TOP 1+log

log

-----------------------------------=

TCNTn

OCRnx / TOP Update

and TOVn Interrupt Flag

Set and OCnA Interrupt

Flag Set or ICFn

Interrupt Flag Set

(Interrupt on TOP)

1 7

Period 2 3 4 5 6 8

OCnx

(COMnx1:0 = 2)

(COMnx1:0 = 3)

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actively changed (by changing the TOP value), using the OCRnA as TOP is clearly a better choice due to its

double buffer feature.

In fast PWM mode, the compare units allow generation of PWM waveforms on the OCnx pins. Setting the

COMnx1:0 bits to two will produce a non-inverted PWM and an inverted PWM output can be generated by

setting the COMnx1:0 to three.

Refer to Table 14-1 on page 131, Table 14-2 on page 132, and Table 14-3 on page 132.

The actual OCnx value will only be visible on the port pin if the data direction for the port pin is set as output

(DDR_OCnx). The PWM waveform is generated by setting (or clearing) the OCnx Register at the compare

match between OCRnx and TCNTn, and clearing (or setting) the OCnx Register at the timer clock cycle the

counter is cleared (changes from TOP to BOTTOM).

The PWM frequency for the output can be calculated by the following equation:

The N variable represents the prescaler divider (1, 8, 64, 256, or 1024).

The extreme values for the OCRnx Register represents special cases when generating a PWM waveform

output in the fast PWM mode. If the OCRnx is set equal to BOTTOM (0x0000) the output will be a narrow spike

for each TOP+1 timer clock cycle. Setting the OCRnx equal to TOP will result in a constant high or low output

(depending on the polarity of the output set by the COMnx1:0 bits).

A frequency (with 50% duty cycle) waveform output in fast PWM mode can be achieved by setting OCnA to

toggle its logical level on each compare match (COMnA1:0 = 1). This applies only if OCR1A is used to define

the TOP value (WGM13:0 = 15). The waveform generated will have a maximum frequency of fOCnA = fclk_I/O/2

when OCRnA is set to zero (0x0000). This feature is similar to the OCnA toggle in CTC mode, except the double

buffer feature of the Output Compare unit is enabled in the fast PWM mode.

14.8.4 Phase Correct PWM Mode

The phase correct Pulse Width Modulation or phase correct PWM mode (WGMn3:0 = 1, 2, 3, 10, or 11)

provides a high resolution phase correct PWM waveform generation option. The phase correct PWM mode is,

like the phase and frequency correct PWM mode, based on a dual-slope operation. The counter counts

repeatedly from BOTTOM (0x0000) to TOP and then from TOP to BOTTOM. In non-inverting Compare Output

mode, the Output Compare (OCnx) is cleared on the compare match between TCNTn and OCRnx while

upcounting, and set on the compare match while downcounting. In inverting Output Compare mode, the

operation is inverted. The dual-slope operation has lower maximum operation frequency than single slope

operation. However, due to the symmetric feature of the dual-slope PWM modes, these modes are preferred for

motor control applications.

The PWM resolution for the phase correct PWM mode can be fixed to 8-, 9-, or 10-bit, or defined by either ICRn

or OCRnA. The minimum resolution allowed is 2-bit (ICRn or OCRnA set to 0x0003), and the maximum

resolution is 16-bit (ICRn or OCRnA set to MAX). The PWM resolution in bits can be calculated by using the

following equation:

In phase correct PWM mode the counter is incremented until the counter value matches either one of the fixed

values 0x00FF, 0x01FF, or 0x03FF (WGMn3:0 = 1, 2, or 3), the value in ICRn (WGMn3:0 = 10), or the value in

OCRnA (WGMn3:0 = 11). The counter has then reached the TOP and changes the count direction. The TCNTn

value will be equal to TOP for one timer clock cycle. The timing diagram for the phase correct PWM mode is

shown on Figure 14-8 on page 126. The figure shows phase correct PWM mode when OCRnA or ICRn is used

to define TOP. The TCNTn value is in the timing diagram shown as a histogram for illustrating the dual-slope

fOCnxPWM fclk_I/O

N1TOP+

-----------------------------------=

RPCPWM TOP 1+log

log

-----------------------------------=

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operation. The diagram includes non-inverted and inverted PWM outputs. The small horizontal line marks on

the TCNTn slopes represent compare matches between OCRnx and TCNTn. The OCnx Interrupt Flag will be

set when a compare match occurs.

Figure 14-8. Phase Correct PWM Mode, Timing Diagram

The Timer/Counter Overflow Flag (TOVn) is set each time the counter reaches BOTTOM. When either OCRnA

or ICRn is used for defining the TOP value, the OCnA or ICFn Flag is set accordingly at the same timer clock

cycle as the OCRnx Registers are updated with the double buffer value (at TOP). The Interrupt Flags can be

used to generate an interrupt each time the counter reaches the TOP or BOTTOM value.

When changing the TOP value the program must ensure that the new TOP value is higher or equal to the value

of all of the Compare Registers. If the TOP value is lower than any of the Compare Registers, a compare match

will never occur between the TCNTn and the OCRnx. Note that when using fixed TOP values, the unused bits

are masked to zero when any of the OCRnx Registers are written. As the third period shown in Figure 14-8 on

page 126 illustrates, changing the TOP actively while the Timer/Counter is running in the phase correct mode

can result in an unsymmetrical output. The reason for this can be found in the time of update of the OCRnx

the length of the falling slope is determined by the previous TOP value, while the length of the rising slope is

determined by the new TOP value. When these two values differ the two slopes of the period will differ in length.

The difference in length gives the unsymmetrical result on the output.

It is recommended to use the phase and frequency correct mode instead of the phase correct mode when

changing the TOP value while the Timer/Counter is running. When using a static TOP value there are practically

no differences between the two modes of operation.

In phase correct PWM mode, the compare units allow generation of PWM waveforms on the OCnx pins. Setting

the COMnx1:0 bits to two will produce a non-inverted PWM and an inverted PWM output can be generated by

setting the COMnx1:0 to three.

Refer to Table 14-1 on page 131, Table 14-2 on page 132, and Table 14-3 on page 132

The actual OCnx value will only be visible on the port pin if the data direction for the port pin is set as output

(DDR_OCnx). The PWM waveform is generated by setting (or clearing) the OCnx Register at the compare

match between OCRnx and TCNTn when the counter increments, and clearing (or setting) the OCnx Register at

OCRnx/TOP Update and

OCnA Interrupt Flag Set

or ICFn Interrupt Flag Set

(Interrupt on TOP)

1 2 3 4

TOVn Interrupt Flag Set

(Interrupt on Bottom)

TCNTn

Period

OCnx

(COMnx1:0 = 2)

(COMnx1:0 = 3)

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compare match between OCRnx and TCNTn when the counter decrements. The PWM frequency for the output

when using phase correct PWM can be calculated by the following equation:

The N variable represents the prescaler divider (1, 8, 64, 256, or 1024).

The extreme values for the OCRnx Register represent special cases when generating a PWM waveform output

in the phase correct PWM mode. If the OCRnx is set equal to BOTTOM the output will be continuously low and

if set equal to TOP the output will be continuously high for non-inverted PWM mode. For inverted PWM the

output will have the opposite logic values. If OCR1A is used to define the TOP value (WGM13:0 = 11) and

COM1A1:0 = 1, the OC1A output will toggle with a 50% duty cycle.

14.8.5 Phase and Frequency Correct PWM Mode

The Phase and Frequency Correct PWM Mode (PWM4x = 1 and WGM40 = 1) provides a high resolution Phase

and Frequency Correct PWM waveform generation option. The Phase and Frequency Correct PWM mode is

based on a dual-slope operation. The counter counts repeatedly from BOTTOM to TOP (defined as OCR4C)

and then from TOP to BOTTOM. In noninverting Compare Output Mode, and in complimentary Compare Output

Mode, the Waveform Output (OCW4x) is cleared on the Compare Match between TCNT4 and OCR4x while

upcounting, and set on the Compare Match while down-counting. In inverting Output Compare mode, the

operation is inverted. The dual-slope operation has lower maximum operation frequency than single slope

operation. However, due to the symmetric feature of the dual-slope PWM modes, these modes are preferred for

motor control applications.

The main difference between the phase correct, and the phase and frequency correct PWM mode is the time

the OCRnx Register is updated by the OCRnx Buffer Register, (see Figure 14-8 on page 126 and Figure 14-9

on page 128).

The PWM resolution for the phase and frequency correct PWM mode can be defined by either ICRn or OCRnA.

The minimum resolution allowed is 2-bit (ICRn or OCRnA set to 0x0003), and the maximum resolution is 16-bit

(ICRn or OCRnA set to MAX). The PWM resolution in bits can be calculated using the following equation:

In phase and frequency correct PWM mode the counter is incremented until the counter value matches either

the value in ICRn (WGMn3:0 = 8), or the value in OCRnA (WGMn3:0 = 9). The counter has then reached the

TOP and changes the count direction. The TCNTn value will be equal to TOP for one timer clock cycle. The

timing diagram for the phase correct and frequency correct PWM mode is shown on Figure 14-9. The figure

shows phase and frequency correct PWM mode when OCRnA or ICRn is used to define TOP. The TCNTn

value is in the timing diagram shown as a histogram for illustrating the dual-slope operation. The diagram

includes non-inverted and inverted PWM outputs. The small horizontal line marks on the TCNTn slopes

represent compare matches between OCRnx and TCNTn. The OCnx Interrupt Flag will be set when a compare

match occurs.

fOCnxPCPWM fclk_I/O

2NTOP

----------------------------=

RPFCPWM TOP 1+log

log

-----------------------------------=

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Figure 14-9. Phase and Frequency Correct PWM Mode, Timing Diagram

The Timer/Counter Overflow Flag (TOVn) is set at the same timer clock cycle as the OCRnx Registers are

updated with the double buffer value (at BOTTOM). When either OCRnA or ICRn is used for defining the TOP

value, the OCnA or ICFn Flag set when TCNTn has reached TOP. The Interrupt Flags can then be used to

generate an interrupt each time the counter reaches the TOP or BOTTOM value.

When changing the TOP value the program must ensure that the new TOP value is higher or equal to the value

of all of the Compare Registers. If the TOP value is lower than any of the Compare Registers, a compare match

will never occur between the TCNTn and the OCRnx.

As Figure 14-9 on page 128 shows the output generated is, in contrast to the phase correct mode, symmetrical

in all periods. Since the OCRnx Registers are updated at BOTTOM, the length of the rising and the falling

slopes will always be equal. This gives symmetrical output pulses and is therefore frequency correct.

Using the ICRn Register for defining TOP works well when using fixed TOP values. By using ICRn, the OCRnA

actively changed by changing the TOP value, using the OCRnA as TOP is clearly a better choice due to its

double buffer feature.

In phase and frequency correct PWM mode, the compare units allow generation of PWM waveforms on the

OCnx pins. Setting the COMnx1:0 bits to two will produce a non-inverted PWM and an inverted PWM output can

be generated by setting the COMnx1:0 to three.

Refer to Table 14-1 on page 131, Table 14-2 on page 132, and Table 14-3 on page 132.

The actual OCnx value will only be visible on the port pin if the data direction for the port pin is set as output

(DDR_OCnx). The PWM waveform is generated by setting (or clearing) the OCnx Register at the compare

match between OCRnx and TCNTn when the counter increments, and clearing (or setting) the OCnx Register at

compare match between OCRnx and TCNTn when the counter decrements. The PWM frequency for the output

when using phase and frequency correct PWM can be calculated by the following equation:

The N variable represents the prescaler divider (1, 8, 64, 256, or 1024).

The extreme values for the OCRnx Register represents special cases when generating a PWM waveform

output in the phase correct PWM mode. If the OCRnx is set equal to BOTTOM the output will be continuously

low and if set equal to TOP the output will be set to high for non-inverted PWM mode. For inverted PWM the

OCRnx/TOP Updateand

TOVn Interrupt Flag Set

(Interrupt on Bottom)

OCnA Interrupt Flag Set

or ICFn Interrupt Flag Set

(Interrupt on TOP)

1 2 3 4

TCNTn

Period

OCnx

(COMnx1:0 = 2)

(COMnx1:0 = 3)

fOCnxPFCPWM fclk_I/O

2NTOP

----------------------------=

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output will have the opposite logic values. If OCR1A is used to define the TOP value (WGM13:0 = 9) and

COM1A1:0 = 1, the OC1A output will toggle with a 50% duty cycle.

14.9 Timer/Counter Ti ming Diagrams

The Timer/Counter is a synchronous design and the timer clock (clkTn) is therefore shown as a clock enable

signal in the following figures. The figures include information on when Interrupt Flags are set, and when the

OCRnx Register is updated with the OCRnx buffer value (only for modes utilizing double buffering). Figure 14-

10 shows a timing diagram for the setting of OCFnx.

Figure 14-10. Timer/Counter Timing Dia gram, Setti ng of OCFnx, no Pres c alin g

Figure 14-11 shows the same timing data, but with the prescaler enabled.

Figure 14-11. Timer/Counter Timing Diagram, Setting of OCFnx, with Prescaler (fclk_I/O/8)

Figure 14-12 shows the count sequence close to TOP in various modes. When using phase and frequency

correct PWM mode the OCRnx Register is updated at BOTTOM. The timing diagrams will be the same, but

TOP should be replaced by BOTTOM, TOP-1 by BOTTOM+1 and so on. The same renaming applies for modes

that set the TOVn Flag at BOTTOM.

clk

(clkI/O/1)

OCFnx

clk

I/O

OCRnx

TCNTn

OCRnx V alue

OCRnx - 1 OCRnx OCRnx + 1 OCRnx + 2

OCFnx

OCRnx

TCNTn

OCRnx V alue

OCRnx - 1 OCRnx OCRnx + 1 OCRnx + 2

clk

I/O

clk

(clk

I/O

/8)

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Figure 14-12. Timer/Counter Timing Diagram, no Prescaling

Figure 14-13 shows the same timing data, but with the prescaler enabled.

Figure 14-13. Timer/Counter Timing Diagram, with Prescaler (fclk_I/O/8)

TOVn (FPWM)

and ICFn (if used

as T OP)

OCRnx

(Update at TOP)

TCNTn

(CTC and FPWM)

TCNTn

(PC and PFC PWM) TOP - 1 TOP TOP - 1 TOP - 2

Old OCRnx Value New OCRnx Value

TOP - 1 TOP BOTTOM BOTTOM + 1

clk

(clk

I/O

/1)

clkI/O

TOVn

(FPWM)

and ICFn

(if used

as T OP)

OCRnx

(Update at TOP)

TCNTn

(CTC and FPWM)

TCNTn

(PC and PFC PWM)

TOP - 1 TOP TOP - 1 TOP - 2

Old OCRnx Value New OCRnx Value

TOP - 1 TOP BOTTOM BOTTOM + 1

clk

I/O

clk

(clk

I/O

/8)

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14.10 16-bit Timer/Counter Register Description

14.10.1 Timer/Counter1 Control Register A – TCCR1A

14.10.2 Timer/Counter3 Control Register A – TCCR3A

•Bit 7:6 – COMnA1:0: Compare Output Mode for Channel A

•Bit 5:4 – COMnB1:0: Compare Output Mode for Channel B

•Bit 3:2 – COMnC1:0: Compare Output Mode for Channel C

The COMnA1:0, COMnB1:0, and COMnC1:0 control the output compare pins (OCnA, OCnB, and OCnC

respectively) behavior. If one or both of the COMnA1:0 bits are written to one, the OCnA output overrides the

normal port functionality of the I/O pin it is connected to. If one or both of the COMnB1:0 bits are written to one,

the OCnB output overrides the normal port functionality of the I/O pin it is connected to. If one or both of the

COMnC1:0 bits are written to one, the OCnC output overrides the normal port functionality of the I/O pin it is

connected to. However, note that the Data Direction Register (DDR) bit corresponding to the OCnA, OCnB or

OCnC pin must be set in order to enable the output driver.

When the OCnA, OCnB or OCnC is connected to the pin, the function of the COMnx1:0 bits is dependent of the

WGMn3:0 bits setting. The table shows the COMnx1:0 bit functionality when the WGMn3:0 bits are set to a

normal or a CTC mode (non-PWM).

Bit 76543210

COM1A1 COM1A0 COM1B1 COM1B0 COM1C1 COM1C0 WGM11 WGM10 TCCR1

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value 0 0 0 0 0 0 0 0

Bit 76543210

COM3A1 COM3A0 COM3B1 COM3B0 COM3C1 COM3C0 WGM31 WGM30 TCCR3A

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value 0 0 0 0 0 0 0 0

Table 14-1. Compare Output Mode, non-PWM

COMnA1/COMnB1/COMnC1 COMnA0/COMnB0/COMnC0 Description

0 0 Normal port operation, OCnA/OCnB/OCnC

disconnected

0 1 Toggle OCnA/OCnB/OCnC on compare match

1 0 Clear OCnA/OCnB/OCnC on compare match (set

output to low level)

1 1 Set OCnA/OCnB/OCnC on compare match (set output

to high level)

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The table shows the COMnx1:0 bit functionality when the WGMn3:0 bits are set to the fast PWM mode.

Note: 1. A special case occurs when OCRnA/OCRnB/OCRnC equals TOP and COMnA1/COMnB1/COMnC1 is set. In

this case the compare match is ignored, but the set or clear is done at TOP. See “Fast PWM Mode” on

page 100. for more details.

The table shows the COMnx1:0 bit functionality when the WGMn3:0 bits are set to the phase correct and

frequency correct PWM mode.

Note: 1. A special case occurs when OCRnA/OCRnB/OCRnC equals TOP and COMnA1/COMnB1//COMnC1 is set.

See “Phase Correct PWM Mode” on page 101. for more details.

• Bit 1:0 – WGMn1:0: Waveform Generation Mode

Combined with the WGMn3:2 bits found in the TCCRnB Register, these bits control the counting sequence of

the counter, the source for maximum (TOP) counter value, and what type of waveform generation to be used,

see the table below. Modes of operation supported by the Timer/Counter unit are: Normal mode (counter), Clear

Timer on Compare match (CTC) mode, and three types of Pulse Width Modulation (PWM) modes (“Modes of

Operation” on page 98).

Table 14-2. Compare Outpu t Mode, Fast PWM(1)

COMnA1/COMnB1/COMnC0 COMnA0/COMnB0/COMnC0 Description

0 0 Normal port operation, OCnA/OCnB/OCnC

disconnected

0 1

WGM13:0 = 14 or 15: Toggle OC1A on Compare

Match, OC1B and OC1C disconnected (normal port

operation). For all other WGM1 settings, normal port

operation, OC1A/OC1B/OC1C disconnected.

1 0 Clear OCnA/OCnB/OCnC on compare match, set

OCnA/OCnB/OCnC at TOP

1 1 Set OCnA/OCnB/OCnC on compare match, clear

OCnA/OCnB/OCnC at TOP

Table 14-3. Compare Output Mode, Phase Correct and Phase and Frequency Corre ct PWM (1)

COMnA1/COMnB/COMnC1 COMnA0/COMnB0/COMnC0 Description

0 0 Normal port operation, OCnA/OCnB/OCnC

disconnected.

0 1

WGM13:0 = 8, 9, 10, or 11: Toggle OC1A on Compare

Match, OC1B and OC1C disconnected (normal port

operation). For all other WGM1 settings, normal port

operation, OC1A/OC1B/OC1C disconnected.

1 0

Clear OCnA/OCnB/OCnC on compare match when up-

counting. Set OCnA/OCnB/OCnC on compare match

when down-counting.

1 1

Set OCnA/OCnB/OCnC on compare match when up-

counting. Clear OCnA/OCnB/OCnC on compare match

when down-counting.

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Note: 1. The CTCn and PWMn1:0 bit definition names are obsolete. Use the WGMn2:0 definitions. However, the functionality

and location of these bits are compatible with previous versions of the timer.

14.10.3 Timer/Counter1 Control Register B – TCCR1B

14.10.4 Timer/Counter3 Control Register B – TCCR3B

• Bit 7 – ICNCn: Input Capture Noise Canceler

Setting this bit (to one) activates the Input Capture Noise Canceler. When the Noise Canceler is activated, the

input from the Input Capture Pin (ICPn) is filtered. The filter function requires four successive equal valued

samples of the ICPn pin for changing its output. The input capture is therefore delayed by four Oscillator cycles

when the noise canceler is enabled.

Table 14-4. Waveform Generation Mode Bit Description

Mode WGMn3 WGMn2

(CTCn) WGMn1

(PWMn1) WGMn0

(PWMn0) Timer/Counter Mode of Operation TOP Update of

OCRnx at

TOVn

Flag Set

0 0 0 0 0 Normal 0xFFFF Immediate MAX

1 0 0 0 1 PWM, Phase Correct, 8-bit 0x00FF TOP BOTTOM

2 0 0 1 0 PWM, Phase Correct, 9-bit 0x01FF TOP BOTTOM

3 0 0 1 1 PWM, Phase Correct, 10-bit 0x03FF TOP BOTTOM

4 0 1 0 0 CTC OCRnA Immediate MAX

5 0 1 0 1 Fast PWM, 8-bit 0x00FF TOP TOP

6 0 1 1 0 Fast PWM, 9-bit 0x01FF TOP TOP

7 0 1 1 1 Fast PWM, 10-bit 0x03FF TOP TOP

8 1 0 0 0 PWM, Phase and Frequency Correct ICRn BOTTOM BOTTOM

9 1 0 0 1 PWM, Phase and Frequency Correct OCRnA BOTTOM BOTTOM

10 1 0 1 0 PWM, Phase Correct ICRn TOP BOTTOM

11 1 0 1 1 PWM, Phase Correct OCRnA TOP BOTTOM

12 1 1 0 0 CTC ICRn Immediate MAX

13 1 1 0 1 (Reserved) – – –

14 1 1 1 0 Fast PWM ICRn TOP TOP

15 1 1 1 1 Fast PWM OCRnA TOP TOP

Bit 76543210

ICNC1 ICES1 – WGM13 WGM12 CS12 CS11 CS10 TCCR1B

Read/Write R/W R/W R R/W R/W R/W R/W R/W

Initial Value 0 0 0 0 0 0 0 0

Bit 76543210

ICNC3 ICES3 – WGM33 WGM32 CS32 CS31 CS30 TCCR3B

Read/Write R/W R/W R R/W R/W R/W R/W R/W

Initial Value 0 0 0 0 0 0 0 0

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• Bit 6 – ICESn: Input Capture Edge Select

This bit selects which edge on the Input Capture Pin (ICPn) that is used to trigger a capture event. When the

ICESn bit is written to zero, a falling (negative) edge is used as trigger, and when the ICESn bit is written to one,

a rising (positive) edge will trigger the capture.

When a capture is triggered according to the ICESn setting, the counter value is copied into the Input Capture

Capture Interrupt, if this interrupt is enabled.

When the ICRn is used as TOP value (see description of the WGMn3:0 bits located in the TCCRnA and the

TCCRnB Register), the ICPn is disconnected and consequently the input capture function is disabled.

• Bit 5 – Reserved Bit

This bit is reserved for future use. For ensuring compatibility with future devices, this bit must be written to zero

when TCCRnB is written.

• Bit 4:3 – WGMn3:2: Waveform Generation Mode

See TCCRnA Register description.

• Bit 2:0 – CSn2:0: Clock Select

The three clock select bits select the clock source to be used by the Timer/Counter, see Figure 13-8 on page

102 and Figure 13-9 on page 103.

If external pin modes are used for the Timer/Countern, transitions on the Tn pin will clock the counter even if the

pin is configured as an output. This feature allows software control of the counting.

14.10.5 Timer/Counter1 Control Register C – TCCR1C

Table 14-5. Clock Select Bit Description

CSn2 CSn1 CSn0 Description

000No clock source. (Timer/Counter stopped)

001clkI/O/1 (No prescaling

010clkI/O/8 (From prescaler)

011clkI/O/64 (From prescaler)

100clkI/O/256 (From prescaler)

101clkI/O/1024 (From prescaler)

110External clock source on Tn pin. Clock on falling edge

111External clock source on Tn pin. Clock on rising edge

Bit 7654 3 210

FOC1A FOC1B FOC1C – – – – – TCCR1C

Read/WriteWWWR R R R R

Initial Value00000 000

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14.10.6 Timer/Counter3 Control Register C – TCCR3C

• Bit 7 – FOCnA: Force Out put Compare for Channel A

The FOCnA/FOCnB/FOCnC bits are only active when the WGMn3:0 bits specifies a non-PWM mode. When

writing a logical one to the FOCnA/FOCnB/FOCnC bit, an immediate compare match is forced on the waveform

generation unit. The OCnA/OCnB/OCnC output is changed according to its COMnx1:0 bits setting. Note that the

FOCnA/FOCnB/FOCnC bits are implemented as strobes. Therefore it is the value present in the COMnx1:0 bits

that determine the effect of the forced compare.

A FOCnA/FOCnB/FOCnC strobe will not generate any interrupt nor will it clear the timer in Clear Timer on

Compare Match (CTC) mode using OCRnA as TOP.

The FOCnA/FOCnB/FOCnB bits are always read as zero.

• Bit 4:0 – Reserved Bits

These bits are reserved for future use. For ensuring compatibility with future devices, these bits must be written

to zero when TCCRnC is written.

14.10.7 Timer/Counter1 – TCNT1H and TCNT1L

14.10.8 Timer/Counter3 – TCNT3H and TCNT3L

The two Timer/Counter I/O locations (TCNTnH and TCNTnL, combined TCNTn) give direct access, both for

read and for write operations, to the Timer/Counter unit 16-bit counter. To ensure that both the high and low

bytes are read and written simultaneously when the CPU accesses these registers, the access is performed

using an 8-bit temporary High Byte Register (TEMP). This temporary register is shared by all the other 16-bit

registers. See “Accessing 16-bit Registers” on page 113.

Modifying the counter (TCNTn) while the counter is running introduces a risk of missing a compare match

between TCNTn and one of the OCRnx Registers.

Writing to the TCNTn Register blocks (removes) the compare match on the following timer clock for all compare

units.

Bit 7654 3 210

FOC3A – – – – – – – TCCR3C

Read/Write W R R R R R R R

Initial Value 0 0 0 0 0 0 0 0

Bit 76543210

TCNT1[15:8] TCNT1H

TCNT1[7:0] TCNT1L

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value00000000

Bit 76543210

TCNT3[15:8] TCNT3H

TCNT3[7:0] TCNT3L

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value00000000

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14.10.9 Output Compare Register 1 A – OCR1AH and OCR1AL

14.10.10 Output Compar e Register 1 B – OCR1BH and OCR1BL

14.10.11 Output Compare Register 1 C – OCR1CH and OCR1CL

14.10.12 Output Compare Register 3 A – OCR3AH and OCR3AL

14.10.13 Output Compar e Register 3 B – OCR3BH and OCR3BL

14.10.14 Output Compar e Register 3 C – OCR3CH and OCR3CL

The Output Compare Registers contain a 16-bit value that is continuously compared with the counter value

(TCNTn). A match can be used to generate an Output Compare interrupt, or to generate a waveform output on

the OCnx pin.

Bit 76543210

OCR1A[15:8] OCR1AH

OCR1A[7:0] OCR1AL

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value 0 0 0 0 0 0 0 0

Bit 76543210

OCR1B[15:8] OCR1BH

OCR1B[7:0] OCR1BL

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value00000000

Bit 76543210

OCR1C[15:8] OCR1CH

OCR1C[7:0] OCR1CL

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value 0 0 0 0 0 0 0 0

Bit 76543210

OCR3A[15:8] OCR3AH

OCR3A[7:0] OCR3AL

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value 0 0 0 0 0 0 0 0

Bit 76543210

OCR3B[15:8] OCR3BH

OCR3B[7:0] OCR3BL

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value 0 0 0 0 0 0 0 0

Bit 76543210

OCR3C[15:8] OCR3CH

OCR3C[7:0] OCR3CL

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value 0 0 0 0 0 0 0 0

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The Output Compare Registers are 16-bit in size. To ensure that both the high and low bytes are written

simultaneously when the CPU writes to these registers, the access is performed using an 8-bit temporary High

Byte Register (TEMP). This temporary register is shared by all the other 16-bit registers. See “Accessing 16-bit

Registers” on page 113.

14.10.15 Input Capture Register 1 – ICR1H and ICR1L

14.10.16 Input Capture Register 3 – ICR3H and ICR3L

The Input Capture is updated with the counter (TCNTn) value each time an event occurs on the ICPn pin (or

optionally on the Analog Comparator output for Timer/Counter1). The Input Capture can be used for defining the

counter TOP value.

The Input Capture Register is 16-bit in size. To ensure that both the high and low bytes are read simultaneously

when the CPU accesses these registers, the access is performed using an 8-bit temporary High Byte Register

(TEMP). This temporary register is shared by all the other 16-bit registers. See “Accessing 16-bit Registers” on

page 113.

14.10.17Timer/Counter1 Interrupt Mask Registe r – TIMSK1

14.10.18Timer/Counter3 Interrupt Mask Registe r – TIMSK3

• Bit 5 – ICIEn: Timer/Countern, Input Capture Interrupt Enable

When this bit is written to one, and the I-flag in the Status Register is set (interrupts globally enabled), the

Timer/Countern Input Capture interrupt is enabled. The corresponding Interrupt Vector (See “Interrupts” on

page 63.) is executed when the ICFn Flag, located in TIFRn, is set.

Bit 76543210

ICR1[15:8] ICR1H

ICR1[7:0] ICR1L

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value 0 0 0 0 0 0 0 0

Bit 76543210

ICR3[15:8] ICR3H

ICR3[7:0] ICR3L

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value 0 0 0 0 0 0 0 0

Bit 7654321 0

––ICIE1– OCIE1C OCIE1B OCIE1A TOIE1 TIMSK1

Read/Write R R R/W R R/W R/W R/W R/W

Initial Value 0 0 0 0 0 0 0 0

Bit 7654321 0

––ICIE3– OCIE3C OCIE3B OCIE3A TOIE3 TIMSK3

Read/Write R R R/W R R/W R/W R/W R/W

Initial Value 0 0 0 0 0 0 0 0

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• Bit 3 – OCIEnC: Timer/Countern, Output Compare C Match Interrupt Enable

When this bit is written to one, and the I-flag in the Status Register is set (interrupts globally enabled), the

Timer/Countern Output Compare C Match interrupt is enabled. The corresponding Interrupt Vector (See

“Interrupts” on page 63.) is executed when the OCFnC Flag, located in TIFRn, is set.

• Bit 2 – OCIEnB: Timer/Countern, Output Compare B Match Interrupt Enable

When this bit is written to one, and the I-flag in the Status Register is set (interrupts globally enabled), the

Timer/Countern Output Compare B Match interrupt is enabled. The corresponding Interrupt Vector (See

“Interrupts” on page 63.) is executed when the OCFnB Flag, located in TIFRn, is set.

• Bit 1 – OCIEnA: Timer/Countern, Output Compare A Match Interrupt Enable

When this bit is written to one, and the I-flag in the Status Register is set (interrupts globally enabled), the

Timer/Countern Output Compare A Match interrupt is enabled. The corresponding Interrupt Vector (See

“Interrupts” on page 63.) is executed when the OCFnA Flag, located in TIFRn, is set.

• Bit 0 – TOIEn: Timer/Countern, Overflow Interrupt Enable

When this bit is written to one, and the I-flag in the Status Register is set (interrupts globally enabled), the

Timer/Countern Overflow interrupt is enabled. The corresponding Interrupt Vector (See “Interrupts” on page 63.)

is executed when the TOVn Flag, located in TIFRn, is set.

14.10.19Timer/Counter1 Interrupt Flag Register – TIFR1

14.10.20Timer/Counter3 Interrupt Flag Register – TIFR3

• Bit 5 – ICFn: T imer/Countern, Input Capture Flag

This flag is set when a capture event occurs on the ICPn pin. When the Input Capture Register (ICRn) is set by

the WGMn3:0 to be used as the TOP value, the ICFn Flag is set when the counter reaches the TOP value.

ICFn is automatically cleared when the Input Capture Interrupt Vector is executed. Alternatively, ICFn can be

cleared by writing a logic one to its bit location.

• Bit 3– OCFnC: Timer/Countern, Output Compare C Match Flag

This flag is set in the timer clock cycle after the counter (TCNTn) value matches the Output Compare Register C

(OCRnC).

Note that a Forced Output Compare (FOCnC) strobe will not set the OCFnC Flag.

OCFnC is automatically cleared when the Output Compare Match C Interrupt Vector is executed. Alternatively,

OCFnC can be cleared by writing a logic one to its bit location.

Bit 76543210

––ICF1– OCF1C OCF1B OCF1A TOV1 TIFR1

Read/Write R R R/W R R/W R/W R/W R/W

Initial Value00000000

Bit 76543210

––ICF3– OCF3C OCF3B OCF3A TOV3 TIFR3

Read/Write R R R/W R R/W R/W R/W R/W

Initial Value00000000

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• Bit 2 – OCFnB: Timer/Counter1, Output Compare B Match Flag

This flag is set in the timer clock cycle after the counter (TCNTn) value matches the Output Compare Register B

(OCRnB).

Note that a Forced Output Compare (FOCnB) strobe will not set the OCFnB Flag.

OCFnB is automatically cleared when the Output Compare Match B Interrupt Vector is executed. Alternatively,

OCFnB can be cleared by writing a logic one to its bit location.

• Bit 1 – OCF1A: Timer/Counter1, Outpu t Compare A Match Flag

This flag is set in the timer clock cycle after the counter (TCNTn value matches the Output Compare Register A

(OCRnA).

Note that a Forced Output Compare (FOCnA) strobe will not set the OCFnA Flag.

OCFnA is automatically cleared when the Output Compare Match A Interrupt Vector is executed. Alternatively,

OCFnA can be cleared by writing a logic one to its bit location.

• Bit 0 – TOVn: Timer/Countern, Overflow Flag

The setting of this flag is dependent of the WGMn3:0 bits setting. In Normal and CTC modes, the TOVn Flag is

set when the timer overflows. Refer to Table 14-4 on page 133 for the TOVn Flag behavior when using another

WGMn3:0 bit setting.

TOVn is automatically cleared when the Timer/Countern Overflow Interrupt Vector is executed. Alternatively,

TOVn can be cleared by writing a logic one to its bit location.

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15. 10-bit High Speed Timer/Counter4

15.1 Features

•Up to 10-Bit Accuracy

•Three Independent Output Compare Units

•Clear Timer on Compare Match (Auto Reload)

•Glitch Free, Phase and Frequ ency Correct Pulse Width Modulator (PWM)

•Enhanced PWM mode: one optional additional accuracy bit without effect on output frequency

•Variable PWM Period

•Independent Dead Time Generators for each PWM channels

•Synchronous update of PWM registers

•Five Independent Interrupt Sources (TOV4, OCF 4A, OCF4B, OCF4D, FPF4)

•High Speed Asynchronous and Synchronous Clocking Modes

•Separate Prescaler Unit

15.2 Overview

Timer/Counter4 is a general purpose high speed Timer/Counter module, with three independent Output

Compare Units, and with enhanced PWM support.

The Timer/Counter4 features a high resolution and a high accuracy usage with the lower prescaling

opportunities. It can also support three accurate and high speed Pulse Width Modulators using clock speeds up

to 64MHz. In PWM mode Timer/Counter4 and the output compare registers serve as triple stand-alone PWMs

with non-overlapping, non-inverted and inverted outputs. The enhanced PWM mode allows to get one more

accuracy bit while keeping the frequency identical to normal mode (a PWM 8 bits accuracy in enhanced mode

outputs the same frequency that a PWM 7 bits accuracy in normal mode). Similarly, the high prescaling

opportunities make this unit useful for lower speed functions or exact timing functions with infrequent actions. A

lock feature allows user to update the PWM registers and

A simplified block diagram of the Timer/Counter4 is shown in Figure 15-1 on page 141. For actual placement of

the I/O pins, refer to “Pinout” on page 3. The device-specific I/O register and bit locations are listed in the

“Register Description” on page 164.

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Figure 15-1. Timer/Counter4 Block Diagram

15.2.1 Speed

The maximum speed of the Timer/Counter4 is 64MHz. However, if a supply voltage below 4V is used, it is

recommended to decrease the input frequency, because the Timer/Counter4 is not running fast enough on low

voltage levels.

15.2.2 Accuracy

The Timer/Counter4 is a 10-bit Timer/Counter module that can alternatively be used as an 8-bit Timer/Counter.

The Timer/Counter4 registers are basically 8-bit registers, but on top of that there is a 2-bit High Byte Register

(TC4H) that can be used as a common temporary buffer to access the two MSBs of the 10-bit Timer/Counter4

registers by the AVR CPU via the 8-bit data bus, if the 10-bit accuracy is used. Whereas, if the two MSBs of the

10-bit registers are written to zero the Timer/Counter4 is working as an 8-bit Timer/Counter. When reading the

low byte of any 8-bit register the two MSBs are written to the TC4H register, and when writing the low byte of

any 8-bit register the two MSBs are written from the TC4H register. Special procedures must be followed when

accessing the 10-bit Timer/Counter4 values via the 8-bit data bus. These procedures are described in the

section “Accessing 10-bit Registers” on page 160.

The Enhanced PWM mode allows to add a resolution bit to each Compare register A/B/D, while the output

frequency remains identical to a Normal PWM mode. That means that the TC4H register contains one more bit

that will be the MSB in a 11-bits enhanced PWM operation. See the section “Enhanced Compare/PWM mode”

on page 150 for details about this feature and how to use it.

15.2.3 Registers

The Timer/Counter (TCNT4) and Output Compare Registers (OCR4A, OCR4B, OCR4C and OCR4D) are 8-bit

registers that are used as a data source to be compared with the TCNT4 contents. The OCR4A, OCR4B and

OCR4D registers determine the action on the OC4A, OC4B and OC4D pins and they can also generate the

8-BIT DATABUS

T/C INT. FLAG

10-BIT COMPARATOR

8-BIT OUTPUT COMPARE

T/C INT. MASK

TIMER/COUNTER4

(TCNT4)

DIRECTION

TIMER/COUNTER4 CONTROL LOGIC

OCF4B

TOV4

TOIE4

OCIE4B

OCIE4A

OCF4A

TOV4 OCF4B OC4AOCF4A

T/C CONTROL

COM4B1

PWM4A

PWM4B

COM4B0

FOC4A

FOC4B

10-BIT COMPARATOR

8-BIT OUTPUT COMPARE

COM4A1

COM4A0

T/C CONTROL

CS42

PSR4

CS41

CS40

CS43

OC4A OC4B OC4B

DEAD TIME GENERATOR DEAD TIME GENERATOR

10-BIT COMPARATOR

8-BIT OUTPUT COMPARE

10-BIT COMPARATOR

8-BIT OUTPUT COMPARE

COM4A1

PWM4D

COM4A0

FOC4D

T/C CONTROL

OCF4D

OCIE4D

OC4D OC4D

DEAD TIME GENERATOR

PSR4

OCW4A

OCW4B

OCW4D

2-BIT HIGH BYTE

WGM41

FPEN4

T/C CONTROL

COM4B1

COM4B0

COM4D1

COM4D0

FPNC4

FPES4

FPAC4

10-BIT OUTPUT

COMPARE REGISTER B 10-BIT OUTPUT

COMPARE REGISTER C 10-BIT OUTPUT

COMPARE REGISTER D

10-BIT OUTPUT

COMPARE REGISTER A

CLEAR

COUNT

CLK

T/C CONTROL

FAULT_PROTECTION

WGM40

FPIE4

FPF4

OC4OE5

OC4OE4

OC4OE3

OC4OE2

OC4OE1

OC4OE0

FPIE4

FPF4

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compare match interrupts. The OCR4C holds the Timer/Counter TOP value, i.e. the clear on compare match

value. The Timer/Counter4 High Byte Register (TC4H) is a 2-bit register that is used as a common temporary

buffer to access the MSB bits of the Timer/Counter4 registers, if the 10-bit accuracy is used.

Interrupt request (overflow TOV4, compare matches OCF4A, OCF4B, OCF4D and fault protection FPF4)

signals are visible in the Timer Interrupt Flag Register (TIFR4) and Timer/Counter4 Control Register D

(TCCR4D). The interrupts are individually masked with the Timer Interrupt Mask Register (TIMSK4) and the

FPIE4 bit in the Timer/Counter4 Control Register D (TCCR4D).

Control signals are found in the Timer/Counter Control Registers TCCR4A, TCCR4B, TCCR4C, TCCR4D, and

TCCR4E.

15.2.4 Synchronization

In asynchronous clocking mode the Timer/Counter4 and the prescaler allow running the CPU from any clock

source while the prescaler is operating on the fast peripheral clock (PCK) having frequency up to 64MHz. This is

possible because there is a synchronization boundary between the CPU clock domain and the fast peripheral

clock domain. Figure 15-2 on page 143 shows Timer/Counter 4 synchronization register block diagram and

describes synchronization delays in between registers. Note that all clock gating details are not shown in the

figure.

The Timer/Counter4 register values go through the internal synchronization registers, which cause the input

synchronization delay, before affecting the counter operation. The registers TCCR4A, TCCR4B, TCCR4C,

TCCR4D, OCR4A, OCR4B, OCR4C, and OCR4D can be read back right after writing the register. The read

back values are delayed for the Timer/Counter4 (TCNT4) register, Timer/Counter4 High Byte Register (TC4H)

and flags (OCF4A, OCF4B, OCF4D, and TOV4), because of the input and output synchronization.

The system clock frequency must be lower than half of the PCK frequency, because the synchronization

mechanism of the asynchronous Timer/Counter4 needs at least two edges of the PCK when the system clock is

high. If the frequency of the system clock is too high, it is a risk that data or control values are lost.

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Figure 15-2. Timer/Counter4 Synchronization Register Blo ck Diagram

15.2.5 Definitions

Many register and bit references in this section are written in general form. A lower case “n” replaces the

Timer/Counter number, in this case 0. A lower case “x” replaces the Output Compare Unit, in this case Compare

Unit A, B, C or D. However, when using the register or bit defines in a program, the precise form must be used,

i.e., TCNT4 for accessing Timer/Counter4 counter value and so on.

The definitions in the table are used extensively throughout the document.

8-BIT DATABUS

OCR4A OCR4A_SI

TCNT4_SO

OCR4B OCR4B_SI

OCR4C OCR4C_SI

TCCR4A TCCR4A_SI

TCCR4B TCCR4B_SI

TCNT4 TCNT4_SI

OCF4A OCF4A_SI

OCF4B OCF4B_SI

TOV4 TOV4_SI TOV4_SO

OCF4B_SO

OCF4A_SO

TCNT4

PLLTM1:0

!= '00'

PCK

(clkTMR)

IO-registers Input synchronization

registers

Timer/Counter4 Output synchronization

registers

SYNC

MODE

ASYNC

MODE

1 CK Delay 1/2 CK Delay

~1/2 CK Delay 1 PCK Delay 1 PCK Delay ~1 CK Delay

TCNT4

OCF4

OCF4B

TOV4

1/2 CK Delay 1 CK Delay

OCR4D OCR4D_SI

TC4H TC4H_SI

TCCR4C TCCR4C_SI

TCCR4D

OCF4D OCF4D_SI

OCF4D_SO

OCF4D

TC4H_SO

TC4H

TCCR4D_SI

BOTTOM The counter reaches the BOTTOM when it becomes 0.

MAX The counter reaches its MAXimum value when it becomes 0x3FF (decimal 1023).

TOP The counter reaches the TOP value (stored in the OCR1C) when it becomes equal to the

highest value in the count sequence. The TOP has a value 0x0FF as default after reset.

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15.3 Counter Unit

The main part of the Timer/Counter4 is the programmable bi-directional counter unit. Figure 15-3 shows a block

diagram of the counter and its surroundings.

Figure 15-3. Counter Un it Block Diagram

Signal description (internal signals):

count: TCNT4 increment or decrement enable.

direction: Select between increment and decrement.

clear: Clear TCNT4 (set all bits to zero).

clkTn: Timer/Counter clock, referred to as clkT4 in the following.

top: Signalize that TCNT4 has reached maximum value.

bottom: Signalize that TCNT4 has reached minimum value (zero).

Depending of the mode of operation used, the counter is cleared, incremented, or decremented at each timer

clock (clkT4). The timer clock is generated from an synchronous system clock or an asynchronous PLL clock

using the Clock Select bits (CS4<3:0>) and the PLL Postscaler for High Speed Timer bits (PLLTM1:0). When no

clock source is selected (CS4<3:0> = 0) the timer is stopped. However, the TCNT4 value can be accessed by

the CPU, regardless of whether clkT1 is present or not. A CPU write overrides (has priority over) all counter clear

or count operations.

The counting sequence of the Timer/Counter4 is determined by the setting of the WGM10 and PWM4x bits

located in the Timer/Counter4 Control Registers (TCCR4A, TCCR4C, and TCCR4D). For more details about

advanced counting sequences and waveform generation, see “Modes of Operation” on page 151. The

Timer/Counter Overflow Flag (TOV4) is set according to the mode of operation selected by the PWM4x and

WGM40 bits. The Overflow Flag can be used for generating a CPU interrupt.

15.3.1 Counter Initialization for Asynchronous Mode

To change Timer/Counter4 to the asynchronous mode follow the procedure below:

1. Enable PLL.

2. Wait 100µs for PLL to stabilize.

3. Poll the PLOCK bit until it is set.

4. Configure the PLLTM1:0 bits in the PLLFRQ register to enable the asynchronous mode (different from

0:0 value).

DATA BUS

TCNT4 Control Logic

count

TOV4

top

Timer/Counter4 Count Enable

( From Prescaler )

bottom

direction

clear PCK

PLLTM1:0

clk

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15.4 Output Compare Unit

The comparator continuously compares TCNT4 with the Output Compare Registers (OCR4A, OCR4B, OCR4C,

and OCR4D). Whenever TCNT4 equals to the Output Compare Register, the comparator signals a match. A

match will set the Output Compare Flag (OCF4A, OCF4B, or OCF4D) at the next timer clock cycle. If the

corresponding interrupt is enabled, the Output Compare Flag generates an Output Compare interrupt. The

Output Compare Flag is automatically cleared when the interrupt is executed. Alternatively, the flag can be

cleared by software by writing a logical one to its I/O bit location. The Waveform Generator uses the match

signal to generate an output according to operating mode set by the PWM4x, WGM40, and Compare Output

mode (COM4x1:0) bits. The top and bottom signals are used by the Waveform Generator for handling the

special cases of the extreme values in some modes of operation (See “Modes of Operation” on page 151.)

Figure 15-4 shows a block diagram of the Output Compare unit.

Figure 15-4. Output Com pare Unit, Block Diagram

The OCR4x Registers are double buffered when using any of the Pulse Width Modulation (PWM) modes. For

the normal mode of operation, the double buffering is disabled. The double buffering synchronizes the update of

the OCR4x Compare Registers to either top or bottom of the counting sequence. The synchronization prevents

the occurrence of odd-length, non-symmetrical PWM pulses, thereby making the output glitch-free. See Figure

15-5 on page 146 for an example. During the time between the write and the update operation, a read from

OCR4A, OCR4B, OCR4C, or OCR4D will read the contents of the temporary location. This means that the most

recently written value always will read out of OCR4A, OCR4B, OCR4C, or OCR4D.

OCFnx (Int.Req.)

(10-bit Comparator )

8-BIT DATA BUS

TCNTn

WGMn0

Waveform Generator

COMnX1:0

PWMnx

TCnH

OCWnx

10-BIT TCNTn10-BIT OCRnx

OCRnx

FOCn

TOP

BOTTOM

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Figure 15-5. Effects of Unsynchronized OCR Latching

15.4.1 Force Output Compare

In non-PWM waveform generation modes, the match output of the comparator can be forced by writing a one to

the Force Output Compare (FOC4x) bit. Forcing Compare Match will not set the OCF4x Flag or reload/clear the

timer, but the Waveform Output (OCW4x) will be updated as if a real Compare Match had occurred (the

COM4x1:0 bits settings define whether the Waveform Output (OCW4x) is set, cleared or toggled).

15.4.2 Compare Match Blocking by TCNT4 Write

All CPU write operations to the TCNT4 Register will block any Compare Match that occur in the next timer clock

cycle, even when the timer is stopped. This feature allows OCR4x to be initialized to the same value as TCNT4

without triggering an interrupt when the Timer/Counter clock is enabled.

15.4.3 Using the Output Compare Unit

Since writing TCNT4 in any mode of operation will block all Compare Matches for one timer clock cycle, there

are risks involved when changing TCNT4 when using the Output Compare Unit, independently of whether the

Timer/Counter is running or not. If the value written to TCNT4 equals the OCR4x value, the Compare Match will

be missed, resulting in incorrect waveform generation. Similarly, do not write the TCNT4 value equal to

BOTTOM when the counter is down-counting.

The setup of the Waveform Output (OCW4x) should be performed before setting the Data Direction Register for

the port pin to output. The easiest way of setting the OCW4x value is to use the Force Output Compare (FOC4x)

strobe bits in Normal mode. The OC4x keeps its value even when changing between Waveform Generation

modes.

Be aware that the COM4x1:0 bits are not double buffered together with the compare value. Changing the

COM4x1:0 bits will take effect immediately.

Output Compare

Waveform OCWnx

Output Compare

Wafeform OCWnx

Unsynchronized WFnx Latch

Synchronized WFnx Latch

Counter Value

Compare Value

Counter Value

Compare Value

Compare Value changes

Glitch

Compare Value changes

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15.5 Dead Time Generator

The Dead Time Generator is provided for the Timer/Counter4 PWM output pairs to allow driving external power

control switches safely. The Dead Time Generator is a separate block that can be used to insert dead times

(non-overlapping times) for the Timer/Counter4 complementary output pairs OC4x and OC4x when the PWM

mode is enabled and the COM4x1:0 bits are set to “01”. The sharing of tasks is as follows: the Waveform

Generator generates the Waveform Output (OCW4x) and the Dead Time Generator generates the non-

overlapping PWM output pair from the Waveform Output. Three Dead Time Generators are provided, one for

each PWM output. The non-overlap time is adjustable and the PWM output and it’s complementary output are

adjusted separately, and independently for both PWM outputs.

Figure 15-6. Output Com pare Unit, Block Diagram

The Dead Time Generation is based on the 4-bit down counters that count the dead time, as shown in Figure

15-7. There is a dedicated prescaler in front of the Dead Time Generator that can divide the Timer/Counter4

clock (PCK or CK) by 1, 2, 4, or 8. This provides for large range of dead times that can be generated. The

prescaler is controlled by two control bits DTPS41..40. The block has also a rising and falling edge detector that

is used to start the dead time counting period. Depending on the edge, one of the transitions on the rising

edges, OC4x or OC4x is delayed until the counter has counted to zero. The comparator is used to compare the

counter with zero and stop the dead time insertion when zero has been reached. The counter is loaded with a 4-

bit DT4H or DT4L value from DT4 I/O register, depending on the edge of the Waveform Output (OCW4x) when

the dead time insertion is started. The Output Compare Output are delayed by one timer clock cycle at minimum

from the Waveform Output when the Dead Time is adjusted to zero. The outputs OC4x and OC4x are inverted,

if the PWM Inversion Mode bit PWM4X is set. This will also cause both outputs to be high during the dead time.

Figure 15-7. Dead Time Generator

The length of the counting period is user adjustable by selecting the dead time prescaler setting by using the

DTPS41:40 control bits, and selecting then the dead time value in I/O register DT4. The DT4 register consists of

two 4-bit fields, DT4H and DT4L that control the dead time periods of the PWM output and its' complementary

output separately in terms of the number of prescaled dead time generator clock cycles. Thus the rising edge of

OCnx

WGMn0

Waveform Generator

top

FOCn

COMnx

bottom

PWMnx

OCWnx

Dead Time Generator

OCnx

DTnH DTnL

DTPSn

CK OR PCK

CLOCK

OCnx

CLOCK CONTROL

OCnx

CK OR PCK

CLOCK

OCWnx

4-BIT COUNTER

COMPARATOR

DTnL

DTnH

DEAD TIME

PRE-SCALER

DTPSn

DTn I/O REGISTER

DATA BUS (8-bit)

TCCRnB REGISTER

PWMnX

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OC4x and OC4x can have different dead time periods as the tnon-overlap / rising edge is adjusted by the 4-bit DT4H

value and the tnon-overlap / falling edge is adjusted by the 4-bit DT4L value.

Figure 15-8. The Complementary Output Pair, COM4x1:0 = 1

15.6 Compa re Match Output Unit

The Compare Output Mode (COM4x1:0) bits have two functions. The Waveform Generator uses the COM4x1:0

bits for defining the inverted or non-inverted Waveform Output (OCW4x) at the next Compare Match. Also, the

COM4x1:0 bits control the OC4x and OC4x pin output source. Figure 15-9 shows a simplified schematic of the

logic affected by the COM4x1:0 bit setting. The I/O Registers, I/O bits, and I/O pins in the figure are shown in

bold. Only the parts of the general I/O Port Control Registers (DDR and PORT) that are affected by the

COM4x1:0 bits are shown.

In Normal Mode (non-PWM) the Dead Time Generator is disabled and it is working like a synchronizer: the

Output Compare (OC4x) is delayed from the Waveform Output (OCW4x) by one timer clock cycle. Whereas in

Fast PWM Mode and in Phase and Frequency Correct PWM Mode when the COM4x1:0 bits are set to “01” both

the non-inverted and the inverted Output Compare output are generated, and an user programmable Dead

Time delay is inserted for these complementary output pairs (OC4x and OC4x). The functionality in PWM

modes is similar to Normal mode when any other COM4x1:0 bit setup is used. When referring to the OC4x

state, the reference is for the Output Compare output (OC4x) from the Dead Time Generator, not the OC4x pin.

If a system reset occur, the OC4x is reset to “0”.

The general I/O port function is overridden by the Output Compare (OC4x / OC4x) from the Dead Time

Generator if either of the COM4x1:0 bits are set. However, the OC4x pin direction (input or output) is still

controlled by the Data Direction Register (DDR) for the port pin. The Data Direction Register bit for the OC4x

and OC4x pins (DDR_OC4x and DDR_OC4x) must be set as output before the OC4x and OC4x values are

visible on the pin. The port override function is independent of the Output Compare mode.

The design of the Output Compare Pin Configuration logic allows initialization of the OC4x state before the

output is enabled. Note that some COM4x1:0 bit settings are reserved for certain modes of operation. For

Output Compare Pin Configurations refer to Table 15-1 on page 152, Table 15-2 on page 154, Table 15-3 on

page 155, and Table 15-4 on page 157.

OCnx

(COMnx = 1)

tnon-overlap / rising edge tnon-overlap / falling edge

OCnx

OCWnx

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Figure 15-9. Compare Match Output Unit, Schematic

15.6.1 Compare Output Mode and Waveform Generation

The Waveform Generator uses the COM4x1:0 bits differently in Normal mode and PWM modes. For all modes,

setting the COM4x1:0 = 0 tells the Waveform Generator that no action on the OCW4x Output is to be performed

on the next Compare Match. For compare output actions in the non-PWM modes refer to Table 15-5 on

page 164. For fast PWM mode, refer to Table 15-6 on page 164, and for the Phase and Frequency Correct

PWM refer to Table 15-7 on page 165. A change of the COM4x1:0 bits state will have effect at the first Compare

Match after the bits are written. For non-PWM modes, the action can be forced to have immediate effect by

using the FOC4x strobe bits.

DATA BUS

PORTC6

DDRC6

DDRC7

PORTC7

clkI/O

PORTB5

DDRB5

DDRB6

PORTB6

PORTD6

DDRD6

DDRD7

PORTD7

OC4D

Dead Time

Generator D Q

OCW4D

clk Tn OC4D

Output Compare

Pin Configuration

COM4D1:0

WGM41

OC4OE5:4

OC4B

Dead Time

Generator B Q

OCW4B

clk Tn OC4B

Output Compare

Pin Configuration

COM4B1:0

WGM41

OC4OE3:2

OC4A

Dead Time

Generator A Q

OCW4A

clk Tn OC4A

Output Compare

Pin Configuration

COM4A1:0

WGM41

OC4OE1:0

OC4A

OC4B

OC4

OC4D

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15.6.2 Enhanced Compare/PWM mode

When the bit ENHC4 of TCCR4E register is set, the Enhanced Compare/PWM mode is enabled. This mode

allows user to add an accuracy bit to Output Compare Register OCR4A, OCR4B, and OCR4D. Like explained

previously, a compare condition appears when one of the three Output Compare Registers (OCR4A/B/D)

matches the value of TCNT4 (10-bits resolution). In basic PWM Mode, the corresponding enabled output

toggles on the Compare Match. The Enhanced Compare/PWM mode introduces a bit that determines on which

internal clock edge the Compare Match condition is actually signalled. That means that the corresponding

outputs will toggle on the standard clock edge (like in Normal mode) if the LSB of OCR4A/B/D is ‘0’, or on the

opposite (next) edge if the LSB is ‘1’.

User will notice that between Normal and Enhanced PWM modes, the output frequency will be identical, while

the PWM resolution will be better in second case.

Writing to the Output Compare registers OCR4A/B/D or reading them will be identical in both modes. In

Enhanced mode, user must just consider that the TC4H register can be up to 3-bits wide (and have the same

behavior than during 2-bits operation). That will concern OCR4A, OCR4B and OCR4D registers accesses only.

Indeed, the OCR4C register must not include the additional accuracy bit, and remains in the resolution that

determines the output signal period.

Figure 15-10. How Register Access Works in Enhanced Mode

Figure 15-10 shows that the true OCR4A/B/D value corresponds to the value loaded by the user shifted on the

right in order to transfer the least significant bit directly to the Waveform generation module.

The maximum available resolution is 11-bits, but any other resolution can be specified. For example, a 8-bits

resolution will allow to obtain the same frequency than a Normal PWM mode with 7-bits resolution.

Example:

PLL Postcaler output = 64MHz, No Prescaler on Timer/Counter4.

Setting OCR4C = 0x7F determines a full 7-bits theoretical resolution, and so a 500kHz output

frequency.

Setting OCR4A = 0x85 (= b’10000101’) signifies that the true value of “Compare A” register is 0x42

(b’01000010’) and that the Enhanced bit is set. That means that the duty cycle obtained (51.95%)

will be the intermediate value between duty cycles that can be obtained by 0x42 and 0x43 Compare

values (51.56%, 52.34%).

9 6 3 15

7420

9 6 3 15

7420

(TC4H) (OCR4A/B/D)

Output Compare Module A/B/D

Waveform Generation

Enhanced

Mode

Pin Toggle

True

OCR4A/B/D

User Interface Side

Timer Logic Side

ENHC4

TCNT4<9:0>

OCR4C<9:0>

Configuration

bits

(LSB)

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15.7 Synchronous update

To avoid unasynchronous and incoherent values in a cycle, if a synchronous update of one of several values is

necessary, all values can be updated at the same time at the end of the PWM cycle by the Timer controller. The

new set of values is calculated by software and the effective update can be initiated by software.

Figure 15-11. Lock Feature and Synchronous update

In normal operation, each write to a Compare register is effective at the end of the current cycle. But some

cases require that two or more Compare registers are updated synchronously, and that may not be always

possible, mostly at high speed PWM frequencies. That may result in some PWM periods with incoherent values.

When using the Lock feature (TLOCK4=1), the values written to the Compare registers are not effective and

temporarily buffered. When releasing the TLOCK4 bit, the update is initiated and the new whole set of values

will be loaded at the end of the current PWM cycle.

Refer to “TCCR4E – Timer/Counter4 Control Register E” on page 171.

15.8 Modes of Operation

The mode of operation, i.e., the behavior of the Timer/Counter and the Output Compare pins, is defined by the

combination of the Waveform Generation mode (bits PWM4x and WGM40) and Compare Output mode

(COM4x1:0) bits. The Compare Output mode bits do not affect the counting sequence, while the Waveform

Generation mode bits do. The COM4x1:0 bits control whether the PWM output generated should be inverted,

non-inverted or complementary. For non-PWM modes the COM4x1:0 bits control whether the output should be

set, cleared, or toggled at a Compare Match.

15.8.1 Normal Mode

The simplest mode of operation is the Normal mode (PWM4x = 0), the counter counts from BOTTOM to TOP

(defined as OCR4C) then restarts from BOTTOM. The OCR4C defines the TOP value for the counter, hence

also its resolution, and allows control of the Compare Match output frequency. In toggle Compare Output Mode

the Waveform Output (OCW4x) is toggled at Compare Match between TCNT4 and OCR4x. In non-inverting

Compare Output Mode the Waveform Output is cleared on the Compare Match. In inverting Compare Output

Mode the Waveform Output is set on Compare Match.

The timing diagram for the Normal mode is shown in Figure 15-12. The counter value (TCNT4) that is shown as

a histogram in the timing diagram is incremented until the counter value matches the TOP value. The counter is

then cleared at the following clock cycle The diagram includes the Waveform Output (OCW4x) in toggle

Compare Mode. The small horizontal line marks on the TCNT4 slopes represent Compare Matches between

OCR4x and TCNT4.

Regulation Loop

Calculation

Writing to Timer

Registers Set j

Request for an

Update

Cycle with

Set i

Cycle with

Set i

Cycle with

Set i

Cycle with

Set i

Cycle with

Set j

TLOCK4=1 TLOCK4=0

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Figure 15-12. Normal Mode, Timing Diagram

The Timer/Counter Overflow Flag (TOV4) is set in the same clock cycle as the TCNT4 becomes zero. The

TOV4 Flag in this case behaves like a 11th bit, except that it is only set, not cleared. However, combined with

the timer overflow interrupt, that automatically clears the TOV4 Flag, the timer resolution can be increased by

software. There are no special cases to consider in the Normal mode, a new counter value can be written

anytime.

The Output Compare Unit can be used to generate interrupts at some given time. Using the Output Compare to

generate waveforms in Normal mode is not recommended, since this will occupy too much of the CPU time. For

generating a waveform, the OCW4x output can be set to toggle its logical level on each Compare Match by

setting the Compare Output mode bits to toggle mode (COM4x1:0 = 1). The OC4x value will not be visible on

the port pin unless the data direction for the pin is set to output. The waveform generated will have a maximum

frequency of fOC4x = fclkT4/4 when OCR4C is set to zero. The waveform frequency is defined by the following

equation:

Resolution shows how many bit is required to express the value in the OCR4C register. It is calculated by

following equation:

ResolutionPWM = log2(OCR4C + 1).

The Output Compare Pin configurations in Normal Mode are described in the table below.

15.8.2 Fast PWM Mode

The fast Pulse Width Modulation or fast PWM mode (PWM4x = 1 and WGM40 = 0) provides a high frequency

PWM waveform generation option. The fast PWM differs from the other PWM option by its single-slope

Table 15-1. Output Compare Pin Configurations in Normal Mode

COM4x1 COM4x0 OC4x Pin OC4x Pin

00Disconnected Disconnected

01Disconnected OC4x

10Disconnected OC4x

11Disconnected OC4x

TCNTn

OCWnx

(COMnx=1)

OCnx Interrupt Flag Set

1 4

Period

2 3

TOVn Interrupt Flag Set

fOC4xfclkT4

21OCR4C+

-------------------------------------------=

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operation. The counter counts from BOTTOM to TOP (defined as OCR4C) then restarts from BOTTOM. In non-

inverting Compare Output mode the Waveform Output (OCW4x) is cleared on the Compare Match between

TCNT4 and OCR4x and set at BOTTOM. In inverting Compare Output mode, the Waveform Output is set on

Compare Match and cleared at BOTTOM. In complementary Compare Output mode the Waveform Output is

cleared on the Compare Match and set at BOTTOM.

Due to the single-slope operation, the operating frequency of the fast PWM mode can be twice as high as the

Phase and Frequency Correct PWM mode that use dual-slope operation. This high frequency makes the fast

PWM mode well suited for power regulation, rectification, and DAC applications. High frequency allows

physically small sized external components (coils, capacitors), and therefore reduces total system cost.

The timing diagram for the fast PWM mode is shown in Figure 15-13. The counter is incremented until the

counter value matches the TOP value. The counter is then cleared at the following timer clock cycle. The

TCNT4 value is in the timing diagram shown as a histogram for illustrating the single-slope operation. The

diagram includes the Waveform Output in non-inverted and inverted Compare Output modes. The small

horizontal line marks on the TCNT4 slopes represent Compare Matches between OCR4x and TCNT4.

Figure 15-13. Fast PWM Mode, Timing Diagram

The Timer/Counter Overflow Flag (TOV4) is set each time the counter reaches TOP. If the interrupt is enabled,

the interrupt handler routine can be used for updating the compare value. In fast PWM mode, the compare unit

allows generation of PWM waveforms on the OC4x pins. Setting the COM4x1:0 bits to two will produce a non-

inverted PWM and setting the COM4x1:0 to three will produce an inverted PWM output. Setting the COM4x1:0

bits to one will enable complementary Compare Output mode and produce both the non-inverted (OC4x) and

inverted output (OC4x). The actual value will only be visible on the port pin if the data direction for the port pin is

set as output. The PWM waveform is generated by setting (or clearing) the Waveform Output (OCW4x) at the

Compare Match between OCR4x and TCNT4, and clearing (or setting) the Waveform Output at the timer clock

cycle the counter is cleared (changes from TOP to BOTTOM).

The PWM frequency for the output can be calculated by the following equation:

The N variable represents the number of steps in single-slope operation. The value of N equals either to the

TOP value.

The extreme values for the OCR4C Register represents special cases when generating a PWM waveform

output in the fast PWM mode. If the OCR4C is set equal to BOTTOM, the output will be a narrow spike for each

MAX+1 timer clock cycle. Setting the OCR4C equal to MAX will result in a constantly high or low output

(depending on the polarity of the output set by the COM4x1:0 bits.)

TCNTn

OCRnx Update and

TOVn Interrupt Flag Set

Period

2 3

OCWnx

(COMnx1:0 = 2)

(COMnx1:0 = 3)

OCRnx Interrupt Flag Set

4 5 6 7

fOCnxPWM fclkT4

-------------=

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A frequency (with 50% duty cycle) waveform output in fast PWM mode can be achieved by setting the

Waveform Output (OCW4x) to toggle its logical level on each Compare Match (COM4x1:0 = 1). The waveform

generated will have a maximum frequency of fOC4 = fclkT4/4 when OCR4C is set to three.

The general I/O port function is overridden by the Output Compare value (OC4x / OC4x) from the Dead Time

Generator, if either of the COM4x1:0 bits are set and the Data Direction Register bits for the OC4X and OC4X

pins are set as an output. If the COM4x1:0 bits are cleared, the actual value from the port register will be visible

on the port pin. The Output Compare Pin configurations are described in the table below.

15.8.3 Phase and Frequency Correct PWM Mode

The Phase and Frequency Correct PWM Mode (PWM4x = 1 and WGM40 = 1) provides a high resolution Phase

and Frequency Correct PWM waveform generation option. The Phase and Frequency Correct PWM mode is

based on a dual-slope operation. The counter counts repeatedly from BOTTOM to TOP (defined as OCR4C)

and then from TOP to BOTTOM. In non-inverting Compare Output Mode, and in complimentary Compare

Output Mode, the Waveform Output (OCW4x) is cleared on the Compare Match between TCNT4 and OCR4x

while upcounting, and set on the Compare Match while down-counting. In inverting Output Compare mode, the

operation is inverted. The dual-slope operation has lower maximum operation frequency than single slope

operation. However, due to the symmetric feature of the dual-slope PWM modes, these modes are preferred for

motor control applications.

The timing diagram for the Phase and Frequency Correct PWM mode is shown on Figure 15-14 in which the

TCNT4 value is shown as a histogram for illustrating the dual-slope operation. The counter is incremented until

the counter value matches TOP. When the counter reaches TOP, it changes the count direction. The TCNT4

value will be equal to TOP for one timer clock cycle. The diagram includes the Waveform Output (OCW4x) in

non-inverted and inverted Compare Output Mode. The small horizontal line marks on the TCNT4 slopes

represent Compare Matches between OCR4x and TCNT4.

Table 15-2. Output Compare Pin Configurations in Fast PWM Mode

COM4x1 COM4x0 OC4x Pin OC4x Pin

00Disconnected Disconnected

01OC4x OC4x

10Disconnected OC4x

11Disconnected OC4x

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Figure 15-14. Ph ase and Frequency Correct PWM Mode, Timing Diagram

The Timer/Counter Overflow Flag (TOV4) is set each time the counter reaches BOTTOM. The Interrupt Flag

can be used to generate an interrupt each time the counter reaches the BOTTOM value.

In the Phase and Frequency Correct PWM mode, the compare unit allows generation of PWM waveforms on

the OC4x pins. Setting the COM4x1:0 bits to two will produce a non-inverted PWM and setting the COM4x1:0 to

three will produce an inverted PWM output. Setting the COM4A1:0 bits to one will enable complementary

Compare Output mode and produce both the non-inverted (OC4x) and inverted output (OC4x). The actual

values will only be visible on the port pin if the data direction for the port pin is set as output. The PWM

waveform is generated by clearing (or setting) the Waveform Output (OCW4x) at the Compare Match between

OCR4x and TCNT4 when the counter increments, and setting (or clearing) the Waveform Output at Compare

Match when the counter decrements. The PWM frequency for the output when using the Phase and Frequency

Correct PWM can be calculated by the following equation:

The N variable represents the number of steps in dual-slope operation. The value of N equals to the TOP value.

The extreme values for the OCR4C Register represent special cases when generating a PWM waveform output

in the Phase and Frequency Correct PWM mode. If the OCR4C is set equal to BOTTOM, the output will be

continuously low and if set equal to MAX the output will be continuously high for non-inverted PWM mode. For

inverted PWM the output will have the opposite logic values.

The general I/O port function is overridden by the Output Compare value (OC4x / OC4x) from the Dead Time

Generator, if either of the COM4x1:0 bits are set and the Data Direction Register bits for the OC4X and OC4X

pins are set as an output. If the COM4x1:0 bits are cleared, the actual value from the port register will be visible

on the port pin. The configurations of the Output Compare Pins are described in the table below.

Table 15-3. Output Compare pin co nfigurations in Phase and Frequency Correct PWM Mode

COM4x1 COM4x0 OC4x Pin OC4x Pin

00Disconnected Disconnected

01OC4x OC4x

10Disconnected OC4x

11Disconnected OC4x

TOVn Interrupt Flag Set

OCnx Interrupt Flag Set

1 2 3

TCNTn

Period

OCWnx

(COMnx = 2)

(COMnx = 3)

OCRnx Update

fOCnxPCPWM fclkT4

-------------=

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15.8.4 PWM6 Mode

The PWM6 Mode (PWM4A = 1, WGM41 = 1, and WGM40 = x) provide PWM waveform generation option e.g.

for controlling Brushless DC (BLDC) motors. In the PWM6 Mode the OCR4A Register controls all six Output

Compare waveforms as the same Waveform Output (OCW4A) from the Waveform Generator is used for

generating all waveforms. The PWM6 Mode also provides an Output Compare Override Enable Register

(OC4OE) that can be used with an instant response for disabling or enabling the Output Compare pins. If the

Output Compare Override Enable bit is cleared, the actual value from the port register will be visible on the port

pin.

The PWM6 Mode provides two counter operation modes, a single-slope operation and a dual-slope operation. If

the single-slope operation is selected (the WGM40 bit is set to 0), the counter counts from BOTTOM to TOP

(defined as OCR4C) then restart from BOTTOM like in Fast PWM Mode. The PWM waveform is generated by

setting (or clearing) the Waveform Output (OCW4A) at the Compare Match between OCR4A and TCNT4, and

clearing (or setting) the Waveform Output at the timer clock cycle the counter is cleared (changes from TOP to

BOTTOM). The Timer/Counter Overflow Flag (TOV4) is set each time the counter reaches the TOP and, if the

interrupt is enabled, the interrupt handler routine can be used for updating the compare value.

Whereas, if the dual-slope operation is selected (the WGM40 bit is set to 1), the counter counts repeatedly from

BOTTOM to TOP (defined as OCR4C) and then from TOP to BOTTOM like in Phase and Frequency Correct

PWM Mode. The PWM waveform is generated by setting (or clearing) the Waveform Output (OCW4A) at the

Compare Match between OCR4A and TCNT4 when the counter increments, and clearing (or setting) the

Waveform Output at the he Compare Match between OCR4A and TCNT4 when the counter decrements. The

Timer/Counter Overflow Flag (TOV4) is set each time the counter reaches the BOTTOM and, if the interrupt is

enabled, the interrupt handler routine can be used for updating the compare value.

The timing diagram for the PWM6 Mode in single-slope operation (WGM41 = 0) when the COM4A1:0 bits are

set to “10” is shown in Figure 15-15 on page 157. The counter is incremented until the counter value matches

the TOP value. The counter is then cleared at the following timer clock cycle. The TCNT4 value is in the timing

diagram shown as a histogram for illustrating the single-slope operation. The timing diagram includes Output

Compare pins OC4A and OC4A, and the corresponding Output Compare Override Enable bits

(OC4OE1..OC4OE0).

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Figure 15-15. PWM6 Mode, Single-slope Operation, Timing Diagram

The general I/O port function is overridden by the Output Compare value (OC4x / OC4x) from the Dead Time

Generator if either of the COM4x1:0 bits are set. The Output Compare pins can also be overridden by the

Output Compare Override Enable bits OC4OE5..OC4OE0. If an Override Enable bit is cleared, the actual value

from the port register will be visible on the port pin and, if the Override Enable bit is set, the Output Compare pin

is allowed to be connected on the port pin. The Output Compare Pin configurations are described in the table.

Table 15-4. Output Compare Pin configurations in PWM6 Mode

COM4A1 COM4A0 OC4A Pin (PC6) OC4A Pin (PC7)

00Disconnected Disconnected

01OC4A • OC4OE0 OC4A • OC4OE1

10OC4A • OC4OE0 OC4A • OC4OE1

11OC4A • OC4OE0 OC4A • OC4OE1

COM4B1 COM4B0 OC4B Pin (PB5) OC4B Pin (PB6)

00Disconnected Disconnected

01OC4A • OC4OE2 OC4A • OC4OE3

10OC4A • OC4OE2 OC4A • OC4OE3

11OC4A • OC4OE2 OC4A • OC4OE3

COM4D1 COM4D0 OC4D Pin (PD6) OC4D Pin (PD7)

TCNT4

OC4A Pin

OC4B Pin

OC4D Pin

OC4OE0

OC4OE1

OC4OE2

OC4OE3

OC4OE4

OC4OE5

OCW4A

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15.9 Timer/Counter Ti ming Diagrams

The Timer/Counter is a synchronous design and the timer clock (clkT4) is therefore shown as a clock enable

signal in the following figures. The figures include information on when Interrupt Flags are set.

Figure 15-16 contains timing data for basic Timer/Counter operation. The figure shows the count sequence

close to the MAX value in all modes other than Phase and Frequency Correct PWM Mode. Figure 15-17 shows

the same timing data, but with the prescaler enabled, in all modes other than Phase and Frequency Correct

PWM Mode. Figure 15-18 on page 159 shows the setting of OCF4A, OCF4B, and OCF4D in all modes, and

Figure 15-19 on page 159 shows the setting of TOV4 in Phase and Frequency Correct PWM Mode.

Figure 15-16. Timer/Counter Timing Diagram, no Prescaling

Figure 15-17. Timer/Counter Timing Diagram, with Prescaler (fclkT4/8)

00Disconnected Disconnected

01OC4A • OC4OE4 OC4A • OC4OE5

10OC4A • OC4OE4 OC4A • OC4OE5

11OC4A • OC4OE4 OC4A • OC4OE5

Table 15-4. Output Compare Pin configurations in PWM6 Mode

COM4A1 COM4A0 OC4A Pin (PC6) OC4A Pin (PC7)

clk

(clk

PCK

/1)

TOVn

clk

PCK

TCNTn TOP - 1 TOP BOTTOM BOTTOM + 1

TOVn

TCNTn TOP - 1 TOP BOTTOM BOTTOM + 1

clk

PCK

clk

(clk

PCK

/8)

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Figure 15-18. Timer/Counter Timing Diagram, Setting of OCF1x, with Prescaler (fclkT4/8)

Figure 15-19. Timer/Counter Timing Diagram, with Prescaler (fclkT4/8)

15.10 Fault Protection Unit

The Timer/Counter4 incorporates a Fault Protection unit that can disable the PWM output pins, if an external

event is triggered. The external signal indicating an event can be applied via the external interrupt INT0 pin or

alternatively, via the analog-comparator unit. The Fault Protection unit is illustrated by the block diagram shown

in Figure 15-20. The elements of the block diagram that are not directly a part of the Fault Protection unit are

gray shaded.

Figure 15-20. Fault Protection Unit Block Diagram

When the Fault Protection mode is enabled by the Fault Protection Enable (FPEN4) bit and a change of the

logic level (an event) occurs on the external interrupt pin (INT0), alternatively on the Analog Comparator output

(ACO), and this change confirms to the setting of the edge detector, a Fault Protection mode will be triggered.

When a Fault Protection is triggered, the COM4x bits are cleared, Output Comparators are disconnected from

the PWM output pins and the PORTB register bits are connected on the PWM output pins. The Fault Protection

Enable (FPEN4) is automatically cleared at the same system clock as the COM4nx bits are cleared. If the Fault

Protection Interrupt Enable bit (FPIE4) is set, a Fault Protection interrupt is generated and the FPEN4 bit is

cleared. Alternatively the FPEN4 bit can be polled by software to figure out when the Timer/Counter has entered

to Fault Protection mode.

OCFnx

OCRnx

TCNTn

OCRnx Value

OCRnx - 1 OCRnx OCRnx + 1 OCRnx + 2

clkPCK

clkTn

(clkPCK

/8)

TOVn

TCNTn BOTTOM + 1 BOTTOM + 1 BOTTOM BOTTOM + 1

clkPCK

clkTn

(clk

PCK

/8)

Analog

Comparator Noise

Canceler

INT0

Edge

Detector

FPAC4 FPNC4 FPES4

ACO* FPEN4

Timer/Counter4

FAULT_PROTECTION (Int. Req.)

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15.10.1 Fault Protection Trigger Source

The main trigger source for the Fault Protection unit is the external interrupt pin (INT0). Alternatively the Analog

Comparator output can be used as trigger source for the Fault Protection unit. The Analog Comparator is

selected as trigger source by setting the Fault Protection Analog Comparator (FPAC4) bit in the Timer/Counter4

Control Register (TCCR4D). Be aware that changing trigger source can trigger a Fault Protection mode.

Therefore it is recommended to clear the FPF4 flag after changing trigger source, setting edge detector or

enabling the Fault Protection.

Both the external interrupt pin (INT0) and the Analog Comparator output (ACO) inputs are sampled using the

same technique as for the T0 pin (Figure 12-1 on page 92). The edge detector is also identical. However, when

the noise canceler is enabled, additional logic is inserted before the edge detector, which increases the delay by

four system clock cycles. An Input Capture can also be triggered by software by controlling the port of the INT0

pin.

15.10.2 Noise Ca nc e ler

The noise canceler improves noise immunity by using a simple digital filtering scheme. The noise canceler input

is monitored over four samples, and all four must be equal for changing the output that in turn is used by the

edge detector.

The noise canceler is enabled by setting the Fault Protection Noise Canceler (FPNC4) bit in Timer/Counter4

Control Register D (TCCR4D). When enabled the noise canceler introduces additional four system clock cycles

of delay from a change applied to the input. The noise canceler uses the system clock and is therefore not

affected by the prescaler.

15.11 Accessing 10-bit Registers

If 10-bit values are written to the TCNTn and OCRnA/B/C/D registers, the 10-bit registers can be byte accessed

by the AVR CPU via the 8-bit data bus using two read or write operations. The 10-bit registers have a common

2-bit Timer/Counter4 High Byte Register (TC4H) that is used for temporary storing of the two MSBs of the 10-bit

access. The same TC4H register is shared between all 10-bit registers. Accessing the low byte triggers the 10-

bit read or write operation. When the low byte of a 10-bit register is written by the CPU, the high byte stored in

the TC4H register, and the low byte written are both copied into the 10-bit register in the same clock cycle.

When the low byte of a 10-bit register is read by the CPU, the high byte of the 10-bit register is copied into the

TC4H register in the same clock cycle as the low byte is read.

To do a 10-bit write, the high byte must be written to the TC4H register before the low byte is written. For a 10-

bit read, the low byte must be read before the high byte.

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The following code examples show how to access the 10-bit timer registers assuming that no interrupts updates

the TC4H register. The same principle can be used directly for accessing the OCRnA/B/C/C/D registers.

Note: 1. The example code assumes that the part specific header file is included.

For I/O registers located in extended I/O map, “IN”, “OUT”, “SBIS”, “SBIC”, “CBI”, and “SBI” instructions must be

replaced with instructions that allow access to extended I/O. Typically “LDS” and “STS” combined with “SBRS”,

“SBRC”, “SBR”, and “CBR”.

The assembly code example returns the TCNTn value in the r17:r16 register pair.

It is important to notice that accessing 10-bit registers are atomic operations. If an interrupt occurs between the

two instructions accessing the 10-bit register, and the interrupt code updates the TC4H register by accessing

the same or any other of the 10-bit timer registers, then the result of the access outside the interrupt will be

corrupted. Therefore, when both the main code and the interrupt code update the TC4H register, the main code

must disable the interrupts during the 16-bit access.

Assembly Code Example

...

; Set TCNTn to 0x01FF

ldi r17,0x01

ldi r16,0xFF

out TCnH,r17

out TCNTn,r16

; Read TCNTn into r17:r16

in r16,TCNTn

in r17,TCnH

...

C Code Example

unsigned int i;

...

/* Set TCNTn to 0x01FF */

TCnH = 0x01;

TCNTn = 0xFF;

/* Read TCNTn into i */

i = TCNTn;

i |= ((unsigned int)TCnH << 8);

...

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The following code examples show how to do an atomic read of the TCNTn register contents. Reading any of

the OCRnA/B/C/D registers can be done by using the same principle.

Note: 1. The example code assumes that the part specific header file is included.

For I/O registers located in extended I/O map, “IN”, “OUT”, “SBIS”, “SBIC”, “CBI”, and “SBI” instructions must be

replaced with instructions that allow access to extended I/O. Typically “LDS” and “STS” combined with “SBRS”,

“SBRC”, “SBR”, and “CBR”.

The assembly code example returns the TCNTn value in the r17:r16 register pair.

Assembly Code Example

TIM1_ReadTCNTn:

; Save global interrupt flag

in r18,SREG

; Disable interrupts

cli

; Read TCNTn into r17:r16

in r16,TCNTn

in r17,TCnH

; Restore global interrupt flag

out SREG,r18

ret

C Code Example

unsigned int TIM1_ReadTCNTn( void )

{

unsigned char sreg;

unsigned int i;

/* Save global interrupt flag */

sreg = SREG;

/* Disable interrupts */

_CLI();

/* Read TCNTn into i */

i = TCNTn;

i |= ((unsigned int)TCnH << 8);

/* Restore global interrupt flag

SREG = sreg;

return i;

}

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The following code examples show how to do an atomic write of the TCNTn register contents. Writing any of the

OCRnA/B/C/D registers can be done by using the same principle.

Note: 1. The example code assumes that the part specific header file is included.

For I/O registers located in extended I/O map, “IN”, “OUT”, “SBIS”, “SBIC”, “CBI”, and “SBI” instructions must be

replaced with instructions that allow access to extended I/O. Typically “LDS” and “STS” combined with “SBRS”,

“SBRC”, “SBR”, and “CBR”.

The assembly code example requires that the r17:r16 register pair contains the value to be written to TCNTn.

15.11.1 Reusing the Temporary High Byte Register

If writing to more than one 10-bit register where the high byte is the same for all registers written, then the high

byte only needs to be written once. However, note that the same rule of atomic operation described previously

also applies in this case.

Assembly Code Example

TIM1_WriteTCNTn:

; Save global interrupt flag

in r18,SREG

; Disable interrupts

cli

; Set TCNTn to r17:r16

out TCnH,r17

out TCNTn,r16

; Restore global interrupt flag

out SREG,r18

ret

C Code Example

void TIM1_WriteTCNTn( unsigned int i )

{

unsigned char sreg;

unsigned int i;

/* Save global interrupt flag */

sreg = SREG;

/* Disable interrupts */

_CLI();

/* Set TCNTn to i */

TCnH = (i >> 8);

TCNTn = (unsigned char)i;

/* Restore global interrupt flag */

SREG = sreg;

}

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15.12 Register Description

15.12.1 TCCR4A – Timer/Counter4 Control Register A

• Bits 7, 6 - COM4A1, COM4A0: Comparator A Output Mode, Bits 1 and 0

These bits control the behavior of the Waveform Output (OCW4A) and the connection of the Output Compare

pin (OC4A). If one or both of the COM4A1:0 bits are set, the OC4A output overrides the normal port functionality

of the I/O pin it is connected to. The complementary OC4B output is connected only in PWM modes when the

COM4A1:0 bits are set to “01”. Note that the Data Direction Register (DDR) bit corresponding to the OC4A and

OC4A pins must be set in order to enable the output driver.

The function of the COM4A1:0 bits depends on the PWM4A, WGM40 and WGM41 bit settings. The table shows

the COM4A1:0 bit functionality when the PWM4A bit is set to Normal Mode (non-PWM).

The table shows the COM4A1:0 bit functionality when the PWM4A, WGM40 and WGM41 bits are set to fast

PWM mode.

The table shows the COM4A1:0 bit functionality when the PWM4A, WGM40, and WGM41 bits are set to Phase

and Frequency Correct PWM Mode.

Bit 76543210

COM4A1 COM4A0 COM4B1 COM4B0 FOC4A FOC4B PWM4A PWM4B TCCR4A

Read/Write R/W R/W R/W R/W W W R/W R/W

Initial value00000000

Table 15-5. Compare Output Mode, Normal Mode (non-PWM)

COM4A1..0 OCW4A Behavior OC4A Pin OC4A Pin

00 Normal port operation Disconnected Disconnected

01 Toggle on Compare Match Connected Disconnected

10 Clear on Compare Match Connected Disconnected

11 Set on Compare Match Connected Disconnected

Table 15-6. Compare Output Mode, Fast PWM Mode

COM4A1..0 OCW4A Behavior OC4A OC4A

00 Normal port operation Disconnected Disconnected

01 Cleared on Compare Match.

Set when TCNT4 = 0x000. Connected Connected

10 Cleared on Compare Match.

Set when TCNT4 = 0x000. Connected Disconnected

11 Set on Compare Match.

Cleared when TCNT4 = 0x000. Connected Disconnected

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The table shows the COM4A1:0 bit functionality when the PWM4A, WGM40, and WGM41 bits are set to single-

slope PWM6 Mode. In the PWM6 Mode the same Waveform Output (OCW4A) is used for generating all

waveforms and the Output Compare values OC4A and OC4A are connected on OC4x and OC4x pins as

described below.

The table shows the COM4A1:0 bit functionality when the PWM4A, WGM40, and WGM41 bits are set to dual-

slope PWM6 Mode.

• Bits 5,4 - COM4B1, COM4B0: Comparator B Output Mode, Bits 1 and 0

These bits control the behavior of the Waveform Output (OCW4B) and the connection of the Output Compare

pin (OC4B). If one or both of the COM4B1:0 bits are set, the OC4B output overrides the normal port functionality

Table 15-7. Compare Outpu t Mode, Phase and Frequency Correct PWM Mode

COM1A1..0 OCW1A Behavior OC4A Pin OC4A Pin

00 Normal port operation. Disconnected Disconnected

01 Cleared on Compare Match when up-counting.

Set on Compare Match when down-counting. Connected Connected

10 Cleared on Compare Match when up-counting.

Set on Compare Match when down-counting. Connected Disconnected

11 Set on Compare Match when up-counting.

Cleared on Compare Match when down-counting. Connected Disconnected

Table 15-8. Compare Output Mode, Single-Slope PWM6 Mode

COM4A1..0 OCW4A Behavior OC4x Pin OC4x Pin

00 Normal port operation Disconnected Disconnected

01 Cleared on Compare Match.

Set when TCNT4 = 0x000. OC4A OC4A

10 Cleared on Compare Match.

Set when TCNT4 = 0x000. OC4A OC4A

11 Set on Compare Match.

Cleared when TCNT4 = 0x000. OC4A OC4A

Table 15-9. Compare Output Mo de, Dual-Slope PWM6 Mode

COM4A1..0 OCW4A Behavior OC4x Pin OC4x Pin

00 Normal port operation Disconnected Disconnected

01 Cleared on Compare Match when up-counting.

Set on Compare Match when down-counting. OC4A OC4A

10 Cleared on Compare Match when up-counting.

Set on Compare Match when down-counting. OC4A OC4A

11 Set on Compare Match when up-counting.

Cleared on Compare Match when down-counting. OC4A OC4A

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of the I/O pin it is connected to. The complementary OC4B output is connected only in PWM modes when the

COM4B1:0 bits are set to “01”. Note that the Data Direction Register (DDR) bit corresponding to the OC4B pin

must be set in order to enable the output driver.

The function of the COM4B1:0 bits depends on the PWM4B and WGM40 bit settings.

The table shows the COM4B1:0 bit functionality when the PWM4B bit is set to Normal Mode (non-PWM).

The table shows the COM4B1:0 bit functionality when the PWM4B and WGM40 bits are set to Fast PWM Mode.

The table shows the COM4B1:0 bit functionality when the PWM4B and WGM40 bits are set to Phase and

Frequency Correct PWM Mode.

• Bit 3 - FOC4A: Force Output Compare Match 4A

The FOC4A bit is only active when the PWM4A bit specify a non-PWM mode.

Table 15-10. Compare Output Mode, Normal Mode (non-PWM)

COM4B1..0 OCW4B Behavior OC4B Pin OC4B Pin

00 Normal port operation Disconnected Disconnected

01 Toggle on Compare Match Connected Disconnected

10 Clear on Compare Match Connected Disconnected

11 Set on Compare Match Connected Disconnected

Table 15-11. Compare Output Mode, Fast PWM Mode

COM4B1..0 OCW4B Behavior OC4B Pin OC4B Pin

00 Normal port operation Disconnected Disconnected

01 Cleared on Compare Match.

Set when TCNT4 = 0x000. Connected Connected

10 Cleared on Compare Match.

Set when TCNT4 = 0x000. Connected Disconnected

11 Set on Compare Match.

Cleared when TCNT4 = 0x000. Connected Disconnected

Table 15-12. Co mpare Output Mode, Phase and Frequency Correct PWM Mode

COM4B1..0 OCW4B Behavior OC4B Pin OC4B Pin

00 Normal port operation Disconnected Disconnected

01 Cleared on Compare Match when up-counting.

Set on Compare Match when down-counting. Connected Connected

10 Cleared on Compare Match when up-counting.

Set on Compare Match when down-counting. Connected Disconnected

11 Set on Compare Match when up-counting.

Cleared on Compare Match when down-counting. Connected Disconnected

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Writing a logical one to this bit forces a change in the Waveform Output (OCW4A) and the Output Compare pin

(OC4A) according to the values already set in COM4A1 and COM4A0. If COM4A1 and COM4A0 written in the

same cycle as FOC4A, the new settings will be used. The Force Output Compare bit can be used to change the

output pin value regardless of the timer value. The automatic action programmed in COM4A1 and COM4A0

takes place as if a compare match had occurred, but no interrupt is generated. The FOC4A bit is always read as

zero.

• Bit 2 - FOC4B: Force Output Compare Match 4B

The FOC4B bit is only active when the PWM4B bit specify a non-PWM mode.

Writing a logical one to this bit forces a change in the Waveform Output (OCW4B) and the Output Compare pin

(OC4B) according to the values already set in COM4B1 and COM4B0. If COM4B1 and COM4B0 written in the

same cycle as FOC4B, the new settings will be used. The Force Output Compare bit can be used to change the

output pin value regardless of the timer value. The automatic action programmed in COM4B1 and COM4B0

takes place as if a compare match had occurred, but no interrupt is generated.

The FOC4B bit is always read as zero.

• Bit 1 - PWM4A: Pulse Width Modulator A Enable

When set (one) this bit enables PWM mode based on comparator OCR4A.

• Bit 0 - PWM4B: Pulse Width Modulator B Enable

When set (one) this bit enables PWM mode based on comparator OCR4B.

15.12.2 TCCR4B – Timer/Counter4 Control Register B

• Bit 7 - PWM4X: PWM Inversion Mode

When this bit is set (one), the PWM Inversion Mode is selected and the Dead Time Generator outputs, OC4x

and OC4x are inverted.

• Bit 6 - PSR4: Prescaler Reset Timer/Counter4

When this bit is set (one), the Timer/Counter4 prescaler (TCNT4 is unaffected) will be reset. The bit will be

cleared by hardware after the operation is performed. Writing a zero to this bit will have no effect. This bit will

always read as zero.

• Bits 5,4 - DTPS41, DTPS40: Dead Time Prescaler Bits

The Timer/Counter4 Control Register B is a 8-bit read/write register.

The dedicated Dead Time prescaler in front of the Dead Time Generator can divide the Timer/Counter4 clock

(PCK or CK) by 1, 2, 4, or 8 providing a large range of dead times that can be generated. The Dead Time

prescaler is controlled by two bits DTPS41 and DTPS40 from the Dead Time Prescaler register. These bits

define the division factor of the Dead Time prescaler. The division factors are given in the table below.

Bit 76543210

PWM4X PSR4 DTPS41 DTPS40 CS43 CS42 CS41 CS40 TCCR4B

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial value00000000

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• Bits 3..0 - CS43, CS42, CS41, CS40: Clock Select Bits 3, 2, 1, and 0

The Clock Select bits 3, 2, 1, and 0 define the prescaling source of Timer/Counter4.

The Stop condition provides a Timer Enable/Disable function.

Table 15-13. Division factors of the Dead Time prescaler

DTPS41 DTPS40 Prescaler divides the T/C4 clock by

001x (no division)

012x

104x

118x

Table 15-14. Time r/Counter4 Prescaler Select

CS43 CS42 CS41 CS40 Asynchronous Clocking Mode Synchronou s Clocking Mode

0000T/C4 stopped T/C4 stopped

0001PCK CK

0010PCK/2 CK/2

0011PCK/4 CK/4

0100PCK/8 CK/8

0101PCK/16 CK/16

0110PCK/32 CK/32

0111PCK/64 CK/64

1000PCK/128 CK/128

1001PCK/256 CK/256

1010PCK/512 CK/512

1011PCK/1024 CK/1024

1100PCK/2048 CK/2048

1101PCK/4096 CK/4096

1110PCK/8192 CK/8192

1111PCK/16384 CK/16384

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15.12.3 TCCR4C – Timer/Counter4 Control Register C

• Bits 7,6 - COM4A1S, COM4A0S: Comparator A Output Mode, Bits 1 and 0

These bits are the shadow bits of the COM4A1 and COM4A0 bits that are described in the section “TCCR4A –

Timer/Counter4 Control Register A” on page 164.

• Bits 5,4 - COM4B1S, COM4B0S: Comparator B Output Mode, Bits 1 and 0

These bits are the shadow bits of the COM4A1 and COM4A0 bits that are described in the section “TCCR4A –

Timer/Counter4 Control Register A” on page 164.

• Bits 3,2 - COM4D1, COM4D0: Comparator D Output Mode, Bits 1 and 0

These bits control the behavior of the Waveform Output (OCW4D) and the connection of the Output Compare

pin (OC4D). If one or both of the COM4D1:0 bits are set, the OC4D output overrides the normal port

functionality of the I/O pin it is connected to. The complementary OC4D output is connected only in PWM modes

when the COM4D1:0 bits are set to “01”. Note that the Data Direction Register (DDR) bit corresponding to the

OC4D pin must be set in order to enable the output driver.

The function of the COM4D1:0 bits depends on the PWM4D and WGM40 bit settings.

The table shows the COM4D1:0 bit functionality when the PWM4D bit is set to a Normal Mode (non-PWM).

The table shows the COM4D1:0 bit functionality when the PWM4D and WGM40 bits are set to Fast PWM Mode.

Bit 76543210

COM4A1S COM4A0S COM4B1S COMAB0S COM4D1 COM4D0 FOC4D PWM4D TCCR4C

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial value 00000000

Table 15-15. Compare Output Mode, Normal Mode (non-PWM)

COM4D1..0 OCW4D Behavior OC4 D Pin OC4D Pin

00 Normal port operation Disconnected Disconnected

01 Toggle on Compare Match Connected Disconnected

10 Clear on Compare Match Connected Disconnected

11 Set on Compare Match Connected Disconnected

Table 15-16. Compare Output Mode, Fast PW M Mode

COM4D1..0 OCW4D Behavior OC4D Pin OC4D Pin

00 Normal port operation Disconnected Disconnected

01 Cleared on Compare Match

Set when TCNT4 = 0x000 Connected Connected

10 Cleared on Compare Match

Set when TCNT4 = 0x000 Connected Disconnected

11 Set on Compare Match

Clear when TCNT4 = 0x000 Connected Disconnected

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The table shows the COM4D1:0 bit functionality when the PWM4D and WGM40 bits are set to Phase and

Frequency Correct PWM Mode

• Bit 1 - FOC4D: Force Output Compare Match 4D

The FOC4D bit is only active when the PWM4D bit specify a non-PWM mode.

Writing a logical one to this bit forces a change in the Waveform Output (OCW4D) and the Output Compare pin

(OC4D) according to the values already set in COM4D1 and COM4D0. If COM4D1 and COM4D0 written in the

same cycle as FOC4D, the new settings will be used. The Force Output Compare bit can be used to change the

output pin value regardless of the timer value. The automatic action programmed in COM4D1 and COM4D0

takes place as if a compare match had occurred, but no interrupt is generated. The FOC4D bit is always read as

zero.

• Bit 0 - PWM4D: Pulse Width Modulator D Enable

When set (one) this bit enables PWM mode based on comparator OCR4D.

15.12.4 TCCR4D – Timer/Counter4 Control Register D

• Bit 7 - FPIE4: Fault Protection Interrupt Enable

Setting this bit (to one) enables the Fault Protection Interrupt.

• Bit 6– FPEN4: Fault Protection Mode Enable

Setting this bit (to one) activates the Fault Protection Mode.

• Bit 5 – FPNC4: Fault Protection Noise Canceler

Setting this bit activates the Fault Protection Noise Canceler. When the noise canceler is activated, the input

from the Fault Protection Pin (INT0) is filtered. The filter function requires four successive equal valued samples

of the INT0 pin for changing its output. The Fault Protection is therefore delayed by four Oscillator cycles when

the noise canceler is enabled.

• Bit 4 – FPES4: Fault Protection Edge Select

This bit selects which edge on the Fault Protection pin (INT0) is used to trigger a fault event. When the FPES4

bit is written to zero, a falling (negative) edge is used as trigger, and when the FPES4 bit is written to one, a

rising (positive) edge will trigger the fault.

Table 15-17. Co mpare Output Mode, Phase and Frequency Correct PWM Mode

COM4D1..0 OCW4D Behavior OC4D Pin OC4D Pin

00 Normal port operation Disconnected Disconnected

01 Cleared on Compare Match when up-counting

Set on Compare Match when down-counting Connected Connected

10 Cleared on Compare Match when up-counting

Set on Compare Match when down-counting Connected Disconnected

11 Set on Compare Match when up-counting

Cleared on Compare Match when down-counting Connected Disconnected

Bit 76543210

FPIE4 FPEN4 FPNC4 FPES4 FPAC4 FPF4 WGM41 WGM40 TCCR4D

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial value 00000000

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• Bit 3 - FPAC4: Fault Protection Analog Comparator Enable

When written logic one, this bit enables the Fault Protection function in Timer/Counter4 to be triggered by the

Analog Comparator. The comparator output is in this case directly connected to the Fault Protection front-end

logic, making the comparator utilize the noise canceler and edge select features of the Timer/Counter4 Fault

Protection interrupt. When written logic zero, no connection between the Analog Comparator and the Fault

Protection function exists. To make the comparator trigger the Timer/Counter4 Fault Protection interrupt, the

FPIE4 bit in the Timer/Counter4 Control Register D (TCCR4D) must be set.

• Bit 2- FPF4: Fault Protection Interru pt Flag

When the FPIE4 bit is set (one), the Fault Protection Interrupt is enabled. Activity on the pin will cause an

interrupt request even, if the Fault Protection pin is configured as an output. The corresponding interrupt of Fault

Protection Interrupt Request is executed from the Fault Protection Interrupt Vector. The bit FPF4 is cleared by

hardware when executing the corresponding interrupt handling vector. Alternatively, FPF4 is cleared after a

synchronization clock cycle by writing a logical one to the flag. When the SREG I-bit, FPIE4 and FPF4 are set,

the Fault Interrupt is executed.

• Bits 1:0 - WGM41, WGM40: Waveform Generation Mode Bits

This bit associated with the PWM4x bits control the counting sequence of the counter, the source for type of

waveform generation to be used, see the table below.

The Modes of operation supported by the Timer/Counter4 are: Normal mode (counter), Fast PWM Mode, Phase

and Frequency Correct PWM and PWM6 Modes.

15.12.5 TCCR4E – Timer/Counter4 Control Register E

• Bit 7 - TLOCK4: Register Update Lock

This bit controls the Compare registers update. When this bit is set, writing to the Compare registers will not

affect the output, however the values are stored and will be updated to the Compare registers when the

TLOCK4 bit will be cleared.

Refer to “Synchronous update” on page 151 for more details.

Table 15-18. Wavefo rm Generation Mode Bit Description

PWM4x WGM41..40 Timer/Counter Mode of Operation TOP Update of

OCR4x at TOV 4 Flag

Set on

0 xx Normal OCR4C Immediate TOP

100 Fast PWM OCR4C TOP TOP

101 Phase and Frequency Correct PWM OCR4C BOTTOM BOTTOM

110 PWM6 / Single-slope OCR4C TOP TOP

111 PWM6 / Dual-slope OCR4C BOTTOM BOTTOM

Bit 76543210

TLOCK4 ENHC4 OC4OE5 OC4OE4 OC4OE3 OC4OE2 OC4OE1 OC4OE0 TCCR4E

Read/Write R R R/W R/W R/W R/W R/W R/W

Initial value 00000000

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• Bit 6- ENHC4: Enhanced Comp are/PWM Mode

When this bit is set, the Waveform Generation Module works in enhanced mode: the compare registers

OCR4A/B/D can welcome one more accuracy bit, while the LSB determines on which clock edge the Compare

condition is signalled and the output pin level is updated.

• Bits 5:0 – OC4OE5:OC4OE0: Outpu t Compare Override Enable Bits

These bits are the Output Compare Override Enable bits that are used to connect or disconnect the Output

Compare Pins in PWM6 Modes with an instant response on the corresponding Output Compare Pins. The

actual value from the port register will be visible on the port pin, when the Output Compare Override Enable Bit

is cleared. The table shows the Output Compare Override Enable Bits and their corresponding Output Compare

pins.

15.12.6 TCNT4 – Timer/Counter4

This 8-bit register contains the value of Timer/Counter4.

The Timer/Counter4 is realized as a 10-bit up/down counter with read and write access. Due to synchronization

of the CPU, Timer/Counter4 data written into Timer/Counter4 is delayed by one and half CPU clock cycles in

synchronous mode and at most one CPU clock cycles for asynchronous mode. When a 10-bit accuracy is

preferred, special procedures must be followed for accessing the 10-bit TCNT4 register via the 8-bit AVR data

bus. These procedures are described in section “Accessing 10-bit Registers” on page 160. Alternatively the

Timer/Counter4 can be used as an 8-bit Timer/Counter. Note that the Timer/Counter4 always starts counting up

after writing the TCNT4 register.

15.12.7 TC4H – Timer/Counter4 High Byte

The temporary Timer/Counter4 register is an 2-bit read/write register.

• Bits 7:3- Res: Reserved Bits

These bits are reserved bits and always reads as zero.

• Bits 2- TC410: Additional MSB bits for 11-bit accesses in Enhanced PWM mode

If 10-bit accuracy is used, the Timer/Counter4 High Byte Register (TC4H) is used for temporary storing the MSB

bits (TC49, TC48) of the 10-bit accesses. The same TC4H register is shared between all 10-bit registers within

the Timer/Counter4. Note that special procedures must be followed when accessing the 10-bit TCNT4 register

via the 8-bit AVR data bus. These procedures are described in section “Accessing 10-bit Registers” on

page 160.

OC4OE0 OC4OE1 OC4OE2 OC4OE3 OC4OE4 OC4OE5

OC4A (PC6) OC4A (PC7) OC4B (PB5) OC4B (PB6) OC4D (PD6) OC4D (PD7)

Bit 76543210

4MSB LSB TCNT4

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial value00000000

Bit 76543210

-----TC410TC49TC48TC4H

Read/WriteRRRRRRR/WR/W

Initial value00000000

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• Bits 1:0 - TC49, TC48: Two MSB bits of the 10-bit accesses

If 10-bit accuracy is used, the Timer/Counter4 High Byte Register (TC4H) is used for temporary storing the MSB

bits (TC49, TC48) of the 10-bit accesses. The same TC4H register is shared between all 10-bit registers within

the Timer/Counter4. Note that special procedures must be followed when accessing the 10-bit TCNT4 register

via the 8-bit AVR data bus. These procedures are described in section “Accessing 10-bit Registers” on

page 160.

15.12.8 OCR4A – Timer/Counter4 Output Compare Register A

The output compare register A is an 8-bit read/write register.

The Timer/Counter Output Compare Register A contains data to be continuously compared with

Timer/Counter4. Actions on compare matches are specified in TCCR4A. A compare match does only occur if

Timer/Counter4 counts to the OCR4A value. A software write that sets TCNT4 and OCR4A to the same value

does not generate a compare match.

A compare match will set the compare interrupt flag OCF4A after a synchronization delay following the compare

event.

Note that, if 10-bit accuracy is used special procedures must be followed when accessing the internal 10-bit

Output Compare Registers via the 8-bit AVR data bus. These procedures are described in section “Accessing

10-bit Registers” on page 160.

15.12.9 OCR4B – Timer/Counter4 Output Compare Register B

The output compare register B is an 8-bit read/write register.

The Timer/Counter Output Compare Register B contains data to be continuously compared with

Timer/Counter4. Actions on compare matches are specified in TCCR4. A compare match does only occur if

Timer/Counter4 counts to the OCR4B value. A software write that sets TCNT4 and OCR4B to the same value

does not generate a compare match.

A compare match will set the compare interrupt flag OCF4B after a synchronization delay following the compare

event.

Note that, if 10-bit accuracy is used special procedures must be followed when accessing the internal 10-bit

Output Compare Registers via the 8-bit AVR data bus. These procedures are described in section “Accessing

10-bit Registers” on page 160.

15.12.10OCR4C – T imer/Counter4 Output Compare Register C

The output compare register C is an 8-bit read/write register.

Bit 76543210

MSB LSB OCR4A

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial value00000000

Bit 76543210

MSB LSB OCR4B

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial value00000000

Bit 76543210

MSB LSB OCR44C

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial value11111111

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The Timer/Counter Output Compare Register C contains data to be continuously compared with

Timer/Counter4, and a compare match will clear TCNT4. This register has the same function in Normal mode

and PWM modes.

Note that, if a smaller value than three is written to the Output Compare Register C, the value is automatically

replaced by three as it is a minimum value allowed to be written to this register.

Note that, if 10-bit accuracy is used special procedures must be followed when accessing the internal 10-bit

Output Compare Registers via the 8-bit AVR data bus. These procedures are described in section “Accessing

10-bit Registers” on page 160.

15.12.11OCR4D – Timer/Counter4 Output Compare Register D

The output compare register D is an 8-bit read/write register.

The Timer/Counter Output Compare Register D contains data to be continuously compared with

Timer/Counter4. Actions on compare matches are specified in TCCR4A. A compare match does only occur if

Timer/Counter4 counts to the OCR4D value. A software write that sets TCNT4 and OCR4D to the same value

does not generate a compare match.

A compare match will set the compare interrupt flag OCF4D after a synchronization delay following the compare

event.

Note that, if 10-bit accuracy is used special procedures must be followed when accessing the internal 10-bit

Output Compare Registers via the 8-bit AVR data bus. These procedures are described in section “Accessing

10-bit Registers” on page 160.

15.12.12TIMSK4 – Timer/Counter4 Inter r upt Mask Register

• Bit 7- OCIE4D: Timer/Counter4 Output Compare Interrupt Enable

When the OCIE4D bit is set (one) and the I-bit in the Status Register is set (one), the Timer/Counter4 Compare

Match D interrupt is enabled. The corresponding interrupt at vector $010 is executed if a compare match D

occurs. The Compare Flag in Timer/Counter4 is set (one) in the Timer/Counter Interrupt Flag Register.

• Bit 6 - OCIE4A: Timer/Counter4 Output Comp are Interrupt Enable

When the OCIE4A bit is set (one) and the I-bit in the Status Register is set (one), the Timer/Counter4 Compare

Match A interrupt is enabled. The corresponding interrupt at vector $003 is executed if a compare match A

occurs. The Compare Flag in Timer/Counter4 is set (one) in the Timer/Counter Interrupt Flag Register.

• Bit 5 - OCIE4B: Timer/Counter4 Output Comp are Interrupt Enable

When the OCIE4B bit is set (one) and the I-bit in the Status Register is set (one), the Timer/Counter4 Compare

Match B interrupt is enabled. The corresponding interrupt at vector $009 is executed if a compare match B

occurs. The Compare Flag in Timer/Counter4 is set (one) in the Timer/Counter Interrupt Flag Register.

Bit 76543210

MSB LSB OCR4D

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial value00000000

Bit 76543210

OCIE4D OCIE4A OCIE4B TOIE4 TIMSK4

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial value00000000

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• Bit 2 - TOIE4: Timer/Counter4 Overflow Interrupt Enable

When the TOIE4 bit is set (one) and the I-bit in the Status Register is set (one), the Timer/Counter4 Overflow

interrupt is enabled. The corresponding interrupt (at vector $004) is executed if an overflow in Timer/Counter4

occurs. The Overflow Flag (Timer4) is set (one) in the Timer/Counter Interrupt Flag Register - TIFR4.

15.12.13TIFR4 – Timer/Counter4 In terrupt Flag Register

• Bit 7- OCF4D: Output Comp are Flag 4D

The OCF4D bit is set (one) when compare match occurs between Timer/Counter4 and the data value in

OCR4D - Output Compare Register 4D. OCF4D is cleared by hardware when executing the corresponding

interrupt handling vector. Alternatively, OCF4D is cleared, after synchronization clock cycle, by writing a logic

one to the flag. When the I-bit in SREG, OCIE4D, and OCF4D are set (one), the Timer/Counter4 D compare

match interrupt is executed.

• Bit 6 - OCF4A: Output Compare Flag 4A

The OCF4A bit is set (one) when compare match occurs between Timer/Counter4 and the data value in OCR4A

- Output Compare Register 4A. OCF4A is cleared by hardware when executing the corresponding interrupt

handling vector. Alternatively, OCF4A is cleared, after synchronization clock cycle, by writing a logic one to the

flag. When the I-bit in SREG, OCIE4A, and OCF4A are set (one), the Timer/Counter4 A compare match

interrupt is executed.

• Bit 5 - OCF4B: Output Compare Flag 4B

The OCF4B bit is set (one) when compare match occurs between Timer/Counter4 and the data value in OCR4B

- Output Compare Register 4B. OCF4B is cleared by hardware when executing the corresponding interrupt

handling vector. Alternatively, OCF4B is cleared, after synchronization clock cycle, by writing a logic one to the

flag. When the I-bit in SREG, OCIE4B, and OCF4B are set (one), the Timer/Counter4 B compare match

interrupt is executed.

• Bit 2 - TOV4: Timer/Counter4 Overflow Flag

In Normal Mode and Fast PWM Mode the TOV4 bit is set (one) each time the counter reaches TOP at the same

clock cycle when the counter is reset to BOTTOM. In Phase and Frequency Correct PWM Mode the TOV4 bit is

set (one) each time the counter reaches BOTTOM at the same clock cycle when zero is clocked to the counter.

The bit TOV4 is cleared by hardware when executing the corresponding interrupt handling vector. Alternatively,

TOV4 is cleared, after synchronization clock cycle, by writing a logical one to the flag. When the SREG I-bit, and

TOIE4 (Timer/Counter4 Overflow Interrupt Enable), and TOV4 are set (one), the Timer/Counter4 Overflow

interrupt is executed.

15.12.14DT4 – Timer/Counter4 Dead Time Value

The dead time value register is an 8-bit read/write register.

Bit 76543210

OCF4D OCF4A OCF4B TOV4 TIFR4

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial value00000000

Bit 76543210

DT4H3 DT4H2 DT4H1 DT4H0 DT4L3 DT4L2 DT4L1 DT4L0 DT4

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial value 00000000

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The dead time delay of all Timer/Counter4 channels are adjusted by the dead time value register, DT4. The

different dead time delay can be adjusted for the rising edge of OC4x and the rising edge of OC4x.

• Bits 7:4- DT4H3:DT4H0: Dead T ime Value for OC4x Output

The dead time value for the OC1x output. The dead time delay is set as a number of the prescaled timer/counter

clocks. The minimum dead time is zero and the maximum dead time is the prescaled time/counter clock period

multiplied by 15.

• Bits 3:0- DT4L3: DT4L0: Dead Time Value for OC4x Output

The dead time value for the OC4x output. The dead time delay is set as a number of the prescaled timer/counter

clocks. The minimum dead time is zero and the maximum dead time is the prescaled time/counter clock period

multiplied by 15.

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16. Output Compare Mo dulator (OCM1C0A )

16.1 Overview

The Output Compare Modulator (OCM) allows generation of waveforms modulated with a carrier frequency.

The modulator uses the outputs from the Output Compare Unit C of the 16-bit Timer/Counter1 and the Output

Compare Unit of the 8-bit Timer/Counter0. For more details about these Timer/Counters see “Timer/Counter0,

Timer/Counter1, and Timer/Counter3 Prescalers” on page 92.

Figure 16-1. Output Com pare Modulator, Block Diagram

When the modulator is enabled, the two output compare channels are modulated together as shown in the block

diagram (Figure 16-1).

16.2 Description

The Output Compare unit 1C and Output Compare unit 2 shares the PB7 port pin for output. The outputs of the

Output Compare units (OC1C and OC0A) overrides the normal PORTB7 Register when one of them is enabled

(i.e., when COMnx1:0 is not equal to zero). When both OC1C and OC0A are enabled at the same time, the

modulator is automatically enabled.

The functional equivalent schematic of the modulator is shown on Figure 16-2. The schematic includes part of

the Timer/Counter units and the port B pin 7 output driver circuit.

Figure 16-2. Output Compare Modulator, Schemati c

When the modulator is enabled the type of modulation (logical AND or OR) can be selected by the PORTB7

(see Figure 16-3 on page 178).

Note that the DDRB7 controls the direction of the port independent of the COMnx1:0 bit setting.

OC1C

OC1C /

OC0A / PB7

Timer/Counter 1

Timer/Counter 0

OC0A

PORTB7 DDRB7

DQDQ

COMA01

COMA00

DATABUS

OC1C /

OC0A/ PB7

COM1C1

COM1C0

Modulator

OC1C

OC0A

( From Waveform Generator )

Vcc

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16.2.1 Timing Example

Figure 16-3 illustrates the modulator in action. In this example the Timer/Counter1 is set to operate in fast PWM

mode (non-inverted) and Timer/Counter0 uses CTC waveform mode with toggle Compare Output mode

(COMnx1:0 = 1).

Figure 16-3. Output Compare Modulator, Timing Diagram

In this example, Timer/Counter0 provides the carrier, while the modulating signal is generated by the Output

Compare unit C of the Timer/Counter1.

The resolution of the PWM signal (OC1C) is reduced by the modulation. The reduction factor is equal to the

number of system clock cycles of one period of the carrier (OC0A). In this example the resolution is reduced by

a factor of two. The reason for the reduction is illustrated in Figure 16-3 at the second and third period of the

PB7 output when PORTB7 equals zero. The period 2 high time is one cycle longer than the period 3 high time,

but the result on the PB7 output is equal in both periods.

1 2

OC0A

(CTC Mode)

OC1C

(FPWM Mode)

PB7

(PORTB7 = 0)

PB7

(PORTB7 = 1)

(Period) 3

clk

I/O

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17. Serial Peripheral Interface – SPI

The Serial Peripheral Interface (SPI) allows high-speed synchronous data transfer between the

ATmega16U4/ATmega32U4 and peripheral devices or between several AVR devices.

The SPI includes the following features:

•Full-duplex, Three-wire Synchronous Data Transfer

•Master or Slave Operation

•LSB First or MSB First Data Transfer

•Seven Programmable Bit Rates

•End of Transmission Interrupt Fl ag

•Write Collision Flag Protection

•Wake-up from Idle Mode

•Double Speed (CK/2) Master SPI Mode

USART can also be used in Master SPI mode, see “USART in SPI Mode” on page 214.

The Power Reduction SPI bit, PRSPI, in “Power Reduction Register 0 - PRR0” on page 47 must be written to

zero to enable SPI module.

Figure 17-1. SPI Block Diagram(1)

Note: 1. Refer to “Pinout” on page 3, and Table 10-3 on page 74 for SPI pin placement.

The interconnection between Master and Slave CPUs with SPI is shown in Figure 17-2 on page 180. The

system consists of two shift Registers, and a Master clock generator. The SPI Master initiates the

communication cycle when pulling low the Slave Select SS pin of the desired Slave. Master and Slave prepare

the data to be sent in their respective shift Registers, and the Master generates the required clock pulses on the

SPI2X

DIVIDER

/2/4/8/16/32/64/128

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SCK line to interchange data. Data is always shifted from Master to Slave on the Master Out – Slave In, MOSI,

line, and from Slave to Master on the Master In – Slave Out, MISO, line. After each data packet, the Master will

synchronize the Slave by pulling high the Slave Select, SS, line.

When configured as a Master, the SPI interface has no automatic control of the SS line. This must be handled

by user software before communication can start. When this is done, writing a byte to the SPI Data Register

starts the SPI clock generator, and the hardware shifts the eight bits into the Slave. After shifting one byte, the

SPI clock generator stops, setting the end of Transmission Flag (SPIF). If the SPI Interrupt Enable bit (SPIE) in

the SPCR Register is set, an interrupt is requested. The Master may continue to shift the next byte by writing it

into SPDR, or signal the end of packet by pulling high the Slave Select, SS line. The last incoming byte will be

kept in the Buffer Register for later use.

When configured as a Slave, the SPI interface will remain sleeping with MISO tri-stated as long as the SS pin is

driven high. In this state, software may update the contents of the SPI Data Register, SPDR, but the data will not

be shifted out by incoming clock pulses on the SCK pin until the SS pin is driven low. As one byte has been

completely shifted, the end of Transmission Flag, SPIF is set. If the SPI Interrupt Enable bit, SPIE, in the SPCR

before reading the incoming data. The last incoming byte will be kept in the Buffer Register for later use.

Figure 17-2. SPI Master-slave Interconnection

The system is single buffered in the transmit direction and double buffered in the receive direction. This means

that bytes to be transmitted cannot be written to the SPI Data Register before the entire shift cycle is completed.

When receiving data, however, a received character must be read from the SPI Data Register before the next

character has been completely shifted in. Otherwise, the first byte is lost.

In SPI Slave mode, the control logic will sample the incoming signal of the SCK pin. To ensure correct sampling

of the clock signal, the frequency of the SPI clock should never exceed fosc/4.

When the SPI is enabled, the data direction of the MOSI, MISO, SCK, and SS pins is overridden according to

the table below. For more details on automatic port overrides, refer to “Alternate Port Functions” on page 72.

Note: 1. See “Alternate Functions of Port B” on page 74 for a detailed description of how to define the direction of the

user defined SPI pins.

Table 17-1. SPI Pin Overrides(1)

Pin Direction, Master SPI Direction, Slave SPI

MOSI User Defined Input

MISO Input User Defined

SCK User Defined Input

SS User Defined Input

SHIFT

ENABLE

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The following code examples show how to initialize the SPI as a Master and how to perform a simple

transmission. DDR_SPI in the examples must be replaced by the actual Data Direction Register controlling the

SPI pins. DD_MOSI, DD_MISO and DD_SCK must be replaced by the actual data direction bits for these pins.

E.g. if MOSI is placed on pin PB5, replace DD_MOSI with DDB5 and DDR_SPI with DDRB.

Note: 1. See “Code Examples” on page 8.

Assembly Code Example(1)

SPI_MasterInit:

; Set MOSI and SCK output, all others input

ldi r17,(1<<DD_MOSI)|(1<<DD_SCK)

out DDR_SPI,r17

; Enable SPI, Master, set clock rate fck/16

ldi

r17,(1<<SPE)|(1<<MSTR)|(1<<SPR0)

out SPCR,r17

ret

SPI_MasterTransmit:

; Start transmission of data (r16)

out SPDR,r16

Wait_Transmit:

; Wait for transmission complete

sbis SPSR,SPIF

rjmp Wait_Transmit

ret

C Code Example(1)

void SPI_MasterInit(void)

{/* Set MOSI and SCK output, all others input */

DDR_SPI = (1<<DD_MOSI)|(1<<DD_SCK);

/* Enable SPI, Master, set clock rate fck/16 */

SPCR = (1<<SPE)|(1<<MSTR)|(1<<SPR0);

}

void SPI_MasterTransmit(char cData)

{/* Start transmission */

SPDR = cData;

/* Wait for transmission complete */

while(!(SPSR & (1<<SPIF)))

;

}

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The following code examples show how to initialize the SPI as a Slave and how to perform a simple reception.

Note: 1. See “Code Examples” on page 8.

17.1 SS Pin Functionality

17.1.1 Slave Mode

When the SPI is configured as a Slave, the Slave Select (SS) pin is always input. When SS is held low, the SPI

is activated, and MISO becomes an output if configured so by the user. All other pins are inputs. When SS is

driven high, all pins are inputs, and the SPI is passive, which means that it will not receive incoming data. Note

that the SPI logic will be reset once the SS pin is driven high.

Assembly Code Example(1)

SPI_SlaveInit:

; Set MISO output, all others input

ldi r17,(1<<DD_MISO)

out DDR_SPI,r17

; Enable SPI

ldi r17,(1<<SPE)

out SPCR,r17

ret

SPI_SlaveReceive:

; Wait for reception complete

sbis SPSR,SPIF

rjmp SPI_SlaveReceive

; Read received data and return

in r16,SPDR

ret

C Code Example(1)

void SPI_SlaveInit(void)

{/* Set MISO output, all others input */

DDR_SPI = (1<<DD_MISO);

/* Enable SPI */

SPCR = (1<<SPE);

}

char SPI_SlaveReceive(void)

{/* Wait for reception complete */

while(!(SPSR & (1<<SPIF)))

;

/* Return Data Register */

return SPDR;

}

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The SS pin is useful for packet/byte synchronization to keep the slave bit counter synchronous with the master

clock generator. When the SS pin is driven high, the SPI slave will immediately reset the send and receive logic,

and drop any partially received data in the Shift Register.

17.1.2 Master Mode

When the SPI is configured as a Master (MSTR in SPCR is set), the user can determine the direction of the SS

pin.

If SS is configured as an output, the pin is a general output pin which does not affect the SPI system. Typically,

the pin will be driving the SS pin of the SPI Slave.

If SS is configured as an input, it must be held high to ensure Mas ter SPI opera tion. If the SS pin is driven

low by peripheral circuitry when the SPI is configured as a Master with the SS pin defined as an input, the SPI

system interprets this as another master selecting the SPI as a slave and starting to send data to it. To avoid

bus contention, the SPI system takes the following actions:

1. The MSTR bit in SPCR is cleared and the SPI system becomes a Slave. As a result of the SPI becom-

ing a Slave, the MOSI and SCK pins become inputs.

2. The SPIF Flag in SPSR is set, and if the SPI interrupt is enabled, and the I-bit in SREG is set, the interrupt

routine will be executed.

Thus, when interrupt-driven SPI transmission is used in Master mode, and there exists a possibility that SS is

driven low, the interrupt should always check that the MSTR bit is still set. If the MSTR bit has been cleared by

a slave select, it must be set by the user to re-enable SPI Master mode.

17.1.3 Data Modes

There are four combinations of SCK phase and polarity with respect to serial data, which are determined by

control bits CPHA and CPOL. The SPI data transfer formats are shown in Figure 17-3 and Figure 17-4 on page

184. Data bits are shifted out and latched in on opposite edges of the SCK signal, ensuring sufficient time for

data signals to stabilize. This is clearly seen by summarizing Table 17-3 and Table 17-4 on page 185, as done

below:

Table 17-2. CPOL and CPHA Functio na lity

Leading Edge Trailing eDge SPI Mode

CPOL=0, CPHA=0 Sample (Rising) Setup (Falling) 0

CPOL=0, CPHA=1 Setup (Rising) Sample (Falling) 1

CPOL=1, CPHA=0 Sample (Falling) Setup (Rising) 2

CPOL=1, CPHA=1 Setup (Falling) Sample (Rising) 3

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Figure 17-3. SPI Transfer Format with CPHA = 0

Figure 17-4. SPI Transfer Format with CPHA = 1

Bit 1

Bit 6

LSB

MSB

SCK (CPOL = 0)

mode 0

SAMPLE I

MOSI/MISO

CHANGE 0

MOSI PIN

CHANGE 0

MISO PIN

SCK (CPOL = 1)

mode 2

MSB

LSB

Bit 6

Bit 1

Bit 5

Bit 2

Bit 4

Bit 3

Bit 4

Bit 2

Bit 5

MSB first (DORD = 0)

LSB first (DORD = 1)

SCK (CPOL = 0)

mode 1

SAMPLE I

MOSI/MISO

CHANGE 0

MOSI PIN

CHANGE 0

MISO PIN

SCK (CPOL = 1)

mode 3

MSB

LSB

Bit 6

Bit 1

Bit 5

Bit 2

Bit 4

Bit 3

Bit 4

Bit 2

Bit 5

Bit 1

Bit 6

LSB

MSB

MSB first (DORD = 0)

LSB first (DORD = 1)

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17.2 Register Description

17.2.1 SPI Control Register – SPCR

• Bit 7 – SPIE: SPI Interrupt Enable

This bit causes the SPI interrupt to be executed if SPIF bit in the SPSR Register is set and the if the Global

Interrupt Enable bit in SREG is set.

• Bit 6 – SPE: SPI Enable

When the SPE bit is written to one, the SPI is enabled. This bit must be set to enable any SPI operations.

• Bit 5 – DORD: Data Order

When the DORD bit is written to one, the LSB of the data word is transmitted first.

When the DORD bit is written to zero, the MSB of the data word is transmitted first.

• Bit 4 – MSTR: Master/Slave Select

This bit selects Master SPI mode when written to one, and Slave SPI mode when written logic zero. If SS is

configured as an input and is driven low while MSTR is set, MSTR will be cleared, and SPIF in SPSR will

become set. The user will then have to set MSTR to re-enable SPI Master mode.

• Bit 3 – CPOL: Clock Polarity

When this bit is written to one, SCK is high when idle. When CPOL is written to zero, SCK is low when idle.

Refer to Figure 17-3 and Figure 17-4 for an example. The CPOL functionality is summarized below:

• Bit 2 – CPHA: Clock Phase

The settings of the Clock Phase bit (CPHA) determine if data is sampled on the leading (first) or trailing (last)

edge of SCK. Refer to Figure 17-3 on page 184 and Figure 17-4 on page 184 for an example. The CPOL

functionality is summarized below:

• Bits 1, 0 – SPR1, SPR0: SPI Clock Rate Select 1 and 0

These two bits control the SCK rate of the device configured as a Master. SPR1 and SPR0 have no effect on

the Slave. The relationship between SCK and the Oscillator Clock frequency fosc is shown below.

Bit 76543210

SPIE SPE DORD MSTR CPOL CPHA SPR1 SPR0 SPCR

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value 0 0 0 0 0 0 0 0

Table 17-3. CPOL Functionality

CPOL Leading Edge Trailing Edge

0Rising Falling

1Falling Rising

Table 17-4. CPHA Functionality

CPHA Leading Edge Trailing Edge

0Sample Setup

1Setup Sample

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17.2.2 SPI Status Register – SPSR

• Bit 7 – SPIF: SPI Interrupt Flag

When a serial transfer is complete, the SPIF Flag is set. An interrupt is generated if SPIE in SPCR is set and

global interrupts are enabled. If SS is an input and is driven low when the SPI is in Master mode, this will also

set the SPIF Flag. SPIF is cleared by hardware when executing the corresponding interrupt handling vector.

Alternatively, the SPIF bit is cleared by first reading the SPI Status Register with SPIF set, then accessing the

SPI Data Register (SPDR).

• Bit 6 – WCOL: Write COLlision Flag

The WCOL bit is set if the SPI Data Register (SPDR) is written during a data transfer. The WCOL bit (and the

SPIF bit) are cleared by first reading the SPI Status Register with WCOL set, and then accessing the SPI Data

• Bit 5..1 – Res: Reserved Bits

These bits are reserved and will always read as zero.

• Bit 0 – SPI2X: Double SPI Speed Bit

When this bit is written logic one the SPI speed (SCK Frequency) will be doubled when the SPI is in Master

mode (see Table 17-2 on page 183). This means that the minimum SCK period will be two CPU clock periods.

When the SPI is configured as Slave, the SPI is only guaranteed to work at fosc/4 or lower.

The SPI interface on the device is also used for program memory and EEPROM downloading or uploading. See

page 367 for serial programming and verification.

17.2.3 SPI Data Register – SPDR

Table 17-5. Relationship Between SCK and the Oscillator Frequency

SPI2X SPR1 SPR0 SCK Frequency

0 0 0 fosc/4

0 0 1 fosc/16

0 1 0 fosc/64

0 1 1 fosc/128

1 0 0 fosc/2

1 0 1 fosc/8

1 1 0 fosc/32

1 1 1 fosc/64

Bit 76543210

SPIFWCOL–––––SPI2XSPSR

Read/WriteRRRRRRRR/W

Initial Value00000000

Bit 76543210

MSB LSB SPDR

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial ValueXXXXXXXXUndefined

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The SPI Data Register is a read/write register used for data transfer between the Register File and the SPI Shift

Receive buffer to be read.

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18. USART

The Universal Synchronous and Asynchronous serial Receiver and Transmitter (USART) is a highly flexible

serial communication device. The main features are:

•Full Duplex Op eration (Independent Serial Receive and Transmit Registers)

•Asynchronous o r Synchronous Operation

•Flow control CTS/RTS signals hardware management

•Master or Slave Clocked Synchronous Operation

•High Resolution Baud Rate Generator

•Supports Serial Frames with 5, 6, 7, 8, or 9 Data Bits and 1 or 2 Stop Bits

•Odd or Even Parity Generation and Parity Check Supported by Hard ware

•Data OverRun Detection

•Framing Error Detection

•Noise Filtering Includes False Start Bit Detection and Digital Low Pas s Fil t e r

•Three Separate Interrupts on TX Complete, TX Data Register Empty, and RX Complete

•Multi-process or Communication Mode

•Double Speed Asynchronous Communication Mode

18.1 Overview

A simplified block diagram of the USART Transmitter is shown in Figure 18-1 on page 189. CPU accessible I/O

Registers and I/O pins are shown in bold.

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Figure 18-1. USART Block Diagram(1)

Note: 1. See “Pinout” on page 3, Table 10-8 on page 78 and for USART pin placement.

The dashed boxes in the block diagram separate the three main parts of the USART (listed from the top): Clock

Generator, Transmitter and Receiver. Control Registers are shared by all units. The Clock Generation logic

consists of synchronization logic for external clock input used by synchronous slave operation, and the baud

rate generator. The XCKn (Transfer Clock) pin is only used by synchronous transfer mode. The Transmitter

consists of a single write buffer, a serial Shift Register, Parity Generator, and Control logic for handling different

serial frame formats. The write buffer allows a continuous transfer of data without any delay between frames.

The Receiver is the most complex part of the USART module due to its clock and data recovery units. The

recovery units are used for asynchronous data reception. In addition to the recovery units, the Receiver includes

a Parity Checker, Control logic, a Shift Register and a two level receive buffer (UDRn). The Receiver supports

the same frame formats as the Transmitter, and can detect Frame Error, Data OverRun and Parity Errors.

18.2 Clock Generation

The Clock Generation logic generates the base clock for the Transmitter and Receiver. The USARTn supports

four modes of clock operation: Normal asynchronous, Double Speed asynchronous, Master synchronous and

Slave synchronous mode. The UMSELn bit in USART Control and Status Register C (UCSRnC) selects

between asynchronous and synchronous operation. Double Speed (asynchronous mode only) is controlled by

the U2Xn found in the UCSRnA Register. When using synchronous mode (UMSELn = 1), the Data Direction

(Slave mode). The XCKn pin is only active when using synchronous mode.

Figure 18-2 shows a block diagram of the clock generation logic.

PARITY

GENERATOR

UBRR[H:L]

UDR (Transmit)

UCSRA UCSRB UCSRC

BAUD RATE GENERATOR

TRANSMIT SHIFT REGISTER

RECEIVE SHIFT REGISTER RxD

TxD

CONTROL

UDR (Receive)

CONTROL

XCK

DATA

RECOVERY

CLOCK

RECOVERY

CONTROL

PARITY

CHECKER

DATA BUS

OSC

SYNC LOGIC

Clock Generator

Transmitter

Receiver

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Figure 18-2. Clock Generation Logic, Block Diagram

Signal description:

18.2.1 Internal Clock Generation – The Baud Rate Generator

Internal clock generation is used for the asynchronous and the synchronous master modes of operation. The

description in this section refers to Figure 18-2 on page 190.

The USART Baud Rate Register (UBRRn) and the down-counter connected to it function as a programmable

prescaler or baud rate generator. The down-counter, running at system clock (fosc), is loaded with the UBRRn

value each time the counter has counted down to zero or when the UBRRLn Register is written. A clock is

generated each time the counter reaches zero. This clock is the baud rate generator clock output

(= fosc/(UBRRn+1)). The Transmitter divides the baud rate generator clock output by 2, 8, or 16 depending on

mode. The baud rate generator output is used directly by the Receiver’s clock and data recovery units.

However, the recovery units use a state machine that uses 2, 8, or 16 states depending on mode set by the

state of the UMSELn, U2Xn, and DDR_XCKn bits.

The following table contains equations for calculating the baud rate (in bits per second) and for calculating the

UBRRn value for each mode of operation using an internally generated clock source.

txclk Transmitter clock (Internal Signal)

rxclk Receiver base clock (Internal Signal)

xcki Input from XCK pin (internal Signal). Used for synchronous slave operation

xcko Clock output to XCK pin (Internal Signal). Used for synchronous master operation

fOSC XTAL pin frequency (System Clock)

Prescaling

Down-Counter /2

UBRR

/4 /2

fosc

UBRR+1

Sync

OSC

XCK

txclk

U2X

UMSEL

DDR_XCK

xcki

xcko

DDR_XCK rxclk

Edge

Detector

UCPOL

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Note: 1. The baud rate is defined to be the transfer rate in bit per second (bps)

Some examples of UBRRn values for some system clock frequencies are found in Table 18-6 on page 208.

18.2.2 Double Speed Operation (U2Xn)

The transfer rate can be doubled by setting the U2Xn bit in UCSRnA. Setting this bit only has effect for the

asynchronous operation. Set this bit to zero when using synchronous operation.

Setting this bit will reduce the divisor of the baud rate divider from 16 to 8, effectively doubling the transfer rate

for asynchronous communication. Note however that the Receiver will in this case only use half the number of

samples (reduced from 16 to 8) for data sampling and clock recovery, and therefore a more accurate baud rate

setting and system clock are required when this mode is used. For the Transmitter, there are no downsides.

18.2.3 External Clock

External clocking is used by the synchronous slave modes of operation. The description in this section refers to

Figure 18-2 on page 190 for details.

External clock input from the XCKn pin is sampled by a synchronization register to minimize the chance of meta-

stability. The output from the synchronization register must then pass through an edge detector before it can be

used by the Transmitter and Receiver. This process introduces a two CPU clock period delay and therefore the

maximum external XCKn clock frequency is limited by the following equation:

Note that fosc depends on the stability of the system clock source. It is therefore recommended to add some

margin to avoid possible loss of data due to frequency variations.

18.2.4 Synchronous Clock Operation

When synchronous mode is used (UMSELn = 1), the XCKn pin will be used as either clock input (Slave) or clock

output (Master). The dependency between the clock edges and data sampling or data change is the same. The

Table 18-1. Equations for Calculating Baud Rate Register Setting

Operating Mod e Equation for Cal culating Baud Rate(1) Equation for Calculating UBRR Value

Asynchronous Normal

mode (U2Xn = 0)

Asynchronous Double

Speed mode (U2Xn = 1)

Synchronous Master

mode

BAUD Baud rate (in bits per second, bps)

fOSC System Oscillator clock frequency

UBRRn Contents of the UBRRHn and UBRRLn Registers, (0-4095)

BAUD fOSC

16 UBRRn1+

------------------------------------------=

UBRRnfOSC

16BAUD

------------------------1–=

BAUD fOSC

8UBRRn1+

---------------------------------------=

UBRRnfOSC

8BAUD

-------------------- 1–=

BAUD fOSC

2UBRRn1+

---------------------------------------=

UBRRnfOSC

2BAUD

-------------------- 1–=

fXCK fOSC

-----------

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basic principle is that data input (on RxDn) is sampled at the opposite XCKn clock edge of the edge the data

output (TxDn) is changed.

Figure 18-3. Synchronous Mode XCKn Timing

The UCPOLn bit UCRSC selects which XCKn clock edge is used for data sampling and which is used for data

change. As the above figure shows, when UCPOLn is zero the data will be changed at rising XCKn edge and

sampled at falling XCKn edge. If UCPOLn is set, the data will be changed at falling XCKn edge and sampled at

rising XCKn edge.

18.3 Frame Formats

A serial frame is defined to be one character of data bits with synchronization bits (start and stop bits), and

optionally a parity bit for error checking. The USART accepts all 30 combinations of the following as valid frame

formats:

1 start bit

5, 6, 7, 8, or 9 data bits

no, even or odd parity bit

1 or 2 stop bits

A frame starts with the start bit followed by the least significant data bit. Then the next data bits, up to a total of

nine, are succeeding, ending with the most significant bit. If enabled, the parity bit is inserted after the data bits,

before the stop bits. When a complete frame is transmitted, it can be directly followed by a new frame, or the

communication line can be set to an idle (high) state. The following figure illustrates the possible combinations

of the frame formats. Bits inside brackets are optional.

Figure 18-4. Frame Formats

RxD / TxD

XCK

RxD / TxD

XCK

UCPOL = 0

UCPOL = 1

Sample

St Start bit, always low

(n) Data bits (0 to 8)

P Parity bit. Can be odd or even

Sp Stop bit, always high

IDLE No transfers on the communication line (RxDn or TxDn). An IDLE line must be high

10 2 3 4 [5] [6] [7] [8] [P]St Sp1 [Sp2] (St / IDLE)(IDLE)

FRAME

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The frame format used by the USART is set by the UCSZn2:0, UPMn1:0, and USBSn bits in UCSRnB and

UCSRnC. The Receiver and Transmitter use the same setting. Note that changing the setting of any of these

bits will corrupt all ongoing communication for both the Receiver and Transmitter.

The USART Character SiZe (UCSZn2:0) bits select the number of data bits in the frame. The USART Parity

mode (UPMn1:0) bits enable and set the type of parity bit. The selection between one or two stop bits is done by

the USART Stop Bit Select (USBSn) bit. The Receiver ignores the second stop bit. An FE (Frame Error) will

therefore only be detected in the cases where the first stop bit is zero.

18.3.1 Parity Bit Calculation

The parity bit is calculated by doing an exclusive-or of all the data bits. If odd parity is used, the result of the

exclusive or is inverted. The relation between the parity bit and data bits is as follows:

If used, the parity bit is located between the last data bit and first stop bit of a serial frame.

18.4 USART Initialization

The USART has to be initialized before any communication can take place. The initialization process normally

consists of setting the baud rate, setting frame format and enabling the Transmitter or the Receiver depending

on the usage. For interrupt driven USART operation, the Global Interrupt Flag should be cleared (and interrupts

globally disabled) when doing the initialization.

Before doing a re-initialization with changed baud rate or frame format, be sure that there are no ongoing

transmissions during the period the registers are changed. The TXCn Flag can be used to check that the

Transmitter has completed all transfers, and the RXC Flag can be used to check that there are no unread data

in the receive buffer. Note that the TXCn Flag must be cleared before each transmission (before UDRn is

written) if it is used for this purpose.

The following simple USART initialization code examples show one assembly and one C function that are equal

in functionality. The examples assume asynchronous operation using polling (no interrupts enabled) and a fixed

Peven Start bit, always low

Podd Parity bit using odd parity

dnData bit n of the character

Peven dn1–d3d2d1d00

Podd



dn1–d3d2d1d01

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frame format. The baud rate is given as a function parameter. For the assembly code, the baud rate parameter

is assumed to be stored in the r17:r16 Registers.

Note: 1. See “Code Examples” on page 8.

More advanced initialization routines can be made that include frame format as parameters, disable interrupts

and so on. However, many applications use a fixed setting of the baud and control registers, and for these types

of applications the initialization code can be placed directly in the main routine, or be combined with initialization

code for other I/O modules.

18.5 Data Transmission – The USART Transmitter

The USART Transmitter is enabled by setting the Transmit Enable (TXEN) bit in the UCSRnB Register. When

the Transmitter is enabled, the normal port operation of the TxDn pin is overridden by the USART and given the

function as the Transmitter’s serial output. The baud rate, mode of operation and frame format must be set up

once before doing any transmissions. If synchronous operation is used, the clock on the XCKn pin will be

overridden and used as transmission clock.

18.5.1 Sending Frames with 5 to 8 Data Bit

A data transmission is initiated by loading the transmit buffer with the data to be transmitted. The CPU can load

the transmit buffer by writing to the UDRn I/O location. The buffered data in the transmit buffer will be moved to

the Shift Register when the Shift Register is ready to send a new frame. The Shift Register is loaded with new

data if it is in idle state (no ongoing transmission) or immediately after the last stop bit of the previous frame is

transmitted. When the Shift Register is loaded with new data, it will transfer one complete frame at the rate

given by the Baud Register, U2Xn bit or by XCKn depending on mode of operation.

Assembly Code Example(1)

USART_Init:

; Set baud rate

out UBRRHn, r17

out UBRRLn, r16

; Enable receiver and transmitter

ldi r16, (1<<RXENn)|(1<<TXENn)

out UCSRnB,r16

; Set frame format: 8data, 2stop bit

ldi r16, (1<<USBSn)|(3<<UCSZn0)

out UCSRnC,r16

ret

C Code Example(1)

void USART_Init( unsigned int baud )

{/* Set baud rate */

UBRRHn = (unsigned char)(baud>>8);

UBRRLn = (unsigned char)baud;

/* Enable receiver and transmitter */

UCSRnB = (1<<RXENn)|(1<<TXENn);

/* Set frame format: 8data, 2stop bit */

UCSRnC = (1<<USBSn)|(3<<UCSZn0);

}

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The following code examples show a simple USART transmit function based on polling of the Data Register

Empty (UDREn) Flag. When using frames with less than eight bits, the most significant bits written to the UDRn

are ignored. The USART has to be initialized before the function can be used. For the assembly code, the data

to be sent is assumed to be stored in Register R16.

Note: 1. See “Code Examples” on page 8.

The function simply waits for the transmit buffer to be empty by checking the UDREn Flag, before loading it with

new data to be transmitted. If the Data Register Empty interrupt is utilized, the interrupt routine writes the data

into the buffer.

18.5.2 Sending Frames with 9 Data Bit

If 9-bit characters are used (UCSZn = 7), the ninth bit must be written to the TXB8 bit in UCSRnB before the low

byte of the character is written to UDRn. The following code examples show a transmit function that handles 9-

bit characters. For the assembly code, the data to be sent is assumed to be stored in registers R17:R16.

Assembly Code Example(1)

USART_Transmit:

; Wait for empty transmit buffer

sbis UCSRnA,UDREn

rjmp USART_Transmit

; Put data (r16) into buffer, sends the data

out UDRn,r16

ret

C Code Example(1)

void USART_Transmit( unsigned char data )

{/* Wait for empty transmit buffer */

while ( !( UCSRnA & (1<<UDREn)) )

;

/* Put data into buffer, sends the data */

UDRn = data;

}

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Notes: 1. These transmit functions are written to be general functions. They can be optimized if the contents of the

UCSRnB is static. For example, only the TXB8 bit of the UCSRnB Register is used after initialization.

2. See “Code Examples” on page 8.

The 9th bit can be used for indicating an address frame when using multi processor communication mode or for

other protocol handling as for example synchronization.

18.5.3 Transmitter Flags and Interrupts

The USART Transmitter has two flags that indicate its state: USART Data Register Empty (UDREn) and

Transmit Complete (TXCn). Both flags can be used for generating interrupts.

The Data Register Empty (UDREn) Flag indicates whether the transmit buffer is ready to receive new data. This

bit is set when the transmit buffer is empty, and cleared when the transmit buffer contains data to be transmitted

that has not yet been moved into the Shift Register. For compatibility with future devices, always write this bit to

zero when writing the UCSRnA Register.

When the Data Register Empty Interrupt Enable (UDRIEn) bit in UCSRnB is written to one, the USART Data

UDREn is cleared by writing UDRn. When interrupt-driven data transmission is used, the Data Register Empty

interrupt routine must either write new data to UDRn in order to clear UDREn or disable the Data Register

Empty interrupt, otherwise a new interrupt will occur once the interrupt routine terminates.

The Transmit Complete (TXCn) Flag bit is set one when the entire frame in the Transmit Shift Register has been

shifted out and there are no new data currently present in the transmit buffer. The TXCn Flag bit is automatically

Assembly Code Example(1)(2)

USART_Transmit:

; Wait for empty transmit buffer

sbis UCSRnA,UDREn

rjmp USART_Transmit

; Copy 9th bit from r17 to TXB8

cbi UCSRnB,TXB8

sbrc r17,0

sbi UCSRnB,TXB8

; Put LSB data (r16) into buffer, sends the data

out UDRn,r16

ret

C Code Example(1)(2)

void USART_Transmit( unsigned int data )

{/* Wait for empty transmit buffer */

while ( !( UCSRnA & (1<<UDREn))) )

;

/* Copy 9th bit to TXB8 */

UCSRnB &= ~(1<<TXB8);

if ( data & 0x0100 )

UCSRnB |= (1<<TXB8);

/* Put data into buffer, sends the data */

UDRn = data;

}

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cleared when a transmit complete interrupt is executed, or it can be cleared by writing a one to its bit location.

The TXCn Flag is useful in half-duplex communication interfaces (like the RS-485 standard), where a

transmitting application must enter receive mode and free the communication bus immediately after completing

the transmission.

When the Transmit Compete Interrupt Enable (TXCIEn) bit in UCSRnB is set, the USART Transmit Complete

Interrupt will be executed when the TXCn Flag becomes set (provided that global interrupts are enabled). When

the transmit complete interrupt is used, the interrupt handling routine does not have to clear the TXCn Flag, this

is done automatically when the interrupt is executed.

18.5.4 Parity Generator

The Parity Generator calculates the parity bit for the serial frame data. When parity bit is enabled (UPMn1 = 1),

the transmitter control logic inserts the parity bit between the last data bit and the first stop bit of the frame that is

sent.

18.5.5 Disabling the Transmitter

The disabling of the Transmitter (setting the TXEN to zero) will not become effective until ongoing and pending

transmissions are completed, i.e., when the Transmit Shift Register and Transmit Buffer Register do not contain

data to be transmitted. When disabled, the Transmitter will no longer override the TxDn pin.

18.6 Data Reception – The USART Receiver

The USART Receiver is enabled by writing the Receive Enable (RXENn) bit in the

UCSRnB Register to one. When the Receiver is enabled, the normal pin operation of the RxDn pin is overridden

by the USART and given the function as the Receiver’s serial input. The baud rate, mode of operation and

frame format must be set up once before any serial reception can be done. If synchronous operation is used,

the clock on the XCKn pin will be used as transfer clock.

18.6.1 Receiving Frames with 5 to 8 Data Bits

The Receiver starts data reception when it detects a valid start bit. Each bit that follows the start bit will be

sampled at the baud rate or XCKn clock, and shifted into the Receive Shift Register until the first stop bit of a

frame is received. A second stop bit will be ignored by the Receiver. When the first stop bit is received, i.e., a

complete serial frame is present in the Receive Shift Register, the contents of the Shift Register will be moved

into the receive buffer. The receive buffer can then be read by reading the UDRn I/O location.

The following code example shows a simple USART receive function based on polling of the Receive Complete

(RXCn) Flag. When using frames with less than eight bits the most significant bits of the data read from the

UDRn will be masked to zero. The USART has to be initialized before the function can be used.

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Note: 1. See “Code Examples” on page 8.

The function simply waits for data to be present in the receive buffer by checking the RXCn Flag, before reading

the buffer and returning the value.

18.6.2 Receiving Frames with 9 Data Bits

If 9-bit characters are used (UCSZn=7) the ninth bit must be read from the RXB8n bit in UCSRnB before

reading the low bits from the UDRn. This rule applies to the FEn, DORn, and UPEn Status Flags as well. Read

status from UCSRnA, then data from UDRn. Reading the UDRn I/O location will change the state of the receive

buffer FIFO and consequently the TXB8n, FEn, DORn, and UPEn bits, which all are stored in the FIFO, will

change.

The following code example shows a simple USART receive function that handles both nine bit characters and

the status bits.

Assembly Code Example(1)

USART_Receive:

; Wait for data to be received

sbis UCSRnA, RXCn

rjmp USART_Receive

; Get and return received data from buffer

in r16, UDRn

ret

C Code Example(1)

unsigned char USART_Receive( void )

{/* Wait for data to be received */

while ( !(UCSRnA & (1<<RXCn)) )

;

/* Get and return received data from buffer */

return UDRn;

}

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Note: 1. See “Code Examples” on page 8.

The receive function example reads all the I/O Registers into the Register File before any computation is done.

This gives an optimal receive buffer utilization since the buffer location read will be free to accept new data as

early as possible.

18.6.3 Receive Compet e Fl ag an d Int erru pt

The USART Receiver has one flag that indicates the Receiver state.

The Receive Complete (RXCn) Flag indicates if there are unread data present in the receive buffer. This flag is

one when unread data exist in the receive buffer, and zero when the receive buffer is empty (i.e., does not

Assembly Code Example(1)

USART_Receive:

; Wait for data to be received

sbis UCSRnA, RXCn

rjmp USART_Receive

; Get status and 9th bit, then data from buffer

in r18, UCSRnA

in r17, UCSRnB

in r16, UDRn

; If error, return -1

andi

r18,(1<<FEn)|(1<<DORn)|(1<<UPEn)

breq USART_ReceiveNoError

ldi r17, HIGH(-1)

ldi r16, LOW(-1)

USART_ReceiveNoError:

; Filter the 9th bit, then return

lsr r17

andi r17, 0x01

ret

C Code Example(1)

unsigned int USART_Receive( void )

{unsigned char status, resh, resl;

/* Wait for data to be received */

while ( !(UCSRnA & (1<<RXCn)) )

;

/* Get status and 9th bit, then data */

/* from buffer */

status = UCSRnA;

resh = UCSRnB;

resl = UDRn;

/* If error, return -1 */

if ( status & (1<<FEn)|(1<<DORn)|(1<<UPEn) )

return -1;

/* Filter the 9th bit, then return */

resh = (resh >> 1) & 0x01;

return ((resh << 8) | resl);

}

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contain any unread data). If the Receiver is disabled (RXENn = 0), the receive buffer will be flushed and

consequently the RXCn bit will become zero.

When the Receive Complete Interrupt Enable (RXCIEn) in UCSRnB is set, the USART Receive Complete

interrupt will be executed as long as the RXCn Flag is set (provided that global interrupts are enabled). When

interrupt-driven data reception is used, the receive complete routine must read the received data from UDRn in

order to clear the RXCn Flag, otherwise a new interrupt will occur once the interrupt routine terminates.

18.6.4 Receiver Error Flags

The USART Receiver has three Error Flags: Frame Error (FEn), Data OverRun (DORn), and Parity Error

(UPEn). All can be accessed by reading UCSRnA. Common for the Error Flags is that they are located in the

receive buffer together with the frame for which they indicate the error status. Due to the buffering of the Error

Flags, the UCSRnA must be read before the receive buffer (UDRn), since reading the UDRn I/O location

changes the buffer read location. Another equality for the Error Flags is that they can not be altered by software

doing a write to the flag location. However, all flags must be set to zero when the UCSRnA is written for upward

compatibility of future USART implementations. None of the Error Flags can generate interrupts.

The Frame Error (FEn) Flag indicates the state of the first stop bit of the next readable frame stored in the

receive buffer. The FEn Flag is zero when the stop bit was correctly read (as one), and the FEn Flag will be one

when the stop bit was incorrect (zero). This flag can be used for detecting out-of-sync conditions, detecting

break conditions and protocol handling. The FEn Flag is not affected by the setting of the USBSn bit in UCSRnC

since the Receiver ignores all, except for the first, stop bits. For compatibility with future devices, always set this

bit to zero when writing to UCSRnA.

The Data OverRun (DORn) Flag indicates data loss due to a receiver buffer full condition. A Data OverRun

occurs when the receive buffer is full (two characters), it is a new character waiting in the Receive Shift Register,

and a new start bit is detected. If the DORn Flag is set there was one or more serial frame lost between the

frame last read from UDRn, and the next frame read from UDRn. For compatibility with future devices, always

write this bit to zero when writing to UCSRnA. The DORn Flag is cleared when the frame received was

successfully moved from the Shift Register to the receive buffer.

The Parity Error (UPEn) Flag indicates that the next frame in the receive buffer had a Parity Error when

received. If Parity Check is not enabled the UPEn bit will always be read zero. For compatibility with future

devices, always set this bit to zero when writing to UCSRnA. For more details see “Parity Bit Calculation” on

page 193 and “Parity Checker” on page 200.

18.6.5 Parity Checker

The Parity Checker is active when the high USART Parity mode (UPMn1) bit is set. Type of Parity Check to be

performed (odd or even) is selected by the UPMn0 bit. When enabled, the Parity Checker calculates the parity

of the data bits in incoming frames and compares the result with the parity bit from the serial frame. The result of

the check is stored in the receive buffer together with the received data and stop bits. The Parity Error (UPEn)

Flag can then be read by software to check if the frame had a Parity Error.

The UPEn bit is set if the next character that can be read from the receive buffer had a Parity Error when

received and the Parity Checking was enabled at that point (UPMn1 = 1). This bit is valid until the receive buffer

(UDRn) is read.

18.6.6 Disabling t he Receiver

In contrast to the Transmitter, disabling of the Receiver will be immediate. Data from ongoing receptions will

therefore be lost. When disabled (i.e., the RXENn is set to zero) the Receiver will no longer override the normal

function of the RxDn port pin. The Receiver buffer FIFO will be flushed when the Receiver is disabled.

Remaining data in the buffer will be lost

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18.6.7 Flushing the Receive Buffer

The receiver buffer FIFO will be flushed when the Receiver is disabled, i.e., the buffer will be emptied of its

contents. Unread data will be lost. If the buffer has to be flushed during normal operation, due to for instance an

error condition, read the UDRn I/O location until the RXCn Flag is cleared. The following code example shows

how to flush the receive buffer.

Note: 1. See “Code Examples” on page 8.

18.7 Asynchronous Data Reception

The USART includes a clock recovery and a data recovery unit for handling asynchronous data reception. The

clock recovery logic is used for synchronizing the internally generated baud rate clock to the incoming

asynchronous serial frames at the RxDn pin. The data recovery logic samples and low pass filters each

incoming bit, thereby improving the noise immunity of the Receiver. The asynchronous reception operational

range depends on the accuracy of the internal baud rate clock, the rate of the incoming frames, and the frame

size in number of bits.

18.7.1 Asynchronous Clock Recovery

The clock recovery logic synchronizes internal clock to the incoming serial frames. Figure 18-5 on page 201

illustrates the sampling process of the start bit of an incoming frame. The sample rate is 16 times the baud rate

for Normal mode, and eight times the baud rate for Double Speed mode. The horizontal arrows illustrate the

synchronization variation due to the sampling process. Note the larger time variation when using the Double

Speed mode (U2Xn = 1) of operation. Samples denoted zero are samples done when the RxDn line is idle (i.e.,

no communication activity).

Figure 18-5. Start Bit Sampling

When the clock recovery logic detects a high (idle) to low (start) transition on the RxDn line, the start bit

detection sequence is initiated. Let sample 1 denote the first zero-sample as shown in Figure 18-5. The clock

Assembly Code Example(1)

USART_Flush:

sbis UCSRnA, RXCn

ret

in r16, UDRn

rjmp USART_Flush

C Code Example(1)

void USART_Flush( void )

{unsigned char dummy;

while ( UCSRnA & (1<<RXCn) ) dummy = UDRn;

}

12345678 9 10 11 12 13 14 15 16 12

STARTIDLE

BIT 0

1234 5 678120

RxD

Sample

(U2X = 0)

Sample

(U2X = 1)

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recovery logic then uses samples 8, 9, and 10 for Normal mode, and samples 4, 5, and 6 for Double Speed

mode (indicated with sample numbers inside boxes on the figure), to decide if a valid start bit is received. If two

or more of these three samples have logical high levels (the majority wins), the start bit is rejected as a noise

spike and the Receiver starts looking for the next high to low-transition. If however, a valid start bit is detected,

the clock recovery logic is synchronized and the data recovery can begin. The synchronization process is

repeated for each start bit.

18.7.2 Asynchronous Data Recovery

When the receiver clock is synchronized to the start bit, the data recovery can begin. The data recovery unit

uses a state machine that has 16 states for each bit in Normal mode and eight states for each bit in Double

Speed mode. Figure 18-6 shows the sampling of the data bits and the parity bit. Each of the samples is given a

number that is equal to the state of the recovery unit.

Figure 18-6. Sampling of Data and Parity Bit

The decision of the logic level of the received bit is taken by doing a majority voting of the logic value to the three

samples in the center of the received bit. The center samples are emphasized on the figure by having the

sample number inside boxes. The majority voting process is done as follows: If two or all three samples have

high levels, the received bit is registered to be a logic 1. If two or all three samples have low levels, the received

bit is registered to be a logic 0. This majority voting process acts as a low pass filter for the incoming signal on

the RxDn pin. The recovery process is then repeated until a complete frame is received. Including the first stop

bit. Note that the Receiver only uses the first stop bit of a frame.

Figure 18-7 on page 202 shows the sampling of the stop bit and the earliest possible beginning of the start bit of

the next frame.

Figure 18-7. Stop Bit Sampling and Next Start Bit Sampling

The same majority voting is done to the stop bit as done for the other bits in the frame. If the stop bit is

registered to have a logic 0 value, the Frame Error (FEn) Flag will be set.

A new high to low transition indicating the start bit of a new frame can come right after the last of the bits used

for majority voting. For Normal Speed mode, the first low level sample can be at point marked (A) in Figure 18-

7. For Double Speed mode the first low level must be delayed to (B). (C) marks a stop bit of full length. The early

start bit detection influences the operational range of the Receiver.

12345678 9 10 11 12 13 14 15 16 1

BIT n

1234 5 6781

RxD

Sample

(U2X = 0)

Sample

(U2X = 1)

12345678 9 10 0/1 0/1 0/1

STOP 1

1234 5 6 0/1

RxD

Sample

(U2X = 0)

Sample

(U2X = 1)

(A) (B) (C)

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18.7.3 Asynchronous Operational Range

The operational range of the Receiver is dependent on the mismatch between the received bit rate and the

internally generated baud rate. If the Transmitter is sending frames at too fast or too slow bit rates, or the

internally generated baud rate of the Receiver does not have a similar (see Table 18-6 on page 208) base

frequency, the Receiver will not be able to synchronize the frames to the start bit.

The following equations can be used to calculate the ratio of the incoming data rate and internal receiver baud

rate.

The following tables list the maximum receiver baud rate error that can be tolerated. Note that Normal Speed

mode has higher toleration of baud rate variations.

The recommendations of the maximum receiver baud rate error was made under the assumption that the

Receiver and Transmitter equally divides the maximum total error.

D Sum of character size and parity size (D = 5 to 10 bit)

S Samples per bit. S = 16 for Normal Speed mode and S = 8 for Double Speed mode

SFFirst sample number used for majority voting. SF = 8 for normal speed and SF = 4 for Double Speed mode

SMMiddle sample number used for majority voting. SM = 9 for normal speed and SM = 5 for Double Speed mode

Rslow is the ratio of the slowest incoming data rate that can be accepted in relation to the receiver baud rate

Rfast is the ratio of the fastest incoming data rate that can be accepted in relation to the receiver baud rate

Table 18-2. Recommended Maximum Receiver Baud Rate Error for Normal Speed Mode (U2X n = 0)

# (Data+Pari ty Bit) Rslow [%] Rfast [%] Max. tota l error [%] Recommended max. receiver

error [%]

593.20 106.67 +6.67/-6.8 ±3.0

694.12 105.79 +5.79/-5.88 ±2.5

794.81 105.11 +5.11/-5.19 ±2.0

895.36 104.58 +4.58/-4.54 ±2.0

995.81 104.14 +4.14/-4.19 ±1.5

10 96.17 103.78 +3.78/-3.83 ±1.5

Table 18-3. Recommended Maximum Receiver Baud Rate Er ror for Double Speed Mode (U2Xn = 1)

# (Data+Parity Bit) Rslow [%] Rfast [%] Max. total error [%] Recommended max. receiver

error [%]

594.12 105.66 +5.66/-5.88 ±2.5

694.92 104.92 +4.92/-5.08 ±2.0

795.52 104,35 +4.35/-4.48 ±1.5

896.00 103.90 +3.90/-4.00 ±1.5

996.39 103.53 +3.53/-3.61 ±1.5

10 96.70 103.23 +3.23/-3.30 ±1.0

Rslow D1+S

S1–DSSF

-------------------------------------------=

Rfast D2+S

D1+SS

-----------------------------------=

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There are two possible sources for the receivers baud rate error. The Receiver’s system clock (XTAL) will

always have some minor instability over the supply voltage range and the temperature range. When using a

crystal to generate the system clock, this is rarely a problem, but for a resonator the system clock may differ

more than 2% depending of the resonators tolerance. The second source for the error is more controllable. The

baud rate generator can not always do an exact division of the system frequency to get the baud rate wanted. In

this case an UBRR value that gives an acceptable low error can be used if possible.

18.8 Multi-processor Communication Mode

Setting the Multi-processor Communication mode (MPCMn) bit in UCSRnA enables a filtering function of

incoming frames received by the USART Receiver. Frames that do not contain address information will be

ignored and not put into the receive buffer. This effectively reduces the number of incoming frames that has to

be handled by the CPU, in a system with multiple MCUs that communicate via the same serial bus. The

Transmitter is unaffected by the MPCMn setting, but has to be used differently when it is a part of a system

utilizing the Multi-processor Communication mode.

If the Receiver is set up to receive frames that contain 5 to 8 data bits, then the first stop bit indicates if the frame

contains data or address information. If the Receiver is set up for frames with nine data bits, then the ninth bit

(RXB8n) is used for identifying address and data frames. When the frame type bit (the first stop or the ninth bit)

is one, the frame contains an address. When the frame type bit is zero the frame is a data frame.

The Multi-processor Communication mode enables several slave MCUs to receive data from a master MCU.

This is done by first decoding an address frame to find out which MCU has been addressed. If a particular slave

MCU has been addressed, it will receive the following data frames as normal, while the other slave MCUs will

ignore the received frames until another address frame is received.

18.8.1 Using MPCMn

For an MCU to act as a master MCU, it can use a 9-bit character frame format (UCSZn = 7). The ninth bit

(TXB8n) must be set when an address frame (TXB8n = 1) or cleared when a data frame (TXB = 0) is being

transmitted. The slave MCUs must in this case be set to use a 9-bit character frame format.

The following procedure should be used to exchange data in Multi-processor Communication mode:

1. All Slave MCUs are in Multi-processor Communication mode (MPCMn in UCSRnA is set).

2. The Master MCU sends an address frame, and all slaves receive and read this frame. In the Slave MCUs,

the RXCn Flag in UCSRnA will be set as normal.

3. Each Slave MCU reads the UDRn Register and determines if it has been selected. If so, it clears the

MPCMn bit in UCSRnA, otherwise it waits for the next address byte and keeps the MPCMn setting.

4. The addressed MCU will receive all data frames until a new address frame is received. The other Slave

MCUs, which still have the MPCMn bit set, will ignore the data frames.

5. When the last data frame is received by the addressed MCU, the addressed MCU sets the MPCMn bit

and waits for a new address frame from master. The process then repeats from 2.

Using any of the 5- to 8-bit character frame formats is possible, but impractical since the Receiver must change

between using n and n+1 character frame formats. This makes full-duplex operation difficult since the

Transmitter and Receiver uses the same character size setting. If 5- to 8-bit character frames are used, the

Transmitter must be set to use two stop bit (USBSn = 1) since the first stop bit is used for indicating the frame

type.

Do not use Read-Modify-Write instructions (SBI and CBI) to set or clear the MPCMn bit. The MPCMn bit shares

the same I/O location as the TXCn Flag and this might accidentally be cleared when using SBI or CBI

instructions.

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18.9 Hardware Flow Control

The hardware flow control can be enabled by software.

CTS: (Clear to Send)

RTS: (Request to Send)

18.9.1 Receiver Flow Control

The reception flow can be controlled by hardware using the RTS pin. The aim of the flow control is to inform the

external transmitter when the internal receive Fifo is full. Thus the transmitter can stop sending characters. RTS

usage and so associated flow control is enabled using RTSEN bit in UCSRnD. Figure 18-8. shows a reception

example.

Figure 18-8. Reception Flow Cont rol Waveform Example

Figure 18-9. RTS behavior

RTS will rise at 2/3 of the last received stop bit if the receive fifo is full.

To ensure reliable transmissions, even after a RTS rise, an extra-data can still be received and stored in the

Receive Shift Register.

18.9.2 Transmission Flow Control

The transmission flow can be controlled by hardware using the CTS pin controlled by the external receiver. The

aim of the flow control is to stop transmission when the receiver is full of data (CTS = 1). CTS usage and so

associated flow control is enabled using CTSEN bit in UCSRnD. The CTS pin is sampled at each CPU write and

at the middle of the last stop bit that is currently being sent.

TXD

RTS

TXD

RXD

HOST

RXD

CTS

RTS

ATmega16U4/ATm

RTS

RXD

C1 C2

0 1 2

FIFO

CPU Read

Index

RTS

RXD Start Byte0 Stop Start Byte1 Stop

Read from CPU

Start Byte2

1 additional byte may be sent

if the transmitter misses the RTS trig

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Figure 18-10. CTS behavior

18.10 Examples of Baud Rate Setting

For standard crystal and resonator frequencies, the most commonly used baud rates for asynchronous

operation can be generated by using the UBRR settings in Table 18-4 on page 206 to Table 18-11 on page 212.

UBRR values which yield an actual baud rate differing less than 0.5% from the target baud rate, are bold in the

table. Higher error ratings are acceptable, but the Receiver will have less noise resistance when the error

ratings are high, especially for large serial frames (see “Asynchronous Operational Range” on page 203). The

error values are calculated using the following equation:

Note: 1. UBRR = 0, Error = 0.0%

CTS

TXD Start Byte0 Stop Start Byte1 Stop Start Byte2

sample sample

Write from CPU

sample

Error[%] BaudRateClosest Match

BaudRate

-------------------------------------------------------- 1–





100%=

Table 18-4. Examples of UBRRn Settings for Commonly Used Oscillator F requencies

Baud

Rate

[bps]

fosc = 1.0000MHz fosc = 1.8432MHz fosc = 2.0000MHz

U2Xn = 0 U2Xn = 1 U2Xn = 0 U2Xn = 1 U2Xn = 0 U2Xn = 1

UBRR Error UBRR Error UBRR Error UBRR Error UBRR Error UBRR Error

2400 25 0.2% 51 0.2% 47 0.0% 95 0.0% 51 0.2% 103 0.2%

4800 12 0.2% 25 0.2% 23 0.0% 47 0.0% 25 0.2% 51 0.2%

9600 6-7.0% 12 0.2% 11 0.0% 23 0.0% 12 0.2% 25 0.2%

14.4k 38.5% 8-3.5% 70.0% 15 0.0% 8-3.5% 16 2.1%

19.2k 28.5% 6-7.0% 50.0% 11 0.0% 6-7.0% 12 0.2%

28.8k 18.5% 38.5% 30.0% 70.0% 38.5% 8-3.5%

38.4k 1-18.6% 28.5% 20.0% 50.0% 28.5% 6-7.0%

57.6k 08.5% 18.5% 10.0% 30.0% 18.5% 38.5%

76.8k ––1-18.6% 1-25.0% 20.0% 1-18.6% 28.5%

115.2k ––08.5% 00.0% 10.0% 08.5% 18.5%

230.4k ––––––00.0% – – – –

250k ––––––––––00.0%

Max.(1) 62.5kbps 125kbps 115.2kbps 230.4kbps 125kbps 250kbps

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Note: 1. UBRR = 0, Error = 0.0%

Table 18-5. Examples of UBRRn Settings for Commonly Used Oscillator F requencies

Baud

Rate

[bps]

fosc = 3.6864MHz fosc = 4.0000MHz fosc = 7.3728MHz

U2Xn = 0 U2Xn = 1 U2Xn = 0 U2Xn = 1 U2Xn = 0 U2Xn = 1

UBRR Error UBRR Error UBRR Error UBRR Error UBRR Error UBRR Error

2400 95 0.0% 191 0.0% 103 0.2% 207 0.2% 191 0.0% 383 0.0%

4800 47 0.0% 95 0.0% 51 0.2% 103 0.2% 95 0.0% 191 0.0%

9600 23 0.0% 47 0.0% 25 0.2% 51 0.2% 47 0.0% 95 0.0%

14.4k 15 0.0% 31 0.0% 16 2.1% 34 -0.8% 31 0.0% 63 0.0%

19.2k 11 0.0% 23 0.0% 12 0.2% 25 0.2% 23 0.0% 47 0.0%

28.8k 70.0% 15 0.0% 8-3.5% 16 2.1% 15 0.0% 31 0.0%

38.4k 50.0% 11 0.0% 6-7.0% 12 0.2% 11 0.0% 23 0.0%

57.6k 30.0% 70.0% 38.5% 8-3.5% 70.0% 15 0.0%

76.8k 20.0% 50.0% 28.5% 6-7.0% 50.0% 11 0.0%

115.2k 10.0% 30.0% 18.5% 38.5% 30.0% 70.0%

230.4k 00.0% 10.0% 08.5% 18.5% 10.0% 30.0%

250k 0-7.8% 1-7.8% 00.0% 10.0% 1-7.8% 3-7.8%

0.5M ––0-7.8% – – 0 0.0% 0-7.8% 1-7.8%

1M ––––––––––0-7.8%

Max.(1) 230.4kbps 460.8kbps 250kbps 0.5Mbps 460.8kbps 921.6kbps

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Note: 1. UBRR = 0, Error = 0.0%

Table 18-6. Examples of UBRRn Settings for Commonly Used Oscillator F requencies

Baud

Rate

[bps]

fosc = 8.0000MHz fosc = 11.0592MHz fosc = 14.7456MHz

U2Xn = 0 U2Xn = 1 U2Xn = 0 U2Xn = 1 U2Xn = 0 U2Xn = 1

UBRR Error UBRR Error UBRR Error UBRR Error UBRR Error UBRR Error

2400 207 0.2% 416 -0.1% 287 0.0% 575 0.0% 383 0.0% 767 0.0%

4800 103 0.2% 207 0.2% 143 0.0% 287 0.0% 191 0.0% 383 0.0%

9600 51 0.2% 103 0.2% 71 0.0% 143 0.0% 95 0.0% 191 0.0%

14.4k 34 -0.8% 68 0.6% 47 0.0% 95 0.0% 63 0.0% 127 0.0%

19.2k 25 0.2% 51 0.2% 35 0.0% 71 0.0% 47 0.0% 95 0.0%

28.8k 16 2.1% 34 -0.8% 23 0.0% 47 0.0% 31 0.0% 63 0.0%

38.4k 12 0.2% 25 0.2% 17 0.0% 35 0.0% 23 0.0% 47 0.0%

57.6k 8-3.5% 16 2.1% 11 0.0% 23 0.0% 15 0.0% 31 0.0%

76.8k 6-7.0% 12 0.2% 80.0% 17 0.0% 11 0.0% 23 0.0%

115.2k 38.5% 8-3.5% 50.0% 11 0.0% 70.0% 15 0.0%

230.4k 18.5% 38.5% 20.0% 50.0% 30.0% 70.0%

250k 10.0% 30.0% 2-7.8% 5-7.8% 3-7.8% 65.3%

0.5M 00.0% 10.0% – – 2 -7.8% 1-7.8% 3-7.8%

1M ––00.0% ––––0-7.8% 1-7.8%

Max.(1) 0.5Mbps 1Mbps 691.2kbps 1.3824Mbps 921.6kbps 1.8432Mbps

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18.11 USART Register Description

18.11.1 USART I/O Data Register n– UDRn

The USART Transmit Data Buffer Register and USART Receive Data Buffer Registers share the same I/O

address referred to as USART Data Register or UDRn. The Transmit Data Buffer Register (TXB) will be the

destination for data written to the UDRn Register location. Reading the UDRn Register location will return the

contents of the Receive Data Buffer Register (RXB).

For 5-, 6-, or 7-bit characters the upper unused bits will be ignored by the Transmitter and set to zero by the

Receiver.

The transmit buffer can only be written when the UDREn Flag in the UCSRnA Register is set. Data written to

UDRn when the UDREn Flag is not set, will be ignored by the USART Transmitter. When data is written to the

transmit buffer, and the Transmitter is enabled, the Transmitter will load the data into the Transmit Shift Register

when the Shift Register is empty. Then the data will be serially transmitted on the TxDn pin.

The receive buffer consists of a two level FIFO. The FIFO will change its state whenever the receive buffer is

accessed. Due to this behavior of the receive buffer, do not use Read-Modify-Write instructions (SBI and CBI)

on this location. Be careful when using bit test instructions (SBIC and SBIS), since these also will change the

state of the FIFO.

18.11.2 USART Control and Status Register A – UCSRnA

• Bit 7 – RXCn: USART Receive Complete

This flag bit is set when there are unread data in the receive buffer and cleared when the receive buffer is empty

(i.e., does not contain any unread data). If the Receiver is disabled, the receive buffer will be flushed and

consequently the RXCn bit will become zero. The RXCn Flag can be used to generate a Receive Complete

interrupt (see description of the RXCIEn bit).

• Bit 6 – TXCn: USART Transmit Complete

This flag bit is set when the entire frame in the Transmit Shift Register has been shifted out and there are no

new data currently present in the transmit buffer (UDRn). The TXCn Flag bit is automatically cleared when a

transmit complete interrupt is executed, or it can be cleared by writing a one to its bit location. The TXCn Flag

can generate a Transmit Complete interrupt (see description of the TXCIEn bit).

• Bit 5 – UDREn: USART Data Register Empty

The UDREn Flag indicates if the transmit buffer (UDRn) is ready to receive new data. If UDREn is one, the

buffer is empty, and therefore ready to be written. The UDREn Flag can generate a Data Register Empty

interrupt (see description of the UDRIEn bit).

UDREn is set after a reset to indicate that the Transmitter is ready.

Bit 76543210

RXB[7:0] UDRn (Read)

TXB[7:0] UDRn (Write)

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value 0 0 0 0 0 0 0 0

Bit 76543210

RXCn TXCn UDREn FEn DORn UPEn U2Xn MPCMn UCSRnA

Read/Write R R/W R R R R R/W R/W

Initial Value00100000

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• Bit 4 – FEn: Frame Error

This bit is set if the next character in the receive buffer had a Frame Error when received. I.e., when the first stop

bit of the next character in the receive buffer is zero. This bit is valid until the receive buffer (UDRn) is read. The

FEn bit is zero when the stop bit of received data is one. Always set this bit to zero when writing to UCSRnA.

• Bit 3 – DORn: Data OverRun

This bit is set if a Data OverRun condition is detected. A Data OverRun occurs when the receive buffer is full

(two characters), it is a new character waiting in the Receive Shift Register, and a new start bit is detected. This

bit is valid until the receive buffer (UDRn) is read. Always set this bit to zero when writing to UCSRnA.

• Bit 2 – UPEn: USART Parity Error

This bit is set if the next character in the receive buffer had a Parity Error when received and the Parity Checking

was enabled at that point (UPMn1 = 1). This bit is valid until the receive buffer (UDRn) is read. Always set this

bit to zero when writing to UCSRnA.

• Bit 1 – U2Xn: Double the USART Transmission Speed

This bit only has effect for the asynchronous operation. Write this bit to zero when using synchronous operation.

Writing this bit to one will reduce the divisor of the baud rate divider from 16 to 8 effectively doubling the transfer

rate for asynchronous communication.

• Bit 0 – MPCMn: Multi-processor Communication Mode

This bit enables the Multi-processor Communication mode. When the MPCMn bit is written to one, all the

incoming frames received by the USART Receiver that do not contain address information will be ignored. The

Transmitter is unaffected by the MPCMn setting. For more detailed information see “Multi-processor

Communication Mode” on page 204.

18.11. 3 USART Contro l and Status Register n B – UCSRnB

• Bit 7 – RXCIEn: RX Complete Interrupt Enable n

Writing this bit to one enables interrupt on the RXCn Flag. A USART Receive Complete interrupt will be

generated only if the RXCIEn bit is written to one, the Global Interrupt Flag in SREG is written to one and the

RXCn bit in UCSRnA is set.

• Bit 6 – TXCIEn: TX Complete Interrupt Enable n

Writing this bit to one enables interrupt on the TXCn Flag. A USART Transmit Complete interrupt will be

generated only if the TXCIEn bit is written to one, the Global Interrupt Flag in SREG is written to one and the

TXCn bit in UCSRnA is set.

• Bit 5 – UDRIEn: USART Data Register Empty Interrupt Enable n

Writing this bit to one enables interrupt on the UDREn Flag. A Data Register Empty interrupt will be generated

only if the UDRIEn bit is written to one, the Global Interrupt Flag in SREG is written to one and the UDREn bit in

UCSRnA is set.

Bit 7 6 5 4 3 2 1 0

RXCIEn TXCIEn UDRIEn RXENn TXENn UCSZn2 RXB8n TXB8n UCSRnB

Read/Write R/W R/W R/W R/W R/W R/W R R/W

Initial Value 0 0 0 0 0 0 0 0

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• Bit 4 – RXENn: Receiver Enable n

Writing this bit to one enables the USART Receiver. The Receiver will override normal port operation for the

RxDn pin when enabled. Disabling the Receiver will flush the receive buffer invalidating the FEn, DORn, and

UPEn Flags.

• Bit 3 – TXENn: Transmitter Enable n

Writing this bit to one enables the USART Transmitter. The Transmitter will override normal port operation for

the TxDn pin when enabled. The disabling of the Transmitter (writing TXENn to zero) will not become effective

until ongoing and pending transmissions are completed, i.e., when the Transmit Shift Register and Transmit

Buffer Register do not contain data to be transmitted. When disabled, the Transmitter will no longer override the

TxDn port.

• Bit 2 – UCSZn2: Character Size n

The UCSZn2 bits combined with the UCSZn1:0 bit in UCSRnC sets the number of data bits (Character SiZe) in

a frame the Receiver and Transmitter use.

• Bit 1 – RXB8n: Receive Data Bit 8 n

RXB8n is the ninth data bit of the received character when operating with serial frames with nine data bits. Must

be read before reading the low bits from UDRn.

• Bit 0 – TXB8n: Transmit Dat a Bit 8 n

TXB8n is the ninth data bit in the character to be transmitted when operating with serial frames with nine data

bits. Must be written before writing the low bits to UDRn.

18.11. 4 USART Contro l and Status Register n C – UCSRnC

• Bits 7:6 – UMSELn1:0 USART Mode Select

These bits select the mode of operation of the USARTn as shown below.

Note: 1. See “USART in SPI Mode” on page 214 for full description of the Master SPI Mode (MSPIM) operation

• Bits 5:4 – UPMn1:0: Parity Mode

These bits enable and set type of parity generation and check. If enabled, the Transmitter will automatically

generate and send the parity of the transmitted data bits within each frame. The Receiver will generate a parity

value for the incoming data and compare it to the UPMn setting. If a mismatch is detected, the UPEn Flag in

UCSRnA will be set.

Bit 7 6 5432 1 0

UMSELn1 UMSELn0 UPMn1 UPMn0 USBSn UCSZn1 UCSZn0 UCPOLn UCSRnC

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Val-

0 0 0001 1 0

Table 18-7. UMSELn Bit Settings

UMSELn1 UMSELn0 Mode

0 0 Asynchronous USART

0 1 Synchronous USART

1 0 (Reserved)

1 1 Master SPI (MSPIM)(1)

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• Bit 3 – USBSn: Stop Bit Select

This bit selects the number of stop bits to be inserted by the Transmitter. The Receiver ignores this setting.

• Bit 2:1 – UCSZn1:0: Character Size

The UCSZn1:0 bits combined with the UCSZn2 bit in UCSRnB sets the number of data bits (Character SiZe) in

a frame the Receiver and Transmitter use.

• Bit 0 – UCPOLn: Clock Polarity

This bit is used for synchronous mode only. Write this bit to zero when asynchronous mode is used. The

UCPOLn bit sets the relationship between data output change and data input sample, and the synchronous

clock (XCKn).

Table 18-8. UPMn Bit Settings

UPMn1 UPMn0 Parity Mode

0 0 Disabled

0 1 Reserved

1 0 Enabled, Even Parity

1 1 Enabled, Odd Parity

Table 18-9. USBS Bit Settings

USBSn Stop Bit(s)

01-bit

12-bit

Table 18-10. UCSZn Bit Settings

UCSZn2 UCSZn1 UCSZn0 Character Size

0005-bit

0016-bit

0107-bit

0118-bit

100Reserved

101Reserved

110Reserved

1119-bit

Table 18-11. UCPOLn Bit Settings

UCPOLn Transmitted Dat a Chang ed (Output of Tx Dn Pin) Received Data Sampled (Input on RxDn Pin)

0Rising XCKn Edge Falling XCKn Edge

1Falling XCKn Edge Rising XCKn Edge

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18.11. 5 USART Contro l and Status Register n D– UCSRnD

• Bits 7:2 – Reserved bits

These bits are reserved and will be read as ‘0’. Do not set these bits.

• Bits 1 – CTSEN: UART CTS Signal Enable

Set this bit by firmware to enable the transmission flow control signal (CTS). Transmission will be enabled only if

CTS input = 0. Clear this bit to disable the transmission flow control signal. Transmission will occur without

hardware condition. Data Direction Register bit must be correctly clear to enable the pin as an input.

• Bits 0 – RTSEN: UART RTS Signal Enable

Set this bit by firmware to enable the reception flow control signal (RTS). In this case the RTS line will

automatically rise when the FIFO is full. Clear this bit to disable the reception flow control signal. Data Direction

18.11.6 USART Baud Rate Registers – UBRRLn and UBRRHn

• Bit 15:12 – Reserved Bits

These bits are reserved for future use. For compatibility with future devices, these bit must be written to zero

when UBRRH is written.

• Bit 11:0 – UBRR11:0: USART Baud Rate Register

This is a 12-bit register which contains the USART baud rate. The UBRRH contains the four most significant

bits, and the UBRRL contains the eight least significant bits of the USART baud rate. Ongoing transmissions by

the Transmitter and Receiver will be corrupted if the baud rate is changed. Writing UBRRL will trigger an

immediate update of the baud rate prescaler.

Bit 7 6 5432 1 0

– – – – – – CTSEN RTSEN UCSRnD

Read/WriteR R RRRR R/WR/W

Initial Val-

0 0 0000 0 0

Bit 1514131211109 8

–––– UBRR[11:8] UBRRHn

UBRR[7:0] UBRRLn

76543210

Read/WriteRRRRR/WR/WR/WR/W

R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value00000000

00000000

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19. USART in SPI Mode

The Universal Synchronous and Asynchronous serial Receiver and Transmitter (USART) can be set to a master

SPI compliant mode of operation. The Master SPI Mode (MSPIM) has the following features:

•Full Duplex, Three-wire Synchronous Data Transfer

•Master Operation

•Supports all four SPI Modes of Operation (Mode 0, 1, 2, and 3)

•LSB First or MSB First Data Transfer (Configurable Data Order)

•Queued Operation (Double Buffered)

•High Resolution Baud Rate Generator

•High Speed Operation (fXCKmax = fCK/2)

•Flexible Interrupt Generation

19.1 Overview

Setting both UMSELn1:0 bits to one enables the USART in MSPIM logic. In this mode of operation the SPI

master control logic takes direct control over the USART resources. These resources include the transmitter

and receiver shift register and buffers, and the baud rate generator. The parity generator and checker, the data

and clock recovery logic, and the RX and TX control logic is disabled. The USART RX and TX control logic is

replaced by a common SPI transfer control logic. However, the pin control logic and interrupt generation logic is

identical in both modes of operation.

The I/O register locations are the same in both modes. However, some of the functionality of the control

registers changes when using MSPIM.

19.2 Clock Generation

The Clock Generation logic generates the base clock for the Transmitter and Receiver. For USART MSPIM

mode of operation only internal clock generation (i.e. master operation) is supported. The Data Direction

operate correctly. Preferably the DDR_XCKn should be set up before the USART in MSPIM is enabled (i.e.

TXENn and RXENn bit set to one).

The internal clock generation used in MSPIM mode is identical to the USART synchronous master mode. The

baud rate or UBRRn setting can therefore be calculated using the same equations in the following table:

Note: 1. The baud rate is defined to be the transfer rate in bit per second (bps).

BAUD: Baud rate (in bits per second, bps).

fOSC: System Oscillator clock frequency.

UBRRn: Contents of the UBRRnH and UBRRnL Registers, (0-4095).

19.3 SPI Data Modes and Timing

There are four combinations of XCKn (SCK) phase and polarity with respect to serial data, which are

determined by control bits UCPHAn and UCPOLn. The data transfer timing diagrams are shown in Figure 19-1.

Operating Mod e Equation for Calculating Baud

Rate(1) Equation for Calculating UBRRn

Value

Synchronous Master

mode

BAUD fOSC

2UBRRn1+

---------------------------------------=

UBRRnfOSC

2BAUD

-------------------- 1–=

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Data bits are shifted out and latched in on opposite edges of the XCKn signal, ensuring sufficient time for data

signals to stabilize. The UCPOLn and UCPHAn functionality is summarized in the following table.

Note that changing the setting of any of these bits will corrupt all ongoing communication for both the Receiver

and Transmitter.

Figure 19-1. UCPHAn and UCPOLn data transfer timing diagrams.

19.4 Frame Formats

A serial frame for the MSPIM is defined to be one character of 8 data bits. The USART in MSPIM mode has two

valid frame formats:

8-bit data with MSB first

8-bit data with LSB first

A frame starts with the least or most significant data bit. Then the next data bits, up to a total of eight, are

succeeding, ending with the most or least significant bit accordingly. When a complete frame is transmitted, a

new frame can directly follow it, or the communication line can be set to an idle (high) state.

The UDORDn bit in UCSRnC sets the frame format used by the USART in MSPIM mode. The Receiver and

Transmitter use the same setting. Note that changing the setting of any of these bits will corrupt all ongoing

communication for both the Receiver and Transmitter.

16-bit data transfer can be achieved by writing two data bytes to UDRn. A UART transmit complete interrupt will

then signal that the 16-bit value has been shifted out.

19.4.1 USART MSPIM Initialization

The USART in MSPIM mode has to be initialized before any communication can take place. The initialization

process normally consists of setting the baud rate, setting master mode of operation (by setting DDR_XCKn to

one), setting frame format and enabling the Transmitter and the Receiver. Only the transmitter can operate

independently. For interrupt driven USART operation, the Global Interrupt Flag should be cleared (and thus

interrupts globally disabled) when doing the initialization.

Table 19-1. UCPOLn and UCPHAn Functionality

UCPOLn UCPHAn SPI Mode Leading Edge Trailing Edge

000Sample (Rising) Setup (Falling)

011Setup (Rising) Sample (Falling)

102Sample (Falling) Setup (Rising)

113Setup (Falling) Sample (Rising)

XCK

Data setup (TXD)

Data sample (RXD)

XCK

Data setup (TXD)

Data sample (RXD)

XCK

Data setup (TXD)

Data sample (RXD)

XCK

Data setup (TXD)

Data sample (RXD)

UCPOL=0 UCPOL=1

UCPHA=0 UCPHA=1

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Note: To ensure immediate initialization of the XCKn output the baud-rate register (UBRRn) must be zero at the time the

transmitter is enabled. Contrary to the normal mode USART operation the UBRRn must then be written to the

desired value after the transmitter is enabled, but before the first transmission is started. Setting UBRRn to zero

before enabling the transmitter is not necessary if the initialization is done immediately after a reset since UBRRn is

reset to zero.

Before doing a re-initialization with changed baud rate, data mode, or frame format, be sure that there is no

ongoing transmissions during the period the registers are changed. The TXCn Flag can be used to check that

the Transmitter has completed all transfers, and the RXCn Flag can be used to check that there are no unread

data in the receive buffer. Note that the TXCn Flag must be cleared before each transmission (before UDRn is

written) if it is used for this purpose.

The following simple USART initialization code examples show one assembly and one C function that are equal

in functionality. The examples assume polling (no interrupts enabled). The baud rate is given as a function

parameter. For the assembly code, the baud rate parameter is assumed to be stored in the r17:r16 registers.

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Note: 1. See “Code Examples” on page 8.

Assembly Code Example(1)

USART_Init:

clr r18

out UBRRnH,r18

out UBRRnL,r18

; Setting the XCKn port pin as output, enables

master mode.

sbi XCKn_DDR, XCKn

; Set MSPI mode of operation and SPI data mode 0.

ldi r18,

(1<<UMSELn1)|(1<<UMSELn0)|(0<<UCPHAn)|(0<<UCPOLn)

out UCSRnC,r18

; Enable receiver and transmitter.

ldi r18, (1<<RXENn)|(1<<TXENn)

out UCSRnB,r18

; Set baud rate.

; IMPORTANT: The Baud Rate must be set after the

transmitter is enabled!

out UBRRnH, r17

out UBRRnL, r18

ret

C Code Example(1)

void USART_Init( unsigned int baud )

{UBRRn = 0;

/* Setting the XCKn port pin as output, enables

master mode. */

XCKn_DDR |= (1<<XCKn);

/* Set MSPI mode of operation and SPI data mode 0.

*/ UCSRnC =

(1<<UMSELn1)|(1<<UMSELn0)|(0<<UCPHAn)|(0<<UCPOLn);

/* Enable receiver and transmitter. */

UCSRnB = (1<<RXENn)|(1<<TXENn);

/* Set baud rate. */

/* IMPORTANT: The Baud Rate must be set after the

transmitter is enabled */

UBRRn = baud;

}

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19.5 Data Transfer

Using the USART in MSPI mode requires the Transmitter to be enabled, i.e. the TXENn bit in the UCSRnB

and given the function as the Transmitter's serial output. Enabling the receiver is optional and is done by setting

the RXENn bit in the UCSRnB register to one. When the receiver is enabled, the normal pin operation of the

RxDn pin is overridden and given the function as the Receiver's serial input. The XCKn will in both cases be

used as the transfer clock.

After initialization the USART is ready for doing data transfers. A data transfer is initiated by writing to the UDRn

I/O location. This is the case for both sending and receiving data since the transmitter controls the transfer

clock. The data written to UDRn is moved from the transmit buffer to the shift register when the shift register is

ready to send a new frame.

Note: To keep the input buffer in sync with the number of data bytes transmitted, the UDRn register must be read once for

each byte transmitted. The input buffer operation is identical to normal USART mode, i.e. if an overflow occurs the

character last received will be lost, not the first data in the buffer. This means that if four bytes are transferred, byte

1 first, then byte 2, 3, and 4, and the UDRn is not read before all transfers are completed, then byte 3 to be received

will be lost, and not byte 1.

The following code examples show a simple USART in MSPIM mode transfer function based on polling of the

Data Register Empty (UDREn) Flag and the Receive Complete (RXCn) Flag. The USART has to be initialized

before the function can be used. For the assembly code, the data to be sent is assumed to be stored in Register

R16 and the data received will be available in the same register (R16) after the function returns.

The function simply waits for the transmit buffer to be empty by checking the UDREn Flag, before loading it with

new data to be transmitted. The function then waits for data to be present in the receive buffer by checking the

RXCn Flag, before reading the buffer and returning the value.

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Note: 1. See “Code Examples” on page 8.

19.5.1 Transmitter and Receiver Flags and Interrupts

The RXCn, TXCn, and UDREn flags and corresponding interrupts in USART in MSPIM mode are identical in

function to the normal USART operation. However, the receiver error status flags (FE, DOR, and PE) are not in

use and is always read as zero.

19.5.2 Disabling the Transmitter or Receiver

The disabling of the transmitter or receiver in USART in MSPIM mode is identical in function to the normal

USART operation.

Assembly Code Example(1)

USART_MSPIM_Transfer:

; Wait for empty transmit buffer

sbis UCSRnA, UDREn

rjmp USART_MSPIM_Transfer

; Put data (r16) into buffer, sends the data

out UDRn,r16

; Wait for data to be received

USART_MSPIM_Wait_RXCn:

sbis UCSRnA, RXCn

rjmp USART_MSPIM_Wait_RXCn

; Get and return received data from buffer

in r16, UDRn

ret

C Code Example(1)

unsigned char USART_Receive( void )

{/* Wait for empty transmit buffer */

while ( !( UCSRnA & (1<<UDREn)) );

/* Put data into buffer, sends the data */

UDRn = data;

/* Wait for data to be received */

while ( !(UCSRnA & (1<<RXCn)) );

/* Get and return received data from buffer */

return UDRn;

}

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19.6 AVR USART MSPIM vs. AVR SPI

The USART in MSPIM mode is fully compatible with the AVR SPI regarding:

Master mode timing diagram

The UCPOLn bit functionality is identical to the SPI CPOL bit

The UCPHAn bit functionality is identical to the SPI CPHA bit

The UDORDn bit functionality is identical to the SPI DORD bit

However, since the USART in MSPIM mode reuses the USART resources, the use of the USART in MSPIM

mode is somewhat different compared to the SPI. In addition to differences of the control register bits, and that

only master operation is supported by the USART in MSPIM mode, the following features differ between the two

modules:

The USART in MSPIM mode includes (double) buffering of the transmitter. The SPI has no buffer.

The USART in MSPIM mode receiver includes an additional buffer level

The SPI WCOL (Write Collision) bit is not included in USART in MSPIM mode

The SPI double speed mode (SPI2X) bit is not included. However, the same effect is achieved by setting

UBRRn accordingly

Interrupt timing is not compatible

Pin control differs due to the master only operation of the USART in MSPIM mode

A comparison of the USART in MSPIM mode and the SPI pins is shown in the table below.

Table 19-2. Comparison of USART in MSPIM mode and SPI pins

USART_MSPIM SPI Comment

TxDn MOSI Master Out only

RxDn MISO Master In only

XCKn SCK (Functionally identical)

(N/A) SS Not supported by USART in MSPIM

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19.7 USART MSPIM Register Description

The following section describes the registers used for SPI operation using the USART.

19.7.1 USART MSPIM I/O Data Register - UDRn

The function and bit description of the USART data register (UDRn) in MSPI mode is identical to normal USART

operation. See “USART I/O Data Register n– UDRn” on page 209.

19.7.2 USART MSPIM Control and Status Register n A - UCSRnA

• Bit 7 - RXCn: USART Receive Complete

This flag bit is set when there are unread data in the receive buffer and cleared when the receive buffer is empty

(i.e., does not contain any unread data). If the Receiver is disabled, the receive buffer will be flushed and

consequently the RXCn bit will become zero. The RXCn Flag can be used to generate a Receive Complete

interrupt (see description of the RXCIEn bit).

• Bit 6 - TXCn: USART Transmit Complete

This flag bit is set when the entire frame in the Transmit Shift Register has been shifted out and there are no

new data currently present in the transmit buffer (UDRn). The TXCn Flag bit is automatically cleared when a

transmit complete interrupt is executed, or it can be cleared by writing a one to its bit location. The TXCn Flag

can generate a Transmit Complete interrupt (see description of the TXCIEn bit).

• Bit 5 - UDREn: USART Data Register Empty

The UDREn Flag indicates if the transmit buffer (UDRn) is ready to receive new data. If UDREn is one, the

buffer is empty, and therefore ready to be written. The UDREn Flag can generate a Data Register Empty

interrupt (see description of the UDRIE bit). UDREn is set after a reset to indicate that the Transmitter is ready.

• Bit 4:0 - Reserved Bits in MSPI mode

When in MSPI mode, these bits are reserved for future use. For compatibility with future devices, these bits

must be written to zero when UCSRnA is written.

Bit 7 6 5 4 3 2 1 0

RXCn TXCn UDREn - - - - - UCSRnA

Read/Write R/W R/W R/W R R R R R

Initial Value 0 0 0 0 0 1 1 0

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19.7.3 USART MSPIM Control and Status Register n B - UCSRnB

• Bit 7 - RXCIEn: RX Complete Interrupt Enable

Writing this bit to one enables interrupt on the RXCn Flag. A USART Receive Complete interrupt will be

generated only if the RXCIEn bit is written to one, the Global Interrupt Flag in SREG is written to one and the

RXCn bit in UCSRnA is set.

• Bit 6 - TXCIEn: TX Complete Interrupt Enable

Writing this bit to one enables interrupt on the TXCn Flag. A USART Transmit Complete interrupt will be

generated only if the TXCIEn bit is written to one, the Global Interrupt Flag in SREG is written to one and the

TXCn bit in UCSRnA is set.

• Bit 5 - UDRIE: USART Data Register Empty Interrupt Enable

Writing this bit to one enables interrupt on the UDREn Flag. A Data Register Empty interrupt will be generated

only if the UDRIE bit is written to one, the Global Interrupt Flag in SREG is written to one and the UDREn bit in

UCSRnA is set.

• Bit 4 - RXENn: Receiver Enable

Writing this bit to one enables the USART Receiver in MSPIM mode. The Receiver will override normal port

operation for the RxDn pin when enabled. Disabling the Receiver will flush the receive buffer. Only enabling the

receiver in MSPI mode (i.e. setting RXENn=1 and TXENn=0) has no meaning since it is the transmitter that

controls the transfer clock and since only master mode is supported.

• Bit 3 - TXENn: Transmitter Enable

Writing this bit to one enables the USART Transmitter. The Transmitter will override normal port operation for

the TxDn pin when enabled. The disabling of the Transmitter (writing TXENn to zero) will not become effective

until ongoing and pending transmissions are completed, i.e., when the Transmit Shift Register and Transmit

Buffer Register do not contain data to be transmitted. When disabled, the Transmitter will no longer override the

TxDn port.

• Bit 2:0 - Reserved Bits in MSPI mode

When in MSPI mode, these bits are reserved for future use. For compatibility with future devices, these bits

must be written to zero when UCSRnB is written.

Bit 7 6 5 4 3 2 1 0

RXCIEn TXCIEn UDRIE RXENn TXENn - - - UCSRnB

Read/Write R/W R/W R/W R/W R/W R R R

Initial Value 0 0 0 0 0 1 1 0

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19.7.4 USART MSPIM Control and Status Register n C - UCSRnC

• Bit 7:6 - UMSELn1:0: USART Mode Select

These bits select the mode of operation of the USART as shown in the table. See “USART Control and Status

enabled when both UMSELn bits are set to one. The UDORDn, UCPHAn, and UCPOLn can be set in the same

write operation where the MSPIM is enabled.

• Bit 5:3 - Reserved

When in MSPI mode, these bits are reserved for future use. For compatibility with future devices, these bits

must be written to zero when UCSRnC is written.

• Bit 2 - UDORDn: Data Order

When set to one the LSB of the data word is transmitted first. When set to zero the MSB of the data word is

transmitted first. For details, see “Frame Formats” on page 192.

• Bit 1 - UCPHAn: Clock Phase

The UCPHAn bit setting determine if data is sampled on the leasing edge (first) or tailing (last) edge of XCKn.

For details, see “SPI Data Modes and Timing” on page 214.

• Bit 0 - UCPOLn: Clock Polarity

The UCPOLn bit sets the polarity of the XCKn clock. The combination of the UCPOLn and UCPHAn bit settings

determine the timing of the data transfer. For details, see “SPI Data Modes and Timing” on page 214.

19.7.5 USART MSPIM Baud Rate Registers - UBRRnL and UBRRnH

The function and bit description of the baud rate registers in MSPI mode is identical to normal USART operation.

See “USART Baud Rate Registers – UBRRLn and UBRRHn” on page 213.

Bit 7 6 5 4 3 2 1 0

UMSELn1 UMSELn0 - - - UDORDn UCPHAn UCPOLn UCSRnC

Read/Write R/W R/W R R R R/W R/W R/W

Initial Value 0 0 0 0 0 1 1 0

Table 19-3. UMSELn Bits Settings

UMSELn1 UMSELn0 Mode

0 0 Asynchronous USART

0 1 Synchronous USART

1 0 (Reserved)

1 1 Master SPI (MSPIM)

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20. 2-wire Serial Interface

20.1 Features

•Simple Yet Powerful and Flexible Communication Interface, only two Bus Lines Needed

•Both Master and Slave Operation Supported

•Device can Operate as Transmitter or Receiver

•7-bit Address Space Allows up to 128 Different Slave Addre sses

•Multi-master Arbitration Support

•Up to 400kHz Data Transfer Speed

•Slew-rate Lim i ted Output Drivers

•Noise Suppression Circuitry Rejects Spikes on Bus Lines

•Fully Programmable Slave Address with General Call Support

•Address Recognition Causes Wake-up When AVR is in Sleep Mode

20.2 2-wire Serial Interface Bus Definition

The 2-wire Serial Interface (TWI) is ideally suited for typical microcontroller applications. The TWI protocol

allows the systems designer to interconnect up to 128 different devices using only two bi-directional bus lines,

one for clock (SCL) and one for data (SDA). The only external hardware needed to implement the bus is a single

pull-up resistor for each of the TWI bus lines. All devices connected to the bus have individual addresses, and

mechanisms for resolving bus contention are inherent in the TWI protocol.

Figure 20-1. TWI Bus Interconnection

20.2.1 TWI Terminology

The following definitions are frequently encountered in this section.

Device 1 Device 2 Device 3 Device n

SDA

SCL

........ R1 R2

Term Description

Master The device that initiates and terminates a transmission. The Master also generates the

SCL clock.

Slave The device addressed by a Master

Transmitter The device placing data on the bus

Receiver The device reading data from the bus

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The Power Reduction TWI bit, PRTWI bit in “Power Reduction Register 0 - PRR0” on page 47 must be written to

zero to enable the 2-wire Serial Interface.

20.2.2 Electrical Interconnection

As depicted in Figure 20-1 on page 225, both bus lines are connected to the positive supply voltage through

pull-up resistors. The bus drivers of all TWI-compliant devices are open-drain or open-collector. This

implements a wired-AND function which is essential to the operation of the interface. A low level on a TWI bus

line is generated when one or more TWI devices output a zero. A high level is output when all TWI devices trim-

state their outputs, allowing the pull-up resistors to pull the line high. Note that all AVR devices connected to the

TWI bus must be powered in order to allow any bus operation.

The number of devices that can be connected to the bus is only limited by the bus capacitance limit of 400pF

and the 7-bit slave address space. A detailed specification of the electrical characteristics of the TWI is given in

“SPI Timing Characteristics” on page 388. Two different sets of specifications are presented there, one relevant

for bus speeds below 100kHz, and one valid for bus speeds up to 400kHz.

20.3 Data Transfer and Frame Format

20.3.1 Transferring Bits

Each data bit transferred on the TWI bus is accompanied by a pulse on the clock line. The level of the data line

must be stable when the clock line is high. The only exception to this rule is for generating start and stop

conditions.

Figure 20-2. Data Validity

20.3.2 START and STOP Conditions

The Master initiates and terminates a data transmission. The transmission is initiated when the Master issues a

START condition on the bus, and it is terminated when the Master issues a STOP condition. Between a START

and a STOP condition, the bus is considered busy, and no other master should try to seize control of the bus. A

special case occurs when a new START condition is issued between a START and STOP condition. This is

referred to as a REPEATED START condition, and is used when the Master wishes to initiate a new transfer

without relinquishing control of the bus. After a REPEATED START, the bus is considered busy until the next

STOP. This is identical to the START behavior, and therefore START is used to describe both START and

REPEATED START for the remainder of this datasheet, unless otherwise noted. As depicted below, START

and STOP conditions are signalled by changing the level of the SDA line when the SCL line is high.

SDA

SCL

Data Stable Data Stable

Data Change

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Figure 20-3. START, REPEATED START and STOP Conditions

20.3.3 Address Packet Format

All address packets transmitted on the TWI bus are nine bits long, consisting of seven address bits, one

READ/WRITE control bit and an acknowledge bit. If the READ/WRITE bit is set, a read operation is to be

performed, otherwise a write operation should be performed. When a Slave recognizes that it is being

addressed, it should acknowledge by pulling SDA low in the ninth SCL (ACK) cycle. If the addressed Slave is

busy, or for some other reason can not service the Master’s request, the SDA line should be left high in the ACK

clock cycle. The Master can then transmit a STOP condition, or a REPEATED START condition to initiate a new

transmission. An address packet consisting of a slave address and a READ or a WRITE bit is called SLA+R or

SLA+W, respectively.

The MSB of the address byte is transmitted first. Slave addresses can freely be allocated by the designer, but

the address 0000 000 is reserved for a general call.

When a general call is issued, all slaves should respond by pulling the SDA line low in the ACK cycle. A general

call is used when a Master wishes to transmit the same message to several slaves in the system. When the

general call address followed by a Write bit is transmitted on the bus, all slaves set up to acknowledge the

general call will pull the SDA line low in the ack cycle. The following data packets will then be received by all the

slaves that acknowledged the general call. Note that transmitting the general call address followed by a Read bit

is meaningless, as this would cause contention if several slaves started transmitting different data.

All addresses of the format 1111 xxx should be reserved for future purposes.

Figure 20-4. Address Packet Format

20.3.4 Data Packet Format

All data packets transmitted on the TWI bus are nine bits long, consisting of one data byte and an acknowledge

bit. During a data transfer, the Master generates the clock and the START and STOP conditions, while the

Receiver is responsible for acknowledging the reception. An Acknowledge (ACK) is signalled by the Receiver

pulling the SDA line low during the ninth SCL cycle. If the Receiver leaves the SDA line high, a NACK is

signalled. When the Receiver has received the last byte, or for some reason cannot receive any more bytes, it

SDA

SCL

START STOPREPEATED START

STOP START

SDA

SCL

START

12 789

Addr MSB Addr LSB R/W ACK

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should inform the Transmitter by sending a NACK after the final byte. The MSB of the data byte is transmitted

first.

Figure 20-5. Data Packet Format

20.3.5 Combining Address and Data Packets into a Transmission

A transmission basically consists of a START condition, a SLA+R/W, one or more data packets and a STOP

condition. An empty message, consisting of a START followed by a STOP condition, is illegal. Note that the

Wired-ANDing of the SCL line can be used to implement handshaking between the Master and the Slave. The

Slave can extend the SCL low period by pulling the SCL line low. This is useful if the clock speed set up by the

Master is too fast for the Slave, or the Slave needs extra time for processing between the data transmissions.

The Slave extending the SCL low period will not affect the SCL high period, which is determined by the Master.

As a consequence, the Slave can reduce the TWI data transfer speed by prolonging the SCL duty cycle.

Figure 20-6 shows a typical data transmission. Note that several data bytes can be transmitted between the

SLA+R/W and the STOP condition, depending on the software protocol implemented by the application

software.

Figure 20-6. Typical Data Transmission

20.4 Multi-master Bus Systems, Arbitration and Synchronization

The TWI protocol allows bus systems with several masters. Special concerns have been taken in order to

ensure that transmissions will proceed as normal, even if two or more masters initiate a transmission at the

same time. Two problems arise in multi-master systems:

An algorithm must be implemented allowing only one of the masters to complete the transmission. All

other masters should cease transmission when they discover that they have lost the selection process.

This selection process is called arbitration. When a contending master discovers that it has lost the

arbitration process, it should immediately switch to Slave mode to check whether it is being addressed by

12 789

Data MSB Data LSB ACK

Aggregate

SDA

SDA from

Transmitter

SDA from

Receiver

SCL from

Master

SLA+R/W Data Byte

STOP, REPEATED

START or Next

Data Byte

12 789

Data Byte

Data MSB Data LSB ACK

SDA

SCL

START

12 789

Addr MSB Addr LSB R/W ACK

SLA+R/W STOP

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the winning master. The fact that multiple masters have started transmission at the same time should not

be detectable to the slaves, i.e. the data being transferred on the bus must not be corrupted.

Different masters may use different SCL frequencies. A scheme must be devised to synchronize the serial

clocks from all masters, in order to let the transmission proceed in a lockstep fashion. This will facilitate

the arbitration process.

The wired-ANDing of the bus lines is used to solve both these problems. The serial clocks from all masters will

be wired-ANDed, yielding a combined clock with a high period equal to the one from the Master with the

shortest high period. The low period of the combined clock is equal to the low period of the Master with the

longest low period. Note that all masters listen to the SCL line, effectively starting to count their SCL high and

low time-out periods when the combined SCL line goes high or low, respectively.

Figure 20-7. SCL Synchronization Between Multiple Masters

Arbitration is carried out by all masters continuously monitoring the SDA line after outputting data. If the value

read from the SDA line does not match the value the Master had output, it has lost the arbitration. Note that a

Master can only lose arbitration when it outputs a high SDA value while another Master outputs a low value. The

losing Master should immediately go to Slave mode, checking if it is being addressed by the winning Master.

The SDA line should be left high, but losing masters are allowed to generate a clock signal until the end of the

current data or address packet. Arbitration will continue until only one Master remains, and this may take many

bits. If several masters are trying to address the same Slave, arbitration will continue into the data packet.

low

high

SCL from

Master A

SCL from

Master B

SCL Bus

Line

low

high

Masters Start

Counting Low Period

Masters Start

Counting High Period

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Figure 20-8. Arbitration Between Two Masters

Note that arbitration is not allowed between:

A REPEATED START condition and a data bit

A STOP condition and a data bit

A REPEATED START and a STOP condition

It is the user software’s responsibility to ensure that these illegal arbitration conditions never occur. This implies

that in multi-master systems, all data transfers must use the same composition of SLA+R/W and data packets.

In other words: All transmissions must contain the same number of data packets, otherwise the result of the

arbitration is undefined.

20.5 Overview of the TWI Module

The TWI module is comprised of several submodules, as shown in Figure 20-9 on page 231. All registers drawn

in a thick line are accessible through the AVR data bus.

SDA from

Master A

SDA from

Master B

SDA Line

Synchronized

SCL Line

START Master A Loses

Arbitration, SDA

SDA

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Figure 20-9. Overview of the TWI Modul e

20.5.1 SCL and SDA Pins

These pins interface the AVR TWI with the rest of the MCU system. The output drivers contain a slew-rate

limiter in order to conform to the TWI specification. The input stages contain a spike suppression unit removing

spikes shorter than 50ns. Note that the internal pull-ups in the AVR pads can be enabled by setting the PORT

bits corresponding to the SCL and SDA pins, as explained in the I/O Port section. The internal pull-ups can in

some systems eliminate the need for external ones.

20.5.2 Bit Rate Generator Unit

This unit controls the period of SCL when operating in a Master mode. The SCL period is controlled by settings

in the TWI Bit Rate Register (TWBR) and the Prescaler bits in the TWI Status Register (TWSR). Slave operation

does not depend on Bit Rate or Prescaler settings, but the CPU clock frequency in the Slave must be at least 16

times higher than the SCL frequency. Note that slaves may prolong the SCL low period, thereby reducing the

average TWI bus clock period. The SCL frequency is generated according to the following equation:

TWBR = Value of the TWI Bit Rate Register

TWPS = Value of the prescaler bits in the TWI Status Register

Note: TWBR should be 10 or higher if the TWI operates in Master mode. If TWBR is lower than 10, the Master may

produce an incorrect output on SDA and SCL for the reminder of the byte. The problem occurs when operating the

TWI in Master mode, sending Start + SLA + R/W to a Slave (a Slave does not need to be connected to the bus for

the condition to happen).

TWI Unit

Address Register

(TWAR)

Address Match Unit

Address Comparator

Control Unit

Control Register

(TWCR)

Status Register

(TWSR)

State Machine and

Status control

SCL

Slew-rate

Control

Spike

Filter

SDA

Slew-rate

Control

Spike

Filter

Bit Rate Generator

Bit Rate Register

(TWBR)

Prescaler

Bus Interface Unit

START / STOP

Control

Arbitration detection Ack

Spike Suppression

Address/Data Shift

SCL frequency CPU Clock frequency

16 2(TWBR) 4TWPS

+

-----------------------------------------------------------=

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20.5.3 Bus Interface Unit

This unit contains the Data and Address Shift Register (TWDR), a START/STOP Controller and Arbitration

detection hardware. The TWDR contains the address or data bytes to be transmitted, or the address or data

bytes received. In addition to the 8-bit TWDR, the Bus Interface Unit also contains a register containing the

(N)ACK bit to be transmitted or received. This (N)ACK Register is not directly accessible by the application

software. However, when receiving, it can be set or cleared by manipulating the TWI Control Register (TWCR).

When in Transmitter mode, the value of the received (N)ACK bit can be determined by the value in the TWSR.

The START/STOP Controller is responsible for generation and detection of START, REPEATED START, and

STOP conditions. The START/STOP controller is able to detect START and STOP conditions even when the

AVR MCU is in one of the sleep modes, enabling the MCU to wake up if addressed by a Master.

If the TWI has initiated a transmission as Master, the Arbitration Detection hardware continuously monitors the

transmission trying to determine if arbitration is in process. If the TWI has lost an arbitration, the Control Unit is

informed. Correct action can then be taken and appropriate status codes generated.

20.5.4 Address Match Unit

The Address Match unit checks if received address bytes match the seven-bit address in the TWI Address

incoming address bits will also be compared against the General Call address. Upon an address match, the

Control Unit is informed, allowing correct action to be taken. The TWI may or may not acknowledge its address,

depending on settings in the TWCR. The Address Match unit is able to compare addresses even when the AVR

MCU is in sleep mode, enabling the MCU to wake up if addressed by a Master. If another interrupt (e.g., INT0)

occurs during TWI Power-down address match and wakes up the CPU, the TWI aborts operation and return to

it’s idle state. If this cause any problems, ensure that TWI Address Match is the only enabled interrupt when

entering Power-down.

20.5.5 Control Unit

The Control unit monitors the TWI bus and generates responses corresponding to settings in the TWI Control

Interrupt Flag (TWINT) is asserted. In the next clock cycle, the TWI Status Register (TWSR) is updated with a

status code identifying the event. The TWSR only contains relevant status information when the TWI Interrupt

Flag is asserted. At all other times, the TWSR contains a special status code indicating that no relevant status

information is available. As long as the TWINT Flag is set, the SCL line is held low. This allows the application

software to complete its tasks before allowing the TWI transmission to continue.

The TWINT Flag is set in the following situations:

After the TWI has transmitted a START/REPEATED START condition

After the TWI has transmitted SLA+R/W

After the TWI has transmitted an address byte

After the TWI has lost arbitration

After the TWI has been addressed by own slave address or general call

After the TWI has received a data byte

After a STOP or REPEATED START has been received while still addressed as a Slave

When a bus error has occurred due to an illegal START or STOP condition.

20.6 Using the TWI

The AVR TWI is byte-oriented and interrupt based. Interrupts are issued after all bus events, like reception of a

byte or transmission of a START condition. Because the TWI is interrupt-based, the application software is free

to carry on other operations during a TWI byte transfer. Note that the TWI Interrupt Enable (TWIE) bit in TWCR

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together with the Global Interrupt Enable bit in SREG allow the application to decide whether or not assertion of

the TWINT Flag should generate an interrupt request. If the TWIE bit is cleared, the application must poll the

TWINT Flag in order to detect actions on the TWI bus.

When the TWINT Flag is asserted, the TWI has finished an operation and awaits application response. In this

case, the TWI Status Register (TWSR) contains a value indicating the current state of the TWI bus. The

application software can then decide how the TWI should behave in the next TWI bus cycle by manipulating the

TWCR and TWDR Registers.

Figure 20-10 on page 233 is a simple example of how the application can interface to the TWI hardware. In this

example, a Master wishes to transmit a single data byte to a Slave. This description is quite abstract, a more

detailed explanation follows later in this section. A simple code example implementing the desired behavior is

also presented.

Figure 20-10. Interfacing the Application to the TWI in a Typ ic al Tran smission

1. The first step in a TWI transmission is to transmit a START condition. This is done by writing a specific

value into TWCR, instructing the TWI hardware to transmit a START condition. Which value to write is

described later on. However, it is important that the TWINT bit is set in the value written. Writing a one

to TWINT clears the flag. The TWI will not start any operation as long as the TWINT bit in TWCR is set.

Immediately after the application has cleared TWINT, the TWI will initiate transmission of the START

condition.

2. When the START condition has been transmitted, the TWINT Flag in TWCR is set, and TWSR is updated

with a status code indicating that the START condition has successfully been sent.

3. The application software should now examine the value of TWSR, to make sure that the START condition

was successfully transmitted. If TWSR indicates otherwise, the application software might take some

special action, like calling an error routine. Assuming that the status code is as expected, the application

must load SLA+W into TWDR. Remember that TWDR is used both for address and data. After TWDR has

been loaded with the desired SLA+W, a specific value must be written to TWCR, instructing the TWI

hardware to transmit the SLA+W present in TWDR. Which value to write is described later on. However, it

is important that the TWINT bit is set in the value written. Writing a one to TWINT clears the flag. The TWI

will not start any operation as long as the TWINT bit in TWCR is set. Immediately after the application has

cleared TWINT, the TWI will initiate transmission of the address packet.

4. When the address packet has been transmitted, the TWINT Flag in TWCR is set, and TWSR is updated

with a status code indicating that the address packet has successfully been sent. The status code will also

reflect whether a Slave acknowledged the packet or not.

START SLA+W A Data A STOP

1. Application

writes to TWCR to

initiate

transmission of

START

2. TWINT set.

Status code indicates

START condition sent

4. TWINT set.

Status code indicates

SLA+W sent, ACK

received

6. TWINT set.

Status code indicates

data sent, ACK received

3. Check TWSR to see if START was

sent. Application loads SLA+W into

TWDR, and loads appropriate control

signals into TWCR, makin sure that

TWINT is written to one,

and TWSTA is written to zero.

5. Check TWSR to see if SLA+W was

sent and ACK received.

Application loads data into TWDR, and

loads appropriate control signals into

TWCR, making sure that TWINT is

written to one

7. Check TWSR to see if data was sent

and ACK received.

Application loads appropriate control

signals to send STOP into TWCR,

making sure that TWINT is written to one

TWI bus

Indicates

TWINT set

Application

Action

TWI

Hardware

Action

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5. The application software should now examine the value of TWSR, to make sure that the address packet

was successfully transmitted, and that the value of the ACK bit was as expected. If TWSR indicates

otherwise, the application software might take some special action, like calling an error routine. Assuming

that the status code is as expected, the application must load a data packet into TWDR. Subsequently, a

specific value must be written to TWCR, instructing the TWI hardware to transmit the data packet present

in TWDR. Which value to write is described later on. However, it is important that the TWINT bit is set in

the value written. Writing a one to TWINT clears the flag. The TWI will not start any operation as long as

the TWINT bit in TWCR is set. Immediately after the application has cleared TWINT, the TWI will initiate

transmission of the data packet.

6. When the data packet has been transmitted, the TWINT Flag in TWCR is set, and TWSR is updated with

a status code indicating that the data packet has successfully been sent. The status code will also reflect

whether a Slave acknowledged the packet or not.

7. The application software should now examine the value of TWSR, to make sure that the data packet was

successfully transmitted, and that the value of the ACK bit was as expected. If TWSR indicates otherwise,

the application software might take some special action, like calling an error routine. Assuming that the

status code is as expected, the application must write a specific value to TWCR, instructing the TWI

hardware to transmit a STOP condition. Which value to write is described later on. However, it is important

that the TWINT bit is set in the value written. Writing a one to TWINT clears the flag. The TWI will not start

any operation as long as the TWINT bit in TWCR is set. Immediately after the application has cleared

TWINT, the TWI will initiate transmission of the STOP condition. Note that TWINT is NOT set after a

STOP condition has been sent.

Even though this example is simple, it shows the principles involved in all TWI transmissions. These can be

summarized as follows:

When the TWI has finished an operation and expects application response, the TWINT Flag is set. The

SCL line is pulled low until TWINT is cleared.

When the TWINT Flag is set, the user must update all TWI Registers with the value relevant for the next

TWI bus cycle. As an example, TWDR must be loaded with the value to be transmitted in the next bus

cycle.

After all TWI Register updates and other pending application software tasks have been completed, TWCR

is written. When writing TWCR, the TWINT bit should be set. Writing a one to TWINT clears the flag. The

TWI will then commence executing whatever operation was specified by the TWCR setting.

In the following an assembly and C implementation of the example is given. Note that the code below assumes

that several definitions have been made, for example by using include-files.

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Assembly Code Example C Example Comments

ldi

r16,

(1<<TWINT)|(1<<TWSTA)|

(1<<TWEN)

out

TWCR, r16

TWCR =

(1<<TWINT)|(1<<TWSTA)

|(1<<TWEN) Send START condition

wait1:

r16,TWCR

sbrs

r16,TWINT

rjmp

wait1

while (!(TWCR &

(1<<TWINT))) ;Wait for TWINT Flag set. This

indicates that the START

condition has been transmitted

r16,TWSR

andi

r16, 0xF8

cpi

r16, START

brne

ERROR

if ((TWSR & 0xF8) !=

START)

ERROR(); Check value of TWI Status

status different from START go to

ERROR

ldi

r16, SLA_W

out

TWDR, r16

ldi

r16, (1<<TWINT) |

(1<<TWEN)

out

TWCR, r16

TWDR = SLA_W;

TWCR = (1<<TWINT) |

(1<<TWEN); Load SLA_W into TWDR

TWCR to start transmission of

address

wait2:

r16,TWCR

sbrs

r16,TWINT

rjmp

wait2

while (!(TWCR &

(1<<TWINT))) ;Wait for TWINT Flag set. This

indicates that the SLA+W has

been transmitted, and

ACK/NACK has been received.

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20.7 Transmission Modes

The TWI can operate in one of four major modes. These are named Master Transmitter (MT), Master Receiver

(MR), Slave Transmitter (ST) and Slave Receiver (SR). Several of these modes can be used in the same

application. As an example, the TWI can use MT mode to write data into a TWI EEPROM, MR mode to read the

data back from the EEPROM. If other masters are present in the system, some of these might transmit data to

the TWI, and then SR mode would be used. It is the application software that decides which modes are legal.

The following sections describe each of these modes. Possible status codes are described along with figures

detailing data transmission in each of the modes. These figures contain the following abbreviations:

S: START condition

r16,TWSR

andi

r16, 0xF8

cpi

r16, MT_SLA_ACK

brne

ERROR

if ((TWSR & 0xF8) !=

MT_SLA_ACK)

ERROR(); Check value of TWI Status

status different from

MT_SLA_ACK go to ERROR

ldi

r16, DATA

out

TWDR, r16

ldi

r16, (1<<TWINT) |

(1<<TWEN)

out

TWCR, r16

TWDR = DATA;

TWCR = (1<<TWINT) |

(1<<TWEN);

Load DATA into TWDR Register.

Clear TWINT bit in TWCR to start

transmission of data

wait3:

r16,TWCR

sbrs

r16,TWINT

rjmp

wait3

while (!(TWCR &

(1<<TWINT))) ;Wait for TWINT Flag set. This

indicates that the DATA has been

transmitted, and ACK/NACK has

been received.

r16,TWSR

andi

r16, 0xF8

cpi

r16, MT_DATA_ACK

brne

ERROR

if ((TWSR & 0xF8) !=

MT_DATA_ACK)

ERROR(); Check value of TWI Status

status different from

MT_DATA_ACK go to ERROR

ldi

r16,

(1<<TWINT)|(1<<TWEN)|

(1<<TWSTO)

out

TWCR, r16

TWCR =

(1<<TWINT)|(1<<TWEN)|

(1<<TWSTO);

Transmit STOP condition

Assembly Code Example C Example Comments

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Rs: REPEATED START condition

R: Read bit (high level at SDA)

W: Write bit (low level at SDA)

A: Acknowledge bit (low level at SDA)

A: Not acknowledge bit (high level at SDA)

Data: 8-bit data byte

P: STOP condition

SLA: Slave Address

In Figure 20-12 on page 240 to Figure 20-18 on page 248, circles are used to indicate that the TWINT Flag is

set. The numbers in the circles show the status code held in TWSR, with the prescaler bits masked to zero. At

these points, actions must be taken by the application to continue or complete the TWI transfer. The TWI

transfer is suspended until the TWINT Flag is cleared by software.

When the TWINT Flag is set, the status code in TWSR is used to determine the appropriate software action. For

each status code, the required software action and details of the following serial transfer are given in Table 20-1

on page 238 to Table 20-4 on page 248. Note that the prescaler bits are masked to zero in these tables.

20.7.1 Master Transmitter Mode

In the Master Transmitter mode, a number of data bytes are transmitted to a Slave Receiver (see Figure 20-11).

In order to enter a Master mode, a START condition must be transmitted. The format of the following address

packet determines whether Master Transmitter or Master Receiver mode is to be entered. If SLA+W is

transmitted, MT mode is entered, if SLA+R is transmitted, MR mode is entered. All the status codes mentioned

in this section assume that the prescaler bits are zero or are masked to zero.

Figure 20-11. Data Transfer in Master Transmitter Mode

A START condition is sent by writing the following value to TWCR:

TWEN must be set to enable the 2-wire Serial Interface, TWSTA must be written to one to transmit a START

condition and TWINT must be written to one to clear the TWINT Flag. The TWI will then test the 2-wire Serial

Bus and generate a START condition as soon as the bus becomes free. After a START condition has been

transmitted, the TWINT Flag is set by hardware, and the status code in TWSR will be 0x08 (see Table 20-1 on

page 238). In order to enter MT mode, SLA+W must be transmitted. This is done by writing SLA+W to TWDR.

TWCR TWINT TWEA TWSTA TWSTO TWWC TWEN –TWIE

value 1 X10X10 X

Device 1

MASTER

TRANSMITTER

Device 2

SLAVE

RECEIVER

Device 3 Device n

SDA

SCL

........

R1 R2

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Thereafter the TWINT bit should be cleared (by writing it to one) to continue the transfer. This is accomplished

by writing the following value to TWCR:

When SLA+W have been transmitted and an acknowledgement bit has been received, TWINT is set again and

a number of status codes in TWSR are possible. Possible status codes in Master mode are 0x18, 0x20, or 0x38.

The appropriate action to be taken for each of these status codes is detailed in Table 20-1 on page 238.

When SLA+W has been successfully transmitted, a data packet should be transmitted. This is done by writing

the data byte to TWDR. TWDR must only be written when TWINT is high. If not, the access will be discarded,

and the Write Collision bit (TWWC) will be set in the TWCR Register. After updating TWDR, the TWINT bit

should be cleared (by writing it to one) to continue the transfer. This is accomplished by writing the following

value to TWCR:

This scheme is repeated until the last byte has been sent and the transfer is ended by generating a STOP

condition or a repeated START condition. A STOP condition is generated by writing the following value to

TWCR:

A REPEATED START condition is generated by writing the following value to TWCR:

After a repeated START condition (state 0x10) the 2-wire Serial Interface can access the same Slave again, or

a new Slave without transmitting a STOP condition. Repeated START enables the Master to switch between

Slaves, Master Transmitter mode and Master Receiver mode without losing control of the bus

TWCR TWINT TWEA TWSTA TWSTO TWWC TWEN –TWIE

value 1 X00X10 X

TWCR TWINT TWEA TWSTA TWSTO TWWC TWEN –TWIE

value 1 X00X10 X

TWCR TWINT TWEA TWSTA TWSTO TWWC TWEN –TWIE

value 1 X01X10 X

TWCR TWINT TWEA TWSTA TWSTO TWWC TWEN –TWIE

value 1 X10X10 X

Table 20-1. Status codes for Master Transmitter Mode

Status Code

(TWSR)

Prescaler Bits

are 0

Status of the 2-wire Serial Bus

and 2-wire Serial Interface

Hardware

Application Software Response

Next Action Taken by TWI Hardware

To/from TWDR To TWCR

STA STO TWIN

TWE

0x08 A START condition has been

transmitted

Load SLA+W 0 0 1 X SLA+W will be transmitted;

ACK or NOT ACK will be received

0x10 A repeated START condition

has been transmitted

Load SLA+W or

Load SLA+R

SLA+W will be transmitted;

ACK or NOT ACK will be received

SLA+R will be transmitted;

Logic will switch to Master Receiver mode

0x18 SLA+W has been transmitted;

ACK has been received

Load data byte or

No TWDR action or

No TWDR action

Data byte will be transmitted and ACK or NOT ACK will

be received

Repeated START will be transmitted

STOP condition will be transmitted and

TWSTO Flag will be reset

STOP condition followed by a START condition will be

transmitted and TWSTO Flag will be reset

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0x20 SLA+W has been transmitted;

NOT ACK has been received

Load data byte or

No TWDR action or

No TWDR action

Data byte will be transmitted and ACK or NOT ACK will

be received

Repeated START will be transmitted

STOP condition will be transmitted and

TWSTO Flag will be reset

STOP condition followed by a START condition will be

transmitted and TWSTO Flag will be reset

0x28 Data byte has been transmit-

ted;

ACK has been received

Load data byte or

No TWDR action or

No TWDR action

Data byte will be transmitted and ACK or NOT ACK will

be received

Repeated START will be transmitted

STOP condition will be transmitted and

TWSTO Flag will be reset

STOP condition followed by a START condition will be

transmitted and TWSTO Flag will be reset

0x30 Data byte has been transmit-

ted;

NOT ACK has been received

Load data byte or

No TWDR action or

No TWDR action

Data byte will be transmitted and ACK or NOT ACK will

be received

Repeated START will be transmitted

STOP condition will be transmitted and

TWSTO Flag will be reset

STOP condition followed by a START condition will be

transmitted and TWSTO Flag will be reset

0x38 Arbitration lost in SLA+W or

data bytes

No TWDR action or

No TWDR action

2-wire Serial Bus will be released and not addressed

Slave mode entered

A START condition will be transmitted when the bus

becomes free

Table 20-1. Status codes for Master Transmitter Mode

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Figure 20-12. F ormats and States in the Master Transmitter Mode

20.7.2 Master Receiver Mode

In the Master Receiver mode, a number of data bytes are received from a Slave Transmitter (see Figure 20-13

on page 241). In order to enter a Master mode, a START condition must be transmitted. The format of the

following address packet determines whether Master Transmitter or Master Receiver mode is to be entered. If

SLA+W is transmitted, MT mode is entered, if SLA+R is transmitted, MR mode is entered. All the status codes

mentioned in this section assume that the prescaler bits are zero or are masked to zero.

S SLA W A DATA A P

$08 $18 $28

R SLA W

$10

$20

$30

A or A

$38

Other master

continues

A or A

$38

Other master

continues

$68

Other master

continues

$78 $B0

To corresponding

states in slave mode

Successfull

transmission

to a slave

receiver

Next transfer

started with a

repeated start

condition

Not acknowledge

received after the

slave address

Not acknowledge

received after a data

byte

Arbitration lost in slave

address or data byte

Arbitration lost and

addressed as slave

DATA A

From master to slave

From slave to master

Any number of data bytes

and their associated acknowledge bits

This number (contained in TWSR) corresponds

to a defined state of the Two-Wire Serial Bus. The

prescaler bits are zero or masked to zero

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Figure 20-13. Data Transfer in Master Receiver Mode

A START condition is sent by writing the following value to TWCR:

TWEN must be written to one to enable the 2-wire Serial Interface, TWSTA must be written to one to transmit a

START condition and TWINT must be set to clear the TWINT Flag. The TWI will then test the 2-wire Serial Bus

and generate a START condition as soon as the bus becomes free. After a START condition has been

transmitted, the TWINT Flag is set by hardware, and the status code in TWSR will be 0x08 (See Table 20-1 on

page 238). In order to enter MR mode, SLA+R must be transmitted. This is done by writing SLA+R to TWDR.

Thereafter the TWINT bit should be cleared (by writing it to one) to continue the transfer. This is accomplished

by writing the following value to TWCR:

When SLA+R have been transmitted and an acknowledgement bit has been received, TWINT is set again and a

number of status codes in TWSR are possible. Possible status codes in Master mode are 0x38, 0x40, or 0x48.

The appropriate action to be taken for each of these status codes is detailed in Table 20-2 on page 242.

Received data can be read from the TWDR Register when the TWINT Flag is set high by hardware. This

scheme is repeated until the last byte has been received. After the last byte has been received, the MR should

inform the ST by sending a NACK after the last received data byte. The transfer is ended by generating a STOP

condition or a repeated START condition. A STOP condition is generated by writing the following value to

TWCR:

A REPEATED START condition is generated by writing the following value to TWCR:

After a repeated START condition (state 0x10) the 2-wire Serial Interface can access the same Slave again, or

a new Slave without transmitting a STOP condition. Repeated START enables the Master to switch between

Slaves, Master Transmitter mode and Master Receiver mode without losing control over the bus.

TWCR TWINT TWEA TWSTA TWSTO TWWC TWEN –TWIE

value 1 X10X10 X

TWCR TWINT TWEA TWSTA TWSTO TWWC TWEN –TWIE

value 1 X00X10 X

TWCR TWINT TWEA TWSTA TWSTO TWWC TWEN –TWIE

value 1 X01X10 X

TWCR TWINT TWEA TWSTA TWSTO TWWC TWEN –TWIE

value 1 X10X10 X

Device 1

MASTER

RECEIVER

Device 2

SLAVE

TRANSMITTER

Device 3 Device n

SDA

SCL

........ R1 R2

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Table 20-2. Status codes for Master Receiver Mode

Status Code

(TWSR)

Prescaler Bits

are 0

Status of the 2-wire Serial Bus

and 2-wire Serial Interface

Hardware

Application Software Response

Next Action Taken by TWI Hardware

To/from TWDR

To TWCR

STA STO TWIN

TWE

0x08 A START condition has been

transmitted

Load SLA+R 0 0 1 X SLA+R will be transmitted

ACK or NOT ACK will be received

0x10 A repeated START condition

has been transmitted

Load SLA+R or

Load SLA+W

SLA+R will be transmitted

ACK or NOT ACK will be received

SLA+W will be transmitted

Logic will switch to Master Transmitter mode

0x38 Arbitration lost in SLA+R or

NOT ACK bit

No TWDR action or

No TWDR action

2-wire Serial Bus will be released and not addressed

Slave mode will be entered

A START condition will be transmitted when the bus

becomes free

0x40 SLA+R has been transmitted;

ACK has been received

No TWDR action or

No TWDR action

Data byte will be received and NOT ACK will be

returned

Data byte will be received and ACK will be returned

0x48 SLA+R has been transmitted;

NOT ACK has been received

No TWDR action or

No TWDR action

Repeated START will be transmitted

STOP condition will be transmitted and TWSTO Flag

will be reset

STOP condition followed by a START condition will be

transmitted and TWSTO Flag will be reset

0x50 Data byte has been received;

ACK has been returned

Read data byte or

Read data byte

Data byte will be received and NOT ACK will be

returned

Data byte will be received and ACK will be returned

0x58 Data byte has been received;

NOT ACK has been returned

Read data byte or

Read data byte

Repeated START will be transmitted

STOP condition will be transmitted and TWSTO Flag

will be reset

STOP condition followed by a START condition will be

transmitted and TWSTO Flag will be reset

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Figure 20-14. Formats and States in the Master Receiver Mode

20.7.3 Slave Receiver Mode

In the Slave Receiver mode, a number of data bytes are received from a Master Transmitter (see Figure 20-15).

All the status codes mentioned in this section assume that the prescaler bits are zero or are masked to zero.

Figure 20-15. Data transfer in Slave Receiver mode

To initiate the Slave Receiver mode, TWAR and TWCR must be initialized as follows:

S SLA R A DATA A

$08 $40 $50

SLA R

$10

$48

A or A

$38

Other master

continues

$38

Other master

continues

$68

Other master

continues

$78 $B0

To corresponding

states in slave mode

Successfull

reception

from a slave

receiver

Next transfer

started with a

repeated start

condition

Not acknowledge

received after the

slave address

Arbitration lost in slave

address or data byte

Arbitration lost and

addressed as slave

DATA A

From master to slave

From slave to master

Any number of data bytes

and their associated acknowledge bits

This number (contained in TWSR) corresponds

to a defined state of the Two-Wire Serial Bus. The

prescaler bits are zero or masked to zero

PDATA A

$58

TWAR TWA6 TWA5 TWA4 TWA3 TWA2 TWA1 TWA0 TWGCE

value Device’s Own Slave Address

Device 3 Device n

SDA

SCL

........

R1 R2

Device 2

MASTER

TRANSMITTER

Device 1

SLAVE

RECEIVER

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The upper seven bits are the address to which the 2-wire Serial Interface will respond when addressed by a

Master. If the LSB is set, the TWI will respond to the general call address (0x00), otherwise it will ignore the

general call address.

TWEN must be written to one to enable the TWI. The TWEA bit must be written to one to enable the

acknowledgement of the device’s own slave address or the general call address. TWSTA and TWSTO must be

written to zero.

When TWAR and TWCR have been initialized, the TWI waits until it is addressed by its own slave address (or

the general call address if enabled) followed by the data direction bit. If the direction bit is “0” (write), the TWI will

operate in SR mode, otherwise ST mode is entered. After its own slave address and the write bit have been

received, the TWINT Flag is set and a valid status code can be read from TWSR. The status code is used to

determine the appropriate software action. The appropriate action to be taken for each status code is detailed in

Table 20-3 on page 245. The Slave Receiver mode may also be entered if arbitration is lost while the TWI is in

the Master mode (see states 0x68 and 0x78).

If the TWEA bit is reset during a transfer, the TWI will return a “Not Acknowledge” (“1”) to SDA after the next

received data byte. This can be used to indicate that the Slave is not able to receive any more bytes. While

TWEA is zero, the TWI does not acknowledge its own slave address. However, the 2-wire Serial Bus is still

monitored and address recognition may resume at any time by setting TWEA. This implies that the TWEA bit

may be used to temporarily isolate the TWI from the 2-wire Serial Bus.

In all sleep modes other than Idle mode, the clock system to the TWI is turned off. If the TWEA bit is set, the

interface can still acknowledge its own slave address or the general call address by using the 2-wire Serial Bus

clock as a clock source. The part will then wake up from sleep and the TWI will hold the SCL clock low during

the wake up and until the TWINT Flag is cleared (by writing it to one). Further data reception will be carried out

as normal, with the AVR clocks running as normal. Observe that if the AVR is set up with a long start-up time,

the SCL line may be held low for a long time, blocking other data transmissions.

Note that the 2-wire Serial Interface Data Register – TWDR does not reflect the last byte present on the bus

when waking up from these Sleep modes.

TWCR TWINT TWEA TWSTA TWSTO TWWC TWEN –TWIE

value 0100010 X

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Table 20-3. Status Codes for Slave Receiver Mode

Status Code

(TWSR)

Prescaler Bits

are 0

Status of the 2-wire Serial Bus

and 2-wire Serial Interface Hard-

ware

Application Software Response

Next Action Taken by TWI Hardware

To/from TWDR

To TWCR

STA STO TWIN

TWE

0x60 Own SLA+W has been received;

ACK has been returned

No TWDR action or

No TWDR action

Data byte will be received and NOT ACK will be

returned

Data byte will be received and ACK will be returned

0x68 Arbitration lost in SLA+R/W as

Master; own SLA+W has been

received; ACK has been returned

No TWDR action or

No TWDR action

Data byte will be received and NOT ACK will be

returned

Data byte will be received and ACK will be returned

0x70 General call address has been

received; ACK has been returned

No TWDR action or

No TWDR action

Data byte will be received and NOT ACK will be

returned

Data byte will be received and ACK will be returned

0x78 Arbitration lost in SLA+R/W as

Master; General call address has

been received; ACK has been re-

turned

No TWDR action or

No TWDR action

Data byte will be received and NOT ACK will be

returned

Data byte will be received and ACK will be returned

0x80 Previously addressed with own

SLA+W; data has been received;

ACK has been returned

Read data byte or

Read data byte

Data byte will be received and NOT ACK will be

returned

Data byte will be received and ACK will be returned

0x88 Previously addressed with own

SLA+W; data has been received;

NOT ACK has been returned

Read data byte or

Read data byte

Switched to the not addressed Slave mode;

no recognition of own SLA or GCA

Switched to the not addressed Slave mode;

own SLA will be recognized;

GCA will be recognized if TWGCE = “1”

Switched to the not addressed Slave mode;

no recognition of own SLA or GCA;

a START condition will be transmitted when the bus

becomes free

Switched to the not addressed Slave mode;

own SLA will be recognized;

GCA will be recognized if TWGCE = “1”;

a START condition will be transmitted when the bus

becomes free

0x90 Previously addressed with gener-

al call; data has been received;

ACK has been returned

Read data byte or

Read data byte

Data byte will be received and NOT ACK will be

returned

Data byte will be received and ACK will be returned

0x98 Previously addressed with gener-

al call; data has been received;

NOT ACK has been returned

Read data byte or

Read data byte

Switched to the not addressed Slave mode;

no recognition of own SLA or GCA

Switched to the not addressed Slave mode;

own SLA will be recognized;

GCA will be recognized if TWGCE = “1”

Switched to the not addressed Slave mode;

no recognition of own SLA or GCA;

a START condition will be transmitted when the bus

becomes free

Switched to the not addressed Slave mode;

own SLA will be recognized;

GCA will be recognized if TWGCE = “1”;

a START condition will be transmitted when the bus

becomes free

0xA0 A STOP condition or repeated

START condition has been re-

ceived while still addressed as

Slave

No action 0

Switched to the not addressed Slave mode;

no recognition of own SLA or GCA

Switched to the not addressed Slave mode;

own SLA will be recognized;

GCA will be recognized if TWGCE = “1”

Switched to the not addressed Slave mode;

no recognition of own SLA or GCA;

a START condition will be transmitted when the bus

becomes free

Switched to the not addressed Slave mode;

own SLA will be recognized;

GCA will be recognized if TWGCE = “1”;

a START condition will be transmitted when the bus

becomes free

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Figure 20-16. Formats and States in the Slave Receiver Mode

20.7.4 Slave Transmitter Mode

In the Slave Transmitter mode, a number of data bytes are transmitted to a Master Receiver (see Figure 20-17).

All the status codes mentioned in this section assume that the prescaler bits are zero or are masked to zero.

Figure 20-17. Data Transfer in Slave Transmitter Mode

To initiate the Slave Transmitter mode, TWAR and TWCR must be initialized as follows:

S SLA W A DATA A

$60 $80

$88

$68

Reception of the own

slave address and one or

more data bytes. All are

acknowledged

Last data byte received

is not acknowledged

Arbitration lost as master

and addressed as slave

Reception of the general call

address and one or more data

bytes

Last data byte received is

not acknowledged

From master to slave

From slave to master

Any number of data bytes

and their associated acknowledge bits

This number (contained in TWSR) corresponds

to a defined state of the Two-Wire Serial Bus. The

prescaler bits are zero or masked to zero

P or SDATA A

$80 $A0

P or SA

ADATAA

$70 $90

$98

$78

P or SDATA A

$90 $A0

P or SA

General Call

Arbitration lost as master and

addressed as slave by general call

DATA A

TWAR TWA6 TWA5 TWA4 TWA3 TWA2 TWA1 TWA0 TWGCE

value Device’s Own Slave Address

Device 3 Device n

SDA

SCL

........

R1 R2

Device 2

MASTER

RECEIVER

Device 1

SLAVE

TRANSMITTER

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The upper seven bits are the address to which the 2-wire Serial Interface will respond when addressed by a

Master. If the LSB is set, the TWI will respond to the general call address (0x00), otherwise it will ignore the

general call address.

TWEN must be written to one to enable the TWI. The TWEA bit must be written to one to enable the

acknowledgement of the device’s own slave address or the general call address. TWSTA and TWSTO must be

written to zero.

When TWAR and TWCR have been initialized, the TWI waits until it is addressed by its own slave address (or

the general call address if enabled) followed by the data direction bit. If the direction bit is “1” (read), the TWI will

operate in ST mode, otherwise SR mode is entered. After its own slave address and the write bit have been

received, the TWINT Flag is set and a valid status code can be read from TWSR. The status code is used to

determine the appropriate software action. The appropriate action to be taken for each status code is detailed in

Table 20-4 on page 248. The Slave Transmitter mode may also be entered if arbitration is lost while the TWI is

in the Master mode (see state 0xB0).

If the TWEA bit is written to zero during a transfer, the TWI will transmit the last byte of the transfer. State 0xC0

or state 0xC8 will be entered, depending on whether the Master Receiver transmits a NACK or ACK after the

final byte. The TWI is switched to the not addressed Slave mode, and will ignore the Master if it continues the

transfer. Thus the Master Receiver receives all “1” as serial data. State 0xC8 is entered if the Master demands

additional data bytes (by transmitting ACK), even though the Slave has transmitted the last byte (TWEA zero

and expecting NACK from the Master).

While TWEA is zero, the TWI does not respond to its own slave address. However, the 2-wire Serial Bus is still

monitored and address recognition may resume at any time by setting TWEA. This implies that the TWEA bit

may be used to temporarily isolate the TWI from the 2-wire Serial Bus.

In all sleep modes other than Idle mode, the clock system to the TWI is turned off. If the TWEA bit is set, the

interface can still acknowledge its own slave address or the general call address by using the 2-wire Serial Bus

clock as a clock source. The part will then wake up from sleep and the TWI will hold the SCL clock will low

during the wake up and until the TWINT Flag is cleared (by writing it to one). Further data transmission will be

carried out as normal, with the AVR clocks running as normal. Observe that if the AVR is set up with a long start-

up time, the SCL line may be held low for a long time, blocking other data transmissions.

Note that the 2-wire Serial Interface Data Register – TWDR does not reflect the last byte present on the bus

when waking up from these sleep modes.

TWCR TWINT TWEA TWSTA TWSTO TWWC TWEN –TWIE

value 0100010 X

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Figure 20-18. Formats and States in the Slave Transmitter Mode

Table 20-4. Status Codes for Slave Transmitter Mode

Status Code

(TWSR)

Prescaler

Bits

are 0

Status of the 2-wire Serial Bus

and 2-wire Serial Interface Hard-

ware

Application Software Response

Next Action Taken by TWI Hardware

To/from TWDR

To TWCR

STA STO TWIN

TWE

0xA8 Own SLA+R has been received;

ACK has been returned

Load data byte or

Load data byte

Last data byte will be transmitted and NOT ACK should

be received

Data byte will be transmitted and ACK should be re-

ceived

0xB0 Arbitration lost in SLA+R/W as

Master; own SLA+R has been re-

ceived; ACK has been returned

Load data byte or

Load data byte

Last data byte will be transmitted and NOT ACK should

be received

Data byte will be transmitted and ACK should be re-

ceived

0xB8 Data byte in TWDR has been

transmitted; ACK has been re-

ceived

Load data byte or

Load data byte

Last data byte will be transmitted and NOT ACK should

be received

Data byte will be transmitted and ACK should be re-

ceived

0xC0 Data byte in TWDR has been

transmitted; NOT ACK has been

received

No TWDR action or

No TWDR action

Switched to the not addressed Slave mode;

no recognition of own SLA or GCA

Switched to the not addressed Slave mode;

own SLA will be recognized;

GCA will be recognized if TWGCE = “1”

Switched to the not addressed Slave mode;

no recognition of own SLA or GCA;

a START condition will be transmitted when the bus

becomes free

Switched to the not addressed Slave mode;

own SLA will be recognized;

GCA will be recognized if TWGCE = “1”;

a START condition will be transmitted when the bus

becomes free

0xC8 Last data byte in TWDR has been

transmitted (TWEA = “0”); ACK

has been received

No TWDR action or

No TWDR action

Switched to the not addressed Slave mode;

no recognition of own SLA or GCA

Switched to the not addressed Slave mode;

own SLA will be recognized;

GCA will be recognized if TWGCE = “1”

Switched to the not addressed Slave mode;

no recognition of own SLA or GCA;

a START condition will be transmitted when the bus

becomes free

Switched to the not addressed Slave mode;

own SLA will be recognized;

GCA will be recognized if TWGCE = “1”;

a START condition will be transmitted when the bus

becomes free

S SLA R A DATA A

$A8 $B8

$B0

Reception of the own

slave address and one or

more data bytes

Last data byte transmitted.

Switched to not addressed

slave (TWEA = '0')

Arbitration lost as master

and addressed as slave

From master to slave

From slave to master

Any number of data bytes

and their associated acknowledge bits

This number (contained in TWSR) corresponds

to a defined state of the Two-Wire Serial Bus. The

prescaler bits are zero or masked to zero

P or SDATA

$C0

DATA A

$C8

P or SAll 1's

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20.7.5 Miscellaneous States

There are two status codes that do not correspond to a defined TWI state, see the table below.

Status 0xF8 indicates that no relevant information is available because the TWINT Flag is not set. This occurs

between other states, and when the TWI is not involved in a serial transfer.

Status 0x00 indicates that a bus error has occurred during a 2-wire Serial Bus transfer. A bus error occurs when

a START or STOP condition occurs at an illegal position in the format frame. Examples of such illegal positions

are during the serial transfer of an address byte, a data byte, or an acknowledge bit. When a bus error occurs,

TWINT is set. To recover from a bus error, the TWSTO Flag must set and TWINT must be cleared by writing a

logic one to it. This causes the TWI to enter the not addressed Slave mode and to clear the TWSTO Flag (no

other bits in TWCR are affected). The SDA and SCL lines are released, and no STOP condition is transmitted.

20.7.6 Combining Several TWI Modes

In some cases, several TWI modes must be combined in order to complete the desired action. Consider for

example reading data from a serial EEPROM. Typically, such a transfer involves the following steps:

1. The transfer must be initiated.

2. The EEPROM must be instructed what location should be read.

3. The reading must be performed.

4. The transfer must be finished.

Note that data is transmitted both from Master to Slave and vice versa. The Master must instruct the Slave what

location it wants to read, requiring the use of the MT mode. Subsequently, data must be read from the Slave,

implying the use of the MR mode. Thus, the transfer direction must be changed. The Master must keep control

of the bus during all these steps, and the steps should be carried out as an atomical operation. If this principle is

violated in a multi master system, another Master can alter the data pointer in the EEPROM between steps 2

and 3, and the Master will read the wrong data location. Such a change in transfer direction is accomplished by

transmitting a REPEATED START between the transmission of the address byte and reception of the data.

After a REPEATED START, the Master keeps ownership of the bus. The following figure shows the flow in this

transfer.

Figure 20-19. Combining Several TWI Modes to Access a Serial EEPROM

Table 20-5. Miscellaneous States

Status Code

(TWSR)

Prescaler Bits

are 0

Status of the 2-wire Serial Bus

and 2-wire Serial Interface

Hardware

Application Software Response

Next Action Taken by TWI Hardware

To/from TWDR

To TWCR

STA STO TWIN

TWE

0xF8 No relevant state information

available; TWINT = “0”

No TWDR action No TWCR action Wait or proceed current transfer

0x00 Bus error due to an illegal

START or STOP condition

No TWDR action 0 1 1 X Only the internal hardware is affected, no STOP condi-

tion is sent on the bus. In all cases, the bus is released

and TWSTO is cleared.

Master Transmitter Master Receiver

S = START Rs = REPEATED START P = STOP

Transmitted from master to slave Transmitted from slave to master

S SLA+W A ADDRESS A Rs SLA+R A DATA A P

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20.8 Multi-master Systems and Arbitration

If multiple masters are connected to the same bus, transmissions may be initiated simultaneously by one or

more of them. The TWI standard ensures that such situations are handled in such a way that one of the masters

will be allowed to proceed with the transfer, and that no data will be lost in the process. An example of an

arbitration situation is depicted below, where two masters are trying to transmit data to a Slave Receiver.

Figure 20-20. An Arbitration Example

Several different scenarios may arise during arbitration, as described below:

Two or more masters are performing identical communication with the same Slave. In this case, neither

the Slave nor any of the masters will know about the bus contention.

Two or more masters are accessing the same Slave with different data or direction bit. In this case,

arbitration will occur, either in the READ/WRITE bit or in the data bits. The masters trying to output a one

on SDA while another Master outputs a zero will lose the arbitration. Losing masters will switch to not

addressed Slave mode or wait until the bus is free and transmit a new START condition, depending on

application software action.

Two or more masters are accessing different slaves. In this case, arbitration will occur in the SLA bits.

Masters trying to output a one on SDA while another Master outputs a zero will lose the arbitration.

Masters losing arbitration in SLA will switch to Slave mode to check if they are being addressed by the

winning Master. If addressed, they will switch to SR or ST mode, depending on the value of the

READ/WRITE bit. If they are not being addressed, they will switch to not addressed Slave mode or wait

until the bus is free and transmit a new START condition, depending on application software action.

This is summarized in Figure 20-21. Possible status values are given in circles.

Device 1

MASTER

TRANSMITTER

Device 2

MASTER

TRANSMITTER

Device 3

SLAVE

RECEIVER

Device n

SDA

SCL

........

R1 R2

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Figure 20-21. Possible Status Codes Caused by Arbitration

Own

Address / General Call

received

Arbitration lost in SLA

TWI bus will be released and not addressed slave mode will be entered

A START condition will be transmitted when the bus becomes free

Arbitration lost in Data

Direction

Ye s

Write Data byte will be received and NOT ACK will be returned

Data byte will be received and ACK will be returned

Last data byte will be transmitted and NOT ACK should be received

Data byte will be transmitted and ACK should be received

Read

68/78

SLASTART Data STOP

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20.9 TWI Register Description

20.9.1 TWI Bit Rate Register – TWBR

• Bits 7..0 – TWI Bit Rate Register

TWBR selects the division factor for the bit rate generator. The bit rate generator is a frequency divider which

generates the SCL clock frequency in the Master modes. See “Bit Rate Generator Unit” on page 231 for

calculating bit rates.

20.9.2 TWI Control Re g ist e r – TW CR

The TWCR is used to control the operation of the TWI. It is used to enable the TWI, to initiate a Master access

by applying a START condition to the bus, to generate a Receiver acknowledge, to generate a stop condition,

and to control halting of the bus while the data to be written to the bus are written to the TWDR. It also indicates

a write collision if data is attempted written to TWDR while the register is inaccessible.

• Bit 7 – TWINT: TWI Interrupt Flag

This bit is set by hardware when the TWI has finished its current job and expects application software response.

If the I-bit in SREG and TWIE in TWCR are set, the MCU will jump to the TWI Interrupt Vector. While the TWINT

Flag is set, the SCL low period is stretched. The TWINT Flag must be cleared by software by writing a logic one

to it. Note that this flag is not automatically cleared by hardware when executing the interrupt routine. Also note

that clearing this flag starts the operation of the TWI, so all accesses to the TWI Address Register (TWAR), TWI

Status Register (TWSR), and TWI Data Register (TWDR) must be complete before clearing this flag.

• Bit 6 – TWEA: TWI Enable Acknowledge Bit

The TWEA bit controls the generation of the acknowledge pulse. If the TWEA bit is written to one, the ACK

pulse is generated on the TWI bus if the following conditions are met:

1. The device’s own slave address has been received.

2. A general call has been received, while the TWGCE bit in the TWAR is set.

3. A data byte has been received in Master Receiver or Slave Receiver mode.

By writing the TWEA bit to zero, the device can be virtually disconnected from the 2-wire Serial Bus temporarily.

Address recognition can then be resumed by writing the TWEA bit to one again.

• Bit 5 – TWSTA: TWI START Condition Bit

The application writes the TWSTA bit to one when it desires to become a Master on the 2-wire Serial Bus. The

TWI hardware checks if the bus is available, and generates a START condition on the bus if it is free. However,

if the bus is not free, the TWI waits until a STOP condition is detected, and then generates a new START

condition to claim the bus Master status. TWSTA must be cleared by software when the START condition has

been transmitted.

Bit 76543210

TWBR7 TWBR6 TWBR5 TWBR4 TWBR3 TWBR2 TWBR1 TWBR0 TWBR

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value00000000

Bit 76543210

TWINT TWEA TWSTA TWSTO TWWC TWEN – TWIE TWCR

Read/Write R/W R/W R/W R/W R R/W R R/W

Initial Value00000000

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• Bit 4 – TWSTO: TWI STOP Condition Bit

Writing the TWSTO bit to one in Master mode will generate a STOP condition on the 2-wire Serial Bus. When

the STOP condition is executed on the bus, the TWSTO bit is cleared automatically. In Slave mode, setting the

TWSTO bit can be used to recover from an error condition. This will not generate a STOP condition, but the TWI

returns to a well-defined unaddressed Slave mode and releases the SCL and SDA lines to a high impedance

state.

• Bit 3 – TWWC: TWI Write Collision Flag

The TWWC bit is set when attempting to write to the TWI Data Register – TWDR when TWINT is low. This flag

is cleared by writing the TWDR Register when TWINT is high.

• Bit 2 – TWEN: TWI Enable Bit

The TWEN bit enables TWI operation and activates the TWI interface. When TWEN is written to one, the TWI

takes control over the I/O pins connected to the SCL and SDA pins, enabling the slew-rate limiters and spike

filters. If this bit is written to zero, the TWI is switched off and all TWI transmissions are terminated, regardless of

any ongoing operation.

• Bit 1 – Res: Reserved Bit

This bit is a reserved bit and will always read as zero.

• Bit 0 – TWIE: TWI Interrupt Enable

When this bit is written to one, and the I-bit in SREG is set, the TWI interrupt request will be activated for as long

as the TWINT Flag is high.

20.9.3 TWI Status Register – TWSR

• Bits 7..3 – TWS: TWI Status

These 5 bits reflect the status of the TWI logic and the 2-wire Serial Bus. The different status codes are

described later in this section. Note that the value read from TWSR contains both the 5-bit status value and the

2-bit prescaler value. The application designer should mask the prescaler bits to zero when checking the Status

bits. This makes status checking independent of prescaler setting. This approach is used in this datasheet,

unless otherwise noted.

• Bit 2 – Res: Reserved Bit

This bit is reserved and will always read as zero.

• Bits 1..0 – TWPS: TWI Prescaler Bits

These bits can be read and written, and control the bit rate prescaler.

Bit 76543210

TWS7 TWS6 TWS5 TWS4 TWS3 – TWPS1 TWPS0 TWSR

Read/WriteRRRRRRR/WR/W

Initial Value11111000

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To calculate bit rates, see “Bit Rate Generator Unit” on page 231. The value of TWPS1..0 is used in the

equation.

20.9.4 TWI Data Register – TWDR

In Transmit mode, TWDR contains the next byte to be transmitted. In Receive mode, the TWDR contains the

last byte received. It is writable while the TWI is not in the process of shifting a byte. This occurs when the TWI

Interrupt Flag (TWINT) is set by hardware. Note that the Data Register cannot be initialized by the user before

the first interrupt occurs. The data in TWDR remains stable as long as TWINT is set. While data is shifted out,

data on the bus is simultaneously shifted in. TWDR always contains the last byte present on the bus, except

after a wake up from a sleep mode by the TWI interrupt. In this case, the contents of TWDR is undefined. In the

case of a lost bus arbitration, no data is lost in the transition from Master to Slave. Handling of the ACK bit is

controlled automatically by the TWI logic, the CPU cannot access the ACK bit directly.

• Bits 7..0 – TWD: TWI Data Register

These eight bits constitute the next data byte to be transmitted, or the latest data byte received on the 2-wire

Serial Bus.

20.9.5 TWI (Slave) Address Register – TWAR

The TWAR should be loaded with the 7-bit Slave address (in the seven most significant bits of TWAR) to which

the TWI will respond when programmed as a Slave Transmitter or Receiver, and not needed in the Master

modes. In multi master systems, TWAR must be set in masters which can be addressed as Slaves by other

Masters.

The LSB of TWAR is used to enable recognition of the general call address (0x00). There is an associated

address comparator that looks for the slave address (or general call address if enabled) in the received serial

address. If a match is found, an interrupt request is generated.

• Bits 7..1 – TWA: TWI (Slave) Address Register

These seven bits constitute the slave address of the TWI unit.

Table 20-6. TWI Prescaler Bits

TWPS1 TWPS0 Prescaler Value

001

014

1016

1164

Bit 76543210

TWD7 TWD6 TWD5 TWD4 TWD3 TWD2 TWD1 TWD0 TWDR

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value11111111

Bit 76543210

TWA6 TWA5 TWA4 TWA3 TWA2 TWA1 TWA0 TWGCE TWAR

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value11111110

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• Bit 0 – TWGCE: TWI General Call Recognition Enable Bit

If set, this bit enables the recognition of a General Call given over the 2-wire Serial Bus.

20.9.6 TWI (Slave) Address Mask Register – TWAMR

• Bits 7..1 – TWAM: TWI Address Mask

The TWAMR can be loaded with a 7-bit Slave Address mask. Each of the bits in TWAMR can mask (disable)

the corresponding address bit in the TWI Address Register (TWAR). If the mask bit is set to one then the

address match logic ignores the compare between the incoming address bit and the corresponding bit in TWAR.

Figure 20-22 shows the address match logic in detail.

Figure 20-22. TWI Address Match Logic, Block Diagram

• Bit 0 – Res: Reserved Bit

This bit is reserved and will always read as zero.

Bit 76543210

TWAM[6:0] – TWAMR

Read/Write R/W R/W R/W R/W R/W R/W R/W R

Initial Value00000000

Address

Match

Address Bit Comparator 0

Address Bit Comparator 6..1

TWAR0

TWAMR0

Address

Bit 0

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21. USB Controller

21.1 Features

•Supports full-speed and low-speed Device role

•Complies with USB Specification v2.0

•Supports ping-pong mode (dual bank)

•832 bytes of DPRAM:

– 1 endpoint 64 bytes max. (default control endpoint)

– 1 endpoints of 256 bytes max., (one or two banks)

– 5 endpoints of 64 bytes max., (one or two banks)

•Crystal-less operation for low-speed mode

21.2 Block Diagram

The USB controller provides the hardware to interface a USB link to a data flow stored in a double port memory

(DPRAM).

The USB controller requires a 48MHz ±0.25% reference clock (for Full-Speed operation), which is the output of

an internal PLL. The on-chip PLL generates the internal high frequency (48MHz) clock for USB interface. The

PLL clock input can be configured to use external low-power crystal oscillator, external source clock or internal

RC (see Section “Crystal-less Operation”, page 259).

The 48MHz clock is used to generate a 12MHz Full-speed (or 1.5MHz Low-Speed) bit clock from the received

USB differential data and to transmit data according to full or low speed USB device tolerance. Clock recovery is

done by a Digital Phase Locked Loop (DPLL) block, which is compliant with the jitter specification of the USB

bus.

To comply with the USB Electrical specification, USB buffers (D+ or D-) should be powered within the 3.0 to

3.6V range. As ATmega16U4/ATmega32U4 can be powered up to 5.5V, an internal regulator provides the USB

buffers power supply.

Figure 21-1. USB controller Block Diagram Overview

CPU

USB Regulator

USB

Interface

PLL clk

8MHz

clk

48MHz

PLL clock

Prescaler

On-Chip

USB DPRAM

DPLL

Clock

Recovery

UCAP

D-

D+

VBUS

UVCC AVCC

Div-by-2

XT1

Clock Mux

IntRC

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21.3 Typical Application Implementation

Depending on the target application power supply, the ATmega16U4/ATmega32U4 requires different hardware

typical implementations.

Figure 21-2. Operating Modes versus Frequency and Power-supply

21.3.1 Bus Powered Device

Figure 21-3. Typical Bus Powered Application with 5V I/O

VCC (V)

VCC min

3.0

3.4

5.5

USB not operational

USB compliant,

without internal regulator

USB compliant,

with internal regulator

4.5

2.7

Max

Operating Frequency (MHz)

8 MHz

16 MHz

2 MHz

3.6

1µF

UDP

UDM

VBUS

UVSS

UID

UCAP

D-

D+

VBUS

UID

UGND

UVCC AVCC VCC

XTAL1 XTAL2 GND GND

Rs=22

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Figure 21-4. Typical Bus Powered Application with 3V I/O

21.3.2 Self Powered Device

Figure 21-5. Typical Self Powered Application with 3.4V to 5.5V I/O

1µF

UVSS

External

3V Regulator

UDP

UDM

VBUS

UVSS

UID

UCAP

D-

D+

VBUS

UID

UGND

UVCC AVCC VCC

XTAL1 XTAL2 GND GND

Rs=22

1µF

External 3.4V - 5.5V

Power Supply

UDP

UDM

VBUS

UVSS

UID

UCAP

D-

D+

VBUS

UID

UGND

UVCC AVCC VCC

XTAL1 XTAL2 GND GND

Rs=22

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Figure 21-6. Typical Self Powered Application with 3.0V to 3.6 I/O

21.4 Crystal-less Operation

To reduce external components count and BOM cost, the USB module can be configured to operate in low-

speed mode with internal RC oscillator as input source clock for the PLL. The internal RC oscillator is factory

calibrated to satisfy the USB low speed frequency accuracy within the 0°C and +40°C temperature range.

For USB full-speed operation only external crystal oscillator or external source clock can be used.

21.5 Design Guidelines

Serial resistors on USB Data lines must have 22 value (±5%)

Traces from the input USB receptable (or from the cable connection in the case of a tethered device) to

the USB microcontroller pads should be as short as possible, and follow differential traces routing rules

(same length, as near as possible, avoid via accumulation)

Voltage transient / ESD suppressors may also be used to prevent USB pads to be damaged by external

disturbances

Ucap capacitor should be 1µF (±10%) for correct operation

A 10µF capacitor is highly recommended on VBUS line

1µF

External 3.0V - 3.6V

Power Supply

UDP

UDM

VBUS

UVSS

UID

UCAP

D-

D+

VBUS

UID

UGND

UVCC AVCC VCC

XTAL1 XTAL2 GND GND

Rs=22

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21.6 General Operating Modes

21.6.1 Introduction

The USB controller is disabled and reset after an hardware reset generated by:

Power on reset

External reset

Watchdog reset

Brown out reset

JTAG reset

But another available and optional CPU reset source is:

USB End Of Reset

In this case, the USB controller is reset, but not disabled (so that the device remains attached).

21.6.2 Power-on and Reset

The next diagram explains the USB controller main states on power-on:

Figure 21-7. USB Controller States after Reset

USB Controller state after an hardware reset is ‘Reset’. In this state:

USBE is not set

the USB controller clock is stopped in order to minimize the power consumption (FRZCLK=1),

the USB controller is disabled,

the USB pad is in the suspend mode,

the Device USB controller internal state is reset.

After setting USBE, the USB Controller enters the Device state. The controller is ‘Idle’.

The USB Controller can at any time be stopped by clearing USBE. In fact, clearing USBE acts as an hardware

reset.

21.6.3 Interrupts

Two interrupts vectors are assigned to USB interface.

Device

Reset

USBE=0 <any other

state>

USBE=1

Clock stopped

FRZCLK=1

Macro off

USBE=0

H W

RESET

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Figure 21-8. USB Interrupt System

The USB hardware module distinguishes between USB General events and USB Endpoint events that are

relevant with data transfers relative to each endpoint.

Figure 21-9. USB General Interrupt Vector Sources

Almost all these interrupts are time-relative events that will be detected only if the USB clock is enabled

(FRZCLK bit set), except for:

VBUS plug-in detection (insert, remove)

WAKEUP interrupt that will trigger each time a state change is detected on the data lines

This asynchronous interrupts allow to wake-up a device that is in power-down mode, generally after that the

USB has entered the Suspend state.

USB General

Interrupt

USB Device

Interrupt

USB General

Interrupt Vector

Endpoint

Interrupt

USB Endpoint/Pipe

Interrupt Vector

VBUSTI

USBINT.0 VBUSTE

USBCON.0

USB General

Interrupt Vector

UPRSMI

UDINT.6 UPRSME

UDIEN.6

EORSMI

UDINT.5 EORSME

UDIEN.5

WAKEUPI

UDINT.4

WAKEUPE

UDIEN.4

EORSTI

UDINT.3 EORSTE

UDIEN.3

SOFI

UDINT.2 SOFE

UDIEN.2

SUSPI

UDINT.0 SUSPE

UDIEN.0

USB Device

Interrupt

USB General

Interrupt Vector

Asynchronous Interrupt source

(allows the CPU to wake up from power down mode)

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Figure 21-10. USB Endpoint Interrupt Vector Sources

Each endpoint has eight interrupts sources associated with flags, and each source can be enabled or not to

trigger the corresponding endpoint interrupt. If, for an endpoint, at least one of the sources is enabled to trigger

interrupt, the corresponding event(s) will make the program branch to the USB Endpoint Interrupt vector. The

user may determine the source (endpoint) of the interrupt by reading the UEINT register, and then handle the

event detected by polling the different flags.

21.7 Power Modes

21.7.1 Idle Mode

In this mode, the CPU core is halted (CPU clock stopped). The Idle mode is taken wether the USB controller is

running or not. The CPU “wakes up” on any USB interrupts.

21.7.2 Power Down

In this mode, the oscillator is stopped and halts all the clocks (CPU and peripherals). The USB controller “wakes

up” when:

the WAKEUPI interrupt is triggered

the VBUSTI interrupt is triggered

21.7.3 Freeze Clock

The firmware has the ability to reduce the power consumption by setting the FRZCLK bit, which freeze the clock

of USB controller. When FRZCLK is set, it is still possible to access to the following registers:

FLERRE

UEIENX.7

OVERFI

UESTAX.6

UNDERFI

UESTAX.5

NAKINI

UEINTX.6 NAKINE

UEIENX.6

NAKOUTI

UEINTX.4 TXSTPE

UEIENX.4

RXSTPI

UEINTX.3 RXSTPE

UEIENX.3

RXOUTI

UEINTX.2 RXOUTE

UEIENX.2

STALLEDI

UEINTX.1 STALLEDE

UEIENX.1

EPINT

UEINT.X

Endpoint 0

Endpoint 1

Endpoint 2

Endpoint 3

Endpoint 4

Endpoint 5

TXINI

UEINTX.0 TXINE

UEIENX.0

USB Endpoint

Interrupt Vector

Endpoint 6

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USBCON, USBSTA, USBINT

UDCON (detach, ..)

UDINT

UDIEN

Moreover, when FRZCLK is set, only the following interrupts may be triggered:

WAKEUPI

VBUSTI

21.8 Speed Control

The speed selection (Full Speed or Low Speed) depends on the D+/D- pull-up. The LSM bit in UDCON register

allows to select an internal pull up on D- (Low Speed mode) or D+ (Full Speed mode) data lines.

Figure 21-11. Device Mode Speed Selection

21.9 Memory Management

The controller only supports the following memory allocation management.

The reservation of a Pipe or an Endpoint can only be made in the increasing order (Pipe/Endpoint 0 to the last

Pipe/Endpoint). The firmware shall thus configure them in the same order.

The reservation of a Pipe or an Endpoint “ki” is done when its ALLOC bit is set. Then, the hardware allocates the

memory and inserts it between the Pipe/Endpoints “ki-1” and “ki+1”. The “ki+1” Pipe/Endpoint memory “slides” up

and its data is lost. Note that the “ki+2” and upper Pipe/Endpoint memory does not slide.

Clearing a Pipe enable (PEN) or an Endpoint enable (EPEN) does not clear either its ALLOC bit, or its

configuration (EPSIZE/PSIZE, EPBK/PBK). To free its memory, the firmware should clear ALLOC. Then, the

“ki+1” Pipe/Endpoint memory automatically “slides” down. Note that the “ki+2” and upper Pipe/Endpoint memory

does not slide.

The following figure illustrates the allocation and reorganization of the USB memory in a typical example:

RPU

DETACH

UDCON.0

D+

D-

RPU

LSM

UDCON.2

UCAP USB

Regulator

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Table 21-1. Allocation and Reorganization USB Memory Flow

First, Endpoint 0 to Endpoint 5 are configured, in the growing order. The memory of each is reserved in

the DPRAM.

Then, the Endpoint 3 is disabled (EPEN=0), but its memory reservation is internally kept by the controller

Its ALLOC bit is cleared: the Endpoint 4 “slides” down, but the Endpoint 5 does not “slide”

Finally, if the firmware chooses to reconfigure the Endpoint 3, with a bigger size. The controller reserved

the memory after the Endpoint 2 memory and automatically “slide” the Endpoint 4. The Endpoint 5 does

not move and a memory conflict appear, in that both Endpoint 4 and 5 use a common area. The data of

those endpoints are potentially lost.

Note that:

the data of Endpoint 0 are never lost whatever the activation or deactivation of the higher Endpoint. Its

data is lost if it is deactivated.

Deactivate and reactivate the same Endpoint with the same parameters does not lead to a “slide” of the

higher endpoints. For those endpoints, the data are preserved.

CFGOK is set by hardware even in the case where there is a “conflict” in the memory allocation

21.10 PAD Suspend

The next figures illustrates the pad behaviour:

In the “idle” mode, the pad is put in low power consumption mode

In the “active” mode, the pad is working

Free memory

EPEN=1

ALLOC=1

Free memory

EPEN=0

(ALLOC=1)

Free memory

Endpoints

activation Endpoint Disable Free its memory

(ALLOC=0)

Free memory

3 (bigger size)

Endpoint

Activatation

Lost memory 4Conflict

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Figure 21-12. Pad Behaviour

The SUSPI flag indicated that a suspend state has been detected on the USB bus. This flag automatically put

the USB pad in Idle. The detection of a non-idle event sets the WAKEUPI flag and wakes-up the USB pad.

Moreover, the pad can also be put in the “idle” mode if the DETACH bit is set. It come back in the active mode

when the DETACH bit is cleared.

21.11 Plug-in Detection

The USB connection is detected by the VBUS pad, thanks to the following architecture:

Figure 21-13. Plug-in Detection Input Block Diagram

The control logic of the VBUS pad outputs a signal regarding the VBUS voltage level:

Idle mode

Active mode

USBE=1

& DETACH=0

& suspend

USBE=0

| DETACH=1

| suspend

SUSPI

Suspend detected = USB pad power down Clear Suspend by software

Resume = USB pad wake-up

Clear Resume by software

WAKEUPI

PAD status

Active

Power Down

Active

VBUSTI

USBINT.0

VBUS VBUS

USBSTA.0

VSS

VDD

Pad logic

Session_valid

RPU

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The “Session_valid” signal is active high when the voltage on the UVBUS pad is higher or equal to 1.4V. If

lower than 1.4V, the signal is not active

The VBUS status bit is set when “Session_valid” signal is active (VBUS > 1.4V)

The VBUSTI flag is set each time the VBUS state changes

The USB peripheral cannot attach to the bus while VBUS bit is not set

21.12 USB Soft ware Operating Modes

Depending on the USB operating mode, the software should perform some the following operations:

Power On the USB interface

Power-On USB pads regulator

Configure PLL interface

Enable PLL

Check PLL lock

Enable USB interface

Configure USB interface (USB speed, Endpoints configuration...)

Wait for USB VBUS information connection

Attach USB device

Power Off the USB interface

Detach USB interface

Disable USB interface

Disable PLL

Disable USB pad regulator

Suspending th e USB in te rfa ce

Clear Suspend Bit

Freeze USB clock

Disable PLL

Be sure to have interrupts enable to exit sleep mode

Make the MCU enter sleep mode

Resuming the USB interface

Enable PLL

Wait PLL lock

Unfreeze USB clock

Clear Resume information

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21.13 Registers Description

21.13.1 USB General Registers

• Bits 7:1 – Reserved

These bits are reserved. Do not modify these bits.

• Bit 0 – UVREGE: USB pad regulator Enable

Set to enable the USB pad regulator. Clear to disable the USB pad regulator.

• Bit 7 – USBE: USB macro Enable Bit

Set to enable the USB controller. Clear to disable and reset the USB controller, to disable the USB transceiver

and to disable the USB controller clock inputs.

• Bit 6 – Reserved

The value read from these bits is always 0. Do not set these bits.

• Bit 5 – FRZCLK: Freeze USB Clock Bit

Set to disable the clock inputs (the ”Resume Detection” is still active). This reduces the power consumption.

Clear to enable the clock inputs.

• Bit 4 – OTGPADE: VBUS Pad Enable

Set to enable the VBUS pad. Clear to disable the VBUS pad.

Note that this bit can be set/cleared even if USBE=0. That allows the VBUS detection even if the USB macro is

disable.

• Bits 3:1 – Reserved

The value read from these bits is always 0. Do not set these bits.

• Bit 0 – VBUSTE: VBUS Transition Interrupt Enable Bit

Set this bit to enable the VBUS Transition interrupt generation.

Clear this bit to disable the VBUS Transition interrupt generation.

Bit 76 5 4 321 0

- - - - - - - UVREGE UHWCON

Read/Write R/W R/W R R/W R R R R/W

Initial Value 0 0 0 0 0 0 0 0

Bit 7 6 5 4 321 0

USBE - FRZCLK OTGPADE - - - VBUSTE USBCON

Read/Write R/W R/W R/W R/W R R R/W R/W

Initial Val-

0 0 1 0 000 0

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• Bits 7:2 - Reserved

The value read from these bits is always 0. Do not set these bits.

• Bit 1 - ID: ID status

This bit is always read as “1”, it has been conserved for compatibility with AT90USB64/128 (in which it indicates

the value of the OTG ID pin).

• Bit 0 – VBUS: VBus Flag

The value read from this bit indicates the state of the VBUS pin. This bit can be used in device mode to monitor

the USB bus connection state of the application. See “Plug-in Detection” on page 265 for more details.

• Bits 7:1 - Reserved

The value read from these bits is always 0. Do not set these bits.

• Bit 0 – VBUSTI: IVBUS Transition Interrupt Flag

Set by hardware when a transition (high to low, low to high) has been detected on the VBUS pad. This shall be

cleared by software (see “USB Software Operating Modes” on page 266).

Bit 7 6 5 4 3 2 1 0

- - - - - - ID VBUS USBSTA

Read/WriteRRRRRRRR

Initial Val-

00000 0 1 0

Bit 76543210

- - - - - - - VBUSTI USBINT

Read/Write R R R R R R R/W R/W

Initial Val-

00000000

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22. USB Device Operating Modes

22.1 Introduction

The USB device controller supports full speed and low speed data transfers. In addition to the default control

endpoint, it provides six other endpoints, which can be configured in control, bulk, interrupt or isochronous

modes:

Endpoint 0:programmable size FIFO up to 64 bytes, default control endpoint

Endpoints 1 programmable size FIFO up to 256 bytes in ping-pong mode

Endpoints 2 to 6: programmable size FIFO up to 64 bytes in ping-pong mode

The controller starts in the “idle” mode. In this mode, the pad consumption is reduced to the minimum.

22.2 Power-on and Reset

The next diagram explains the USB device controller main states on power-on:

Figure 22-1. USB Device Controller States after Reset

The reset state of the Device controller is:

the macro clock is stopped in order to minimize the power consumption (FRZCLK set)

the USB device controller internal state is reset (all the registers are reset to their default value. Note that

DETACH is set.)

the endpoint banks are reset

the D+ or D- pull up are not activated (mode Detach)

The D+ or D- pull-up will be activated as soon as the DETACH bit is cleared and VBUS is present.

The macro is in the ‘Idle’ state after reset with a minimum power consump tion and does not need to have the

PLL activated to enter this state.

The USB device controller can at any time be reset by clearing USBE (disable USB interface).

22.3 Endpoint Reset

An endpoint can be reset at any time by setting in the UERST register the bit corresponding to the endpoint

(EPRSTx). This resets:

the internal state machine on that endpoint

the Rx and Tx banks are cleared and their internal pointers are restored

the UEINTX, UESTA0X and UESTA1X are restored to their reset value

The data toggle field remains unchanged.

The other registers remain unchanged.

The endpoint configuration remains active and the endpoint is still enabled.

Reset

Idle

RESET

USBE=0

<any

other

state>

USBE=0

USBE=1

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The endpoint reset may be associated with a clear of the data toggle command (RSTDT bit) as an answer to the

CLEAR_FEATURE USB command.

22.4 USB Reset

When an USB reset is detected on the USB line (SE0 state with a minimum duration of 2.5µs), the next

operations are performed by the controller:

all the endpoints are disabled

the default control endpoint remains configured (see “Endpoint Reset” on page 270 for more details)

If the CPU hardware reset function is activated (RSTCPU bit set in UDCON register), a reset is generated to the

CPU core without disabling the USB controller (that follows the same behavior than after a standard USB End of

Reset, and remains attached). That feature may be used to enhance device reliability.

22.5 Endpoint Selection

Prior to any operation performed by the CPU, the endpoint must first be selected. This is done by setting the

EPNUM2:0 bits (UENUM register) with the endpoint number which will be managed by the CPU.

The CPU can then access to the various endpoint registers and data.

22.6 Endpoint Activation

The endpoint is maintained under reset as long as the EPEN bit is not set.

The following flow must be respected in order to activate an endpoint:

Figure 22-2. Endpoint Activation Flow:

As long as the endpoint is not correctly configured (CFGOK cleared), the hardware does not acknowledge the

packets sent by the host.

CFGOK is will not be sent if the Endpoint size parameter is bigger than the DPRAM size.

Endpoint

Activation

CFGOK=1

ERROR

Yes

Endpoint activated

Activate the endpoint

Select the endpoint

EPEN=1

UENUM

EPNUM=x

Test the correct endpoint

configuration

UECFG1X

ALLOC

EPSIZE

EPBK

Configure:

- the endpoint size

- the bank parametrization

Allocation and reorganization of

the memory is made on-the-fly

UECFG0X

EPDIR

EPTYPE

...

Configure:

- the endpoint direction

- the endpoint type

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A clear of EPEN acts as an endpoint reset (see “Endpoint Reset” on page 270 for more details). It also performs

the next operation:

The configuration of the endpoint is kept (EPSIZE, EPBK, ALLOC kept)

It resets the data toggle field

The DPRAM memory associated to the endpoint is still reserved

See “Memory Management” on page 263 for more details about the memory allocation/reorganization.

22.7 Address Setup

The USB device address is set up according to the USB protocol:

the USB device, after power-up, responds at address 0

the host sends a SETUP command (SET_ADDRESS(addr))

the firmware handles this request, and records that address in UADD, but keep ADDEN cleared

the USB device firmware sends an IN command of 0 bytes (IN 0 Zero Length Packet)

then, the firmware can enable the USB device address by setting ADDEN. The only accepted address by

the controller is the one stored in UADD.

ADDEN and UADD shall not be written at the same time.

UADD contains the default address 00h after a power-up or USB reset.

ADDEN is cleared by hardware:

after a power-up reset

when an USB reset is received

or when the macro is disabled (USBE cleared)

When this bit is cleared, the default device address 00h is used.

22.8 Suspend, Wake-up and Resume

After a period of 3ms during which the USB line was inactive, the controller switches to the full-speed mode and

triggers (if enabled) the SUSPI (suspend) interrupt. The firmware may then set the FRZCLK bit.

The CPU can also, depending on software architecture, enter in the idle mode to lower again the power

consumption.

There are two ways to recover from the “Suspend” mode:

First one is to clear the FRZCLK bit. This is possible if the CPU is not in the Idle mode.

Second way, if the CPU is “idle”, is to enable the WAKEUPI interrupt (WAKEUPE set). Then, as soon as

an non-idle signal is seen by the controller, the WAKEUPI interrupt is triggered. The firmware shall then

clear the FRZCLK bit to restart the transfer.

There are no relationship between the SUSPI interrupt and the WAKEUPI interrupt: the WAKEUPI interrupt is

triggered as soon as there are non-idle patterns on the data lines. Thus, the WAKEUPI interrupt can occurs

even if the controller is not in the “suspend” mode.

When the WAKEUPI interrupt is triggered, if the SUSPI interrupt bit was already set, it is cleared by hardware.

When the SUSPI interrupt is triggered, if the WAKEUPI interrupt bit was already set, it is cleared by hardware.

22.9 Detach

The reset value of the DETACH bit is 1.

It is possible to re-enumerate a device, simply by setting and clearing the DETACH bit (but firmware must take

in account a debouncing delay of some milliseconds).

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Setting DETACH will disconnect the pull-up on the D+ or D- pad (depending on full or low speed mode

selected). Then, clearing DETACH will connect the pull-up on the D+ or D- pad.

Figure 22-3. Detach a Device in Full-speed

22.10 Remote W ake-up

The “Remote Wake-up” (or “upstream resume”) feature is the only operation allowed to be sent by the device on

its own initiative. Anyway, to do that, the device should first have received a DEVICE_REMOTE_WAKEUP

request from the host.

First, the USB controller must have detected the “suspend” state of the line: the remote wake-up can only

be sent when a SUSPI flag is set

The firmware has then the ability to set RMWKUP to send the “upstream resume” stream. This will

automatically be done by the controller after 5ms of inactivity on the USB line.

When the controller starts to send the “upstream resume”, the UPRSMI interrupt is triggered (if enabled).

SUSPI is cleared by hardware

RMWKUP is cleared by hardware at the end of the “upstream resume”

If the controller detects a good “End Of Resume” signal from the host, an EORSMI interrupt is triggered (if

enabled)

22.11 STALL Request

For each endpoint, the STALL management is performed using two bits:

STALLRQ (enable stall request)

STALLRQC (disable stall request)

STALLEDI (stall sent interrupt)

To send a STALL handshake at the next request, the STALLRQ request bit has to be set. All following requests

will be handshak’ed with a STALL until the STALLRQC bit is set.

Setting STALLRQC automatically clears the STALLRQ bit. The STALLRQC bit is also immediately cleared by

hardware after being set by software. Thus, the firmware will never read this bit as set.

Each time the STALL handshake is sent, the STALLEDI flag is set by the USB controller and the EPINTx

interrupt will be triggered (if enabled).

The incoming packets will be discarded (RXOUTI and RWAL will not be set).

The host will then send a command to reset the STALL: the firmware just has to set the STALLRQC bit and to

reset the endpoint.

22.11.1 Special Consideration for Control Endpoints

A SETUP request is always ACK’ed.

EN=1

D +

UVREF

D -

Detach, then

Attach EN=1

D +

UVREF

D -

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If a STALL request is set for a Control Endpoint and if a SETUP request occurs, the SETUP request has to be

ACK’ed and the STALLRQ request and STALLEDI sent flags are automatically reset (RXSETUPI set, TXIN

cleared, STALLED cleared, TXINI cleared...).

This management simplifies the enumeration process management. If a command is not supported or contains

an error, the firmware set the STALL request flag and can return to the main task, waiting for the next SETUP

request.

This function is compliant with the Chapter 8 test that may send extra status for a GET_DESCRIPTOR. The

firmware sets the STALL request just after receiving the status. All extra status will be automatically STALL’ed

until the next SETUP request.

22.11.2 STALL Handshake and Retry Mechanism

The Retry mechanism has priority over the STALL handshake. A STALL handshake is sent if the STALLRQ

request bit is set and if there is no retry required.

22.12 CONTROL Endpoint Management

A SETUP request is always ACK’ed. When a new setup packet is received, the RXSTPI interrupt is triggered (if

enabled). The RXOUTI interrupt is not triggered.

The FIFOCON and RWAL fields are irrelevant with CONTROL endpoints. The firmware shall thus never use

them on that endpoints. When read, their value is always 0.

CONTROL endpoints are managed by the following bits:

RXSTPI is set when a new SETUP is received. It shall be cleared by firmware to acknowledge the packet

and to clear the endpoint bank.

RXOUTI is set when a new OUT data is received. It shall be cleared by firmware to acknowledge the

packet and to clear the endpoint bank.

TXINI is set when the bank is ready to accept a new IN packet. It shall be cleared by firmware to send the

packet and to clear the endpoint bank.

22.12.1 Control Write

The next figure shows a control write transaction. During the status stage, the controller will not necessary send

a NAK at the first IN token:

If the firmware knows the exact number of descriptor bytes that must be read, it can then anticipate on the

status stage and send a ZLP for the next IN token

or it can read the bytes and poll NAKINI, which tells that all the bytes have been sent by the host, and the

transaction is now in the status stage

SETUP

RXSTPI

RXOUTI

TXINI

USB line

HW SW

OUT

HW SW

OUT

HW SW

IN IN

NAK

DATASETUP STATUS

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22.12.2 Control Read

The next figure shows a control read transaction. The USB controller has to manage the simultaneous write

requests from the CPU and the USB host:

A NAK handshake is always generated at the first status stage command.

When the controller detect the status stage, all the data written by the CPU are erased, and clearing TXINI has

no effects.

The firmware checks if the transmission is complete or if the reception is complete.

The OUT retry is always ack’ed. This reception:

- set the RXOUTI flag (received OUT data)

- set the TXINI flag (data sent, ready to accept new data)

software algorithm:

set transmit ready

wait (transmit complete OR Receive complete)

if receive complete, clear flag and return

if transmit complete, continue

Once the OUT status stage has been received, the USB controller waits for a SETUP request. The SETUP

request have priority over any other request and has to be ACK’ed. This means that any other flag should be

cleared and the fifo reset when a SETUP is received.

WARNING: the byte counter is reset when the OUT Zero Length Packet is received. The firmware has to take

care of this.

22.13 OUT End point Management

OUT packets are sent by the host. All the data can be read by the CPU, which acknowledges or not the bank

when it is empty.

22.13.1 Overview

The Endpoint must be configured first.

Each time the current bank is full, the RXOUTI and the FIFOCON bits are set. This triggers an interrupt if the

RXOUTE bit is set. The firmware can acknowledge the USB interrupt by clearing the RXOUTI bit. The Firmware

read the data and clear the FIFOCON bit in order to free the current bank. If the OUT Endpoint is composed of

multiple banks, clearing the FIFOCON bit will switch to the next bank. The RXOUTI and FIFOCON bits are then

updated by hardware in accordance with the status of the new bank.

RXOUTI shall always be cleared before clearing FIFOCON.

SETUP

RXSTPI

RXOUTI

TXINI

USB line

HW SW

IN OUT OUT

NAK

Wr Enable

HOST

Wr Enable

CPU

DATASETUP STATUS

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The RWAL bit always reflects the state of the current bank. This bit is set if the firmware can read data from the

bank, and cleared by hardware when the bank is empty.

22.13.2 Detailed description

22.13.2.1

The data are read by the CPU, following the next flow:

When the bank is filled by the host, an endpoint interrupt (EPINTx) is triggered, if enabled (RXOUTE set)

and RXOUTI is set. The CPU can also poll RXOUTI or FIFOCON, depending on the software architecture

The CPU acknowledges the interrupt by clearing RXOUTI

The CPU can read the number of byte (N) in the current bank (N=BYCT)

The CPU can read the data from the current bank (“N” read of UEDATX)

The CPU can free the bank by clearing FIFOCON when all the data is read, that is:

after “N” read of UEDATX

as soon as RWAL is cleared by hardware

If the endpoint uses 2 banks, the second one can be filled by the HOST while the current one is being read by

the CPU. Then, when the CPU clear FIFOCON, the next bank may be already ready and RXOUTI is set

immediately.

22.14 IN endpoint management

IN packets are sent by the USB device controller, upon an IN request from the host. All the data can be written

by the CPU, which acknowledge or not the bank when it is full.Overview

The Endpoint must be configured first.

The TXINI bit is set by hardware when the current bank becomes free. This triggers an interrupt if the TXINE bit

is set. The FIFOCON bit is set at the same time. The CPU writes into the FIFO and clears the FIFOCON bit to

allow the USB controller to send the data. If the IN Endpoint is composed of multiple banks, this also switches to

the next data bank. The TXINI and FIFOCON bits are automatically updated by hardware regarding the status

of the next bank.

TXINI shall always be cleared before clearing FIFOCON.

OUT DATA

(to bank 0) ACK

RXOUTI

FIFOCON

OUT DATA

(to bank 0) ACK

Example with 1 OUT data bank

read data from CPU

BANK 0

OUT DATA

(to bank 0) ACK

RXOUTI

FIFOCON

OUT DATA

(to bank 1) ACK

Example with 2 OUT data banks

read data from CPU

BANK 0

read data from CPU

BANK 0

read data from CPU

BANK 1

NAK

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The RWAL bit always reflects the state of the current bank. This bit is set if the firmware can write data to the

bank, and cleared by hardware when the bank is full

22.14.1 Detailed Description

The data are written by the CPU, following the next flow:

When the bank is empty, an endpoint interrupt (EPINTx) is triggered, if enabled (TXINE set) and TXINI is

set. The CPU can also poll TXINI or FIFOCON, depending the software architecture choice

The CPU acknowledges the interrupt by clearing TXINI

The CPU can write the data into the current bank (write in UEDATX)

The CPU can free the bank by clearing FIFOCON when all the data are written, that is:

after “N” write into UEDATX

as soon as RWAL is cleared by hardware

If the endpoint uses two banks, the second one can be read by the HOST while the current is being written by

the CPU. Then, when the CPU clears FIFOCON, the next bank may be already ready (free) and TXINI is set

immediately.

22.14.1.1Abort

An “abort” stage can be produced by the host in some situations:

In a control transaction: ZLP data OUT received during a IN stage

In an isochronous IN transaction: ZLP data OUT received on the OUT endpoint during a IN stage on the

IN endpoint

...

The KILLBK bit is used to kill the last “written” bank. The best way to manage this abort is to perform the

following operations:

IN DATA

(bank 0) ACK

TXINI

FIFOCON

Example with 1 IN data bank

write data from CPU

BANK 0

Example with 2 IN data banks

SW SW

IN DATA

(bank 0) ACK

TXINI

FIFOCON write data from CPU

BANK 0

SW SW

IN DATA

(bank 1) ACK

write data from CPU

BANK 0

write data from CPU

BANK 1

write data from CPU

BANK0

NAK

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Table 22-1. Abort Flow

22.15 Isochronous Mode

22.15.1 Underflow

An underflow can occur during IN stage if the host attempts to read a bank which is empty. In this situation, the

UNDERFI interrupt is triggered.

An underflow can also occur during OUT stage if the host send a packet while the banks are already full.

Typically, the CPU is not fast enough. The packet is lost.

It is not possible to have underflow error during OUT stage, in the CPU side, since the CPU should read only if

the bank is ready to give data (RXOUTI=1 or RWAL=1)

22.15.2 CRC Error

A CRC error can occur during OUT stage if the USB controller detects a bad received packet. In this situation,

the STALLEDI interrupt is triggered. This does not prevent the RXOUTI interrupt from being triggered.

22.16 Overflow

In Control, Isochronous, Bulk or Interrupt Endpoint, an overflow can occur during OUT stage, if the host

attempts to write in a bank that is too small for the packet. In this situation, the OVERFI interrupt is triggered (if

enabled). The packet is acknowledged and the RXOUTI interrupt is also triggered (if enabled). The bank is filled

with the first bytes of the packet.

It is not possible to have overflow error during IN stage, in the CPU side, since the CPU should write only if the

bank is ready to access data (TXINI=1 or RWAL=1).

22.17 Interrupts

Figure 22-4 shows all the interrupts sources.

Endpoint

Abort

Abort done

Abort is based on the fact

that no banks are busy,

meaning that nothing has to

be sent.

Disable the TXINI interrupt.

Endpoint

reset

NBUSYBK

Yes

Clear

UEIENX.

TXINE

KILLBK=1

Yes

Kill the last written

bank.

Wait for the end of the

procedure.

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Figure 22-4. USB Device Controller Interrupt System

There are two kind of interrupts: processing (i.e. their generation are part of the normal processing) and

exception (errors).

Processing interrupts are generated when:

VBUS plug-in detection (insert, remove)(VBUSTI)

Upstream resume(UPRSMI)

End of resume(EORSMI)

Wake up(WAKEUPI)

End of reset (Speed Initialization)(EORSTI)

Start of frame(SOFI, if FNCERR=0)

Suspend detected after 3ms of inactivity(SUSPI)

Exception Interrupts are generated when:

CRC error in frame number of SOF(SOFI, FNCERR=1)

UPRSMI

UDINT.6 UPRSME

UDIEN.6

EORSMI

UDINT.5 EORSME

UDIEN.5

WAKEUPI

UDINT.4

WAKEUPE

UDIEN.4

EORSTI

UDINT.3 EORSTE

UDIEN.3

SOFI

UDINT.2 SOFE

UDIEN.2

SUSPI

UDINT.0 SUSPE

UDIEN.0

USB Device

Interrupt

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Figure 22-5. USB Device Con t roller Endpoint Interrupt System

Processing interrupts are generated when:

Ready to accept IN data(EPINTx, TXINI=1)

Received OUT data(EPINTx, RXOUTI=1)

Received SETUP(EPINTx, RXSTPI=1)

Exception Interrupts are generated when:

Stalled packet(EPINTx, STALLEDI=1)

CRC error on OUT in isochronous mode(EPINTx, STALLEDI=1)

Overflow in isochronous mode(EPINTx, OVERFI=1)

Underflow in isochronous mode(EPINTx, UNDERFI=1)

NAK IN sent(EPINTx, NAKINI=1)

NAK OUT sent(EPINTx, NAKOUTI=1)

EPINT

UEINT.X

Endpoint 0

Endpoint 1

Endpoint 2

Endpoint 3

Endpoint 4

Endpoint 5

Endpoint Interrupt

Endpoint 6

FLERRE

UEIENX.7

OVERFI

UESTAX.6

UNDERFI

UESTAX.5

NAKINI

UEINTX.6 NAKINE

UEIENX.6

NAKOUTI

UEINTX.4 TXSTPE

UEIENX.4

RXSTPI

UEINTX.3 TXOUTE

UEIENX.3

RXOUTI

UEINTX.2 RXOUTE

UEIENX.2

STALLEDI

UEINTX.1

STALLEDE

UEIENX.1

TXINI

UEINTX.0

TXINE

UEIENX.0

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22.18 Registers

22.18.1 USB Device Gen e ra l Re gi st ers

• Bits 7:4 - Reserved

The value read from these bits is always 0. Do not set these bits.

• Bit 3 - RSTCPU - USB Reset CPU bit

Set this bit to 1 by firmware in order to reset the CPU on the detection of a USB End of Reset signal (without

disabling the USB controller and Attached state). This bit is reset when the USB controller is disabled, but is not

affected by the CPU reset generated after a USB End of Reset (remains enabled).

• Bit 2 - LSM - USB Device Low Speed Mode Selection

When configured USB is configured in device mode, this bit allows to select the USB the USB Low Speed or Full

Speed Mod.

Clear to select full speed mode (D+ internal pull-up will be activate with the ATTACH bit will be set).

Set to select low speed mode (D- internal pull-up will be activate with the ATTACH bit will be set). This bit has no

effect when the USB interface is configured in HOST mode.

• Bit 1- RMWKUP - Remote Wake-up Bit

Set to send an “upstream-resume” to the host for a remote wake-up (the SUSPI bit must be set).

Cleared by hardware when signalling finished. Clearing by software has no effect.

See Section 22.10, page 273 for more details.

• Bit 0 - DETACH - Detach Bit

Set to physically detach de device (disconnect internal pull-up on D+ or D-).

Clear to reconnect the device. See Section 22.9, page 272 for more details

Bit 76543 2 1 0

----RSTCPU

LSM RMWKUP DETACH UDCON

Read/WriteRRRRRR/WR/WR/W

Initial Value00000 0 0 1

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• Bit 7 - Reserved

The value read from this bits is always 0. Do not set this bit.

• Bit 6 - UPRSMI - Upstream Resume Interrupt Flag

Set by hardware when the USB controller is sending a resume signal called “Upstream Resume”. This triggers

an USB interrupt if UPRSME is set.

Shall be cleared by software (USB clocks must be enabled before). Setting by software has no effect.

• Bit 5 - EORSMI - End Of Resume Interrupt Flag

Set by hardware when the USB controller detects a good “End Of Resume” signal initiated by the host. This

triggers an USB interrupt if EORSME is set.

Shall be cleared by software. Setting by software has no effect.

• Bit 4 - WAKEUPI - Wake-up CPU Interrupt Flag

Set by hardware when the USB controller is re-activated by a filtered non-idle signal from the lines (not by an

upstream resume). This triggers an interrupt if WAKEUPE is set.

Shall be cleared by software (USB clock inputs must be enabled before). Setting by software has no effect.

See “Suspend, Wake-up and Resume” on page 272 for more details.

•Bit 3 - EORSTI - End Of Reset Interrupt Flag

Set by hardware when an “End Of Reset” has been detected by the USB controller. This triggers an USB

interrupt if EORSTE is set.

Shall be cleared by software. Setting by software has no effect.

• Bit 2 - SOFI - Start Of Frame Interrupt Flag

Set by hardware when an USB “Start Of Frame” PID (SOF) has been detected (every 1ms). This triggers an

USB interrupt if SOFE is set.

• Bit 1 - Reserved

The value read from this bits is always 0. Do not set this bit

• Bit 0 - SUSPI - Suspend Interrupt Flag

Shall be cleared by software. Setting by software has no effect. The interrupt bits are set even if their

corresponding ‘Enable’ bits is not set.

See “Suspend, Wake-up and Resume” on page 272 for more details.

See for more details.

Bit 76543210

-UPRSMI EORSMI WAKEUPI EORSTI SOFI - SUSPI UDINT

Read/Write

Initial Value 0 0 0 0 0 0 0 0

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• Bit 7 - Reserved

The value read from this bits is always 0. Do not set this bit.

• Bit 6 - UPRSME - Upstream Resume Interrupt Enable Bit

Set to enable the UPRSMI interrupt.

Clear to disable the UPRSMI interrupt.

• Bit 5 - EORSME - End Of Resume Interrupt Enable Bit

Set to enable the EORSMI interrupt.

Clear to disable the EORSMI interrupt.

• Bit 4 - WAKEUPE - Wake-up CPU Interrupt Enable Bit

Set to enable the WAKEUPI interrupt.

Clear to disable the WAKEUPI interrupt.

• Bit 3 - EORSTE - End Of Reset Interrupt Enable Bit

Set to enable the EORSTI interrupt. This bit is set after a reset.

Clear to disable the EORSTI interrupt.

• Bit 2 - SOFE - Start Of Frame Interrupt Enable Bit

Set to enable the SOFI interrupt.

Clear to disable the SOFI interrupt.

•Bit 1 - Reserved

The value read from this bits is always 0. Do not set this bit.

• Bit 0 - SUSPE - Suspend Interrupt Enable Bit

Set to enable the SUSPI interrupt.

Clear to disable the SUSPI interrupt.

Bit 7 6 5 4 3 2 1 0

- UPRSME EORSME WAKEUPE EORSTE SOFE - SUSPE UDIEN

Read/Write

Initial Value 0 0 0 0 0 0 0 0

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• Bit 7 - ADDEN - Address Enable Bit

Set to activate the UADD (USB address).

Cleared by hardware. Clearing by software has no effect.

See “Address Setup” on page 272 for more details.

• Bits 6-0 - UADD6:0 - USB Address Bits

Load by software to configure the device address

• 7-3 - Reserved

The value read from these bits is always 0. Do not set these bits.

• 2-0 - FNUM10:8 - Frame Number Upper Value

Set by hardware. These bits are the three MSB of the 11-bits Frame Number information. They are provided in

the last received SOF packet. FNUM is updated if a corrupted SOF is received.

• Bits 7:0 - FNUM7:0 - Frame Number Lower Value

Set by hardware. These bits are the 8 LSB of the 11-bits Frame Number information

Bit 76543210

ADDEN UADD6:0 UDADDR

Read/Write W R/W R/W R/W R/W R/W R/W R/W

Initial Value00000000

Bit 76543 2 1 0

- - - - - FNUM10:8 UDFNUMH

Read/WriteRRRRR R R R

Initial Value 0 0 0 0 0 0 0 0

Bit 76543210

FNUM7:0 UDFNUML

Read/WriteRRRRRRRR

Initial Value 0 0 0 0 0 0 0 0

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• Bits 7:5 - Reserved

The value read from these bits is always 0. Do not set these bits.

• Bit 4 - FNCERR -Frame Number CRC Error Flag

Set by hardware when a corrupted Frame Number in start of frame packet is received.

This bit and the SOFI interrupt are updated at the same time.

• Bits 3:0 - Reserved

The value read from these bits is always 0. Do not set these bits.

22.18.2 USB Device Endpoint Registers

• Bits 7:3 - Reserved

The value read from these bits is always 0. Do not set these bits.

• Bits 2-0 - EPNUM2:0 Endpoint Number Bits

Load by software to select the number of the endpoint which shall be accessed by the CPU. See “Endpoint

Reset” on page 270 for more details.

EPNUM = 111b is forbidden

• Bit 7 - Reserved

The value read from these bits is always 0. Do not set these bits.

• Bits 6-0 - EPRST6:0 - Endpoint FIFO Reset Bits

Set to reset the selected endpoint FIFO prior to any other operation, upon hardware reset or when an USB bus

reset has been received. See “Endpoint Reset” on page 270 for more information

Then, clear by software to complete the reset operation and start using the endpoint.

Bit 76543210

---FNCERR----UDMFN

Read/Wri

Initial

Value

00000000

Bit 76543210

- - - - - EPNUM2:0 UENUM

Read/Write R R R R R R/W R/W R/W

Initial Value 0 0 0 0 0 0 0 0

Bit 76543210

- EPRST6 EPRST5 EPRST4 EPRST3 EPRST2 EPRST1 EPRST0 UERST

Read/Write R R/W R/W R/W R/W R/W R/W R/W

Initial Value 0 0 0 0 0 0 0 0

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• Bits 7:6 - Reserved

The value read from these bits is always 0. Do not set these bits.

• Bit 5 - STALLRQ - STALL Request Handshake Bit

Set to request a STALL answer to the host for the next handshake.

Cleared by hardware when a new SETUP is received. Clearing by software has no effect.

See “STALL Request” on page 273 for more details.

• Bit 4 - STALLRQC - STALL Request Clear Handshake Bit

Set to disable the STALL handshake mechanism.

Cleared by hardware immediately after the set. Clearing by software has no effect.

See “STALL Request” on page 273 for more details.

• Bit 3 - RSTDT - Reset Data Toggle Bit

Set to automatically clear the data toggle sequence:

For OUT endpoint: the next received packet will have the data toggle 0.

For IN endpoint: the next packet to be sent will have the data toggle 0.

Cleared by hardware instantaneously. The firmware does not have to wait that the bit is cleared. Clearing by

software has no effect.

• 2 - Reserved

The value read from these bits is always 0. Do not set these bits.

• Bit 1 - Reserved

The value read from these bits is always 0. Do not set these bits.

• Bit 0 - EPEN - Endpoint Enable Bit

Set to enable the endpoint according to the device configuration. Endpoint 0 shall always be enabled after a

hardware or USB reset and participate in the device configuration.

Clear this bit to disable the endpoint. See “Endpoint Activation” on page 271 for more details.

• Bits 7:6 - EPTYPE1:0 - Endpoint Type Bits

Set this bit according to the endpoint configuration:

00b: Control10b: Bulk

01b: Isochronous11b: Interrupt

Bit 7 6 5 4 3 2 1 0

- - STALLRQ STALLRQC RSTDT - - EPEN UECONX

Read/Write R R W W W R R R/W

Initial Value 0 0 0 0 0 0 0 0

Bit 76543210

EPTYPE1:0 -----EPDIRUECFG0X

Read/Write R/W R/W R R R R R R/W

Initial Value00000000

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• Bits 5:1 - Reserved

The value read from these bits is always 0. Do not set these bits.

• Bit 0 - EPDIR - Endpoint Direction Bit

Set to configure an IN direction for bulk, interrupt or isochronous endpoints.

Clear to configure an OUT direction for bulk, interrupt, isochronous or control endpoints.

• Bit 7 - Reserved

The value read from these bits is always 0. Do not set these bits.

• Bits 6-4 - EPSIZE2:0 - Endpoint Size Bits

Set this bit according to the endpoint size:

000b: 8 bytes100b: 128 bytes

001b: 16 bytes101b: 256 bytes

010b: 32 bytes110b: 512 bytes

011b: 64 bytes111b: Reserved. Do not use this configuration.

• Bits 3:2 - EPBK1:0 - Endpoint Bank Bits

Set this field according to the endpoint size:

00b: One bank

01b: Double bank

1xb: Reserved. Do not use this configuration.

• Bit 1 - ALLOC - Endpoint Allocation Bit

Set this bit to allocate the endpoint memory.

Clear to free the endpoint memory.

See “Endpoint Activation” on page 271 for more details.

• Bit 0 - Reserved

The value read from these bits is always 0. Do not set these bits.

• Bit 7 - CFGOK - Configuration Status Flag

Set by hardware when the endpoint X size parameter (EPSIZE) and the bank parametrization (EPBK) are

correct compared to the maximum FIFO capacity and the maximum number of allowed bank. This bit is updated

when the bit ALLOC is set.

If this bit is cleared, the user should reprogram the UECFG1X register with correct EPSIZE and EPBK values.

Bit 76543210

- EPSIZE2:0 EPBK1:0 ALLOC - UECFG1X

Read/Write R R/W R/W R/W R/W R/W R/W R

Initial Value 0 0 0 0 0 0 0 0

Bit 76543210

CFGOK OVERFI UNDERFI - DTSEQ1:0 NBUSYBK1:0 UESTA0X

Read/Write R R/W R/W R/W R R R R

Initial Value00000000

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• Bit 6 - OVERFI - Overflow Error Interrupt Flag

Set by hardware when an overflow error occurs in an isochronous endpoint. An interrupt (EPINTx) is triggered

(if enabled).

See Section 22.15, page 278 for more details.

Shall be cleared by software. Setting by software has no effect.

• Bit 5 - UNDERFI - Flow Error Interrupt Flag

Set by hardware when an underflow error occurs in an isochronous endpoint. An interrupt (EPINTx) is triggered

(if enabled).

See Section 22.15, page 278 for more details.

Shall be cleared by software. Setting by software has no effect.

• Bit 4 - Reserved

The value read from these bits is always 0. Do not set these bits.

• Bits 3-2 - DTSEQ1:0 - Data Toggle Sequencing Flag

Set by hardware to indicate the PID data of the current bank:

00bData0

01bData1

1xbReserved

For OUT transfer, this value indicates the last data toggle received on the current bank.

For IN transfer, it indicates the Toggle that will be used for the next packet to be sent. This is not relative to the

current bank.

• Bits 1:0 - NBUSYBK1:0 - Busy Bank Flag

Set by hardware to indicate the number of busy bank.

For IN endpoint, it indicates the number of busy bank(s), filled by the user, ready for IN transfer.

For OUT endpoint, it indicates the number of busy bank(s) filled by OUT transaction from the host.

00bAll banks are free

01b1 busy bank

10b2 busy banks

11bReserved

• Bits 7:3 - Reserved

The value read from these bits is always 0. Do not set these bits.

• Bit 2 - CTRLDIR - Control Direction (Flag, and bit for debug purpose)

Set by hardware after a SETUP packet, and gives the direction of the following packet:

Bit 76543 2 10

- - - - - CTRLDIR CURRBK1:0 UESTA1X

Read/WriteRRRRR R RR

Initial Value 0 0 0 0 0 0 0 0

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- 1 for IN endpoint

- 0 for OUT endpoint

Can not be set or cleared by software.

• Bits 1:0 - CURRBK1:0 - Current Bank (all endpoints except Control endp oint) Flag

Set by hardware to indicate the number of the current bank:

00bBank0

01bBank1

1xbReserved

Can not be set or cleared by software.

• Bit 7 - FIFOCON - FIFO Control Bit

For OUT and SETUP Endpoint:

Set by hardware when a new OUT message is stored in the current bank, at the same time than RXOUT or

RXSTP.

Clear to free the current bank and to switch to the following bank. Setting by software has no effect.

For IN Endpoint:

Set by hardware when the current bank is free, at the same time than TXIN.

Clear to send the FIFO data and to switch the bank. Setting by software has no effect.

• Bit 6 - NAKINI - NAK IN Received Interrupt Flag

Set by hardware when a NAK handshake has been sent in response of a IN request from the host. This triggers

an USB interrupt if NAKINE is sent.

Shall be cleared by software. Setting by software has no effect.

• Bit 5 - RWAL - Read/Write Allowed Flag

Set by hardware to signal:

- for an IN endpoint: the current bank is not full i.e. the firmware can push data into the FIFO

- for an OUT endpoint: the current bank is not empty, i.e. the firmware can read data from the FIFO

The bit is never set if STALLRQ is set, or in case of error.

Cleared by hardware otherwise.

This bit shall not be used for the control endpoint.

• Bit 4 - NAKOUTI - NAK OUT Received Interrupt Flag

Set by hardware when a NAK handshake has been sent in response of a OUT/PING request from the host. This

triggers an USB interrupt if NAKOUTE is sent.

Shall be cleared by software. Setting by software has no effect.

Bit 76543210

FIFOCON NAKINI RWAL NAKOUTI RXSTPI RXOUTI STALLEDI TXINI UEINTX

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value 0 0 0 0 0 0 0 0

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• Bit 3 - RXSTPI - Received SETUP Interrupt Flag

Set by hardware to signal that the current bank contains a new valid SETUP packet. An interrupt (EPINTx) is

triggered (if enabled).

Shall be cleared by software to handshake the interrupt. Setting by software has no effect.

This bit is inactive (cleared) if the endpoint is an IN endpoint.

• Bit 2 - RXOUTI / KILLBK - Received OUT Data Interrupt Flag

Set by hardware to signal that the current bank contains a new packet. An interrupt (EPINTx) is triggered (if

enabled).

Shall be cleared by software to handshake the interrupt. Setting by software has no effect.

Kill Bank IN Bit

Set this bit to kill the last written bank.

Cleared by hardware when the bank is killed. Clearing by software has no effect.

See “Abort” on page 277 for more details on the Abort.

• Bit 1 - STALLEDI - STALLEDI Interrupt Flag

Set by hardware to signal that a STALL handshake has been sent, or that a CRC error has been detected in a

OUT isochronous endpoint.

Shall be cleared by software. Setting by software has no effect.

• Bit 0 - TXINI - Transmitter Ready Interrupt Flag

Set by hardware to signal that the current bank is free and can be filled. An interrupt (EPINTx) is triggered (if

enabled).

Shall be cleared by software to handshake the interrupt. Setting by software has no effect.

This bit is inactive (cleared) if the endpoint is an OUT endpoint.

• Bit 7 - FLERRE - Flow Error Interrupt Enable Flag

Set to enable an endpoint interrupt (EPINTx) when OVERFI or UNDERFI are sent.

Clear to disable an endpoint interrupt (EPINTx) when OVERFI or UNDERFI are sent.

• Bit 6 - NAKINE - NAK IN Interrupt Enable Bit

Set to enable an endpoint interrupt (EPINTx) when NAKINI is set.

Clear to disable an endpoint interrupt (EPINTx) when NAKINI is set.

• Bit 5 - Reserved

The value read from these bits is always 0. Do not set these bits.

• Bit 4 - NAKOUTE - NAK OUT Interrupt Enable Bit

Set to enable an endpoint interrupt (EPINTx) when NAKOUTI is set.

Clear to disable an endpoint interrupt (EPINTx) when NAKOUTI is set.

Bit 7 6 5 4 3 2 1 0

FLERRE NAKINE - NAKOUTE RXSTPE RXOUTE STALLEDE TXINE UEIENX

Read/Write R/W R/W R R/W R/W R/W R/W R/W

Initial Value 0 0 0 0 0 0 0 0

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• Bit 3 - RXSTPE - Received SETUP Interrupt Enable Flag

Set to enable an endpoint interrupt (EPINTx) when RXSTPI is sent.

Clear to disable an endpoint interrupt (EPINTx) when RXSTPI is sent.

• Bit 2 - RXOUTE - Received OUT Data Interrupt Enable Flag

Set to enable an endpoint interrupt (EPINTx) when RXOUTI is sent.

Clear to disable an endpoint interrupt (EPINTx) when RXOUTI is sent.

• Bit 1 - STALLEDE - Stalled Interrupt Enable Flag

Set to enable an endpoint interrupt (EPINTx) when STALLEDI is sent.

Clear to disable an endpoint interrupt (EPINTx) when STALLEDI is sent.

• Bit 0 - TXINE - Transmitter Ready Interrupt Enable Flag

Set to enable an endpoint interrupt (EPINTx) when TXINI is sent.

Clear to disable an endpoint interrupt (EPINTx) when TXINI is sent.

• Bits 7:0 - DAT7:0 -Data Bits

Set by the software to read/write a byte from/to the endpoint FIFO selected by EPNUM

• Bits 7:3 - Reserved

The value read from these bits is always 0. Do not set these bits.

• Bits 2:0 - BYCT10:8 - Byte count (high) Bits

Set by hardware. This field is the MSB of the byte count of the FIFO endpoint. The LSB part is provided by the

UEBCLX register

• Bits 7-0 - BYCT7:0 - Byte Count (low) Bits

Set by the hardware. BYCT10:0 is:

(for IN endpoint) increased after each writing into the endpoint and decremented after each byte sent,

(for OUT endpoint) increased after each byte sent by the host, and decremented after each byte read by

the software.

Bit 76543210

DAT D7 DAT D6 DAT D5 DAT D4 DAT D3 DAT D2 DAT D1 DAT D0 UEDATX

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value00000000

Bit 76543 2 1 0

- - - - - BYCT D10 BYCT D9 BYCT D8 UEBCHX

Read/WriteRRRRR R R R

Initial Value 0 0 0 0 0 0 0 0

Bit 76543210

BYCT D7 BYCT D6 BYCT D5 BYCT D4 BYCT D3 BYCT D2 BYCT D1 BYCT D0 UEBCLX

Read/WriteRRRRRRRR

Initial Value00000000

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• Bit 7 - Reserved

The value read from these bits is always 0. Do not set these bits.

• Bits 6:0 - EPINT6:0 - Endpoint Interrupts Bits

Set by hardware when an interrupt is triggered by the UEINTX register and if the corresponding endpoint

interrupt enable bit is set.

Cleared by hardware when the interrupt source is served.

Bit 76543210

- EPINT D6 EPINT D5 EPINT D4 EPINT D3 EPINT D2 EPINT D1 EPINT D0 UEINT

Read/WriteRRRRRRRR

Initial Value 0 0 0 0 0 0 0 0

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23. Analog Comparator

The Analog Comparator compares the input values on the positive pin AIN+ and negative pin AIN-. When the

voltage on the positive pin AIN+ is higher than the voltage on the negative pin AIN-, the Analog Comparator

output, ACO, is set. The comparator’s output can be set to trigger the Timer/Counter1 Input Capture function. In

addition, the comparator can trigger a separate interrupt, exclusive to the Analog Comparator. The user can

select Interrupt triggering on comparator output rise, fall or toggle. A block diagram of the comparator and its

surrounding logic is shown in Figure 23-1. AIN+ can be connected either to the AIN0 (PE6) pin, or to the internal

Bandgap reference. AIN- can only be connected to the ADC multiplexer.

The Power Reduction ADC bit, PRADC, in “Power Reduction Register 0 - PRR0” on page 47 must be disabled

by writing a logical zero to be able to use the ADC input MUX.

Figure 23-1. Analog Comparator Block Diagram(2)

Notes: 1. See Table 23-2 on page 295.

2. Refer to “Pinout” on page 3 and Table 10-3 on page 74 for Analog Comparator pin placement.

ACBG

BANDGAP

REFERENCE

ADC MULTIPLEXER

OUTPUT

ACME

ADEN

(1)

AIN+

AIN-

BANDGAP

REFERENCE

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23.1 Register Description

23.1.1 ADC Control and Status Register B – ADCSRB

• Bit 6 – ACME: Analog Comparator Multiplexer Enable

When this bit is written logic one and the ADC is switched off (ADEN in ADCSRA is zero), the ADC multiplexer

is connected to the negative input to the Analog Comparator. When this bit is written logic zero, the Bandgap

reference is connected to the negative input of the Analog Comparator (See “Internal Voltage Reference” on

page 54.) For a detailed description of this bit, see “Analog Comparator Multiplexed Input” on page 295.

23.1.2 Analog Comparator Control and Status Register – ACSR

• Bit 7 – ACD: Analog Comparator Disable

When this bit is written logic one, the power to the Analog Comparator is switched off. This bit can be set at any

time to turn off the Analog Comparator. This will reduce power consumption in Active and Idle mode. When

changing the ACD bit, the Analog Comparator Interrupt must be disabled by clearing the ACIE bit in ACSR.

Otherwise an interrupt can occur when the bit is changed.

• Bit 6 – ACBG: Analog Comparator Bandgap Sele ct

When this bit is set, a fixed bandgap reference voltage replaces the positive input to the Analog Comparator.

When this bit is cleared, AIN0 is applied to the positive input of the Analog Comparator. See “Internal Voltage

Reference” on page 54.

• Bit 5 – ACO: Analog Comparator Output

The output of the Analog Comparator is synchronized and then directly connected to ACO. The synchronization

introduces a delay of 1 - 2 clock cycles.

• Bit 4 – ACI: Analog Comparator Interrupt Flag

This bit is set by hardware when a comparator output event triggers the interrupt mode defined by ACIS1 and

ACIS0. The Analog Comparator interrupt routine is executed if the ACIE bit is set and the I-bit in SREG is set.

ACI is cleared by hardware when executing the corresponding interrupt handling vector. Alternatively, ACI is

cleared by writing a logic one to the flag.

• Bit 3 – ACIE: Analog Comparator Interrupt Enable

When the ACIE bit is written logic one and the I-bit in the Status Register is set, the Analog Comparator interrupt

is activated. When written logic zero, the interrupt is disabled.

Bit 7 6543210

ADHSM ACME MUX5 –ADTS3 ADTS2 ADTS1 ADTS0 ADCSRB

Read/Write R R/W R/W R R R/W R/W R/W

Initial Value0 0000000

Bit 76543210

ACD ACBG ACO ACI ACIE ACIC ACIS1 ACIS0 ACSR

Read/Write R/W R/W R R/W R/W R/W R/W R/W

Initial Value 0 0 N/A 0 0 0 0 0

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• Bit 2 – ACIC: Analog Comparator Input Capture Enable

When written logic one, this bit enables the input capture function in Timer/Counter1 to be triggered by the

Analog Comparator. The comparator output is in this case directly connected to the input capture front-end

logic, making the comparator utilize the noise canceler and edge select features of the Timer/Counter1 Input

Capture interrupt. When written logic zero, no connection between the Analog Comparator and the input

capture function exists. To make the comparator trigger the Timer/Counter1 Input Capture interrupt, the ICIE1

bit in the Timer Interrupt Mask Register (TIMSK1) must be set.

• Bits 1, 0 – ACIS1, ACIS0: Analog Comparator Interrupt Mode Select

These bits determine which comparator events that trigger the Analog Comparator interrupt. The different

settings are shown in the table below.

When changing the ACIS1/ACIS0 bits, the Analog Comparator Interrupt must be disabled by clearing its

Interrupt Enable bit in the ACSR Register. Otherwise an interrupt can occur when the bits are changed.

23.2 Analog Comparator Multiplexed Input

It is possible to select any of the ADC13..0 pins to replace the negative input to the Analog Comparator. The

ADC multiplexer is used to select this input, and consequently, the ADC must be switched off to utilize this

feature. If the Analog Comparator Multiplexer Enable bit (ACME in ADCSRB) is set and the ADC is switched off

(ADEN in ADCSRA is zero), and MUX2..0 in ADMUX select the input pin to replace the negative input to the

Analog Comparator, as shown in the table. If ACME is cleared or ADEN is set, the Bandgap reference is applied

to the negative input to the Analog Comparator.

Table 23-1. ACIS1/ACIS0 Settings

ACIS1 ACIS0 Interrupt Mode

0 0 Comparator Interrupt on Output Toggle

0 1 Reserved

1 0 Comparator Interrupt on Falling Output Edge

1 1 Comparator Interrupt on Rising Output Edge

Table 23-2. Analog Compara t or Multiplexed Input

ACME ADEN MUX2..0 Analog Comparator Negative Input

0 x xxx Bandgap Ref.

1 1 xxx Bandgap Ref.

1 0 000 ADC0

1 0 001 ADC1

1 0 010

N/A

1 0 011

1 0 100 ADC4

1 0 101 ADC5

1 0 110 ADC6

1 0 111 ADC7

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23.2.1 Digital Input Disable Register 1 – DIDR1

• Bit 0 – AIN0D: AIN0 Digital Input Disable

When this bit is written logic one, the digital input buffer on the AIN0 pin is disabled. The corresponding PIN

the digital input from this pin is not needed, this bit should be written logic one to reduce power consumption in

the digital input buffer.

Bit 76543210

––––––– AIN0D DIDR1

Read/WriteRRRRRRRR/W

Initial Value00000000

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24. Analog to Digital Converter - ADC

24.1 Features

•10/8-bit Resolution

•0.5LSB Integral Non-linearity

•±2LSB Absolute Accuracy

•65 - 260µs Conversion Time

•Up to 15kSPS at Maximum Resolution

•Twelve Multiplexed Single-Ended Input Ch annels

•One Differential amplifier provid ing gain of 1x - 10x - 40x - 200x

•Temperature senso r

•Optional Left Adjustment for ADC Result Readout

•0 - VCC ADC Input Voltage Range

•Selectable 2.56V ADC Reference Voltage

•Free Running or Single Conversion Mode

•ADC Start Conversion by Auto Triggering on Interrupt Sources

•Interrupt on ADC Conversion Complete

•Sleep Mode Noise Canceler

The ATmega16U4/ATmega32U4 features a 10-bit successive approximation ADC. The ADC is connected to an

12-channel Analog Multiplexer which allows six single-ended voltage inputs constructed from several pins of

Port B, D, and F. The single-ended voltage inputs refer to 0V (GND).

The device also supports 32 differential voltage input combinations, thanks to a differential amplifier equipped

with a programmable gain stage, providing amplification steps of 0 dB (1x), 10 dB (10x), 16dB (40x), or 23dB

(200x) on the differential input voltage before the A/D conversion. Two differential analog input channels share a

common negative terminal (ADC0/ADC1), while any other ADC input can be selected as the positive input

terminal. If 1x, 10x, or 40x gain is used, 8-bit resolution can be expected. If 200x gain is used, 7-bit resolution

can be expected.

The ADC contains a Sample and Hold circuit which ensures that the input voltage to the ADC is held at a

constant level during conversion. A block diagram of the ADC is shown in Figure 24-1.

The ADC has a separate analog supply voltage pin, AVCC. AVCC must not differ more than ± 0.3V from VCC. See

the paragraph “ADC Noise Canceler” on page 305 on how to connect this pin.

Internal reference voltages of nominally 2.56V or AVCC are provided On-chip. The voltage reference may be

externally decoupled at the AREF pin by a capacitor for better noise performance.

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Figure 24-1. Analog to Dig ital Converter Block Schematic

ADC CONVERSION

COMPLETE IRQ

8-BIT DATA BUS

15 0

ADC MULTIPLEXER

SELECT (ADMUX) ADC CTRL. & STATUS

(ADCH/ADCL)

MUX2

ADIE

ADATE

ADSC

ADEN

ADIF ADIF

MUX1

MUX0

ADPS0

ADPS1

ADPS2

MUX3

CONVERSION LOGIC

10-BIT DAC +

SAMPLE & HOLD

COMPARATOR

INTERNAL

REFERENCE

MUX DECODER

MUX4

AVCC

ADC9

ADC8

ADC7

ADC6

ADC5

ADC4

ADC1

ADC0

REFS0

REFS1

ADLAR

CHANNEL SELECTION

GAIN SELECTION

ADC[9:0]

ADC MULTIPLEXER

OUTPUT

DIFFERENTIAL

AMPLIFIER

AREF

BANDGAP

REFERENCE

PRESCALER

SINGLE ENDED / DIFFERENTIAL SELECTION

GND

POS.

INPUT

MUX

NEG.

INPUT

MUX

TRIGGER

SELECT

ADTS[3:0]

INTERRUPT

FLAGS

ADHSM

START

ADC13

ADC12

ADC11

ADC10

MUX5

TEMPERATURE

SENSOR

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24.2 Operation

The ADC converts an analog input voltage to a 10-bit digital value through successive approximation. The

minimum value represents GND and the maximum value represents the voltage on the AREF pin minus 1 LSB.

Optionally, AVCC or an internal 2.56V reference voltage may be connected to the AREF pin by writing to the

REFSn bits in the ADMUX Register. The internal voltage reference may thus be decoupled by an external

capacitor at the AREF pin to improve noise immunity.

The analog input channel and differential gain are selected by writing to the MUX bits in ADMUX. Any of the

ADC input pins, as well as GND and a fixed bandgap voltage reference, can be selected as single ended inputs

to the ADC. A selection of ADC input pins can be selected as positive and negative inputs to the differential

amplifier.

The ADC is enabled by setting the ADC Enable bit, ADEN in ADCSRA. Voltage reference and input channel

selections will not go into effect until ADEN is set. The ADC does not consume power when ADEN is cleared, so

it is recommended to switch off the ADC before entering power saving sleep modes.

The ADC generates a 10-bit result which is presented in the ADC Data Registers, ADCH and ADCL. By default,

the result is presented right adjusted, but can optionally be presented left adjusted by setting the ADLAR bit in

ADMUX.

If the result is left adjusted and no more than 8-bit precision is required, it is sufficient to read ADCH. Otherwise,

ADCL must be read first, then ADCH, to ensure that the content of the Data Registers belongs to the same

conversion. Once ADCL is read, ADC access to Data Registers is blocked. This means that if ADCL has been

read, and a conversion completes before ADCH is read, neither register is updated and the result from the

conversion is lost. When ADCH is read, ADC access to the ADCH and ADCL Registers is re-enabled.

The ADC has its own interrupt which can be triggered when a conversion completes. The ADC access to the

Data Registers is prohibited between reading of ADCH and ADCL, the interrupt will trigger even if the result is

lost.

24.3 Starting a Conversion

A single conversion is started by writing a logical one to the ADC Start Conversion bit, ADSC. This bit stays high

as long as the conversion is in progress and will be cleared by hardware when the conversion is completed. If a

different data channel is selected while a conversion is in progress, the ADC will finish the current conversion

before performing the channel change.

Alternatively, a conversion can be triggered automatically by various sources. Auto Triggering is enabled by

setting the ADC Auto Trigger Enable bit, ADATE in ADCSRA. The trigger source is selected by setting the ADC

Trigger Select bits, ADTS in ADCSRB (See description of the ADTS bits for a list of the trigger sources). When

a positive edge occurs on the selected trigger signal, the ADC prescaler is reset and a conversion is started.

This provides a method of starting conversions at fixed intervals. If the trigger signal is still set when the

conversion completes, a new conversion will not be started. If another positive edge occurs on the trigger signal

during conversion, the edge will be ignored. Note that an interrupt flag will be set even if the specific interrupt is

disabled or the Global Interrupt Enable bit in SREG is cleared. A conversion can thus be triggered without

causing an interrupt. However, the interrupt flag must be cleared in order to trigger a new conversion at the next

interrupt event.

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Figure 24-2. ADC Auto Trigger Logic

Using the ADC Interrupt Flag as a trigger source makes the ADC start a new conversion as soon as the ongoing

conversion has finished. The ADC then operates in Free Running mode, constantly sampling and updating the

ADC Data Register. The first conversion must be started by writing a logical one to the ADSC bit in ADCSRA. In

this mode the ADC will perform successive conversions independently of whether the ADC Interrupt Flag, ADIF

is cleared or not.

If Auto Triggering is enabled, single conversions can be started by writing ADSC in ADCSRA to one. ADSC can

also be used to determine if a conversion is in progress. The ADSC bit will be read as one during a conversion,

independently of how the conversion was started.

24.4 Prescaling and Conversion Timing

Figure 24-3. ADC Prescaler

By default, the successive approximation circuitry requires an input clock frequency between 50kHz and

200kHz to get maximum resolution. If a lower resolution than 10 bits is needed, the input clock frequency to the

ADC can be higher than 200kHz to get a higher sample rate. Alternatively, setting the ADHSM bit in ADCSRB

allows an increased ADC clock frequency at the expense of higher power consumption.

The ADC module contains a prescaler, which generates an acceptable ADC clock frequency from any CPU

frequency above 100kHz. The prescaling is set by the ADPS bits in ADCSRA. The prescaler starts counting

from the moment the ADC is switched on by setting the ADEN bit in ADCSRA. The prescaler keeps running for

as long as the ADEN bit is set, and is continuously reset when ADEN is low.

When initiating a single ended conversion by setting the ADSC bit in ADCSRA, the conversion starts at the

following rising edge of the ADC clock cycle. See “Differential Channels” on page 302 for details on differential

conversion timing.

ADSC

ADIF

SOURCE 1

SOURCE n

ADTS[2:0]

CONVERSION

LOGIC

PRESCALER

START CLK

ADC

.EDGE

DETECTOR

ADATE

7-BIT ADC PRESCALER

ADC CLOCK SOURCE

ADPS0

ADPS1

ADPS2

CK/128

CK/2

CK/4

CK/8

CK/16

CK/32

CK/64

Reset

ADEN

START

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A normal conversion takes 13 ADC clock cycles. The first conversion after the ADC is switched on (ADEN in

ADCSRA is set) takes 25 ADC clock cycles in order to initialize the analog circuitry.

The actual sample-and-hold takes place 1.5 ADC clock cycles after the start of a normal conversion and 13.5

ADC clock cycles after the start of an first conversion. When a conversion is complete, the result is written to the

ADC Data Registers, and ADIF is set. In Single Conversion mode, ADSC is cleared simultaneously. The

software may then set ADSC again, and a new conversion will be initiated on the first rising ADC clock edge.

When Auto Triggering is used, the prescaler is reset when the trigger event occurs. This assures a fixed delay

from the trigger event to the start of conversion. In this mode, the sample-and-hold takes place two ADC clock

cycles after the rising edge on the trigger source signal. Three additional CPU clock cycles are used for

synchronization logic.

In Free Running mode, a new conversion will be started immediately after the conversion completes, while

ADSC remains high. For a summary of conversion times, see the table below.

Figure 24-4. ADC Timing Diagram, First Conversion (Single Conversion Mode)

Figure 24-5. ADC Timing Diagram, Single Conversion

Sign and MSB of Result

LSB of Result

ADC Clock

ADSC

Sample & Hold

ADIF

ADCH

ADCL

Cycle Number

ADEN

1212

13 14 15 16 17 18 19 20 21 22 23 24 25 1 2

First Conversion Next

Conversion

MUX and REFS

Update

MUX

and REFS

Update

Conversion

Complete

123456789 10 11 12 13

Sign and MSB of Result

LSB of Result

ADC Clock

ADSC

ADIF

ADCH

ADCL

Cycle Number 12

One Conversion Next Conversion

Sample & Hold

MUX and REFS

Update

Conversion

Complete MUX and REFS

Update

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Figure 24-6. ADC Timi ng Diagram, Auto Trig gered Conversion

Figure 24-7. ADC Timing Diagram, Free Running Conversion

24.4.1 Differential Channels

When using differential channels, certain aspects of the conversion need to be taken into consideration.

Differential conversions are synchronized to the internal clock CKADC2 equal to half the ADC clock frequency.

This synchronization is done automatically by the ADC interface in such a way that the sample-and-hold occurs

at a specific phase of CKADC2. A conversion initiated by the user (i.e., all single conversions, and the first free

running conversion) when CKADC2 is low will take the same amount of time as a single ended conversion (13

ADC clock cycles from the next prescaled clock cycle). A conversion initiated by the user when CKADC2 is high

will take 14 ADC clock cycles due to the synchronization mechanism. In Free Running mode, a new conversion

is initiated immediately after the previous conversion completes, and since CKADC2 is high at this time, all

automatically started (i.e., all but the first) Free Running conversions will take 14 ADC clock cycles.

If differential channels are used and conversions are started by Auto Triggering, the ADC must be switched off

between conversions. When Auto Triggering is used, the ADC prescaler is reset before the conversion is

started. Since the stage is dependent of a stable ADC clock prior to the conversion, this conversion will not be

valid. By disabling and then re-enabling the ADC between each conversion (writing ADEN in ADCSRA to “0”

Table 24-1. ADC Conversio n Time

Condition First Conversion Normal Conversion,

Single Ended Auto Triggered

Conversion

Sample and Hold

(Cycles from Start of Convention) 14.5 1.5 2

Conversion Time

(Cycles) 25 13 13.5

1 2 3 4 5 6 7 8 910 11 12 13

Sign and MSB of Result

LSB of Result

ADC Clock

Trigger

Source

ADIF

ADCH

ADCL

Cycle Number 12

One Conversion Next Conversion

Conversion

Complete

Prescaler

Reset

ADATE

Prescaler

Reset

Sample &

Hold

MUX and REFS

Update

11 12 13

Sign and MSB of Result

LSB of Result

ADC Clock

ADSC

ADIF

ADCH

ADCL

Cycle Number 12

One Conversion Next Conversion

Conversion

Complete Sample & Hold

MUX and REFS

Update

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then to “1”), only extended conversions are performed. The result from the extended conversions will be valid.

See “Prescaling and Conversion Timing” on page 300 for timing details.

The gain stage is optimized for a bandwidth of 4kHz at all gain settings. Higher frequencies may be subjected to

non-linear amplification. An external low-pass filter should be used if the input signal contains higher frequency

components than the gain stage bandwidth. Note that the ADC clock frequency is independent of the gain stage

bandwidth limitation. E.g. the ADC clock period may be 6µs, allowing a channel to be sampled at 12kSPS,

regardless of the bandwidth of this channel.

24.5 Changing Channel or Reference Selection

The MUXn and REFS1:0 bits in the ADMUX Register are single buffered through a temporary register to which

the CPU has random access. This ensures that the channels and reference selection only takes place at a safe

point during the conversion. The channel and reference selection is continuously updated until a conversion is

started. Once the conversion starts, the channel and reference selection is locked to ensure a sufficient

sampling time for the ADC. Continuous updating resumes in the last ADC clock cycle before the conversion

completes (ADIF in ADCSRA is set). Note that the conversion starts on the following rising ADC clock edge after

ADSC is written. The user is thus advised not to write new channel or reference selection values to ADMUX until

one ADC clock cycle after ADSC is written.

If Auto Triggering is used, the exact time of the triggering event can be indeterministic. Special care must be

taken when updating the ADMUX Register, in order to control which conversion will be affected by the new

settings.

If both ADATE and ADEN is written to one, an interrupt event can occur at any time. If the ADMUX Register is

changed in this period, the user cannot tell if the next conversion is based on the old or the new settings.

ADMUX can be safely updated in the following ways:

a. When ADATE or ADEN is cleared.

3. During conversion, minimum one ADC clock cycle after the trigger event.

4. After a conversion, before the interrupt flag used as trigger source is cleared.

When updating ADMUX in one of these conditions, the new settings will affect the next ADC conversion.

Special care should be taken when changing differential channels. Once a differential channel has been

selected, the stage may take as much as 125µs to stabilize to the new value. Thus conversions should not be

started within the first 125µs after selecting a new differential channel. Alternatively, conversion results obtained

within this period should be discarded.

The same settling time should be observed for the first differential conversion after changing ADC reference (by

changing the REFS1:0 bits in ADMUX).

The settling time and gain stage bandwidth is independent of the ADHSM bit setting.

24.5.1 ADC Input Channels

When changing channel selections, the user should observe the following guidelines to ensure that the correct

channel is selected:

In Single Conversion mode, always select the channel before starting the conversion. The channel

selection may be changed one ADC clock cycle after writing one to ADSC. However, the simplest method

is to wait for the conversion to complete before changing the channel selection.

In Free Running mode, always select the channel before starting the first conversion. The channel

selection may be changed one ADC clock cycle after writing one to ADSC. However, the simplest method

is to wait for the first conversion to complete, and then change the channel selection. Since the next

conversion has already started automatically, the next result will reflect the previous channel selection.

Subsequent conversions will reflect the new channel selection.

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When switching to a differential gain channel, the first conversion result may have a poor accuracy due to the

required settling time for the automatic offset cancellation circuitry. The user should preferably disregard the first

conversion result.

24.5.2 ADC Voltage Reference

The reference voltage for the ADC (VREF) indicates the conversion range for the ADC. Single ended channels

that exceed VREF will result in codes close to 0x3FF. VREF can be selected as either AVCC, internal 2.56V

reference, or external AREF pin.

AVCC is connected to the ADC through a passive switch. The internal 2.56V reference is generated from the

internal bandgap reference (VBG) through an internal amplifier. In either case, the external AREF pin is directly

connected to the ADC, and the reference voltage can be made more immune to noise by connecting a capacitor

between the AREF pin and ground. VREF can also be measured at the AREF pin with a high impedance

voltmeter. Note that VREF is a high impudent source, and only a capacitive load should be connected in a

system.

If the user has a fixed voltage source connected to the AREF pin, the user may not use the other reference

voltage options in the application, as they will be shorted to the external voltage. If no external voltage is applied

to the AREF pin, the user may switch between AVCC and 2.56V as reference selection. The first ADC

conversion result after switching reference voltage source may be inaccurate, and the user is advised to discard

this result.

If differential channels are used, the selected reference should not be closer to AVCC than indicated in Table 29-

7 on page 390.

24.6 Temperature Sensor

The ATmega16U4/ATmega32U4 includes an on-chip temperature sensor, whose the value can be read through

the A/D Converter.

The temperature measurement is based on an on-chip temperature sensor that is coupled to a single ended

ADC input. MUX[5..0] bits in ADMUX register enables the temperature sensor. The internal 2.56V voltage

reference must also be selected for the ADC voltage reference source in he temperature sensor measurement.

When the temperature sensor is enabled, the ADC converter can be used in single conversion mode to

measure the voltage over the temperature sensor.

The temperature sensor and its internal driver are enabled when ADMUX value selects the temperature sensor

as ADC input. The propagation delay of this driver is approximately 2µS. Therefore two successive conversions

are required. The correct temperature measurement will be the second one.

One can also reduce this timing to one conversion by setting the ADMUX during the previous conversion.

Indeed the ADMUX can be programmed to select the temperature sensor just after the beginning of the

previous conversion start event and then the driver will be enabled 2µS before sampling and hold phase of

temperature sensor measurement.

24.6.1 Sensor Calibration

The sensor initial tolerance is large (±10°C), but its characteristic is linear. Thus, if the application requires

accuracy, the firmware must include a calibration stage to use the sensor for direct temperature measurement.

Another application of this sensor may concern the Internal Calibrated RC Oscillator, whose the frequency can

be adjusted by the user through the OSCCAL register (see “Oscillator Calibration Register – OSCCAL” on

page 32). During the production, a calibration is done at two temperatures (+25°C and +85°C, with a tolerance

of ±10°C(1)). At each temperature, the temperature sensor value Ti is measured and stored in EEPROM

memory(2), and the OSCCAL calibration value Oi (i.e. the value that should be set in OSCCAL register at this

temperature to have an accurate 8MHz output) is stored in another memory zone.

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Thanks to these four values and the linear characteristics of the temperature sensor and Internal RC Oscillator,

firmware can easily recalibrate the RC Oscillator on-the-go in function of the temperature sensor measure(3) (an

application note describes the operation):

Figure 24-8. Linear Characterization of OSCCAL in Function of T° Measurement from ADC

Notes: 1. The temperature sensor calibration values cannot be used to do accurate temperature measurements since

the calibration temperature during production is not accurate (±10°C)

2. Be aware that if EESAVE fuse is left unprogrammed, any chip erase operation will clear the temperature

sensor calibration values contained in EEPROM memory.

3. Accuracy results after a software recalibration of OSCCAL in function of Temperature is given by

characterization.

24.7 ADC Noise Canceler

The ADC features a noise canceler that enables conversion during sleep mode to reduce noise induced from

the CPU core and other I/O peripherals. The noise canceler can be used with ADC Noise Reduction and Idle

mode. To make use of this feature, the following procedure should be used:

a. Make sure that the ADC is enabled and is not busy converting. Single Conversion mode must be

selected and the ADC conversion complete interrupt must be enabled.

5. Enter ADC Noise Reduction mode (or Idle mode). The ADC will start a conversion once the CPU

has been halted.

6. If no other interrupts occur before the ADC conversion completes, the ADC interrupt will wake up

the CPU and execute the ADC Conversion Complete interrupt routine. If another interrupt wakes up

the CPU before the ADC conversion is complete, that interrupt will be executed, and an ADC

Conversion Complete interrupt request will be generated when the ADC conversion completes. The

CPU will remain in active mode until a new sleep command is executed.

Note that the ADC will not be automatically turned off when entering other sleep modes than Idle mode and

ADC Noise Reduction mode. The user is advised to write zero to ADEN before entering such sleep modes to

avoid excessive power consumption.

If the ADC is enabled in such sleep modes and the user wants to perform differential conversions, the user is

advised to switch the ADC off and on after waking up from sleep to prompt an extended conversion to get a

valid result.

24.7.1 Analog In put Circuitry

The analog input circuitry for single ended channels is illustrated in Figure 24-9. An analog source applied to

ADCn is subjected to the pin capacitance and input leakage of that pin, regardless of whether that channel is

selected as input for the ADC. When the channel is selected, the source must drive the S/H capacitor through

the series resistance (combined resistance in the input path).

OSCCAL

T(ADC

T1T2

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The ADC is optimized for analog signals with an output impedance of approximately 10k or less. If such a

source is used, the sampling time will be negligible. If a source with higher impedance is used, the sampling

time will depend on how long time the source needs to charge the S/H capacitor, with can vary widely. The user

is recommended to only use low impedance sources with slowly varying signals, since this minimizes the

required charge transfer to the S/H capacitor.

If differential gain channels are used, the input circuitry looks somewhat different, although source impedances

of a few hundred k or less is recommended.

Signal components higher than the Nyquist frequency (fADC/2) should not be present for either kind of channels,

to avoid distortion from unpredictable signal convolution. The user is advised to remove high frequency

components with a low-pass filter before applying the signals as inputs to the ADC.

Figure 24-9. Analog Input Circuitry

24.7.2 Analog Noise Canceling Techniques

Digital circuitry inside and outside the device generates EMI which might affect the accuracy of analog

measurements. If conversion accuracy is critical, the noise level can be reduced by applying the following

techniques:

a. Keep analog signal paths as short as possible. Make sure analog tracks run over the analog

ground plane, and keep them well away from high-speed switching digital tracks.

7. The AVCC pin on the device should be connected to the digital VCC supply voltage via an LC network

as shown in Figure 24-10.

8. Use the ADC noise canceler function to reduce induced noise from the CPU.

9. If any ADC port pins are used as digital outputs, it is essential that these do not switch while a

conversion is in progress.

ADCn

1..100 kΩ

S/H

= 14 pF

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Figure 24-10. ADC Power Connections

Note: The same circuitry should be used for AVCC filtering on the ADC8-ADC13 side.

24.7.3 Offset Compensation Schemes

The gain stage has a built-in offset cancellation circuitry that nulls the offset of differential measurements as

much as possible. The remaining offset in the analog path can be measured directly by selecting the same

channel for both differential inputs. This offset residue can be then subtracted in software from the

measurement results. Using this kind of software based offset correction, offset on any channel can be reduced

below one LSB.

24.7.4 ADC Accuracy Definitions

An n-bit single-ended ADC converts a voltage linearly between GND and VREF in 2n steps (LSBs). The lowest

code is read as 0, and the highest code is read as 2n-1.

Several parameters describe the deviation from the ideal behavior:

Offset: The deviation of the first transition (0x000 to 0x001) compared to the ideal transition (at 0.5 LSB).

Ideal value: 0 LSB.

VCC

GND

100nF

Analog Ground Plane

(ADC0) PF0

(ADC7) PF7

(ADC1) PF1

(ADC4) PF4

(ADC5) PF5

(ADC6) PF6

AREF

GND

AVCC

10µH

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Figure 24-11. Offset Error

Gain Error: After adjusting for offset, the Gain Error is found as the deviation of the last transition (0x3FE

to 0x3FF) compared to the ideal transition (at 1.5 LSB below maximum). Ideal value: 0 LSB.

Figure 24-12. Gain Error

Integral Non-linearity (INL): After adjusting for offset and gain error, the INL is the maximum deviation of

an actual transition compared to an ideal transition for any code. Ideal value: 0 LSB.

Output Code

VREF Input Voltage

Ideal ADC

Actual ADC

Offset

Error

Output Code

REF

Input Voltage

Ideal ADC

Actual ADC

Gain

Error

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Figure 24-13. Integ ral Non-linearity (INL)

Differential Non-linearity (DNL): The maximum deviation of the actual code width (the interval between

two adjacent transitions) from the ideal code width (1 LSB). Ideal value: 0 LSB.

Figure 24-14. Differential Non-linearity (DNL)

Quantization Error: Due to the quantization of the input voltage into a finite number of codes, a range of

input voltages (1 LSB wide) will code to the same value. Always ±0.5 LSB.

Absolute Accuracy: The maximum deviation of an actual (unadjusted) transition compared to an ideal

transition for any code. This is the compound effect of offset, gain error, differential error, non-linearity, and

quantization error. Ideal value: ±0.5 LSB.

24.8 ADC Conversion Result

After the conversion is complete (ADIF is high), the conversion result can be found in the ADC Result Registers

(ADCL, ADCH).

For single ended conversion, the result is:

Output Code

REF

Input Voltage

Ideal ADC

Actual ADC

INL

Output Code

0x3FF

0x000

0VREF Input Voltage

DNL

1 LSB

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where VIN is the voltage on the selected input pin and VREF the selected voltage reference (see Table 24-3 on

page 313 and Table 24-4 on page 313). 0x000 represents analog ground, and 0x3FF represents the selected

reference voltage minus one LSB.

If differential channels are used, the result is:

where VPOS is the voltage on the positive input pin, VNEG the voltage on the negative input pin, GAIN the

selected gain factor and VREF the selected voltage reference. The result is presented in two’s complement form,

from 0x200 (-512d) through 0x1FF (+511d). Note that if the user wants to perform a quick polarity check of the

result, it is sufficient to read the MSB of the result (ADC9 in ADCH). If the bit is one, the result is negative, and if

this bit is zero, the result is positive. Figure 24-15 on page 311 shows the decoding of the differential input

range.

Table 82 shows the resulting output codes if the differential input channel pair (ADCn - ADCm) is selected with

a reference voltage of VREF.

ADC VIN 1023

VREF

--------------------------

ADC VPOS VNEG

–GAIN 512

VREF

------------------------------------------------------------------------

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Figure 24-15. Differential Measurement Range

Example 1:

ADMUX = 0xE9, MUX5 = 0 (ADC1 - ADC0, 10x gain, 2.56V reference, left adjusted result)

Voltage on ADC1 is 300mV, voltage on ADC0 is 500mV

ADCR = 512 * 10 * (300 - 500) / 2560 = -400 = 0x270

Table 24-2. Correlation Between Input Voltage and Output Codes

VADCn Read code Corresponding decimal value

VADCm + VREF /GAIN 0x1FF 511

VADCm + 0.999 VREF /GAIN 0x1FF 511

VADCm + 0.998 VREF /GAIN 0x1FE 510

... ... ...

VADCm + 0.001 VREF /GAIN 0x001 1

VADCm 0x000 0

VADCm - 0.001 VREF /GAIN 0x3FF -1

... ... ...

VADCm - 0.999 VREF /GAIN 0x201 -511

VADCm - VREF /GAIN 0x200 -512

Output Code

0x1FF

0x000

REF

Differential Input

Voltage (Volts)

0x3FF

0x200

- V

REF

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ADCL will thus read 0x00, and ADCH will read 0x9C.

Writing zero to ADLAR right adjusts the result: ADCL = 0x70, ADCH = 0x02.

Example 2:

ADMUX = 0xF0, MUX5 = 0 (ADC0 - ADC1, 1x gain, 2.56V reference, left adjusted result)

Voltage on ADC0 is 300mV, voltage on ADC1 is 500mV

ADCR = 512 * 1 * (300 - 500) / 2560 = -41 = 0x029

ADCL will thus read 0x40, and ADCH will read 0x0A.

Writing zero to ADLAR right adjusts the result: ADCL = 0x00, ADCH = 0x29.

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24.9 ADC Register Description

24.9.1 ADC Multiplexer Selection Register – ADMUX

• Bit 7:6 – REFS1:0: Reference Selection Bits

These bits select the voltage reference for the ADC, as shown in the table. If these bits are changed during a

conversion, the change will not go in effect until this conversion is complete (ADIF in ADCSRA is set). The

internal voltage reference options may not be used if an external reference voltage is being applied to the AREF

pin.

• Bit 5 – ADLAR: ADC Left Adjust Result

The ADLAR bit affects the presentation of the ADC conversion result in the ADC Data Register. Write one to

ADLAR to left adjust the result. Otherwise, the result is right adjusted. Changing the ADLAR bit will affect the

ADC Data Register immediately, regardless of any ongoing conversions. For a complete description of this bit,

see “The ADC Data Register – ADCL and ADCH” on page 316.

• Bits 4:0 – MUX4:0: Analog Channel Selection Bits

The value of these bits selects which combination of analog inputs are connected to the ADC. These bits also

select the gain for the differential channels as shown in the table. If these bits are changed during a conversion,

the change will not go in effect until this conversion is complete (ADIF in ADCSRA is set).

Bit 76543210

REFS1 REFS0 ADLAR MUX4 MUX3 MUX2 MUX1 MUX0 ADMUX

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value00000000

Table 24-3. Voltage Reference Selection s for ADC

REFS1 REFS0 Voltage Reference Selection

0 0 AREF, Internal VREF turned off

0 1 AVCC with external capacitor on AREF pin

1 0 Reserved

1 1 Internal 2.56V Voltage Reference with external capacitor on AREF pin

Table 24-4. Input Channel and Gain Selections

MUX5..0(1) Single Ended Input Positive Differential Input Negative Differential Input Gain

000000 ADC0

N/A

000001 ADC1

000010

N/A

000011

000100 ADC4

000101 ADC5

000110 ADC6

000111 ADC7

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001000

N/A

N/A N/A N/A

001001 ADC1 ADC0 10x

001010 N/A N/A N/A

001011 ADC1 ADC0 200x

001100

N/A

001101

001110

001111

010000 ADC0 ADC1 1x

010001

N/A

N/A010010

010011

010100 ADC4 ADC1 1x

010101 ADC5 ADC1 1x

010110 ADC6 ADC1 1x

010111 ADC7 ADC1 1x

011000

N/A

011001

011010

011011

011100

011101

011110 1.1V (VBand Gap)

011111 0V (GND)

100000 ADC8

100001 ADC9

100010 ADC10

100011 ADC11

100100 ADC12

100101 ADC13

100110 N/A ADC1 ADC0 40x

100111 Temperature Sensor

Table 24-4. Input Channel and Gain Selections

MUX5..0(1) Single Ended Input Positive Differential Input Negative Differential Input Gain

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Note: 1. MUX5 bit make part of ADCSRB register.

24.9.2 ADC Control and Status Register A – ADCSRA

• Bit 7 – ADEN: ADC Enable

Writing this bit to one enables the ADC. By writing it to zero, the ADC is turned off. Turning the ADC off while a

conversion is in progress, will terminate this conversion.

101000

N/A

ADC4 ADC0 10x

101001 ADC5 ADC0 10x

101010 ADC6 ADC0 10x

101011 ADC7 ADC0 10x

101100 ADC4 ADC1 10x

101101 ADC5 ADC1 10x

101110 ADC6 ADC1 10x

101111 ADC7 ADC1 10x

110000 ADC4 ADC0 40x

110001 ADC5 ADC0 40x

110010 ADC6 ADC0 40x

110011 ADC7 ADC0 40x

110100

N/A

ADC4 ADC1 40x

110101 ADC5 ADC1 40x

110110 ADC6 ADC1 40x

110111 ADC7 ADC1 40x

111000 ADC4 ADC0 200x

111001 ADC5 ADC0 200x

111010 ADC6 ADC0 200x

111011 ADC7 ADC0 200x

111100 ADC4 ADC1 200x

111101 ADC5 ADC1 200x

111110 ADC6 ADC1 200x

111111 ADC7 ADC1 200x

Table 24-4. Input Channel and Gain Selections

MUX5..0(1) Single Ended Input Positive Differential Input Negative Differential Input Gain

Bit 76543210

ADEN ADSC ADATE ADIF ADIE ADPS2 ADPS1 ADPS0 ADCSRA

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value 0 0 0 0 0 0 0 0

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• Bit 6 – ADSC: ADC Start Conversion

In Single Conversion mode, write this bit to one to start each conversion. In Free Running mode, write this bit to

one to start the first conversion. The first conversion after ADSC has been written after the ADC has been

enabled, or if ADSC is written at the same time as the ADC is enabled, will take 25 ADC clock cycles instead of

the normal 13. This first conversion performs initialization of the ADC.

ADSC will read as one as long as a conversion is in progress. When the conversion is complete, it returns to

zero. Writing zero to this bit has no effect.

• Bit 5 – ADATE: ADC Auto Trigger Enable

When this bit is written to one, Auto Triggering of the ADC is enabled. The ADC will start a conversion on a

positive edge of the selected trigger signal. The trigger source is selected by setting the ADC Trigger Select bits,

ADTS in ADCSRB.

• Bit 4 – ADIF: ADC Interrupt Flag

This bit is set when an ADC conversion completes and the Data Registers are updated. The ADC Conversion

Complete Interrupt is executed if the ADIE bit and the I-bit in SREG are set. ADIF is cleared by hardware when

executing the corresponding interrupt handling vector. Alternatively, ADIF is cleared by writing a logical one to

the flag. Beware that if doing a Read-Modify-Write on ADCSRA, a pending interrupt can be disabled. This also

applies if the SBI and CBI instructions are used.

• Bit 3 – ADIE: ADC Interrupt Enable

When this bit is written to one and the I-bit in SREG is set, the ADC Conversion Complete Interrupt is activated.

• Bits 2:0 – ADPS2:0: ADC Prescaler Select Bits

These bits determine the division factor between the XTAL frequency and the input clock to the ADC.

24.9.3 The ADC Data Register – ADCL and ADCH

24.9.3.1ADLAR = 0

Table 24-5. ADC Prescaler Selections

ADPS2 ADPS1 ADPS0 Division Factor

0 0 0 2

0 0 1 2

0 1 0 4

0 1 1 8

100 16

101 32

110 64

111 128

Bit 151413121110 9 8

– – – – – – ADC9 ADC8 ADCH

ADC7 ADC6 ADC5 ADC4 ADC3 ADC2 ADC1 ADC0 ADCL

Bit 76543210

Read/WriteRRRRRRRR

RRRRRRRR

Initial Value00000000

00000000

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24.9.3.2ADLAR = 1

When an ADC conversion is complete, the result is found in these two registers. If differential channels are

used, the result is presented in two’s complement form.

When ADCL is read, the ADC Data Register is not updated until ADCH is read. Consequently, if the result is left

adjusted and no more than 8-bit precision (7 bit + sign bit for differential input channels) is required, it is

sufficient to read ADCH. Otherwise, ADCL must be read first, then ADCH.

The ADLAR bit in ADMUX, and the MUXn bits in ADMUX affect the way the result is read from the registers. If

ADLAR is set, the result is left adjusted. If ADLAR is cleared (default), the result is right adjusted.

• ADC9:0: ADC Conversion Result

These bits represent the result from the conversion, as detailed in “ADC Conversion Result” on page 309.

24.9.4 ADC Control and Status Register B – ADCSRB

• Bit 7 – ADHSM: ADC High Speed Mode

Writing this bit to one enables the ADC High Speed mode. This mode enables higher conversion rate at the

expense of higher power consumption.

• Bit 5 – MUX5: Analog Channel Additional Selection Bits

This bit make part of MUX5:0 bits of ADRCSRB and ADMUX register, that select the combination of analog

inputs connected to the ADC (including differential amplifier configuration).

• Bit 3:0 – ADTS3:0: ADC Auto Trigger Source

If ADATE in ADCSRA is written to one, the value of these bits selects which source will trigger an ADC

conversion. If ADATE is cleared, the ADTS3:0 settings will have no effect. A conversion will be triggered by the

rising edge of the selected interrupt flag. Note that switching from a trigger source that is cleared to a trigger

source that is set, will generate a positive edge on the trigger signal. If ADEN in ADCSRA is set, this will start a

conversion. Switching to Free Running mode (ADTS[3:0]=0) will not cause a trigger event, even if the ADC

Interrupt Flag is set.

Bit 151413121110 9 8

ADC9 ADC8 ADC7 ADC6 ADC5 ADC4 ADC3 ADC2 ADCH

ADC1ADC0––––––ADCL

Bit 76543210

Read/WriteRRRRRRRR

RRRRRRRR

Initial Value00000000

00000000

Bit 7 6543210

ADHSM ACME MUX5 –ADTS3 ADTS2 ADTS1 ADTS0 ADCSRB

Read/Write R/W R/W R R R R/W R/W R/W

Initial Value00000000

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24.9.5 Digital Input Disable Register 0 – DIDR0

• Bit 7:4, 1:0 – ADC7D..4D - ADC1D..0D: ADC7:4 - ADC1:0 Digital Input Disable

When this bit is written logic one, the digital input buffer on the corresponding ADC pin is disabled. The

corresponding PIN Register bit will always read as zero when this bit is set. When an analog signal is applied to

the ADC7..4 / ADC1..0 pin and the digital input from this pin is not needed, this bit should be written logic one to

reduce power consumption in the digital input buffer.

24.9.6 Digital Input Disable Register 2 – DIDR2

• Bit 5:0 – ADC13D..ADC8D: ADC13:8 Digital Input Disable

When this bit is written logic one, the digital input buffer on the corresponding ADC pin is disabled. The

corresponding PIN Register bit will always read as zero when this bit is set. When an analog signal is applied to

the ADC13..8 pin and the digital input from this pin is not needed, this bit should be written logic one to reduce

power consumption in the digital input buffer.

Table 24-6. ADC Auto Trigger Source Selections

ADTS3 ADTS2 ADTS1 ADTS0 Trigger Source

0 0 0 0 Free Running mode

0 0 0 1 Analog Comparator

0 0 1 0 External Interrupt Request 0

0 0 1 1 Timer/Counter0 Compare Match A

0 1 0 0 Timer/Counter0 Overflow

0 1 0 1 Timer/Counter1 Compare Match B

0 1 1 0 Timer/Counter1 Overflow

0 1 1 1 Timer/Counter1 Capture Event

1 0 0 0 Timer/Counter4 Overflow

1 0 0 1 Timer/Counter4 Compare Match A

1 0 1 0 Timer/Counter4 Compare Match B

1 0 1 1 Timer/Counter4 Compare Match D

Bit 76543210

ADC7D ADC6D ADC5D ADC4D - - ADC1D ADC0D DIDR0

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value00000000

Bit 76543210

- - ADC13D ADC12D ADC11D ADC10D ADC9D ADC8D DIDR2

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value00000000

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25. JTAG Interface and On-chip Debug System

25.0.1 Features

•JTAG (IEEE std. 1149.1 Compliant) Interfa ce

•Boundary-scan Capabilities According to the IEEE st d. 1149.1 (JTAG) Standard

•Debugger Access to:

– All Internal Peripheral Units

– Internal and Exte rna l RAM

– The Internal Register File

– Program Counter

– EEPROM and Flash Memories

•Extensive On-chip Debug Supp ort for Break Conditions, Including

– AVR Break Instruction

– Break on Change of Program Memory Flow

– Single Step Break

– Program Memory Break Points on Single Address or Address Range

– Data Memory Break Points on Single Address or Address Range

•Programming of Flash, EEPROM, Fuses, and Lock Bits through the JTAG Interface

•On-chip Debugging Supported by AVR Studio®

25.1 Overview

The AVR IEEE std. 1149.1 compliant JTAG interface can be used for:

Testing PCBs by using the JTAG Boundary-scan capability

Programming the non-volatile memories, Fuses and Lock bits

On-chip debugging

A brief description is given in the following sections. Detailed descriptions for Programming via the JTAG

interface, and using the Boundary-scan Chain can be found in the sections “Programming via the JTAG

Interface” on page 371 and “IEEE 1149.1 (JTAG) Boundary-scan” on page 325, respectively. The On-chip

Debug support is considered being private JTAG instructions, and distributed within Atmel and to selected third

party vendors only.

Figure 25-1 shows a block diagram of the JTAG interface and the On-chip Debug system. The TAP Controller is

a state machine controlled by the TCK and TMS signals. The TAP Controller selects either the JTAG Instruction

output. The Instruction Register holds JTAG instructions controlling the behavior of a Data Register.

The ID-Register, Bypass Register, and the Boundary-scan Chain are the Data Registers used for board-level

testing. The JTAG Programming Interface (actually consisting of several physical and virtual Data Registers) is

used for serial programming via the JTAG interface. The Internal Scan Chain and Break Point Scan Chain are

used for On-chip debugging only.

25.2 Test Access Port – TAP

The JTAG interface is accessed through four of the AVR’s pins. In JTAG terminology, these pins constitute the

Test Access Port – TAP. These pins are:

TMS: Test mode select. This pin is used for navigating through the TAP-controller state machine.

TCK: Test Clock. JTAG operation is synchronous to TCK.

TDI: Test Data In. Serial input data to be shifted in to the Instruction Register or Data Register (Scan

Chains).

TDO: Test Data Out. Serial output data from Instruction Register or Data Register.

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The IEEE std. 1149.1 also specifies an optional TAP signal; TRST – Test ReSeT – which is not provided.

When the JTAGEN Fuse is unprogrammed, these four TAP pins are normal port pins, and the TAP controller is

in reset. When programmed, the input TAP signals are internally pulled high and the JTAG is enabled for

Boundary-scan and programming. The device is shipped with this fuse programmed.

For the On-chip Debug system, in addition to the JTAG interface pins, the RESET pin is monitored by the

debugger to be able to detect external reset sources. The debugger can also pull the RESET pin low to reset the

whole system, assuming only open collectors on the reset line are used in the application.

Figure 25-1. Block Diagram

TAP

CONTROLLER

TDI

TDO

TCK

TMS

FLASH

MEMORY

AVR CPU

DIGITAL

PERIPHERAL

UNITS

JTAG / AVR CORE

COMMUNICATION

INTERFACE

BREAKPOINT

UNIT FLOW CONTROL

UNIT

OCD STATUS

AND CONTROL

INTERNAL

SCAN

CHAIN

INSTRUCTION

BYPASS

JTAG PROGRAMMING

INTERFACE

Instruction

Address

Data

BREAKPOINT

SCAN CHAIN

ADDRESS

DECODER

ANALOG

PERIPHERIAL

UNITS

I/O PORT 0

I/O PORT n

BOUNDARY SCAN CHAIN

Analog inputs

Control & Clock lines

DEVICE BOUNDARY

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Figure 25-2. TAP Controller State Diagram

25.3 TAP Controller

The TAP controller is a 16-state finite state machine that controls the operation of the Boundary-scan circuitry,

JTAG programming circuitry, or On-chip Debug system. The state transitions depicted in Figure 25-2 depend on

the signal present on TMS (shown adjacent to each state transition) at the time of the rising edge at TCK. The

initial state after a Power-on Reset is Test-Logic-Reset.

As a definition in this document, the LSB is shifted in and out first for all Shift Registers.

Assuming Run-Test/Idle is the present state, a typical scenario for using the JTAG interface is:

At the TMS input, apply the sequence 1, 1, 0, 0 at the rising edges of TCK to enter the Shift Instruction

Instruction Register from the TDI input at the rising edge of TCK. The TMS input must be held low during

input of the three LSBs in order to remain in the Shift-IR state. The MSB of the instruction is shifted in

when this state is left by setting TMS high. While the instruction is shifted in from the TDI pin, the captured

IR-state 0x01 is shifted out on the TDO pin. The JTAG Instruction selects a particular Data Register as

path between TDI and TDO and controls the circuitry surrounding the selected Data Register.

Apply the TMS sequence 1, 1, 0 to re-enter the Run-Test/Idle state. The instruction is latched onto the

parallel output from the Shift Register path in the Update-IR state. The Exit-IR, Pause-IR, and Exit2-IR

states are only used for navigating the state machine.

Test-Logic-Reset

Run-Test/Idle

Shift-DR

Exit1-DR

Pause-DR

Exit2-DR

Update-DR

Select-IR Scan

Capture-IR

Shift-IR

Exit1-IR

Pause-IR

Exit2-IR

Update-IR

Select-DR Scan

Capture-DR

011 1

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At the TMS input, apply the sequence 1, 0, 0 at the rising edges of TCK to enter the Shift Data Register –

Shift-DR state. While in this state, upload the selected Data Register (selected by the present JTAG

instruction in the JTAG Instruction Register) from the TDI input at the rising edge of TCK. In order to

remain in the Shift-DR state, the TMS input must be held low during input of all bits except the MSB. The

MSB of the data is shifted in when this state is left by setting TMS high. While the Data Register is shifted

in from the TDI pin, the parallel inputs to the Data Register captured in the Capture-DR state is shifted out

on the TDO pin.

Apply the TMS sequence 1, 1, 0 to re-enter the Run-Test/Idle state. If the selected Data Register has a

latched parallel-output, the latching takes place in the Update-DR state. The Exit-DR, Pause-DR, and

Exit2-DR states are only used for navigating the state machine.

As shown in the state diagram, the Run-Test/Idle state need not be entered between selecting JTAG instruction

and using Data Registers, and some JTAG instructions may select certain functions to be performed in the Run-

Test/Idle, making it unsuitable as an Idle state.

Note: Independent of the initial state of the TAP Controller, the Test-Logic-Reset state can always be entered by holding

TMS high for five TCK clock periods.

For detailed information on the JTAG specification, refer to the literature listed in “Bibliography” on page 324.

25.4 Using the Boundary-scan Chain

A complete description of the Boundary-scan capabilities are given in the section “IEEE 1149.1 (JTAG)

Boundary-scan” on page 325.

25.5 Using the On-chip Debug System

As shown in Figure 25-1 on page 320, the hardware support for On-chip Debugging consists mainly of

A scan chain on the interface between the internal AVR CPU and the internal peripheral units

Break Point unit

Communication interface between the CPU and JTAG system

All read or modify/write operations needed for implementing the Debugger are done by applying AVR

instructions via the internal AVR CPU Scan Chain. The CPU sends the result to an I/O memory mapped location

which is part of the communication interface between the CPU and the JTAG system.

The Break Point Unit implements Break on Change of Program Flow, Single Step Break, two Program Memory

Break Points, and two combined Break Points. Together, the four Break Points can be configured as either:

Four single Program Memory Break Points

Three Single Program Memory Break Point + one single Data Memory Break Point

Two Single Program Memory Break Points + two single Data Memory Break Points

Two Single Program Memory Break Points + one Program Memory Break Point with mask (“range Break

Point”)

Two Single Program Memory Break Points + 1 Data Memory Break Point with mask (“range Break Point”).

A debugger, like the AVR Studio, may however use one or more of these resources for its internal purpose,

leaving less flexibility to the end-user.

A list of the On-chip Debug specific JTAG instructions is given in “On-chip Debug Specific JTAG Instructions” on

page 323.

The JTAGEN Fuse must be programmed to enable the JTAG Test Access Port. In addition, the OCDEN Fuse

must be programmed and no Lock bits must be set for the On-chip debug system to work. As a security feature,

the On-chip debug system is disabled when either of the LB1 or LB2 Lock bits are set. Otherwise, the On-chip

debug system would have provided a back-door into a secured device.

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The AVR Studio enables the user to fully control execution of programs on an AVR device with On-chip Debug

capability, AVR In-Circuit Emulator, or the built-in AVR Instruction Set Simulator. AVR Studio® supports source

level execution of Assembly programs assembled with Atmel Corporation’s AVR Assembler and C programs

compiled with third party vendors’ compilers.

AVR Studio runs under Microsoft® Windows® 95/98/2000 and Microsoft Windows NT.

For a full description of the AVR Studio, refer to the AVR Studio User Guide. Only highlights are presented in

this document.

All necessary execution commands are available in AVR Studio, both on source level and on disassembly level.

The user can execute the program, single step through the code either by tracing into or stepping over

functions, step out of functions, place the cursor on a statement and execute until the statement is reached, stop

the execution, and reset the execution target. In addition, the user can have an unlimited number of code Break

Points (using the BREAK instruction) and up to two data memory Break Points, alternatively combined as a

mask (range) Break Point.

25.6 On-chip Debug Specific JTAG Instructions

The On-chip debug support is considered being private JTAG instructions, and distributed within Atmel and to

selected third party vendors only. Instruction opcodes are listed for reference.

25.6.1 PRIVATE0; 0x8

Private JTAG instruction for accessing On-chip debug system.

25.6.2 PRIVATE1; 0x9

Private JTAG instruction for accessing On-chip debug system.

25.6.3 PRIVATE2; 0xA

Private JTAG instruction for accessing On-chip debug system.

25.6.4 PRIVATE3; 0xB

Private JTAG instruction for accessing On-chip debug system.

25.7 On-chip Debug Related Register in I/O Memory

25.7.1 On-chip Debug Register – OCDR

The OCDR Register provides a communication channel from the running program in the microcontroller to the

debugger. The CPU can transfer a byte to the debugger by writing to this location. At the same time, an internal

flag; I/O Debug Register Dirty – IDRD – is set to indicate to the debugger that the register has been written.

When the CPU reads the OCDR Register the seven LSB will be from the OCDR Register, while the MSB is the

IDRD bit. The debugger clears the IDRD bit when it has read the information.

In some AVR devices, this register is shared with a standard I/O location. In this case, the OCDR Register can

only be accessed if the OCDEN Fuse is programmed, and the debugger enables access to the OCDR Register.

In all other cases, the standard I/O location is accessed.

Bit 7 6543210

MSB/IDRD LSB OCDR

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value0 0000000

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Refer to the debugger documentation for further information on how to use this register.

25.8 Using the JTAG Programming Capabilities

Programming of AVR parts via JTAG is performed via the 4-pin JTAG port, TCK, TMS, TDI, and TDO. These

are the only pins that need to be controlled/observed to perform JTAG programming (in addition to power pins).

It is not required to apply 12V externally. The JTAGEN Fuse must be programmed and the JTD bit in the

MCUCR Register must be cleared to enable the JTAG Test Access Port.

The JTAG programming capability supports:

Flash programming and verifying

EEPROM programming and verifying

Fuse programming and verifying

Lock bit programming and verifying

The Lock bit security is exactly as in parallel programming mode. If the Lock bits LB1 or LB2 are programmed,

the OCDEN Fuse cannot be programmed unless first doing a chip erase. This is a security feature that ensures

no back-door exists for reading out the content of a secured device.

The details on programming through the JTAG interface and programming specific JTAG instructions are given

in the section “Programming via the JTAG Interface” on page 371.

25.9 Bibliography

For more information about general Boundary-scan, the following literature can be consulted:

IEEE: IEEE Std. 1149.1-1990. IEEE Standard Test Access Port and Boundary-scan Architecture, IEEE,

1993.

Colin Maunder: The Board Designers Guide to Testable Logic Circuits, Addison-Wesley, 1992.

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26. IEEE 1149.1 (JTAG) Boundary-scan

26.1 Features

•JTAG (IEEE std. 1149.1 compliant) Inter face

•Boundary-scan Capabilities According to the JTAG Standard

•Full Scan of all Port Functions as well as Analog Circuitry having Off-chip Connections

•Supports the Optional IDCODE Instruction

•Additional Public AVR_RESET Instruction to Reset the AVR

26.2 System Overview

The Boundary-scan chain has the capability of driving and observing the logic levels on the digital I/O pins, as

well as the boundary between digital and analog logic for analog circuitry having off-chip connections. At system

level, all ICs having JTAG capabilities are connected serially by the TDI/TDO signals to form a long Shift

values received from other devices. The controller compares the received data with the expected result. In this

way, Boundary-scan provides a mechanism for testing interconnections and integrity of components on Printed

Circuits Boards by using the four TAP signals only.

The four IEEE 1149.1 defined mandatory JTAG instructions IDCODE, BYPASS, SAMPLE/PRELOAD, and

EXTEST, as well as the AVR specific public JTAG instruction AVR_RESET can be used for testing the Printed

Circuit Board. Initial scanning of the Data Register path will show the ID-Code of the device, since IDCODE is

the default JTAG instruction. It may be desirable to have the AVR device in reset during test mode. If not reset,

inputs to the device may be determined by the scan operations, and the internal software may be in an

undetermined state when exiting the test mode. Entering reset, the outputs of any port pin will instantly enter the

high impedance state, making the HIGHZ instruction redundant. If needed, the BYPASS instruction can be

issued to make the shortest possible scan chain through the device. The device can be set in the reset state

either by pulling the external RESET pin low, or issuing the AVR_RESET instruction with appropriate setting of

the Reset Data Register.

The EXTEST instruction is used for sampling external pins and loading output pins with data. The data from the

output latch will be driven out on the pins as soon as the EXTEST instruction is loaded into the JTAG IR-

avoid damaging the board when issuing the EXTEST instruction for the first time. SAMPLE/PRELOAD can also

be used for taking a snapshot of the external pins during normal operation of the part.

The JTAGEN Fuse must be programmed and the JTD bit in the I/O Register MCUCR must be cleared to enable

the JTAG Test Access Port.

When using the JTAG interface for Boundary-scan, using a JTAG TCK clock frequency higher than the internal

chip frequency is possible. The chip clock is not required to run.

26.3 Data Registers

The Data Registers relevant for Boundary-scan operations are:

Bypass Register

Device Identification Register

Reset Register

Boundary-scan Chain

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26.3.1 Bypass Regist er

The Bypass Register consists of a single Shift Register stage. When the Bypass Register is selected as path

between TDI and TDO, the register is reset to 0 when leaving the Capture-DR controller state. The Bypass

26.3.2 Device Identification Register

Figure 26-1 shows the structure of the Device Identification Register.

Figure 26-1. The Format of the Device Identification Register

26.3.2.1Version

Version is a 4-bit number identifying the revision of the component. The JTAG version number follows the

revision of the device. Revision A is 0x0, revision B is 0x1 and so on.

26.3.2.2Part Number

The part number is a 16-bit code identifying the component. The JTAG Part Number for

ATmega16U4/ATmega32U4 is listed in this table.

26.3.2.3Manufacturer ID

The Manufacturer ID is a 11-bit code identifying the manufacturer. The JTAG manufacturer ID for Atmel is listed

below.

26.3.3 Reset Regist er

The Reset Register is a test Data Register used to reset the part. Since the AVR tri-states Port Pins when reset,

the Reset Register can also replace the function of the un-implemented optional JTAG instruction HIGHZ.

A high value in the Reset Register corresponds to pulling the external Reset low. The part is reset as long as

there is a high value present in the Reset Register. Depending on the fuse settings for the clock options, the part

will remain reset for a reset time-out period (refer to “Clock Sources” on page 28) after releasing the Reset

Figure 26-2.

MSB LSB

Bit 31 28 27 12 11 1 0

Device ID Version Part Number Manufacturer ID 1

4 bits 16 bits 11 bits 1-bit

Part Number JTAG Part Number (Hex)

AVR USB 0x9782

Manufacturer JTAG Manufacturer ID (Hex)

Atmel 0x01F

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Figure 26-2. Reset Register

26.3.4 Boundary-scan Chain

The Boundary-scan Chain has the capability of driving and observing the logic levels on the digital I/O pins, as

well as the boundary between digital and analog logic for analog circuitry having off-chip connections.

See “Boundary-scan Chain” on page 329 for a complete description.

26.4 Boundary-scan Specific JTAG Instructions

The Instruction Register is 4-bit wide, supporting up to 16 instructions. Listed below are the JTAG instructions

useful for Boundary-scan operation. Note that the optional HIGHZ instruction is not implemented, but all outputs

with tri-state capability can be set in high-impedance state by using the AVR_RESET instruction, since the initial

state for all port pins is tri-state.

As a definition in this datasheet, the LSB is shifted in and out first for all Shift Registers.

The OPCODE for each instruction is shown behind the instruction name in hex format. The text describes which

Data Register is selected as path between TDI and TDO for each instruction.

26.4.1 EXTEST; 0x0

Mandatory JTAG instruction for selecting the Boundary-scan Chain as Data Register for testing circuitry

external to the AVR package. For port-pins, Pull-up Disable, Output Control, Output Data, and Input Data are all

accessible in the scan chain. For Analog circuits having off-chip connections, the interface between the analog

and the digital logic is in the scan chain. The contents of the latched outputs of the Boundary-scan chain is

driven out as soon as the JTAG IR-Register is loaded with the EXTEST instruction.

The active states are:

Capture-DR: Data on the external pins are sampled into the Boundary-scan Chain

Shift-DR: The Internal Scan Chain is shifted by the TCK input

Update-DR: Data from the scan chain is applied to output pins

26.4.2 IDCODE; 0x1

Optional JTAG instruction selecting the 32-bit ID-Register as Data Register. The ID-Register consists of a

version number, a device number and the manufacturer code chosen by JEDEC. This is the default instruction

after power-up.

The active states are:

Capture-DR: Data in the IDCODE Register is sampled into the Boundary-scan Chain

Shift-DR: The IDCODE scan chain is shifted by the TCK input

From

TDI

ClockDR · AVR_RESET

TDO

From Other Internal and

External Reset Sources

Internal reset

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26.4.3 SAMPLE_PREL OA D ; 0x 2

Mandatory JTAG instruction for pre-loading the output latches and taking a snap-shot of the input/output pins

without affecting the system operation. However, the output latches are not connected to the pins. The

Boundary-scan Chain is selected as Data Register.

The active states are:

Capture-DR: Data on the external pins are sampled into the Boundary-scan Chain

Shift-DR: The Boundary-scan Chain is shifted by the TCK input

Update-DR: Data from the Boundary-scan chain is applied to the output latches. However, the output

latches are not connected to the pins.

26.4.4 AVR_RESET; 0xC

The AVR specific public JTAG instruction for forcing the AVR device into the Reset mode or releasing the JTAG

reset source. The TAP controller is not reset by this instruction. The one bit Reset Register is selected as Data

this chain is not latched.

The active states are:

Shift-DR: The Reset Register is shifted by the TCK input

26.4.5 BYPASS; 0xF

Mandatory JTAG instruction selecting the Bypass Register for Data Register.

The active states are:

Capture-DR: Loads a logic “0” into the Bypass Register

Shift-DR: The Bypass Register cell between TDI and TDO is shifted

26.5 Boundary-scan Related Register in I/O Memory

26.5.1 MCU Control Register – MCUCR

The MCU Control Register contains control bits for general MCU functions.

• Bits 7 – JTD: JTAG Interface Disable

When this bit is zero, the JTAG interface is enabled if the JTAGEN Fuse is programmed. If this bit is one, the

JTAG interface is disabled. In order to avoid unintentional disabling or enabling of the JTAG interface, a timed

sequence must be followed when changing this bit: The application software must write this bit to the desired

value twice within four cycles to change its value. Note that this bit must not be altered when using the On-chip

Debug system.

26.5.2 MCU Status Register – MCUSR

The MCU Status Register provides information on which reset source caused an MCU reset.

Bit 76543210

JTD ––PUD – – IVSEL IVCE MCUCR

Read/Write R/W R R R/W R R R/W R/W

Initial Value00000000

Bit 76543210

–––JTRFWDRF BORF EXTRF PORF MCUSR

Read/WriteRRRR/WR/WR/WR/WR/W

Initial Value000See Bit Description

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• Bit 4 – JTRF: JTAG Reset Flag

This bit is set if a reset is being caused by a logic one in the JTAG Reset Register selected by the JTAG

instruction AVR_RESET. This bit is reset by a Power-on Reset, or by writing a logic zero to the flag.

26.6 Boundary-scan Chain

The Boundary-scan chain has the capability of driving and observing the logic levels on the digital I/O pins, as

well as the boundary between digital and analog logic for analog circuitry having off-chip connection.

26.6.1 Scanning the Digital Port Pins

Figure 26-3 on page 330 shows the Boundary-scan Cell for a bi-directional port pin. The pull-up function is

disabled during Boundary-scan when the JTAG IC contains EXTEST or SAMPLE_PRELOAD. The cell consists

of a bi-directional pin cell that combines the three signals Output Control - OCxn, Output Data - ODxn, and Input

Data - IDxn, into only a two-stage Shift Register. The port and pin indexes are not used in the following

description.

The Boundary-scan logic is not included in the figures in the datasheet. Figure 26-4 on page 331 shows a

simple digital port pin as described in the section “I/O-Ports” on page 67. The Boundary-scan details from Figure

26-3 on page 330 replaces the dashed box in Figure 26-4 on page 331.

When no alternate port function is present, the Input Data - ID - corresponds to the PINxn Register value (but ID

has no synchronizer), Output Data corresponds to the PORT Register, Output Control corresponds to the Data

Direction - DD Register, and the Pull-up Enable - PUExn - corresponds to logic expression PUD · DDxn ·

PORTxn.

Digital alternate port functions are connected outside the dotted box in Figure 26-4 on page 331 to make the

scan chain read the actual pin value. For analog function, there is a direct connection from the external pin to

the analog circuit. There is no scan chain on the interface between the digital and the analog circuitry, but some

digital control signal to analog circuitry are turned off to avoid driving contention on the pads.

When JTAG IR contains EXTEST or SAMPLE_PRELOAD the clock is not sent out on the port pins even if the

CKOUT fuse is programmed. Even though the clock is output when the JTAG IR contains SAMPLE_PRELOAD,

the clock is not sampled by the boundary scan.

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Figure 26-3. Boundary-scan Cell for bi-directional Port Pin with Pull-up Function

DQ DQ

Port Pin (PXn)

Vcc

EXTEST

To Next Cell

ShiftDR

Output Control (OC)

Output Data (OD)

Input Data (ID)

From Last Cell UpdateDRClockDR

FF1 LD1

LD0FF0

Pull-up Enable (PUE)

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Figure 26-4. General Port Pin Schematic Diagram

26.6.2 Scanning the RESET Pin

The RESET pin accepts 5V active low logic for standard reset operation, and 12V active high logic for High

Voltage Parallel programming. An observe-only cell as shown in Figure 26-5 is inserted for the 5V reset signal.

Figure 26-5. Observe-only Cell

CLK

RPx

RRx

WRx

RDx

WDx

PUD

SYNCHRONIZER

WDx: WRITE DDRx

WRx: WRITE PORTx

RRx: READ PORTx REGISTER

RPx: READ PORTx PIN

PUD: PULLUP DISABLE

CLK : I/O CLOCK

RDx: READ DDRx

RESET

CLR

PORTxn

CLR

DDxn

PINxn

DATA BUS

SLEEP

SLEEP: SLEEP CONTROL

Pxn

I/O

See Boundary-scan

Description for Details!

PUExn

OCxn

ODxn

IDxn

PUExn: PULLUP ENABLE for pin Pxn

OCxn: OUTPUT CONTROL for pin Pxn

ODxn: OUTPUT DATA to pin Pxn

IDxn: INPUT DATA from pin Pxn

From

Cell

ClockDR

ShiftDR

Cell

From System Pin To System Logic

FF1

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26.7 Boundary-scan Order

The table below shows the Scan order between TDI and TDO when the Boundary-scan chain is selected as

data path. Bit 0 is the LSB; the first bit scanned in, and the first bit scanned out. The scan order follows the pin-

out order as far as possible. Exceptions from the rules are the Scan chains for the analog circuits, which

constitute the most significant bits of the scan chain regardless of which physical pin they are connected to. In

Figure 26-3 on page 330, PXn. Data corresponds to FF0, PXn. Control corresponds to FF1, PXn. Bit 4, 5, 6, and

7 of Port F is not in the scan chain, since these pins constitute the TAP pins when the JTAG is enabled. The

USB pads are not included in the boundary-scan.

Table 26-1. ATmega16U4/ATmega32U4 Boundary-scan Order

Bit Number Signal Name Module

88 PE6.Data

Port E

87 PE6.Control

86 Reserved

85 Reserved

84 Reserved

83 Reserved

82 PB0.Data

Port B

81 PB0.Control

80 PB1.Data

79 PB1.Control

78 PB2.Data

77 PB2.Control

76 PB3.Data

75 PB3.Control

74 PB4.Data

73 PB4.Control

72 PB5.Data

71 PB5.Control

70 PB6.Data

69 PB6.Control

68 PB7.Data

67 PB7.Control

66 Reserved

PORTE

65 Reserved

64 Reserved

63 Reserved

62 RSTT Reset Logic (Observe Only)

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61 PD0.Data

Port D

60 PD0.Control

59 PD1.Data

58 PD1.Control

57 PD2.Data

56 PD2.Control

55 PD3.Data

54 PD3.Control

53 PD4.Data

52 PD4.Control

51 PD5.Data

50 PD5.Control

49 PD6.Data

48 PD6.Control

47 PD7.Data

46 PD7.Control

45 Reserved

Port E

44 Reserved

43 Reserved

42 Reserved

Table 26-1. ATmega16U4/ATmega32U4 Boundary-scan Order

Bit Number Signal Name Module

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41 Reserved

Reserved

40 Reserved

39 Reserved

38 Reserved

37 Reserved

36 Reserved

35 Reserved

34 Reserved

33 Reserved

32 Reserved

31 Reserved

30 Reserved

29 Reserved

28 Reserved

27 Reserved

26 Reserved

25 PE2.Data

Port E

24 PE2.Control

Table 26-1. ATmega16U4/ATmega32U4 Boundary-scan Order

Bit Number Signal Name Module

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26.8 Boundary-scan Description Language Files

Boundary-scan Description Language (BSDL) files describe Boundary-scan capable devices in a standard

format used by automated test-generation software. The order and function of bits in the Boundary-scan Data

23 Reserved

Reserved

22 Reserved

21 Reserved

20 Reserved

19 Reserved

18 Reserved

17 Reserved

16 Reserved

15 Reserved

14 Reserved

13 Reserved

12 Reserved

11 Reserved

10 Reserved

9Reserved

8Reserved

7Reserved

Port F

6Reserved

5Reserved

4Reserved

3PF1.Data

2PF1.Control

1PF0.Data

0PF0.Control

Table 26-1. ATmega16U4/ATmega32U4 Boundary-scan Order

Bit Number Signal Name Module

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27. Boot Loader Support – Read-While-Write Self-Programming

The Boot Loader Support provides a real Read-While-Write Self-Programming mechanism for downloading and

uploading program code by the MCU itself. This feature allows flexible application software updates controlled

by the MCU using a Flash-resident Boot Loader program. The Boot Loader program can use any available data

interface and associated protocol to read code and write (program) that code into the Flash memory, or read the

code from the program memory. The program code within the Boot Loader section has the capability to write

into the entire Flash, including the Boot Loader memory. The Boot Loader can thus even modify itself, and it can

also erase itself from the code if the feature is not needed anymore. The size of the Boot Loader memory is

configurable with fuses and the Boot Loader has two separate sets of Boot Lock bits which can be set

independently. This gives the user a unique flexibility to select different levels of protection.

General information on SPM and ELPM is provided in “AVR CPU Core” on page 9.

27.1 Boot Loader Features

•Read-While-Write Self-Programming

•Flexible Boot Memory Size

•High Security (Separate Boot Lock Bits for a Flexible Protection)

•Sep arate Fuse to Select Reset Vector

•Optimized Page(1) Size

•Code Efficient Algorithm

•Efficient Read-Modify-Write Support

Note: 1. A page is a section in the Flash consisting of several bytes (see Table 28-11 on page 359) used during

programming. The page organization does not affect normal operation.

27.2 Application and Boot Loader Flash Sections

The Flash memory is organized in two main sections, the Application section and the Boot Loader section (see

Figure 27-2 on page 338). The size of the different sections is configured by the BOOTSZ Fuses as shown in

Table 27-8 on page 349 and Figure 27-2 on page 338. These two sections can have different level of protection

since they have different sets of Lock bits.

27.2.1 Application Section

The Application section is the section of the Flash that is used for storing the application code. The protection

level for the Application section can be selected by the application Boot Lock bits (Boot Lock bits 0). The

Application section can never store any Boot Loader code since the SPM instruction is disabled when executed

from the Application section.

Refer to Table 2 7- 2 on page 339 .

27.2.2 BLS – Boot Loader Section

While the Application section is used for storing the application code, the The Boot Loader software must be

located in the BLS since the SPM instruction can initiate a programming when executing from the BLS only. The

SPM instruction can access the entire Flash, including the BLS itself. The protection level for the Boot Loader

section can be selected by the Boot Loader Lock bits (Boot Lock bits 1). Refer to Table 27-3 on page 339.

27.3 Read-While-W rite and No Read-While-Write Flash Sections

Whether the CPU supports Read-While-Write or if the CPU is halted during a Boot Loader software update is

dependent on which address that is being programmed. In addition to the two sections that are configurable by

the BOOTSZ Fuses as described above, the Flash is also divided into two fixed sections, the Read-While-Write

(RWW) section and the No Read-While-Write (NRWW) section.

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The limit between the RWW- and NRWW sections is given in Table 27-1 on page 337 and Figure 27-1 on page

338. The main difference between the two sections is:

When erasing or writing a page located inside the RWW section, the NRWW section can be read during

the operation

When erasing or writing a page located inside the NRWW section, the CPU is halted during the entire

operation

Note that the user software can never read any code that is located inside the RWW section during a Boot

Loader software operation. The syntax “Read-While-Write section” refers to which section that is being

programmed (erased or written), not which section that actually is being read during a Boot Loader software

update.

27.3.1 RWW – Read-While-Write Section

If a Boot Loader software update is programming a page inside the RWW section, it is possible to read code

from the Flash, but only code that is located in the NRWW section. During an on-going programming, the

software must ensure that the RWW section never is being read. If the user software is trying to read code that

is located inside the RWW section (i.e., by load program memory, call, or jump instructions or an interrupt)

during programming, the software might end up in an unknown state. To avoid this, the interrupts should either

be disabled or moved to the Boot Loader section. The Boot Loader section is always located in the NRWW

section. The RWW Section Busy bit (RWWSB) in the Store Program Memory Control and Status Register

(SPMCSR) will be read as logical one as long as the RWW section is blocked for reading. After a programming

is completed, the RWWSB must be cleared by software before reading code located in the RWW section.

Refer to “Store Program Memory Control and Status Register – SPMCSR” on page 341 for details on how to

clear RWWSB.

27.3.2 NRWW – No Read-While-Write Section

The code located in the NRWW section can be read when the Boot Loader software is updating a page in the

RWW section. When the Boot Loader code updates the NRWW section, the CPU is halted during the entire

Page Erase or Page Write operation.

Table 27-1. Read-While-Write Features

Which Section does the Z-pointer

Address During th e Prog ramming ?

Which Section Can

be Read During

Programming? Is the CPU

Halted? Read-While-Write

Supported?

RWW Section NRWW Section No Yes

NRWW Section None Yes No

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Figure 27-1. Read-While-Write vs. No Read-While-Write

Figure 27-2. Memory Sections

Note: 1. The parameters in Figure 27-2 are given in Table 27-8 on page 349.

Read-While-Write

(RWW) Section

No Read-While-Write

(NRWW) Section

Z-pointer

Addresses RWW

Section

Z-pointer

Addresses NRWW

Section

CPU is Halted

During the Operation

Code Located in

NRWW Section

Can be Read During

the Operation

0x0000

Flashend

Program Memory

BOOTSZ = '11'

Application Flash Section

Boot Loader Flash Section Flashend

Program Memory

BOOTSZ = '10'

0x0000

Program Memory

BOOTSZ = '01'

Program Memory

BOOTSZ = '00'

Application Flash Section

Boot Loader Flash Section

0x0000

Flashend

Application Flash Section

Flashend

End RWW

Start NRWW

Application Flash Section

Boot Loader Flash Section

End RWW

Start NRWW

End RWW

Start NRWW

0x0000

End RWW, End Application

Start NRWW, Start Boot Loader

Application Flash SectionApplication Flash Section

Application Flash Section

Read-While-Write SectionNo Read-While-Write Section Read-While-Write SectionNo Read-While-Write Section

Read-While-Write SectionNo Read-While-Write SectionRead-While-Write SectionNo Read-While-Write Section

End Application

Start Boot Loader

End Application

Start Boot Loader

End Application

Start Boot Loader

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27.4 Boot Loader Lock Bits

If no Boot Loader capability is needed, the entire Flash is available for application code. The Boot Loader has

two separate sets of Boot Lock bits which can be set independently. This gives the user a unique flexibility to

select different levels of protection.

The user can select:

To protect the entire Flash from a software update by the MCU

To protect only the Boot Loader Flash section from a software update by the MCU

To protect only the Application Flash section from a software update by the MCU

Allow software update in the entire Flash

See the following two tables for further details. The Boot Lock bits can be set by software and in Serial or in

Parallel Programming mode. They can only be cleared by a Chip Erase command only. The general Write Lock

(Lock Bit mode 2) does not control the programming of the Flash memory by SPM instruction. Similarly, the

general Read/Write Lock (Lock Bit mode 1) does not control reading nor writing by (E)LPM/SPM, if it is

attempted.

Note: 1. “1” means unprogrammed, “0” means programmed

Note: 1. “1” means unprogrammed, “0” means programmed.

Table 27-2. Boot Lock Bit0 Protection Modes (Application Section)(1)

BLB0 Mode BLB02 BLB01 Protection

1 1 1 No restrictions for SPM or (E)LPM accessing the Application section

2 1 0 SPM is not allowed to write to the Application section

3 0 0

SPM is not allowed to write to the Application section, and (E)LPM executing from

the Boot Loader section is not allowed to read from the Application section. If

Interrupt Vectors are placed in the Boot Loader section, interrupts are disabled while

executing from the Application section.

4 0 1

(E)LPM executing from the Boot Loader section is not allowed to read from the

Application section. If Interrupt Vectors are placed in the Boot Loader section,

interrupts are disabled while executing from the Application section.

Table 27-3. Boot Lock Bit1 Protection Modes (Boot Load er Section)(1)

BLB1 Mode BLB12 BLB11 Protection

1 1 1 No restrictions for SPM or (E)LPM accessing the Boot Loader section

2 1 0 SPM is not allowed to write to the Boot Loader section

3 0 0

SPM is not allowed to write to the Boot Loader section, and (E)LPM executing from

the Application section is not allowed to read from the Boot Loader section. If

Interrupt Vectors are placed in the Application section, interrupts are disabled while

executing from the Boot Loader section.

4 0 1

(E)LPM executing from the Application section is not allowed to read from the Boot

Loader section. If Interrupt Vectors are placed in the Application section, interrupts

are disabled while executing from the Boot Loader section.

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27.5 Entering the Boot Loader Program

The bootloader can be executed with three different conditions:

27.5.1 Regular Application Conditions

A jump or call from the application program. This may be initiated by a trigger such as a command received via

USART, SPI, or USB.

27.5.2 Boot Reset Fuse

The Boot Reset Fuse (BOOTRST) can be programmed so that the Reset Vector is pointing to the Boot Flash

start address after a reset. In this case, the Boot Loader is started after a reset. After the application code is

loaded, the program can start executing the application code. Note that the fuses cannot be changed by the

MCU itself. This means that once the Boot Reset Fuse is programmed, the Reset Vector will always point to the

Boot Loader Reset and the fuse can only be changed through the serial or parallel programming interface.

Note: 1. “1” means unprogrammed, “0” means programmed.

27.5.3 External Hardware conditions

The Hardware Boot Enable Fuse (HWBE) can be programmed (see the table below) so that upon special

hardware conditions under reset, the bootloader execution is forced after reset.

Note: 1. “1” means unprogrammed, “0” means programmed.

When the HWBE fuse is enable the ALE/HWB pin is configured as input during reset and sampled during reset

rising edge. When ALE/HWB pin is ‘0’ during reset rising edge, the reset vector will be set as the Boot Loader

Reset address and the Boot Loader will be executed (See Figure 27-3).

Table 27-4. Boot Reset Fuse(1)

BOOTRST Reset Address

1Reset Vector = Application Reset (address 0x0000)

0Reset Vector = Boot Loader Reset (see Table 27-8 on page 349)

Table 27-5. Hardware Boot Enable Fuse(1)

HWBE Reset Address

1ALE/HWB pin can not be used to force Boot Loader execution after reset

0ALE/HWB pin is used during reset to force bootloader execution after reset

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Figure 27-3. Boot Process Description

27.5.4 Store Program Memory Control and Status Register – SPMCSR

The Store Program Memory Control and Status Register contains the control bits needed to control the Boot

Loader operations.

• Bit 7 – SPMIE: SPM Interrupt Enable

When the SPMIE bit is written to one, and the I-bit in the Status Register is set (one), the SPM ready interrupt

will be enabled. The SPM ready Interrupt will be executed as long as the SPMEN bit in the SPMCSR Register is

cleared.

• Bit 6 – RWWSB: Read-While-W rite Section Busy

When a Self-Programming (Page Erase or Page Write) operation to the RWW section is initiated, the RWWSB

will be set (one) by hardware. When the RWWSB bit is set, the RWW section cannot be accessed. The

RWWSB bit will be cleared if the RWWSRE bit is written to one after a Self-Programming operation is

completed. Alternatively the RWWSB bit will automatically be cleared if a page load operation is initiated.

• Bit 5 – SIGRD: Signature Row Read

If this bit is written to one at the same time as SPMEN, the next LPM instruction within three clock cycles will

read a byte from the signature row into the destination register. see “Reading the Signature Row from Software”

HWBE

BOOTRST ?

Ext. Hardware

Conditions ?

Reset Vector = Application Reset Reset Vector =Boot Lhoader Reset

RESET

ALE/HWB

tSHRH tHHRH

Bit 7 6 5 4 3 2 1 0

SPMIE RWWSB SIGRD RWWSRE BLBSET PGWRT PGERS SPMEN SPMCSR

Read/Write R/W R R/W R/W R/W R/W R/W R/W

Initial Value 0 0 0 0 0 0 0 0

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on page 346 for details. An SPM instruction within four cycles after SIGRD and SPMEN are set will have no

effect. This operation is reserved for future use and should not be used.

• Bit 4 – RWWSRE: Read-While-Write Section Read Enable

When programming (Page Erase or Page Write) to the RWW section, the RWW section is blocked for reading

(the RWWSB will be set by hardware). To re-enable the RWW section, the user software must wait until the

programming is completed (SPMEN will be cleared). Then, if the RWWSRE bit is written to one at the same

time as SPMEN, the next SPM instruction within four clock cycles re-enables the RWW section. The RWW

section cannot be re-enabled while the Flash is busy with a Page Erase or a Page Write (SPMEN is set). If the

RWWSRE bit is written while the Flash is being loaded, the Flash load operation will abort and the data loaded

will be lost.

• Bit 3 – BLBSET: Boot Lock Bit Set

If this bit is written to one at the same time as SPMEN, the next SPM instruction within four clock cycles sets

Boot Lock bits, according to the data in R0. The data in R1 and the address in the Z-pointer are ignored. The

BLBSET bit will automatically be cleared upon completion of the Lock bit set, or if no SPM instruction is

executed within four clock cycles.

An (E)LPM instruction within three cycles after BLBSET and SPMEN are set in the SPMCSR Register, will read

either the Lock bits or the Fuse bits (depending on Z0 in the Z-pointer) into the destination register. See

“Reading the Fuse and Lock Bits from Software” on page 345 for details.

• Bit 2 – PGWRT: Page Write

If this bit is written to one at the same time as SPMEN, the next SPM instruction within four clock cycles

executes Page Write, with the data stored in the temporary buffer. The page address is taken from the high part

of the Z-pointer. The data in R1 and R0 are ignored. The PGWRT bit will auto-clear upon completion of a Page

Write, or if no SPM instruction is executed within four clock cycles. The CPU is halted during the entire Page

Write operation if the NRWW section is addressed.

• Bit 1 – PGERS: Page Erase

If this bit is written to one at the same time as SPMEN, the next SPM instruction within four clock cycles

executes Page Erase. The page address is taken from the high part of the Z-pointer. The data in R1 and R0 are

ignored. The PGERS bit will auto-clear upon completion of a Page Erase, or if no SPM instruction is executed

within four clock cycles. The CPU is halted during the entire Page Write operation if the NRWW section is

addressed.

• Bit 0 – SPMEN: Stor e Program Memory Enable

This bit enables the SPM instruction for the next four clock cycles. If written to one together with either

RWWSRE, BLBSET, PGWRT’ or PGERS, the following SPM instruction will have a special meaning, see

description above. If only SPMEN is written, the following SPM instruction will store the value in R1:R0 in the

temporary page buffer addressed by the Z-pointer. The LSB of the Z-pointer is ignored. The SPMEN bit will

auto-clear upon completion of an SPM instruction, or if no SPM instruction is executed within four clock cycles.

During Page Erase and Page Write, the SPMEN bit remains high until the operation is completed.

Writing any other combination than “10001”, “01001”, “00101”, “00011”, or “00001” in the lower five bits will have

no effect.

Note: Only one SPM instruction should be active at any time.

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27.6 Addressing the Flash During Self-Programming

The Z-pointer is used to address the SPM commands. The Z pointer consists of the Z-registers ZL and ZH in the

that the RAMPZ register is only implemented when the program space is larger than 64KB.

Since the Flash is organized in pages, the Program Counter can be treated as having two different sections.

One section, consisting of the least significant bits, is addressing the words within a page, while the most

significant bits are addressing the pages. This is shown in Figure 27-4 on page 343. Note that the Page Erase

and Page Write operations are addressed independently. Therefore it is of major importance that the Boot

Loader software addresses the same page in both the Page Erase and Page Write operation. Once a

programming operation is initiated, the address is latched and the Z-pointer can be used for other operations.

The (E)LPM instruction use the Z-pointer to store the address. Since this instruction addresses the Flash byte-

by-byte, also bit Z0 of the Z-pointer is used.

Figure 27-4. Addressing the Flash During SPM(1)

Note: 1. The different variables used in Figure 27-4 are listed in Table 27-10 on page 350.

Bit2322212019181716

15 14 13 12 11 10 9 8

RAMPZ RAMPZ7 RAMPZ6 RAMPZ5 RAMPZ4 RAMPZ3 RAMPZ2 RAMPZ1 RAMPZ0

ZH (R31) Z15 Z14 Z13 Z12 Z11 Z10 Z9 Z8

ZL (R30) Z7 Z6 Z5 Z4 Z3 Z2 Z1 Z0

76543210

PROGRAM MEMORY

0123

Z - POINTER

BIT

ZPAGEMSB

WORD ADDRESS

WITHIN A PAGE

PAGE ADDRESS

WITHIN THE FLASH

ZPCMSB

INSTRUCTION WORD

PAGE PCWORD[PAGEMSB:0]:

PAGEEND

PAGE

PCWORDPCPAGE

PCMSB PAGEMSB

PROGRAM COUNTER

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27.7 Self-Programming the Flash

The program memory is updated in a page by page fashion. Before programming a page with the data stored in

the temporary page buffer, the page must be erased. The temporary page buffer is filled one word at a time

using SPM and the buffer can be filled either before the Page Erase command or between a Page Erase and a

Page Write operation:

Alternative 1, fill the buffer before a Page Erase

Fill temporary page buffer

Perform a Page Erase

Perform a Page Write

Alternative 2, fill the buffer after Page Erase

Perform a Page Erase

Fill temporary page buffer

Perform a Page Write

If only a part of the page needs to be changed, the rest of the page must be stored (for example in the

temporary page buffer) before the erase, and then be rewritten. When using alternative 1, the Boot Loader

provides an effective Read-Modify-Write feature which allows the user software to first read the page, do the

necessary changes, and then write back the modified data. If alternative 2 is used, it is not possible to read the

old data while loading since the page is already erased. The temporary page buffer can be accessed in a

random sequence. It is essential that the page address used in both the Page Erase and Page Write operation

is addressing the same page. See “Simple Assembly Code Example for a Boot Loader” on page 347 for an

assembly code example.

27.7.1 Performing Page Erase by SPM

To execute Page Erase, set up the address in the Z-pointer, write “X0000011” to SPMCSR and execute SPM

within four clock cycles after writing SPMCSR. The data in R1 and R0 is ignored. The page address must be

written to PCPAGE in the Z-register. Other bits in the Z-pointer will be ignored during this operation.

Page Erase to the RWW section: The NRWW section can be read during the Page Erase

Page Erase to the NRWW section: The CPU is halted during the operation

27.7.2 Filling the Temporary Buffer (Page Loading)

To write an instruction word, set up the address in the Z-pointer and data in R1:R0, write “00000001” to

SPMCSR and execute SPM within four clock cycles after writing SPMCSR. The content of PCWORD in the Z-

Write operation or by writing the RWWSRE bit in SPMCSR. It is also erased after a system reset. Note that it is

not possible to write more than one time to each address without erasing the temporary buffer.

If the EEPROM is written in the middle of an SPM Page Load operation, all data loaded will be lost.

27.7.3 Performing a Page Write

To execute Page Write, set up the address in the Z-pointer, write “X0000101” to SPMCSR and execute SPM

within four clock cycles after writing SPMCSR. The data in R1 and R0 is ignored. The page address must be

written to PCPAGE. Other bits in the Z-pointer must be written to zero during this operation.

Page Write to the RWW section: The NRWW section can be read during the Page Write

Page Write to the NRWW section: The CPU is halted during the operation

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27.7.4 Using the SPM Interrupt

If the SPM interrupt is enabled, the SPM interrupt will generate a constant interrupt when the SPMEN bit in

SPMCSR is cleared. This means that the interrupt can be used instead of polling the SPMCSR Register in

software. When using the SPM interrupt, the Interrupt Vectors should be moved to the BLS section to avoid that

an interrupt is accessing the RWW section when it is blocked for reading. How to move the interrupts is

described in “Interrupts” on page 63.

27.7.5 Consideration While Updating BLS

Special care must be taken if the user allows the Boot Loader section to be updated by leaving Boot Lock bit11

unprogrammed. An accidental write to the Boot Loader itself can corrupt the entire Boot Loader, and further

software updates might be impossible. If it is not necessary to change the Boot Loader software itself, it is

recommended to program the Boot Lock bit11 to protect the Boot Loader software from any internal software

changes.

27.7.6 Prevent Reading the RWW Section During Self-P rogra mmin g

During Self-Programming (either Page Erase or Page Write), the RWW section is always blocked for reading.

The user software itself must prevent that this section is addressed during the self programming operation. The

RWWSB in the SPMCSR will be set as long as the RWW section is busy. During Self-Programming the Interrupt

Vector table should be moved to the BLS as described in “Interrupts” on page 63, or the interrupts must be

disabled. Before addressing the RWW section after the programming is completed, the user software must clear

the RWWSB by writing the RWWSRE. See “Simple Assembly Code Example for a Boot Loader” on page 347

for an example.

27.7.7 Setting the Boot Loader Lock Bits by SPM

To set the Boot Loader Lock bits, write the desired data to R0, write “X0001001” to SPMCSR and execute SPM

within four clock cycles after writing SPMCSR. The only accessible Lock bits are the Boot Lock bits that may

prevent the Application and Boot Loader section from any software update by the MCU.

See Table 27-2 on page 339 and Table 27-3 on page 339 for how the different settings of the Boot Loader bits

affect the Flash access.

If bits 5..2 in R0 are cleared (zero), the corresponding Boot Lock bit will be programmed if an SPM instruction is

executed within four cycles after BLBSET and SPMEN are set in SPMCSR. The Z-pointer is don’t care during

this operation, but for future compatibility it is recommended to load the Z-pointer with 0x0001 (same as used for

reading the lOck bits). For future compatibility it is also recommended to set bits 7, 6, 1, and 0 in R0 to “1” when

writing the Lock bits. When programming the Lock bits the entire Flash can be read during the operation.

27.7.8 EEPROM Write Prevents Writing to SPMCSR

Note that an EEPROM write operation will block all software programming to Flash. Reading the Fuses and

Lock bits from software will also be prevented during the EEPROM write operation. It is recommended that the

user checks the status bit (EEPE) in the EECR Register and verifies that the bit is cleared before writing to the

SPMCSR Register.

27.7.9 Reading the Fuse and Lock Bits from Software

It is possible to read both the Fuse and Lock bits from software. To read the Lock bits, load the Z-pointer with

0x0001 and set the BLBSET and SPMEN bits in SPMCSR. When an (E)LPM instruction is executed within

Bit 76543210

R0 1 1 BLB12 BLB11 BLB02 BLB01 1 1

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three CPU cycles after the BLBSET and SPMEN bits are set in SPMCSR, the value of the Lock bits will be

loaded in the destination register. The BLBSET and SPMEN bits will auto-clear upon completion of reading the

Lock bits or if no (E)LPM instruction is executed within three CPU cycles or no SPM instruction is executed

within four CPU cycles. When BLBSET and SPMEN are cleared, (E)LPM will work as described in the

Instruction set Manual.

The algorithm for reading the Fuse Low byte is similar to the one described above for reading the Lock bits. To

read the Fuse Low byte, load the Z-pointer with 0x0000 and set the BLBSET and SPMEN bits in SPMCSR.

When an (E)LPM instruction is executed within three cycles after the BLBSET and SPMEN bits are set in the

SPMCSR, the value of the Fuse Low byte (FLB) will be loaded in the destination register as shown below. Refer

to Table 27-5 on page 340 for a detailed description and mapping of the Fuse Low byte.

Similarly, when reading the Fuse High byte, load 0x0003 in the Z-pointer. When an (E)LPM instruction is

executed within three cycles after the BLBSET and SPMEN bits are set in the SPMCSR, the value of the Fuse

High byte (FHB) will be loaded in the destination register as shown below. Refer to Table 27-4 on page 340 for

detailed description and mapping of the Fuse High byte.

When reading the Extended Fuse byte, load 0x0002 in the Z-pointer. When an (E)LPM instruction is executed

within three cycles after the BLBSET and SPMEN bits are set in the SPMCSR, the value of the Extended Fuse

byte (EFB) will be loaded in the destination register as shown below. Refer to Table 27-3 on page 339 for

detailed description and mapping of the Extended Fuse byte.

Fuse and Lock bits that are programmed, will be read as zero. Fuse and Lock bits that are unprogrammed, will

be read as one.

27.7.10 Reading the Signature Row from Software

To read the Signature Row from software, load the Z-pointer with the signature byte address given in the table

below and set the SIGRD and SPMEN bits in SPMCSR. When an LPM instruction is executed within three CPU

cycles after the SIGRD and SPMEN bits are set in SPMCSR, the signature byte value will be loaded in the

destination register. The SIGRD and SPMEN bits will auto-clear upon completion of reading the Signature Row

Lock bits or if no LPM instruction is executed within three CPU cycles. When SIGRD and SPMEN are cleared,

LPM will work as described in the Instruction set Manual.

Bit 76543210

Rd – – BLB12 BLB11 BLB02 BLB01 LB2 LB1

Bit 7654 3210

Rd FLB7 FLB6 FLB5 FLB4 FLB3 FLB2 FLB1 FLB0

Bit 7654 3210

Rd FHB7 FHB6 FHB5 FHB4 FHB3 FHB2 FHB1 FHB0

Bit 7654 3210

Rd –––––EFB2EFB1EFB0

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Note: All other addresses are reserved for future use.

27.7.11 Preventing Flash Corruption

During periods of low VCC, the Flash program can be corrupted because the supply voltage is too low for the

CPU and the Flash to operate properly. These issues are the same as for board level systems using the Flash,

and the same design solutions should be applied.

A Flash program corruption can be caused by two situations when the voltage is too low. First, a regular write

sequence to the Flash requires a minimum voltage to operate correctly. Secondly, the CPU itself can execute

instructions incorrectly, if the supply voltage for executing instructions is too low.

Flash corruption can easily be avoided by following these design recommendations (one is sufficient):

1. If there is no need for a Boot Loader update in the system, program the Boot Loader Lock bits to pre-

vent any Boot Loader software updates.

2. Keep the AVR RESET active (low) during periods of insufficient power supply voltage. This can be done

by enabling the internal Brown-out Detector (BOD) if the operating voltage matches the detection level. If

not, an external low VCC reset protection circuit can be used. If a reset occurs while a write operation is in

progress, the write operation will be completed provided that the power supply voltage is sufficient.

3. Keep the AVR core in Power-down sleep mode during periods of low VCC. This will prevent the CPU from

attempting to decode and execute instructions, effectively protecting the SPMCSR Register and thus the

Flash from unintentional writes.

27.7.12 Programming Time for Flash when Using SPM

The calibrated RC Oscillator is used to time Flash accesses. This table shows the typical programming time for

Flash accesses from the CPU.

27.7.13 Simple Assembly Code Example for a Boot Loader

;-the routine writes one page of data from RAM to Flash

; the first data location in RAM is pointed to by the Y pointer

; the first data location in Flash is pointed to by the Z-pointer

;-error handling is not included

;-the routine must be placed inside the Boot space

; (at least the Do_spm sub routine). Only code inside NRWW section can

; be read during Self-Programming (Page Erase and Page Write).

;-registers used: r0, r1, temp1 (r16), temp2 (r17), looplo (r24),

Table 27-6. Signature Row Addressing

Signature Byte Z-Pointer Address

Device Signature Byte 1 0x0000

Device Signature Byte 2 0x0002

Device Signature Byte 3 0x0004

RC Oscillator Calibration Byte 0x0001

Table 27-7. SPM Programming Time

Symbol Min. Programming Time Max. Programming Time

Flash write (Page Erase, Page Write, and write Lock bits by

SPM) 3.7ms 4.5ms

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; loophi (r25), spmcrval (r20)

; storing and restoring of registers is not included in the routine

; register usage can be optimized at the expense of code size

;-It is assumed that either the interrupt table is moved to the Boot

; loader section or that the interrupts are disabled.

.equ PAGESIZEB = PAGESIZE*2 ;PAGESIZEB is page size in BYTES, not words

.org SMALLBOOTSTART

Write_page:

; Page Erase

ldi spmcrval, (1<<PGERS) | (1<<SPMEN)

call Do_spm

; re-enable the RWW section

ldi spmcrval, (1<<RWWSRE) | (1<<SPMEN)

call Do_spm

;transfer data from RAM to Flash page buffer

ldi looplo, low(PAGESIZEB) ;init loop variable

ldi loophi, high(PAGESIZEB) ;not required for PAGESIZEB<=256

Wrloop:

ld r0, Y+

ld r1, Y+

ldi spmcrval, (1<<SPMEN)

call Do_spm

adiw ZH:ZL, 2

sbiw loophi:looplo, 2 ;use subi for PAGESIZEB<=256

brne Wrloop

; execute Page Write

subi ZL, low(PAGESIZEB) ;restore pointer

sbci ZH, high(PAGESIZEB) ;not required for PAGESIZEB<=256

ldi spmcrval, (1<<PGWRT) | (1<<SPMEN)

call Do_spm

; re-enable the RWW section

ldi spmcrval, (1<<RWWSRE) | (1<<SPMEN)

call Do_spm

; read back and check, optional

ldi looplo, low(PAGESIZEB) ;init loop variable

ldi loophi, high(PAGESIZEB) ;not required for PAGESIZEB<=256

subi YL, low(PAGESIZEB) ;restore pointer

sbci YH, high(PAGESIZEB)

Rdloop:

elpm r0, Z+

ld r1, Y+

cpse r0, r1

jmp Error

sbiw loophi:looplo, 1 ;use subi for PAGESIZEB<=256

brne Rdloop

; return to RWW section

; verify that RWW section is safe to read

Return:

in temp1, SPMCSR

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sbrs temp1, RWWSB ; If RWWSB is set, the RWW

section is not ready yet

ret

; re-enable the RWW section

ldi spmcrval, (1<<RWWSRE) | (1<<SPMEN)

call Do_spm

rjmp Return

Do_spm:

; check for previous SPM complete

Wait_spm:

in temp1, SPMCSR

sbrc temp1, SPMEN

rjmp Wait_spm

; input: spmcrval determines SPM action

; disable interrupts if enabled, store status

in temp2, SREG

cli

; check that no EEPROM write access is present

Wait_ee:

sbic EECR, EEPE

rjmp Wait_ee

; SPM timed sequence

out SPMCSR, spmcrval

spm

; restore SREG (to enable interrupts if originally enabled)

out SREG, temp2

ret

27.7.14 Boot Loader Parameters

The parameters used in the description of the Self-Programming are given throughout the following tables.

Note: 1. The different BOOTSZ Fuse configurations are shown in Figure 27-2 on page 338

Table 27-8. Boot Size Configuration (Word Addresses)(1)

Device

BOOTSZ1

BOOTSZ0

Boot Size

Pages

Application

Flash Section

Boot Loader

Flash Section

End

Application

Section

Boot

Reset Address

(Start Boot

Loader Section)

ATmega32U4

1 1 256 words 40x0000 - 0x3EFF 0x3F00 - 0x3FFF 0x3EFF 0x3F00

1 0 512 words 80x0000 - 0x3DFF 0x3E00 - 0x3FFF 0x3DFF 0x3E00

0 1 1024 words 16 0x0000 - 0x3BFF 0x3C00 - 0x3FFF 0x3BFF 0x3C00

0 0 2048 words 32 0x0000 - 0x37FF 0x3800 - 0x3FFF 0x37FF 0x3800

ATmega16U4

1 1 256 words 40x0000 - 0x1EFF 0x1F00 - 0x1FFF 0x1EFF 0x1F00

1 0 512 words 80x0000 - 0x1DFF 0x1E00 - 0x1FFF 0x1DFF 0x1E00

0 1 1024 words 16 0x0000 - 0x1BFF 0x1C00 - 0x1FFF 0x1BFF 0x1C00

0 0 2048 words 32 0x0000 - 0x17FF 0x1800 - 0x1FFF 0x17FF 0x1800

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Note: 1. For details about these two section, see “NRWW – No Read-While-Write Section” on page 337 and “RWW –

Read-While-Write Section” on page 337.

Note: 1. Z0: should be zero for all SPM commands, byte select for the (E)LPM instruction.

Note: See “Addressing the Flash During Self-Programming” on page 343 for details about the use of Z-pointer during

Self-Programming.

Table 27-9. Read-While-Write Limit (Word Add resses)(1)

Device Section Pages Address

ATmega32U4

Read-While-Write section (RWW) 224 0x0000 - 0x37FF

No Read-While-Write section (NRWW) 32 0x3800 - 0x3FFF

ATmega16U4

Read-While-Write section (RWW) 97 0x0000 - 0x17FF

No Read-While-Write section (NRWW) 32 0x1800 - 0x1FFF

Table 27-10. Explana tion of different variables used in Figure 27-4 and the mapping to the Z-pointer

Variable Corresponding

Z-value(1) Description

PCMSB 13 Most significant bit in the Program Counter. (The Program Counter is 14

bits PC[13:0])

PAGEMSB 6Most significant bit which is used to address the words within one page

(64 words in a page requires six bits PC [5:0])

ZPCMSB Z14 Bit in Z-pointer that is mapped to PCMSB. Because Z0 is not used, the

ZPCMSB equals PCMSB + 1

ZPAGEMS

BZ7 Bit in Z-pointer that is mapped to PCMSB. Because Z0 is not used, the

ZPAGEMSB equals PAGEMSB + 1

PCPAGE PC[13:6] Z14:Z7 Program Counter page address: Page select, for Page Erase and Page

Write

PCWORD PC[5:0] Z6:Z1 Program Counter word address: Word select, for filling temporary buffer

(must be zero during Page Write operation)

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28. Memory Programming

28.1 Program And Data Memory Lock Bits

The device provides six Lock bits which can be left unprogrammed (“1”) or can be programmed (“0”) to obtain

the additional features listed in Table 28-2. The Lock bits can only be erased to “1” with the Chip Erase

command.

Note: 1. “1”: unprogrammed, “0”: programmed

Table 28-1. Lock Bit Byte()

Lock Bit Byte Bit No Description Default Value

ATmega16U4/32U4 ATmega16U4RC/32U4RC

7 – 1

6 – 1

BLB12 5Boot Lock bit 1

BLB11 4Boot Lock bit 0 1

BLB02 3Boot Lock bit 1

BLB01 2Boot Lock bit 1

LB2 1Lock bit 0 1

LB1 0Lock bit 0 1

Table 28-2. Lock Bit Protection Modes(1)(2)

Memory Lock Bits Protection Type

LB Mode LB2 LB1

1 1 1 No memory lock features enabled.

2 1 0

Further programming of the Flash and EEPROM is disabled in

Parallel and Serial Programming mode. The Fuse bits are locked in

both Serial and Parallel Programming mode.(1)

3 0 0

Further programming and verification of the Flash and EEPROM is

disabled in Parallel and Serial Programming mode. The Boot Lock

bits and Fuse bits are locked in both Serial and Parallel

Programming mode.(1)

BLB0 Mode BLB02 BLB01

1 1 1 No restrictions for SPM or (E)LPM accessing the Application

section.

2 1 0 SPM is not allowed to write to the Application section.

3 0 0

SPM is not allowed to write to the Application section, and (E)LPM

executing from the Boot Loader section is not allowed to read from

the Application section. If Interrupt Vectors are placed in the Boot

Loader section, interrupts are disabled while executing from the

Application section.

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Notes: 1. Program the Fuse bits and Boot Lock bits before programming the LB1 and LB2.

2. “1” means unprogrammed, “0” means programmed.

28.2 Fuse Bits

The device has three bytes. Table 28-3 to Table 28-5 on page 355 describe briefly the functionality of all the

fuses and how they are mapped into the Fuse bytes. Note that the fuses are read as logical zero, “0”, if they are

programmed.

Notes: 1. See Table 8-1 on page 53 for BODLEVEL Fuse decoding.

2. “1” means unprogrammed, “0” means programmed.

4 0 1

(E)LPM executing from the Boot Loader section is not allowed to

read from the Application section. If Interrupt Vectors are placed in

the Boot Loader section, interrupts are disabled while executing

from the Application section.

BLB1 Mode BLB12 BLB11

1 1 1 No restrictions for SPM or (E)LPM accessing the Boot Loader

section.

2 1 0 SPM is not allowed to write to the Boot Loader section.

3 0 0

SPM is not allowed to write to the Boot Loader section, and (E)LPM

executing from the Application section is not allowed to read from

the Boot Loader section. If Interrupt Vectors are placed in the

Application section, interrupts are disabled while executing from the

Boot Loader section.

4 0 1

(E)LPM executing from the Application section is not allowed to

read from the Boot Loader section. If Interrupt Vectors are placed in

the Application section, interrupts are disabled while executing from

the Boot Loader section.

Table 28-2. Lock Bit Protection Modes(1)(2)

Memory Lock Bits Protection Type

Table 28-3. Extended Fuse Byte(1)(2)

Fuse Low Byte Bit No Description Default Value

ATmega16/32U4 ATmega16/32U4RC

– 7 – 1

– 6 – 1

– 5 – 1

– 4 – 1

HWBE 3Hardware Boot Enable 0 (programmed) 1 (unprogrammed)

BODLEVEL2(1) 2Brown-out Detector trigger

level 0 (programmed)

BODLEVEL1(1) 1Brown-out Detector trigger

level 1 (unprogrammed)

BODLEVEL0(1) 0Brown-out Detector trigger

level 1 (unprogrammed)

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Note: 1. The SPIEN Fuse is not accessible in serial programming mode.

2. The default value of BOOTSZ1..0 results in maximum Boot Size. See Table 28-7 for details.

3. See “Watchdog Timer” on page 55 for details.

4. Never ship a product with the OCDEN Fuse programmed regardless of the setting of Lock bits and JTAGEN

Fuse. A programmed OCDEN Fuse enables some parts of the clock system to be running in all sleep modes.

This may increase the power consumption.

Note: 1. The default setting of CKSEL3..0 results in Low Power Crystal Oscillator for ATmega16U4 and ATmega32U4,

and Internal RC oscillator for ATmega16U4RC and ATmega32U4RC.

2. The CKOUT Fuse allow the system clock to be output on PORTC7. See “CLKPR – Clock Prescaler Register”

on page 39 for details.

3. See “System Clock Prescaler” on page 35 for details.

The status of the Fuse bits is not affected by Chip Erase. Note that the Fuse bits are locked if Lock bit1 (LB1) is

programmed. Program the Fuse bits before programming the Lock bits.

Table 28-4. Fuse High Byte

Fuse High

Byte Bit No Description Default Value

OCDEN(4) 7Enable OCD 1 (unprogrammed, OCD disabled)

JTAGEN 6Enable JTAG 0 (programmed, JTAG enabled)

SPIEN(1) 5Enable Serial Program and Data

Downloading 0 (programmed, SPI prog. enabled)

WDTON(3) 4Watchdog Timer always on 1 (unprogrammed)

EESAVE 3EEPROM memory is preserved

through the Chip Erase

1 (unprogrammed, EEPROM

preserved)

BOOTSZ1 2Select Boot Size (see Table 28-7

for details) 0 (programmed)(2)

BOOTSZ0 1Select Boot Size (see Table 28-7

for details) 0 (programmed)(2)

BOOTRST 0Select Bootloader Address as

Reset Vector

1 (unprogrammed, Reset vector

@0x0000)

Table 28-5. Fuse Low Byte

Fuse Low Byte Bit Nr Description Default Value

ATmega16U4/32U4 ATmega16U4RC/32U4RC

CKDIV8(3) 7Divide clock by 8 0 (programmed)

CKOUT(2) 6Clock output 1 (unprogrammed)

SUT1 5Select start-up time 0 (programmed)

SUT0 4Select start-up time 1 (unprogrammed)

CKSEL3 3Select Clock source 1 (unprogrammed)(1) 0 (programmed)(1)

CKSEL2 2Select Clock source 1 (unprogrammed)(1) 0 (programmed)(1)

CKSEL1 1Select Clock source 1 (unprogrammed)(1) 1 (unprogrammed)(1)

CKSEL0 0Select Clock source 0 (programmed)(1) 0 (programmed)(1)

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28.2.1 Latching of Fuses

The fuse values are latched when the device enters programming mode and changes of the fuse values will

have no effect until the part leaves Programming mode. This does not apply to the EESAVE Fuse which will

take effect once it is programmed. The fuses are also latched on Power-up in Normal mode.

28.3 Signature Bytes

All Atmel microcontrollers have a three-byte signature code which identifies the device. This code can be read in

both serial and parallel mode, also when the device is locked. The three bytes reside in a separate address

space.

ATmega16U4 Signature Bytes:

1. 0x000: 0x1E (indicates manufactured by Atmel).

2. 0x001: 0x94 (indicates 16KB Flash memory).

3. 0x002: 0x88 (indicates ATmega16U4 device).

ATmega32U4 Signature Bytes:

1. 0x000: 0x1E (indicates manufactured by Atmel).

2. 0x001: 0x95 (indicates 32KB Flash memory).

3. 0x002: 0x87 (indicates ATmega32U4 device).

28.4 Calibration Byte

The device has a byte calibration value for the internal RC Oscillator. This byte resides in the high byte of

address 0x000 in the signature address space. During reset, this byte is automatically written into the OSCCAL

28.5 Parallel Programming Parameters, Pin Mapping, and Commands

This section describes how to parallel program and verify Flash Program memory, EEPROM Data memory,

Memory Lock bits, and Fuse bits in the device. Pulses are assumed to be at least 250ns unless otherwise

noted.

28.5.1 Signal Names

In this section, some pins of the device are referenced by signal names describing their functionality during

parallel programming, see Figure 28-1 on page 357 and Table 28-6 on page 357. Pins not described in the

following table are referenced by pin names.

The XA1/XA0 pins determine the action executed when the XTAL1 pin is given a positive pulse. The bit coding

is shown in Table 28-9 on page 358.

When pulsing WR or OE, the command loaded determines the action executed. The different commands are

shown in Table 28-10 on page 358.

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Figure 28-1. Parallel Programming(1)

Note: 1. Unused Pins should be left floating.

Table 28-6. Pin Name Mapping

Signal Name in

Programming Mode Pin Name I/O Function

RDY/BSY PD1 O0: Device is busy programming, 1: Device is ready for new

command

OE PD2 IOutput Enable (Active low)

WR PD3 IWrite Pulse (Active low)

BS1 PD4 IByte Select 1

XA0 PD5 IXTAL Action Bit 0

XA1 PD6 IXTAL Action Bit 1

PAGEL PD7 IProgram Memory and EEPROM data Page Load

BS2 PE6 IByte Select 2

DATA PB7-0 I/O Bi-directional Data bus (Output when OE is low)

VCC

+5V

GND

XTAL1

PD1

PD2

PD3

PD4

PD5

PD6

PB7 - PB0 DATA

RESET

PD7

+12 V

BS1

XA0

XA1

RDY/BSY

PAGEL

PE6

BS2

AVCC

+5V

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Table 28-7. BS2 and BS1 Encoding

BS2 BS1 Flash / EEPROM

Address

Flash Data

Loading /

Reading Fuse

Programming Reading Fuse

and Lock Bits

0 0 Low Byte Low Byte Low Byte Fuse Low Byte

0 1 High Byte High Byte High Byte Lock bits

1 0 Extended High

Byte Reserved Extended Byte Extended Fuse

Byte

1 1 Reserved Reserved Reserved Fuse High Byte

Table 28-8. Pin Values Used to Enter Programming Mode

Pin Symbol Value

PAGEL Prog_enable[3] 0

XA1 Prog_enable[2] 0

XA0 Prog_enable[1] 0

BS1 Prog_enable[0] 0

Table 28-9. XA1 and XA0 Enoding

XA1 XA0 Action when XTAL1 is Pulsed

0 0 Load Flash or EEPROM Address (High or low address byte determined by

BS2 and BS1)

0 1 Load Data (High or Low data byte for Flash determined by BS1)

1 0 Load Command

1 1 No Action, Idle

Table 28-10. Comman d Byte Bit Encoding

Command Byte Command Executed

1000 0000 Chip Erase

0100 0000 Write Fuse bits

0010 0000 Write Lock bits

0001 0000 Write Flash

0001 0001 Write EEPROM

0000 1000 Read Signature Bytes and Calibration byte

0000 0100 Read Fuse and Lock bits

0000 0010 Read Flash

0000 0011 Read EEPROM

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28.6 Parallel Programming

28.6.1 Enter Programming Mode

The following algorithm puts the device in parallel programming mode:

1. Apply 4.5 - 5.5V between VCC and GND.

2. Set RESET to “0” and toggle XTAL1 at least six times.

3. Set the Prog_enable pins listed in Table 28-8 on page 358 to “0000” and wait at least 100ns.

4. Apply 11.5 - 12.5V to RESET. Any activity on Prog_enable pins within 100ns after +12V has been applied

to RESET, will cause the device to fail entering programming mode.

5. Wait at least 50µs before sending a new command.

28.6.2 Considerations for Efficient Programming

The loaded command and address are retained in the device during programming. For efficient programming,

the following should be considered.

The command needs only be loaded once when writing or reading multiple memory locations

Skip writing the data value 0xFF, that is the contents of the entire EEPROM (unless the EESAVE Fuse is

programmed) and Flash after a Chip Erase

Address high byte needs only be loaded before programming or reading a new 256 word window in Flash

or 256 byte EEPROM. This consideration also applies to Signature bytes reading.

28.6.3 Chip Erase

The Chip Erase will erase the Flash and EEPROM(1) memories plus Lock bits. The Lock bits are not reset until

the program memory has been completely erased. The Fuse bits are not changed. A Chip Erase must be

performed before the Flash and/or EEPROM are reprogrammed.

Note: 1. The EEPRPOM memory is preserved during Chip Erase if the EESAVE Fuse is programmed.

Load Command “Chip Erase”:

1. Set XA1, XA0 to “10”. This enables command loading.

2. Set BS1 to “0”.

3. Set DATA to “1000 0000”. This is the command for Chip Erase.

4. Give XTAL1 a positive pulse. This loads the command.

Table 28-11. No. of Words in a Page and No. of Pages in the Flash

Device Flash Size Page Size PCWORD No. of

Pages PCPAGE PCMSB

ATmega16U4 8K words (16KB) 64 words PC[5:0] 128 PC[12:6] 12

ATmega32U4 16K words (32KB) 64 words PC[5:0] 256 PC[13:6] 13

Table 28-12. No. of Words in a Page and No. of Pages in the EEPROM

Device EEPROM Size Page Size PCWORD No. of

Pages PCPAGE EEAMSB

ATmega16U4 512 bytes 4 bytes EEA[1:0] 128 EEA[8:2] 8

ATmega32U4 1KB 4 bytes EEA[1:0] 256 EEA[9:2] 9

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5. Give WR a negative pulse. This starts the Chip Erase. RDY/BSY goes low.

6. Wait until RDY/BSY goes high before loading a new command.

28.6.4 Programming the Flash

The Flash is organized in pages, see Table 28-11 on page 359. When programming the Flash, the program data is

latched into a page buffer. This allows one page of program data to be programmed simultaneously. The

following procedure describes how to program the entire Flash memory:

A. Load Command “Write Flash”

1. Set XA1, XA0 to “10”. This enables command loading.

2. Set BS1 to “0”.

3. Set DATA to “0001 0000”. This is the command for Write Flash.

4. Give XTAL1 a positive pulse. This loads the command.

B. Load Address Low byte (Address bits 7..0)

1. Set XA1, XA0 to “00”. This enables address loading.

2. Set BS2, BS1 to “00”. This selects the address low byte.

3. Set DATA = Address low byte (0x00 - 0xFF).

4. Give XTAL1 a positive pulse. This loads the address low byte.

C. Load Data Low Byte

1. Set XA1, XA0 to “01”. This enables data loading.

2. Set DATA = Data low byte (0x00 - 0xFF).

3. Give XTAL1 a positive pulse. This loads the data byte.

D. Load Data High Byte

1. Set BS1 to “1”. This selects high data byte.

2. Set XA1, XA0 to “01”. This enables data loading.

3. Set DATA = Data high byte (0x00 - 0xFF).

4. Give XTAL1 a positive pulse. This loads the data byte.

E. Latch Data

1. Set BS1 to “1”. This selects high data byte.

2. Give PAGEL a positive pulse. This latches the data bytes. (See Figure 28-3 on page 361 for signal

waveforms)

F. Repeat B through E until the entire buffer is filled or until all data within the page is loaded.

While the lower bits in the address are mapped to words within the page, the higher bits address the pages

within the FLASH. This is illustrated in Figure 28-2 on page 361. Note that if less than eight bits are required to

address words in the page (page size < 256), the most significant bit(s) in the address low byte are used to

address the page when performing a Page Write.

G. Load Address High byte (Address bits15..8)

1. Set XA1, XA0 to “00”. This enables address loading.

2. Set BS2, BS1 to “01”. This selects the address high byte.

3. Set DATA = Address high byte (0x00 - 0xFF).

4. Give XTAL1 a positive pulse. This loads the address high byte.

H. Load Address Extended High byte (Address bits 23..16)

1. Set XA1, XA0 to “00”. This enables address loading.

2. Set BS2, BS1 to “10”. This selects the address extended high byte.

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3. Set DATA = Address extended high byte (0x00 - 0xFF).

4. Give XTAL1 a positive pulse. This loads the address high byte.

I. Program Page

1. Set BS2, BS1 to “00”

2. Give WR a negative pulse. This starts programming of the entire page of data. RDY/BSY goes low.

3. Wait until RDY/BSY goes high (See Figure 28-3 on page 361 for signal waveforms).

J. Repeat B through I until the entire Flash is programmed or until all data has been programmed

K. End Page Programming

1. 1. Set XA1, XA0 to “10”. This enables command loading.

2. Set DATA to “0000 0000”. This is the command for No Operation.

3. Give XTAL1 a positive pulse. This loads the command, and the internal write signals are reset.

Figure 28-2. Addressing the Flash Wh ich is Organized in Pages(1)

Note: 1. PCPAGE and PCWORD are listed in Table 28-11 on page 359.

Figure 28-3. Programming the Flash Waveforms(1)

PROGRAM MEMORY

WORD ADDRESS

WITHIN A PAGE

PAGE ADDRESS

WITHIN THE FLASH

INSTRUCTION WORD

PAG E PCWORD[PAGEMSB:0]:

PAGEEND

PAG E

PCWORDPCPAGE

PCMSB PAGEMSB

PROGRAM

COUNTER

RDY/BSY

RESET +12V

PAGEL

BS2

0x10 ADDR. LOW ADDR. HIGH

DATA DATA LOW DATA HIGH ADDR. LOW DATA LOW DATA HIGH

XA1

XA0

BS1

XTAL1

XX XX XX

ABCDEBCDEG

ADDR. EXT.H

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Note: 1. “XX” is don’t care. The letters refer to the programming description above.

28.6.5 Programming the EEPROM

The EEPROM is organized in pages, see Table 28-12 on page 359. When programming the EEPROM, the

program data is latched into a page buffer. This allows one page of data to be programmed simultaneously. The

programming algorithm for the EEPROM data memory is as follows (refer to “Programming the Flash” on

page 360 for details on Command, Address and Data loading):

1. A: Load Command “0001 0001”.

2. G: Load Address High Byte (0x00 - 0xFF).

3. B: Load Address Low Byte (0x00 - 0xFF).

4. C: Load Data (0x00 - 0xFF).

5. E: Latch data (give PAGEL a positive pulse).

K: Repeat 3 through 5 until the entire buffer is filled

L: Program EEPROM page

1. Set BS2, BS1 to “00”.

2. Give WR a negative pulse. This starts programming of the EEPROM page. RDY/BSY goes low.

3. Wait until to RDY/BSY goes high before programming the next page (See Figure 28-4 on page 362 for

signal waveforms).

Figure 28-4. Programming the EEPROM Waveforms

28.6.6 Reading the Flash

The algorithm for reading the Flash memory is as follows (refer to “Programming the Flash” on page 360 for

details on Command and Address loading):

1. A: Load Command “0000 0010”.

2. H: Load Address Extended Byte (0x00- 0xFF).

3. G: Load Address High Byte (0x00 - 0xFF).

4. B: Load Address Low Byte (0x00 - 0xFF).

5. Set OE to “0”, and BS1 to “0”. The Flash word low byte can now be read at DATA.

6. Set BS to “1”. The Flash word high byte can now be read at DATA.

7. Set OE to “1”.

RDY/BSY

RESET +12V

PAGEL

BS2

0x11 ADDR. HIGH

DATA ADDR. LOW DATA ADDR. LOW DATA XX

XA1

XA0

BS1

XTAL1

AGBCEB C EL

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28.6.7 Reading the EEPROM

The algorithm for reading the EEPROM memory is as follows (refer to “Programming the Flash” on page 360 for

details on Command and Address loading):

1. A: Load Command “0000 0011”.

2. G: Load Address High Byte (0x00 - 0xFF).

3. B: Load Address Low Byte (0x00 - 0xFF).

4. Set OE to “0”, and BS1 to “0”. The EEPROM Data byte can now be read at DATA.

5. Set OE to “1”.

28.6.8 Programming the Fuse Low Bits

The algorithm for programming the Fuse Low bits is as follows (refer to “Programming the Flash” on page 360

for details on Command and Data loading):

1. A: Load Command “0100 0000”.

2. C: Load Data Low Byte. Bit n = “0” programs and bit n = “1” erases the Fuse bit.

3. Give WR a negative pulse and wait for RDY/BSY to go high.

28.6.9 Programming the Fuse High Bits

The algorithm for programming the Fuse High bits is as follows (refer to “Programming the Flash” on page 360

for details on Command and Data loading):

1. A: Load Command “0100 0000”.

2. C: Load Data Low Byte. Bit n = “0” programs and bit n = “1” erases the Fuse bit.

3. Set BS2, BS1 to “01”. This selects high data byte.

4. Give WR a negative pulse and wait for RDY/BSY to go high.

5. Set BS2, BS1 to “00”. This selects low data byte.

28.6.10 Programming the Extended Fuse Bits

The algorithm for programming the Extended Fuse bits is as follows (refer to “Programming the Flash” on

page 360 for details on Command and Data loading):

1. 1. A: Load Command “0100 0000”.

2. 2. C: Load Data Low Byte. Bit n = “0” programs and bit n = “1” erases the Fuse bit.

3. 3. Set BS2, BS1 to “10”. This selects extended data byte.

4. 4. Give WR a negative pulse and wait for RDY/BSY to go high.

5. 5. Set BS2, BS1 to “00”. This selects low data byte.

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Figure 28-5. Progr amming the FUSES Waveforms

28.6.11 Programming the Lock Bits

The algorithm for programming the Lock bits is as follows (refer to “Programming the Flash” on page 360 for

details on Command and Data loading):

1. A: Load Command “0010 0000”.

2. C: Load Data Low Byte. Bit n = “0” programs the Lock bit. If LB mode 3 is programmed (LB1 and LB2 is

programmed), it is not possible to program the Boot Lock bits by any External Programming mode.

3. Give WR a negative pulse and wait for RDY/BSY to go high.

The Lock bits can only be cleared by executing Chip Erase.

28.6.12 Reading the Fuse and Lock Bits

The algorithm for reading the Fuse and Lock bits is as follows (refer to “Programming the Flash” on page 360 for

details on Command loading):

1. A: Load Command “0000 0100”.

2. Set OE to “0”, and BS2, BS1 to “00”. The status of the Fuse Low bits can now be read at DATA (“0” means

programmed).

3. Set OE to “0”, and BS2, BS1 to “11”. The status of the Fuse High bits can now be read at DATA (“0”

means programmed).

4. Set OE to “0”, and BS2, BS1 to “10”. The status of the Extended Fuse bits can now be read at DATA (“0”

means programmed).

5. Set OE to “0”, and BS2, BS1 to “01”. The status of the Lock bits can now be read at DATA (“0” means

programmed).

6. Set OE to “1”.

RDY/BSY

RESET +12V

PAGEL

0x40

DATA

DATA XX

XA1

XA0

BS1

XTAL1

0x40 DATA XX

Write Fuse Low byte Write Fuse high byte

0x40 DATA XX

Write Extended Fuse byte

BS2

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Figure 28-6. Mapping Between BS1, BS2 and the Fuse and Lock Bits During Read

28.6.13 Reading the Signature Bytes

The algorithm for reading the Signature bytes is as follows (refer to “Programming the Flash” on page 360 for

details on Command and Address loading):

1. A: Load Command “0000 1000”.

2. B: Load Address Low Byte (0x00 - 0x02).

3. Set OE to “0”, and BS to “0”. The selected Signature byte can now be read at DATA.

4. Set OE to “1”.

28.6.14 Reading the Calibration Byte

The algorithm for reading the Calibration byte is as follows (refer to “Programming the Flash” on page 360 for

details on Command and Address loading):

1. A: Load Command “0000 1000”.

2. B: Load Address Low Byte, 0x00.

3. Set OE to “0”, and BS1 to “1”. The Calibration byte can now be read at DATA.

4. Set OE to “1”.

28.6.15 Parallel Prog ramming Characteristics

Figure 28-7. Paral lel Programming Tim ing, Including some General Timin g Requirements

Lock Bits 0

BS2

Fuse High Byte

BS1

DATA

Fuse Low Byte 0

BS2

Extended Fuse Byte

Data & Contol

(DATA, XA0/1, BS1, BS2)

XTAL1

XHXL

WLWH

DVXH

XLDX

PLWL

WLRH

RDY/BSY

PAGEL

PHPL

PLBX

BVPH

XLWL

WLBX

BVWL

WLRL

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Figure 28-8. Parallel Programming Timin g, Lo ading Sequence with Timing Requirements(1)

Note: 1. The timing requirements shown in Figure 28-7 (i.e., tDVXH, tXHXL, and tXLDX) also apply to loading operation.

Figure 28-9. Parallel Programming Timing, Reading Sequence (withi n the Same Page) with Timing

Requirements(1)

Note: 1. The timing requirements shown in Figure 28-7 (i.e., tDVXH, tXHXL, and tXLDX) also apply to reading operation.

Table 28-13. Parallel Prog ramming Characteristics, VCC = 5V ± 10%

Symbol Parameter Min. Typ. Max. Units

VPP Programming Enable Voltage 11.5 12.5 V

IPP Programming Enable Current 250 A

XTAL1

PAGEL

PLXH

XLXH

XLPH

ADDR0 (Low Byte) DATA (Low Byte) DATA (High Byte) ADDR1 (Low Byte)

DATA

BS1

XA0

XA1

LOAD ADDRESS

(LOW BYTE) LOAD DATA

(HIGH BYTE)

LOAD DATA

LOAD ADDRESS

(LOW BYTE)

XTAL1

ADDR0 (Low Byte) DATA (Low Byte) DATA (High Byte) ADDR1 (Low Byte)

DATA

BS1

XA0

XA1

LOAD ADDRESS

(LOW BYTE) READ DATA

(HIGH BYTE) LOAD ADDRESS

(LOW BYTE)

BVDV

OLDV

XLOL

OHDZ

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Notes: 1. tWLRH is valid for the Write Flash, Write EEPROM, Write Fuse bits and Write Lock bits commands.

2. tWLRH_CE is valid for the Chip Erase command.

28.7 Serial Downloading

Both the Flash and EEPROM memory arrays can be programmed using a serial programming bus while RESET

is pulled to GND. The serial programming interface consists of pins SCK, PDI (input) and PDO (output). After

RESET is set low, the Programming Enable instruction needs to be executed first before program/erase

operations can be executed. NOTE, in Table 28-14 on page 368, the pin mapping for serial programming is listed.

Not all packages use the SPI pins dedicated for the internal Serial Peripheral Interface - SPI.

tDVXH Data and Control Valid before XTAL1 High 67

tXLXH XTAL1 Low to XTAL1 High 200

tXHXL XTAL1 Pulse Width High 150

tXLDX Data and Control Hold after XTAL1 Low 67

tXLWL XTAL1 Low to WR Low 0

tXLPH XTAL1 Low to PAGEL high 0

tPLXH PAGEL low to XTAL1 high 150

tBVPH BS1 Valid before PAGEL High 67

tPHPL PAGEL Pulse Width High 150

tPLBX BS1 Hold after PAGEL Low 67

tWLBX BS2/1 Hold after WR Low 67

tPLWL PAGEL Low to WR Low 67

tBVWL BS2/1 Valid to WR Low 67

tWLWH WR Pulse Width Low 150

tWLRL WR Low to RDY/BSY Low 0 1 s

tWLRH WR Low to RDY/BSY High(1) 3.7 4.5

tWLRH_CE WR Low to RDY/BSY High for Chip Erase(2) 7.5 9

tXLOL XTAL1 Low to OE Low 0

tBVDV BS1 Valid to DATA valid 0250

tOLDV OE Low to DATA Valid 250

tOHDZ OE High to DATA Tri-stated 250

Table 28-13. Parallel Prog ramming Characteristics, VCC = 5V ± 10%

Symbol Parameter Min. Typ. Max. Units

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28.8 Serial Programming Pin Mapping

Figure 28-10. Serial Programming and Verify(1)

Notes: 1. If the device is clocked by the internal Oscillator, it is no need to connect a clock source to the XTAL1 pin.

2. VCC - 0.3V < AVCC < VCC + 0.3V, however, AVCC should always be within 1.8 - 5.5V

When programming the EEPROM, an auto-erase cycle is built into the self-timed programming operation (in the

Serial mode ONLY) and there is no need to first execute the Chip Erase instruction. The Chip Erase operation

turns the content of every memory location in both the Program and EEPROM arrays into 0xFF.

Depending on CKSEL Fuses, a valid clock must be present. The minimum low and high periods for the serial

clock (SCK) input are defined as follows:

Low:> two CPU clock cycles for fck < 12MHz, three CPU clock cycles for fck >= 12MHz

High:> two CPU clock cycles for fck < 12MHz, three CPU clock cycles for fck >= 12MHz

28.8.1 Serial Programming Algorithm

When writing serial data to the device, data is clocked on the rising edge of SCK.

When reading data from the device, data is clocked on the falling edge of SCK. See Figure 28-11 on page 369

for timing details.

To program and verify the device in the serial programming mode, the following sequence is recommended (see

four byte instruction formats in Table 28-16 on page 370):

1. Power-up sequence:

Apply power between VCC and GND while RESET and SCK are set to “0”. In some systems, the pro-

grammer can not guarantee that SCK is held low during power-up. In this case, RESET must be given

a positive pulse of at least two CPU clock cycles duration after SCK has been set to “0”.

2. Wait for at least 20ms and enable serial programming by sending the Programming Enable serial

instruction to pin PDI.

3. The serial programming instructions will not work if the communication is out of synchronization. When in

sync. the second byte (0x53), will echo back when issuing the third byte of the Programming Enable

Table 28-14. Pin Mapping Serial Programming

Symbol Pins (TQFP-64) I/O Description

PDI PB2 ISerial Data in

PDO PB3 OSerial Data out

SCK PB1 ISerial Clock

VCC

GND

XTAL1

SCK

PDO

PDI

RESET

+1.8 - 5.5V

AVCC

+1.8 - 5.5V

(2)

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instruction. Whether the echo is correct or not, all four bytes of the instruction must be transmitted. If the

0x53 did not echo back, give RESET a positive pulse and issue a new Programming Enable command.

4. The Flash is programmed one page at a time. The memory page is loaded one byte at a time by supplying

the seven LSB of the address and data together with the Load Program Memory Page instruction. To

ensure correct loading of the page, the data low byte must be loaded before data high byte is applied for a

given address. The Program Memory Page is stored by loading the Write Program Memory Page

instruction with the address lines 15..8. Before issuing this command, make sure the instruction Load

Extended Address Byte has been used to define the MSB of the address. The extended address byte is

stored until the command is re-issued, i.e., the command needs only be issued for the first page, and

when crossing the 64KWord boundary. If polling (RDY/BSY) is not used, the user must wait at least

tWD_FLASH before issuing the next page. (See Table 28-15 on page 369.) Accessing the serial programming

interface before the Flash write operation completes can result in incorrect programming.

5. The EEPROM array is programmed one byte at a time by supplying the address and data together with

the appropriate Write instruction. An EEPROM memory location is first automatically erased before new

data is written. If polling is not used, the user must wait at least tWD_EEPROM before issuing the next byte.

(See Table 28-15 on page 369.) In a chip erased device, no 0xFFs in the data file(s) need to be

programmed.

6. Any memory location can be verified by using the Read instruction which returns the content at the

selected address at serial output PDO. When reading the Flash memory, use the instruction Load

Extended Address Byte to define the upper address byte, which is not included in the Read Program

Memory instruction. The extended address byte is stored until the command is re-issued, i.e., the

command needs only be issued for the first page, and when crossing the 64KWord boundary.

7. At the end of the programming session, RESET can be set high to commence normal operation.

8. Power-off sequence (if needed):

Set RESET to “1”.

Turn VCC power off.

Figure 28-1 1. Serial Programming Waveforms

Table 28-15. Minimum Wait Delay Before Writing the Next Flash or EEPROM

Symbol Minimum Wait Delay

tWD_FLASH 4.5ms

tWD_EEPROM 9.0ms

tWD_ERASE 9.0ms

MSB

LSB

SERIAL CLOCK INPUT

(SCK)

SERIAL DATA INPUT

(MOSI)

(MISO)

SAMPLE

SERIAL DATA OUTPUT

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Table 28-16. Serial Programming Instruction Set

Instruction

Instruction Format

OperationByte 1 Byte 2 Byte 3 Byte4

Programming Enable 1010 1100 0101 0011 xxxx xxxx xxxx xxxx Enable Serial Programming after

RESET goes low.

Chip Erase 1010 1100 100x xxxx xxxx xxxx xxxx xxxx Chip Erase EEPROM and Flash.

Load Extended Address Byte

0100 1101 0000 0000 cccc cccc xxxx xxxx Defines Extended Address Byte for

Read Program Memory and Write

Program Memory Page.

Read Program Memory

0010 H000 aaaa aaaa bbbb bbbb oooo oooo Read H (high or low) data o from

Program memory at word address

c:a:b.

Load Program Memory Page

0100 H000 xxxx xxxx xxbb bbbb iiii iiii Write H (high or low) data i to Program

Memory page at word address b. Data

low byte must be loaded before Data

high byte is applied within the same

address.

Write Program Memory Page 0100 1100 aaaa aaaa bbxx xxxx xxxx xxxx Write Program Memory Page at

address c:a:b.

Read EEPROM Memory 1010 0000 0000 aaaa bbbb bbbb oooo oooo Read data o from EEPROM memory at

address a:b.

Write EEPROM Memory 1100 0000 0000 aaaa bbbb bbbb iiii iiii Write data i to EEPROM memory at

address a:b.

Load EEPROM Memory

Page (page access)

1100 0001 0000 0000 0000 00bb iiii iiii Load data i to EEPROM memory page

buffer. After data is loaded, program

EEPROM page.

Write EEPROM Memory

Page (page access)

1100 0010 0000 aaaa bbbb bb00 xxxx xxxx Write EEPROM page at address a:b.

Read Lock bits

0101 1000 0000 0000 xxxx xxxx xxoo oooo Read Lock bits. “0” = programmed, “1”

= unprogrammed. See Table 28-1 on

page 353 for details.

Write Lock bits

1010 1100 111x xxxx xxxx xxxx 11ii iiii Write Lock bits. Set bits = “0” to

program Lock bits. See Table 28-1 on

page 353 for details.

Read Signature Byte 0011 0000 000x xxxx xxxx xxbb oooo oooo Read Signature Byte o at address b.

Write Fuse bits 1010 1100 1010 0000 xxxx xxxx iiii iiii Set bits = “0” to program, “1” to

unprogram.

Write Fuse High bits 1010 1100 1010 1000 xxxx xxxx iiii iiii Set bits = “0” to program, “1” to

unprogram.

Write Extended Fuse Bits

1010 1100 1010 0100 xxxx xxxx iiii iiii Set bits = “0” to program, “1” to

unprogram. See Table 28-3 on

page 354 for details.

Read Fuse bits 0101 0000 0000 0000 xxxx xxxx oooo oooo Read Fuse bits. “0” = programmed, “1”

= unprogrammed.

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Note: a = address high bits, b = address low bits, c = address extended bits, H = 0 - Low byte, 1 - High Byte, o = data out, i = data in,

x = don’t care.

28.8.2 Serial Programming Characteristics

For characteristics of the Serial Programming module see “SPI Timing Characteristics” on page 388.

28.9 Programming via the JTAG Interface

Programming through the JTAG interface requires control of the four JTAG specific pins: TCK, TMS, TDI, and

TDO. Control of the reset and clock pins is not required.

To be able to use the JTAG interface, the JTAGEN Fuse must be programmed. The device is default shipped

with the fuse programmed. In addition, the JTD bit in MCUCSR must be cleared. Alternatively, if the JTD bit is

set, the external reset can be forced low. Then, the JTD bit will be cleared after two chip clocks, and the JTAG

pins are available for programming. This provides a means of using the JTAG pins as normal port pins in

Running mode while still allowing In-System Programming via the JTAG interface. Note that this technique can

not be used when using the JTAG pins for Boundary-scan or On-chip Debug. In these cases the JTAG pins

must be dedicated for this purpose.

During programming the clock frequency of the TCK Input must be less than the maximum frequency of the

chip. The System Clock Prescaler can not be used to divide the TCK Clock Input into a sufficiently low

frequency.

As a definition in this datasheet, the LSB is shifted in and out first of all Shift Registers.

28.9.1 Programming Specific JTAG Instructions

The Instruction Register is 4-bit wide, supporting up to 16 instructions. The JTAG instructions useful for

programming are listed below.

The OPCODE for each instruction is shown behind the instruction name in hex format. The text describes which

Data Register is selected as path between TDI and TDO for each instruction.

The Run-Test/Idle state of the TAP controller is used to generate internal clocks. It can also be used as an idle

state between JTAG sequences. The state machine sequence for changing the instruction word is shown in

Figure 28-12 on page 372.

Read Fuse High bits 0101 1000 0000 1000 xxxx xxxx oooo oooo Read Fuse High bits. “0” = pro-

grammed, “1” = unprogrammed.

Read Extended Fuse Bits

0101 0000 0000 1000 xxxx xxxx oooo oooo Read Extended Fuse bits. “0” = pro-

grammed, “1” = unprogrammed. See

Table 28-3 on page 354 for details.

Read Calibration Byte 0011 1000 000x xxxx 0000 0000 oooo oooo Read Calibration Byte

Poll RDY/BSY

1111 0000 0000 0000 xxxx xxxx xxxx xxxoIf o = “1”, a programming operation is

still busy. Wait until this bit returns to

“0” before applying another command.

Table 28-16. Serial Programming Instruction Set

Instruction

Instruction Format

OperationByte 1 Byte 2 Byte 3 Byte4

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Figure 28-12. State Mach ine Sequence for Changing the Instruction Word

28.9.2 AVR_RESET (0xC)

The AVR specific public JTAG instruction for setting the AVR device in the Reset mode or taking the device out

from the Reset mode. The TAP controller is not reset by this instruction. The one bit Reset Register is selected

as Data Register. Note that the reset will be active as long as there is a logic “one” in the Reset Chain. The

output from this chain is not latched.

The active states are:

Shift-DR: The Reset Register is shifted by the TCK input.

28.9.3 PROG_ENABLE (0x4)

The AVR specific public JTAG instruction for enabling programming via the JTAG port. The 16-bit Programming

Enable Register is selected as Data Register. The active states are the following:

Shift-DR: The programming enable signature is shifted into the Data Register

Update-DR: The programming enable signature is compared to the correct value, and Programming

mode is entered if the signature is valid

28.9.4 PROG_COMMANDS (0x5)

The AVR specific public JTAG instruction for entering programming commands via the JTAG port. The 15-bit

Programming Command Register is selected as Data Register. The active states are the following:

Test-Logic-Reset

Run-Test/Idle

Shift-DR

Exit1-DR

Pause-DR

Exit2-DR

Update-DR

Select-IR Scan

Capture-IR

Shift-IR

Exit1-IR

Pause-IR

Exit2-IR

Update-IR

Select-DR Scan

Capture-DR

011 1

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Capture-DR: The result of the previous command is loaded into the Data Register

Shift-DR: The Data Register is shifted by the TCK input, shifting out the result of the previous command

and shifting in the new command

Update-DR: The programming command is applied to the Flash inputs

Run-Test/Idle: One clock cycle is generated, executing the applied command

28.9.5 PROG_PAGELOAD (0x6)

The AVR specific public JTAG instruction to directly load the Flash data page via the JTAG port. An 8-bit Flash

Data Byte Register is selected as the Data Register. This is physically the 8 LSBs of the Programming

Command Register. The active states are the following:

Shift-DR: The Flash Data Byte Register is shifted by the TCK input

Update-DR: The content of the Flash Data Byte Register is copied into a temporary register. A write

sequence is initiated that within 11 TCK cycles loads the content of the temporary register into the Flash

page buffer. The AVR automatically alternates between writing the low and the high byte for each new

Update-DR state, starting with the low byte for the first Update-DR encountered after entering the

PROG_PAGELOAD command. The Program Counter is pre-incremented before writing the low byte,

except for the first written byte. This ensures that the first data is written to the address set up by

PROG_COMMANDS, and loading the last location in the page buffer does not make the program counter

increment into the next page.

28.9.6 PROG_PAGEREAD (0x7)

The AVR specific public JTAG instruction to directly capture the Flash content via the JTAG port. An 8-bit Flash

Data Byte Register is selected as the Data Register. This is physically the 8 LSBs of the Programming

Command Register. The active states are the following:

Capture-DR: The content of the selected Flash byte is captured into the Flash Data Byte Register. The

AVR automatically alternates between reading the low and the high byte for each new Capture-DR state,

starting with the low byte for the first Capture-DR encountered after entering the PROG_PAGEREAD

command. The Program Counter is post-incremented after reading each high byte, including the first read

byte. This ensures that the first data is captured from the first address set up by PROG_COMMANDS,

and reading the last location in the page makes the program counter increment into the next page.

Shift-DR: The Flash Data Byte Register is shifted by the TCK input

28.9.7 Data Registers

The Data Registers are selected by the JTAG instruction registers described in section “Programming Specific

JTAG Instructions” on page 371. The Data Registers relevant for programming operations are:

Reset Register

Programming Enable Register

Programming Command Register

Flash Data Byte Register

28.9.8 Reset Regist er

The Reset Register is a Test Data Register used to reset the part during programming. It is required to reset the

part before entering Programming mode.

A high value in the Reset Register corresponds to pulling the external reset low. The part is reset as long as

there is a high value present in the Reset Register. Depending on the Fuse settings for the clock options, the

part will remain reset for a Reset Time-out period (refer to “Clock Sources” on page 28) after releasing the Reset

Figure 8-1 on page 51.

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28.9.9 Programming Enable Register

The Programming Enable Register is a 16-bit register. The contents of this register is compared to the

programming enable signature, binary code 0b1010_0011_0111_0000. When the contents of the register is

equal to the programming enable signature, programming via the JTAG port is enabled. The register is reset to

0 on Power-on Reset, and should always be reset when leaving Programming mode.

Figure 28-13. Programming Enable Register

28.9.10 Programming Command Register

The Programming Command Register is a 15-bit register. This register is used to serially shift in programming

commands, and to serially shift out the result of the previous command, if any. The JTAG Programming

Instruction Set is shown in Figure 28-15 on page 378. The state sequence when shifting in the programming

commands is illustrated in Figure 28-15 on page 378.

Figure 28-14. Programming Comm and Register

TDI

TDO

=DQ

ClockDR & PROG_ENABLE

Programming Enable

0xA370

TDI

TDO

Flash

EEPROM

Fuses

Lock Bits

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Table 28-17. JTAG Programming Instruction

Set a = address high bits, b = address low bits, c = address extended bits, H = 0 - Low byte, 1 - High Byte, o = data out,

i = data in, x = don’t care

Instruction TDI Sequence TDO Sequence Notes

1a. Chip Erase

0100011_10000000

0110001_10000000

0110011_10000000

xxxxxxx_xxxxxxxx

1b. Poll for Chip Erase Complete 0110011_10000000 xxxxxox_xxxxxxxx (2)

2a. Enter Flash Write 0100011_00010000 xxxxxxx_xxxxxxxx

2b. Load Address Extended High Byte 0001011_cccccccc xxxxxxx_xxxxxxxx (10)

2c. Load Address High Byte 0000111_aaaaaaaa xxxxxxx_xxxxxxxx

2d. Load Address Low Byte 0000011_bbbbbbbb xxxxxxx_xxxxxxxx

2e. Load Data Low Byte 0010011_iiiiiiii xxxxxxx_xxxxxxxx

2f. Load Data High Byte 0010111_iiiiiiii xxxxxxx_xxxxxxxx

2g. Latch Data

0110111_00000000

1110111_00000000

0110111_00000000

xxxxxxx_xxxxxxxx

(1)

2h. Write Flash Page

0110111_00000000

0110101_00000000

0110111_00000000

xxxxxxx_xxxxxxxx

(1)

2i. Poll for Page Write Complete 0110111_00000000 xxxxxox_xxxxxxxx (2)

3a. Enter Flash Read 0100011_00000010 xxxxxxx_xxxxxxxx

3b. Load Address Extended High Byte 0001011_cccccccc xxxxxxx_xxxxxxxx (10)

3c. Load Address High Byte 0000111_aaaaaaaa xxxxxxx_xxxxxxxx

3d. Load Address Low Byte 0000011_bbbbbbbb xxxxxxx_xxxxxxxx

3e. Read Data Low and High Byte

0110010_00000000

0110110_00000000

0110111_00000000

xxxxxxx_xxxxxxxx

xxxxxxx_oooooooo

Low byte

High byte

4a. Enter EEPROM Write 0100011_00010001 xxxxxxx_xxxxxxxx

4b. Load Address High Byte 0000111_aaaaaaaa xxxxxxx_xxxxxxxx (10)

4c. Load Address Low Byte 0000011_bbbbbbbb xxxxxxx_xxxxxxxx

4d. Load Data Byte 0010011_iiiiiiii xxxxxxx_xxxxxxxx

4e. Latch Data

0110111_00000000

1110111_00000000

0110111_00000000

xxxxxxx_xxxxxxxx

(1)

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4f. Write EEPROM Page

0110011_00000000

0110001_00000000

0110011_00000000

xxxxxxx_xxxxxxxx

(1)

4g. Poll for Page Write Complete 0110011_00000000 xxxxxox_xxxxxxxx (2)

5a. Enter EEPROM Read 0100011_00000011 xxxxxxx_xxxxxxxx

5b. Load Address High Byte 0000111_aaaaaaaa xxxxxxx_xxxxxxxx (10)

5c. Load Address Low Byte 0000011_bbbbbbbb xxxxxxx_xxxxxxxx

5d. Read Data Byte

0110011_bbbbbbbb

0110010_00000000

0110011_00000000

xxxxxxx_xxxxxxxx

xxxxxxx_oooooooo

6a. Enter Fuse Write 0100011_01000000 xxxxxxx_xxxxxxxx

6b. Load Data Low Byte(6) 0010011_iiiiiiii xxxxxxx_xxxxxxxx (3)

6c. Write Fuse Extended Byte

0111011_00000000

0111001_00000000

0111011_00000000

xxxxxxx_xxxxxxxx

(1)

6d. Poll for Fuse Write Complete 0110111_00000000 xxxxxox_xxxxxxxx (2)

6e. Load Data Low Byte(7) 0010011_iiiiiiii xxxxxxx_xxxxxxxx (3)

6f. Write Fuse High Byte

0110111_00000000

0110101_00000000

0110111_00000000

xxxxxxx_xxxxxxxx

(1)

6g. Poll for Fuse Write Complete 0110111_00000000 xxxxxox_xxxxxxxx (2)

6h. Load Data Low Byte(7) 0010011_iiiiiiii xxxxxxx_xxxxxxxx (3)

6i. Write Fuse Low Byte

0110011_00000000

0110001_00000000

0110011_00000000

xxxxxxx_xxxxxxxx

(1)

6j. Poll for Fuse Write Complete 0110011_00000000 xxxxxox_xxxxxxxx (2)

7a. Enter Lock Bit Write 0100011_00100000 xxxxxxx_xxxxxxxx

7b. Load Data Byte(9) 0010011_11iiiiii xxxxxxx_xxxxxxxx (4)

Table 28-17. JTAG Programming Instruction

Set a = address high bits, b = address low bits, c = address extended bits, H = 0 - Low byte, 1 - High Byte, o = data out,

i = data in, x = don’t care

Instruction TDI Sequence TDO Sequence Notes

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Notes: 1. This command sequence is not required if the seven MSB are correctly set by the previous command sequence (which is

normally the case).

2. Repeat until o = “1”.

3. Set bits to “0” to program the corresponding Fuse, “1” to unprogram the Fuse.

4. Set bits to “0” to program the corresponding Lock bit, “1” to leave the Lock bit unchanged.

5. “0” = programmed, “1” = unprogrammed.

6. The bit mapping for Fuses Extended byte is listed in Table 28-3 on page 354.

7. The bit mapping for Fuses High byte is listed in Table 28-4 on page 355.

7c. Write Lock Bits

0110011_00000000

0110001_00000000

0110011_00000000

xxxxxxx_xxxxxxxx

(1)

7d. Poll for Lock Bit Write complete 0110011_00000000 xxxxxox_xxxxxxxx (2)

8a. Enter Fuse/Lock Bit Read 0100011_00000100 xxxxxxx_xxxxxxxx

8b. Read Extended Fuse Byte(6) 0111010_00000000

0111011_00000000

xxxxxxx_xxxxxxxx

xxxxxxx_oooooooo

8c. Read Fuse High Byte(7) 0111110_00000000

0111111_00000000

xxxxxxx_xxxxxxxx

xxxxxxx_oooooooo

8d. Read Fuse Low Byte(8) 0110010_00000000

0110011_00000000

xxxxxxx_xxxxxxxx

xxxxxxx_oooooooo

8e. Read Lock Bits(9) 0110110_00000000

0110111_00000000

xxxxxxx_xxxxxxxx

xxxxxxx_xxoooooo (5)

8f. Read Fuses and Lock Bits

0111010_00000000

0111110_00000000

0110010_00000000

0110110_00000000

0110111_00000000

xxxxxxx_xxxxxxxx

xxxxxxx_oooooooo

(5)

Fuse Ext. byte

Fuse High byte

Fuse Low byte

Lock bits

9a. Enter Signature Byte Read 0100011_00001000 xxxxxxx_xxxxxxxx

9b. Load Address Byte 0000011_bbbbbbbb xxxxxxx_xxxxxxxx

9c. Read Signature Byte 0110010_00000000

0110011_00000000

xxxxxxx_xxxxxxxx

xxxxxxx_oooooooo

10a. Enter Calibration Byte Read 0100011_00001000 xxxxxxx_xxxxxxxx

10b. Load Address Byte 0000011_bbbbbbbb xxxxxxx_xxxxxxxx

10c. Read Calibration Byte 0110110_00000000

0110111_00000000

xxxxxxx_xxxxxxxx

xxxxxxx_oooooooo

11a. Load No Operation Command 0100011_00000000

0110011_00000000

xxxxxxx_xxxxxxxx

Table 28-17. JTAG Programming Instruction

Set a = address high bits, b = address low bits, c = address extended bits, H = 0 - Low byte, 1 - High Byte, o = data out,

i = data in, x = don’t care

Instruction TDI Sequence TDO Sequence Notes

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8. The bit mapping for Fuses Low byte is listed in Table 28-5 on page 355.

9. The bit mapping for Lock bits byte is listed in Table 28-1 on page 353.

10. Address bits exceeding PCMSB and EEAMSB (Table 28-11 on page 359 and Table 28-12 on page 359) are don’t care.

11. All TDI and TDO sequences are represented by binary digits (0b...).

Figure 28-15. State Mach ine Sequence for Changing/Reading th e Data Word

28.9.11 Flash Data Byte Register

The Flash Data Byte Register provides an efficient way to load the entire Flash page buffer before executing

Page Write, or to read out/verify the content of the Flash. A state machine sets up the control signals to the

Flash and senses the strobe signals from the Flash, thus only the data words need to be shifted in/out.

The Flash Data Byte Register actually consists of the 8-bit scan chain and a 8-bit temporary register. During

page load, the Update-DR state copies the content of the scan chain over to the temporary register and initiates

a write sequence that within 11 TCK cycles loads the content of the temporary register into the Flash page

buffer. The AVR automatically alternates between writing the low and the high byte for each new Update-DR

state, starting with the low byte for the first Update-DR encountered after entering the PROG_PAGELOAD

command. The Program Counter is pre-incremented before writing the low byte, except for the first written byte.

This ensures that the first data is written to the address set up by PROG_COMMANDS, and loading the last

location in the page buffer does not make the Program Counter increment into the next page.

During Page Read, the content of the selected Flash byte is captured into the Flash Data Byte Register during

the Capture-DR state. The AVR automatically alternates between reading the low and the high byte for each

Test-Logic-Reset

Run-Test/Idle

Shift-DR

Exit1-DR

Pause-DR

Exit2-DR

Update-DR

Select-IR Scan

Capture-IR

Shift-IR

Exit1-IR

Pause-IR

Exit2-IR

Update-IR

Select-DR Scan

Capture-DR

011 1

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new Capture-DR state, starting with the low byte for the first Capture-DR encountered after entering the

PROG_PAGEREAD command. The Program Counter is post-incremented after reading each high byte,

including the first read byte. This ensures that the first data is captured from the first address set up by

PROG_COMMANDS, and reading the last location in the page makes the program counter increment into the

next page.

Figure 28-16. Flash Data Byte Register

The state machine controlling the Flash Data Byte Register is clocked by TCK. During normal operation in which

eight bits are shifted for each Flash byte, the clock cycles needed to navigate through the TAP controller

automatically feeds the state machine for the Flash Data Byte Register with sufficient number of clock pulses to

complete its operation transparently for the user. However, if too few bits are shifted between each Update-DR

state during page load, the TAP controller should stay in the Run-Test/Idle state for some TCK cycles to ensure

that there are at least 11 TCK cycles between each Update-DR state.

28.9.12 Programming Algorithm

All references below of type “1a”, “1b”, and so on, refer to Table 28-17 on page 375.

28.9.13 Entering Programming Mode

1. Enter JTAG instruction AVR_RESET and shift 1 in the Reset Register.

2. Enter instruction PROG_ENABLE and shift 0b1010_0011_0111_0000 in the Programming Enable

28.9.14 Leavi ng Pro gr amming Mode

1. Enter JTAG instruction PROG_COMMANDS.

2. Disable all programming instructions by using no operation instruction 11a.

3. Enter instruction PROG_ENABLE and shift 0b0000_0000_0000_0000 in the programming Enable

4. Enter JTAG instruction AVR_RESET and shift 0 in the Reset Register.

TDI

TDO

Flash

EEPROM

Fuses

Lock Bits

STROBES

ADDRESS

State

Machine

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28.9.15 Performing Chip Erase

1. Enter JTAG instruction PROG_COMMANDS.

2. Start Chip Erase using programming instruction 1a.

3. Poll for Chip Erase complete using programming instruction 1b, or wait for tWLRH_CE (refer to Table 28-13

on page 366).

28.9.16 Programming the Flash

Before programming the Flash a Chip Erase must be performed, see “Performing Chip Erase” on page 380.

1. Enter JTAG instruction PROG_COMMANDS.

2. Enable Flash write using programming instruction 2a.

3. Load address Extended High byte using programming instruction 2b.

4. Load address High byte using programming instruction 2c.

5. Load address Low byte using programming instruction 2d.

6. Load data using programming instructions 2e, 2f, and 2g.

7. Repeat steps 5 and 6 for all instruction words in the page.

8. Write the page using programming instruction 2h.

9. Poll for Flash write complete using programming instruction 2i, or wait for tWLRH (refer to Table 28-13 on

page 366).

10. Repeat steps 3 to 9 until all data have been programmed.

A more efficient data transfer can be achieved using the PROG_PAGELOAD instruction:

1. Enter JTAG instruction PROG_COMMANDS.

2. Enable Flash write using programming instruction 2a.

3. Load the page address using programming instructions 2b, 2c, and 2d. PCWORD (refer to Table 28-11

on page 359) is used to address within one page and must be written as 0.

4. Enter JTAG instruction PROG_PAGELOAD.

5. Load the entire page by shifting in all instruction words in the page byte-by-byte, starting with the LSB of

the first instruction in the page and ending with the MSB of the last instruction in the page. Use Update-

DR to copy the contents of the Flash Data Byte Register into the Flash page location and to auto-

increment the Program Counter before each new word.

6. Enter JTAG instruction PROG_COMMANDS.

7. Write the page using programming instruction 2h.

8. Poll for Flash write complete using programming instruction 2i, or wait for tWLRH (refer to Table 28-13 on

page 366).

9. Repeat steps 3 to 8 until all data have been programmed.

28.9.17 Reading the Flash

1. Enter JTAG instruction PROG_COMMANDS.

2. Enable Flash read using programming instruction 3a.

3. Load address using programming instructions 3b, 3c and 3d.

4. Read data using programming instruction 3e.

5. Repeat steps 3 and 4 until all data have been read.

A more efficient data transfer can be achieved using the PROG_PAGEREAD instruction:

1. Enter JTAG instruction PROG_COMMANDS.

2. Enable Flash read using programming instruction 3a.

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3. Load the page address using programming instructions 3b, 3c, and 3d. PCWORD (refer to Table 28-11

on page 359) is used to address within one page and must be written as 0.

4. Enter JTAG instruction PROG_PAGEREAD.

5. Read the entire page (or Flash) by shifting out all instruction words in the page (or Flash), starting with the

LSB of the first instruction in the page (Flash) and ending with the MSB of the last instruction in the page

(Flash). The Capture-DR state both captures the data from the Flash, and also auto-increments the

program counter after each word is read. Note that Capture-DR comes before the shift-DR state. Hence,

the first byte which is shifted out contains valid data.

6. Enter JTAG instruction PROG_COMMANDS.

7. Repeat steps 3 to 6 until all data have been read.

28.9.18 Programming the EEPROM

Before programming the EEPROM a Chip Erase must be performed, see “Performing Chip Erase” on page 380.

1. Enter JTAG instruction PROG_COMMANDS.

2. Enable EEPROM write using programming instruction 4a.

3. Load address High byte using programming instruction 4b.

4. Load address Low byte using programming instruction 4c.

5. Load data using programming instructions 4d and 4e.

6. Repeat steps 4 and 5 for all data bytes in the page.

7. Write the data using programming instruction 4f.

8. Poll for EEPROM write complete using programming instruction 4g, or wait for tWLRH (refer to Table 28-13

on page 366).

9. Repeat steps 3 to 8 until all data have been programmed.

Note that the PROG_PAGELOAD instruction can not be used when programming the EEPROM.

28.9.19 Reading the EEPROM

1. Enter JTAG instruction PROG_COMMANDS.

2. Enable EEPROM read using programming instruction 5a.

3. Load address using programming instructions 5b and 5c.

4. Read data using programming instruction 5d.

5. Repeat steps 3 and 4 until all data have been read.

Note that the PROG_PAGEREAD instruction can not be used when reading the EEPROM.

28.9.20 Programming the Fuses

1. Enter JTAG instruction PROG_COMMANDS.

2. Enable Fuse write using programming instruction 6a.

3. Load data high byte using programming instructions 6b. A bit value of “0” will program the corresponding

fuse, a “1” will unprogram the fuse.

4. Write Fuse High byte using programming instruction 6c.

5. Poll for Fuse write complete using programming instruction 6d, or wait for tWLRH (refer to Table 28-13 on

page 366).

6. Load data low byte using programming instructions 6e. A “0” will program the fuse, a “1” will unprogram

the fuse.

7. Write Fuse low byte using programming instruction 6f.

8. Poll for Fuse write complete using programming instruction 6g, or wait for tWLRH (refer to Table 28-13 on

page 366).

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28.9.21 Programming the Lock Bits

1. Enter JTAG instruction PROG_COMMANDS.

2. Enable Lock bit write using programming instruction 7a.

3. Load data using programming instructions 7b. A bit value of “0” will program the corresponding lock bit, a

“1” will leave the lock bit unchanged.

4. Write Lock bits using programming instruction 7c.

5. Poll for Lock bit write complete using programming instruction 7d, or wait for tWLRH (refer to Table 28-13

on page 366).

28.9.22 Reading the Fuses and Lock Bits

1. Enter JTAG instruction PROG_COMMANDS.

2. Enable Fuse/Lock bit read using programming instruction 8a.

3. To read all Fuses and Lock bits, use programming instruction 8e.

To only read Fuse High byte, use programming instruction 8b.

To only read Fuse Low byte, use programming instruction 8c.

To only read Lock bits, use programming instruction 8d.

28.9.23 Reading the Signature Bytes

1. Enter JTAG instruction PROG_COMMANDS.

2. Enable Signature byte read using programming instruction 9a.

3. Load address 0x00 using programming instruction 9b.

4. Read first signature byte using programming instruction 9c.

5. Repeat steps 3 and 4 with address 0x01 and address 0x02 to read the second and third signature bytes,

respectively.

28.9.24 Reading the Calibration Byte

1. Enter JTAG instruction PROG_COMMANDS.

2. Enable Calibration byte read using programming instruction 10a.

3. Load address 0x00 using programming instruction 10b.

4. Read the calibration byte using programming instruction 10c.

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29. Electrical Characteristics

29.1 Absolute Maximum Ratings*

29.2 DC Characteristics

Operating Temperature. . . . . . . . . . . . -40C to +85C*NOTICE: Stresses beyond those listed under “Absolute

Maximum Ratings” may cause permanent dam-

age to the device. This is a stress rating only

and functional operation of the device at these

or other conditions beyond those indicated in

the operational sections of this specification is

not implied. Exposure to absolute maximum rat-

ing conditions for extended periods may affect

device reliability.

Storage Temperature . . . . . . . . . . . . -65°C to +150°C

Voltage on any Pin except RESET and VBUS

with respect to Ground(8) . . . . . . . . -0.5V to VCC+0.5V

Voltage on RESET with respect to Ground-0.5V to +13.0V

Voltage on VBUS with respect to Ground-0.5V to +6.0V

Maximum Operating Voltage . . . . . . . . . . . . . . . . 6.0V

DC Current per I/O Pin . . . . . . . . . . . . . . . . . . 40.0mA

DC Current VCC and GND Pins . . . . . . . . . . 200.0mA

Table 29-1. DC Characteristic, TA = -40°C to 85°C, VCC = 2.7V to 5.5V (unless otherwise noted)

Symbol Parameter Condition Min.(5) Typ. Max.(5) Units

VIL

Input Low Voltage,

Except XTAL1 and

Reset pin

VCC = 2.7V - 5.5V -0.5

0.2VCC-

0.1V(1)

(LVTTL)

VIL1

Input Low Voltage,

XTAL1 pin VCC = 2.7V - 5.5V -0.5 0.1VCC(1)

VIL2

Input Low Voltage,

RESET pin VCC = 2.7V - 5.5V -0.5 0.1VCC(1)

VIH

Input High Voltage,

Except XTAL1 and

RESET pins

VCC = 2.7V - 5.5V

0.2VCC+0.9

V(2)

(LVTTL)

VCC + 0.5

VIH1

Input High Voltage,

XTAL1 pin VCC = 2.7V - 5.5V 0.7VCC(2) VCC + 0.5

VIH2

Input High Voltage,

RESET pin VCC = 2.7V - 5.5V 0.9VCC(2) VCC + 0.5

VOL Output Low Voltage(3), IOL = 10mA, VCC = 5V

IOL = 5mA, VCC = 3V

0.7

0.5

VOH Output High Voltage(4), IOH = -10mA, VCC = 5V

IOH = -5mA, VCC = 3V

4.2

2.3

IIL

Input Leakage

Current I/O Pin

VCC = 5.5V, pin low

(absolute value) 1µA

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Note: 1. "Max" means the highest value where the pin is guaranteed to be read as low

2. "Min" means the lowest value where the pin is guaranteed to be read as high

3. Although each I/O port can sink more than the test conditions (20mA at VCC = 5V, 10mA at VCC = 3V) under steady state

conditions (non-transient), the following must be observed:

ATmega16U4/ATmega32U4:

1.)The sum of all IOL, for ports A0-A7, G2, C4-C7 should not exceed 100mA.

2.)The sum of all IOL, for ports C0-C3, G0-G1, D0-D7 should not exceed 100mA.

3.)The sum of all IOL, for ports G3-G5, B0-B7, E0-E7 should not exceed 100mA.

4.)The sum of all IOL, for ports F0-F7 should not exceed 100mA.

If IOL exceeds the test condition, VOL may exceed the related specification. Pins are not guaranteed to sink current

greater than the listed test condition.

4. Although each I/O port can source more than the test conditions (20mA at VCC = 5V, 10mA at VCC = 3V) under steady

state conditions (non-transient), the following must be observed:

ATmega16U4/ATmega32U4:

IIH

Input Leakage

Current I/O Pin

VCC = 5.5V, pin high

(absolute value) 1µA

RRST Reset Pull-up Resistor 30 60

k

RPU I/O Pin Pull-up Resistor 20 50

ICC

Power Supply Current(6)

Active 4MHz, VCC = 3V

(ATmega16U4/ATmega32U4)5

Active 8MHz, VCC = 5V

(ATmega16U4/ATmega32U4)10 15

Active 16MHz, VCC = 5V

(ATmega16U4/ATmega32U4)27

Idle 4MHz, VCC = 3V

(ATmega16U4/ATmega32U4)2

Idle 8MHz, VCC = 5V

(ATmega16U4/ATmega32U4)6

Power-down mode

WDT enabled, VCC = 3V,

Regulator Disabled <10 12

µA

WDT disabled, VCC =

3V,Regulator Disabled 1 5

VACIO

Analog Comparator

Input Offset Voltage

VCC = 5V

Vin = VCC/2 <10 40 mV

IACLK

Analog Comparator

Input Leakage Current

VCC = 5V

Vin = VCC/2 -50 50 nA

tACID

Analog Comparator

Propagation Delay

VCC = 2.7V

VCC = 4.0V

750

500 ns

Rusb USB Series resistor

(external) 22±5% 

Vreg Regulator Output

Voltage

CUCAP = 1µF ±20%,

UVcc  4.0V, I80mA(7),

UVcc  3.4V, I55mA(7)

3.0 3.3 3.6 V

Table 29-1. DC Characteristic, TA = -40°C to 85°C, VCC = 2.7V to 5.5V (unless otherwise noted)

Symbol Parameter Condition Min.(5) Typ. Max.(5) Units

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1)The sum of all IOH, for ports A0-A7, G2, C4-C7 should not exceed 100mA.

2)The sum of all IOH, for ports C0-C3, G0-G1, D0-D7 should not exceed 100mA.

3)The sum of all IOH, for ports G3-G5, B0-B7, E0-E7 should not exceed 100mA.

4)The sum of all IOH, for ports F0-F7 should not exceed 100mA.

5. All DC Characteristics contained in this datasheet are based on simulation and characterization of other AVR

microcontrollers manufactured in the same process technology. These values are preliminary values representing design

targets, and will be updated after characterization of actual silicon

6. Values with “Power Reduction Register 1 - PRR1” disabled (0x00).

7. Maximum regulator output current should be reduced by the USB buffer current required when USB is active (about

25mA). The remaining regulator output current can be used for the external application.

8. As specified on the USB Electrical chapter, the D+/D- pads can withstand voltages down to -1V applied through a 39

resistor

29.3 External Clock Drive Waveforms

Figure 29-1. External Clock Drive Waveforms

29.4 External Clock Drive

Note: All DC Characteristics contained in this datasheet are based on simulation and characterization of other AVR

microcontrollers manufactured in the same process technology.

IL1

IH1

Table 29-2. External Clock Driv e

Symbol Parameter

VCC=2.7-5.5V VCC=4.5-5.5V

UnitsMin. Max. Min. Max.

1/tCLCL Oscillator Frequency 0 8 0 16 MHz

tCLCL Clock Period 125 62.5 ns

tCHCX High Time 50 25 ns

tCLCX Low Time 50 25 ns

tCLCH Rise Time 1.6 0.5 s

tCHCL Fall Time 1.6 0.5 s

tCLCL

Change in period from one clock

cycle to the next 2 2 %

386

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29.5 System and Reset Characteristics

The Power-on Reset will not work unless the supply voltage has been below VPOT (falling).

29.6 Maximum speed vs. VCC

Maximum frequency is depending on VCC. As shown in Figure 29-2 on page 386, the Maximum Frequency vs.

VCC curve is linear between 2.7V < VCC < 5.5V.

Figure 29-2. Maximum Frequency vs. VCC

29.7 2-wire Serial Interface Characteristics

The following table describes the requirements for devices connected to the 2-wire Serial Bus. The device 2-wire Serial

Interface meets or exceeds these requirements under the noted conditions.

Timing symbols refer to Figure 29-3 on page 388.

Table 29-3. Reset and Brown-out Detection Characteristics

Symbol Parameter Condition Min. Typ. Max. Units

VPOT

Power-on Reset Threshold Voltage (rising) 1.4 2.3 V

Power-on Reset Threshold Voltage (falling)() 1.3 2.3 V

VPOR VCC Start Voltage to ensure internal Power-on Reset signal -0.1 +0.1 V

VCCRR VCC Rise Rate to ensure internal Power_on Reset signal 0.3 V/ms

VRST RESET Pin Threshold Voltage 0.2VCC 0.85VCC V

tRST Minimum pulse width on RESET Pin 5V, 25°C 400 ns

2.7V4.5V5.5V

Safe Operating Area

16 MHz

8 MHz

387

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Notes: 1. In ATmega16U4/ATmega32U4, this parameter is characterized and not 100% tested.

2. Required only for fSCL > 100kHz.

3. Cb = capacitance of one bus line in pF.

Table 29-4. 2-wire Serial Bus Requirements

Symbol Parameter Condition Min. Max. Units

VIL Input Low-voltage -0.5 0.3 VCC

VIH Input High-voltage 0.7VCC VCC + 0.5

Vhys(1) Hysteresis of Schmitt Trigger Inputs 0.05VCC(2) –

VOL(1) Output Low-voltage 3mA sink current 00.4

tr(1) Rise Time for both SDA and SCL 20 + 0.1Cb(3)(2) 300

tof(1) Output Fall Time from VIHmin to VILmax 10pF < Cb < 400pF(3) 20 + 0.1Cb(3)(2) 250

tSP(1) Spikes Suppressed by Input Filter 050(2)

IiInput Current each I/O Pin 0.1VCC < Vi < 0.9VCC -10 10 µA

Ci(1) Capacitance for each I/O Pin –10 pF

fSCL SCL Clock Frequency fCK(4) > max(16fSCL, 250kHz)(5) 0400 kHz

Rp Value of Pull-up resistor

fSCL  100kHz

fSCL > 100kHz

tHD;STA Hold Time (repeated) START Condition

fSCL  100kHz 4.0 –

µs

fSCL > 100kHz 0.6 –

tLOW Low Period of the SCL Clock

fSCL  100kHz(6) 4.7 –

fSCL > 100kHz(7) 1.3 –

tHIGH High period of the SCL clock

fSCL  100kHz 4.0 –

µs

fSCL > 100kHz 0.6 –

tSU;STA Set-up time for a repeated START condition

fSCL  100kHz 4.7 –

fSCL > 100kHz 0.6 –

tHD;DAT Data hold time

fSCL  100kHz 03.45

fSCL > 100kHz 00.9

tSU;DAT Data setup time

fSCL  100kHz 250 –

fSCL > 100kHz 100 –

tSU;STO Setup time for STOP condition

fSCL  100kHz 4.0 –

µs

fSCL > 100kHz 0.6 –

tBUF

Bus free time between a STOP and START

condition

fSCL  100kHz 4.7 –

fSCL > 100kHz 1.3 –

VCC 0.4V–

3mA

----------------------------

1000ns

-------------------



VCC 0.4V–

3mA

----------------------------

300ns

----------------

388

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4. fCK = CPU clock frequency.

5. This requirement applies to all ATmega16U4/ATmega32U4 2-wire Serial Interface operation. Other devices connected to

the 2-wire Serial Bus need only obey the general fSCL requirement.

6. The actual low period generated by the ATmega16U4/ATmega32U4 2-wire Serial Interface is (1/fSCL - 2/fCK), thus fCK

must be greater than 6MHz for the low time requirement to be strictly met at fSCL = 100kHz.

7. The actual low period generated by the ATmega16U4/ATmega32U4 2-wire Serial Interface is (1/fSCL - 2/fCK), thus the low

time requirement will not be strictly met for fSCL > 308kHz when fCK = 8MHz. Still, ATmega16U4/ATmega32U4 devices

connected to the bus may communicate at full speed (400kHz) with other ATmega16U4/ATmega32U4 devices, as well

as any other device with a proper tLOW acceptance margin.

Figure 29-3. 2-wire Serial Bus Timing

29.8 SPI Timing Characteristics

See Figure 29-4 and Figure 29-5 on page 389 for details.

Note: 1. In SPI Programming mode the minimum SCK high/low period is:

- 2 tCLCL for fCK < 12MHz

- 3 tCLCL for fCK > 12MHz

SU;STA

LOW

HIGH

LOW

HD;STA

HD;DAT

SU;DAT

SU;STO

BUF

SCL

SDA

Table 29-5. SPI Timing Parameters

Description Mode Min. Typ. Max.

1SCK period Master See Table 17-2

2SCK high/low Master 50% duty cycle

3Rise/Fall time Master 1600

4Setup Master 10

5Hold Master 10

6Out to SCK Master 0.5 • tsck

7SCK to out Master 10

8SCK to out high Master 10

9SS low to out Slave 15

10 SCK period Slave 4 • tck

11 SCK high/low(1) Slave 2 • tck

12 Rise/Fall time Slave 1600

13 Setup Slave 10

14 Hold Slave tck

15 SCK to out Slave 15

16 SCK to SS high Slave 20

17 SS high to tri-state Slave 10

18 SS low to SCK Slave 20

389

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Figure 29-4. SPI Interface Timing Requirements (Master Mode)

Figure 29-5. SPI Interface Timing Requirements (Slave Mode)

29.9 Hardware Boot Entrance Timing Characteristics

Figure 29-6. Hardware Boot Timing Requirements

MOSI

(Data Output)

SCK

(CPOL = 1)

MISO

(Data Input)

SCK

(CPOL = 0)

MSB LSB

LSBMSB

...

345

MISO

(Data Output)

SCK

(CPOL = 1)

MOSI

(Data Input)

SCK

(CPOL = 0)

MSB LSB

LSBMSB

...

11 11

1213 14

Table 29-6. Hardware Boot Timings

Symbol Parameter Min. Max.

tSHRH HWB low Setup before Reset High 0

tHHRH HWB low Hold after Reset High StartUpTime(SUT) +

Time Out Delay(TOUT)

RESET

ALE/HWB

tSHRH tHHRH

390

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Table 29-7. ADC Characteristics

Symbol Parameter Condition Min. Typ. Max. Units

Resolution

Single Ended Conversion 10

BitsDifferential conversion, gain = 1x/10x/40x 8

Differential conversion, gain = 200x 8

TUE Absolute accuracy

VREF = 4V, VCC = 4V, ADC clock = 200kHz 2.0 3.0

LSB

Gain = 1x/10x/40x, VREF = 4V, VCC = 5V,

ADC clock = 200 kHz 2.0 3.0

Gain = 200x, VREF = 4V, VCC = 5V,

ADC clock = 200kHz 2.0 4.0

INL Integral Non-Linearity

VREF = 4V, VCC = 4V, ADC clock = 200kHz 0.5 1.5

LSB

Gain = 1x/10x/40x, VREF = 4V, VCC = 5V,

ADC clock = 200kHz 0.3 1.5

Gain = 200x, VREF = 4V, VCC = 5V,

ADC clock = 200kHz 0.5 1.5

DNL Differential Non-Linearity

VREF = 4V, VCC = 4V, ADC clock = 200kHz 0.4 0.7

LSB

Gain = 1x/10x/40x, VREF = 4V, VCC = 5V,

ADC clock = 200kHz 0.3 1.0

Gain = 200x, VREF = 4V, VCC = 5V,

ADC clock = 200kHz 0.6 1.0

Gain Error

VREF = 4V, VCC = 4V, ADC clock = 200kHz -2.5 -1.0 2.5

LSB

Gain = 1x/10x/40x, VREF = 4V, VCC = 5V,

ADC clock = 200kHz 0.0 -1.5 -2.5

Gain = 200x, VREF = 4V, VCC = 5V,

ADC clock = 200kHz 0.0 -1.8 -3.0

Offset Error

VREF = 4V, VCC = 4V, ADC clock = 200kHz -2.5 1.5 2.5

LSB

VREF= 4V, VCC = 5V, ADC clock = 200kHz,

Differential mode -2.0 0.0 2.0

VREF Reference Voltage

Single Ended Conversion 2.56 AVCC

Differential Conversion 2.56 AVCC - 0.5

AVCC Analog Supply Voltage VCC -

0.3 VCC + 0.3 V

VIN Input Voltage

Single ended channels GND VREF V

Differential Conversion 0AVCC

Input Bandwidth

Single Ended Channels 38.5

kHz

Differential Channels 4

391

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VINT

Internal Voltage

Reference 2.56V 2.4 2.56 2.8 V

RREF

Reference Input

Resistance 32 k

RAIN Analog Input Resistance 100 M

Table 29-7. ADC Characteristics

Symbol Parameter Condition Min. Typ. Max. Units

392

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30. Typical Characteristics

The following charts show typical behavior. These figures are not tested during manufacturing. All current con-

sumption measurements are performed with all I/O pins configured as inputs and with internal pull-ups enabled.

A sine wave generator with rail-to-rail output is used as clock source.

All Active- and Idle current consumption measurements are done with all bits in the PRR registers set and thus,

the corresponding I/O modules are turned off. Also the Analog Comparator is disabled during these measure-

ments. See “Power Reduction Register” on page 45 for details.

The power consumption in Power-down mode is independent of clock selection.

The current consumption is a function of several factors such as: operating voltage, operating frequency, load-

ing of I/O pins, switching rate of I/O pins, code executed and ambient temperature. The dominating factors are

operating voltage and frequency.

The current drawn from capacitive loaded pins may be estimated (for one pin) as CL*VCC*f where CL = load

capacitance, VCC = operating voltage and f = average switching frequency of I/O pin.

The parts are characterized at frequencies higher than test limits. Parts are not guaranteed to function properly

at frequencies higher than the ordering code indicates.

The difference between current consumption in Power-down mode with Watchdog Timer enabled and Power-

down mode with Watchdog Timer disabled represents the differential current drawn by the Watchdog Timer.

30.1 Active Supply Current

Figure 30-1. Active Supply Current vs. Low F requency (1MHz) and T= 25°C

5.5V

5.0V

4.5V

4.0V

3.6V

2.7V

0.2

0.4

0.6

0.8

1.2

1.4

1.6

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Frequency (MHz)

ICC (mA)

393

ATmega16U4/32U4 [DATASHEET]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 30-2. Active Supply Current vs. Low F requency (1MHz) and T= 85°C

Figure 30-3. Active Supply Current vs. Frequency (1 - 16MHz) and T= -40°C

5.5V

5.0V

4.5V

4.0V

3.6V

2.7V

0.7

0.9

1.1

1.3

1.5

1.7

1.9

2.1

2.3

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Frequency (MHz)

ICC (mA)

246810 12 14 16

Frequency (MHz)

ICC (mA)

5.5V

2.7V

3.6V

4.0V

5.0V

4.5V

394

ATmega16U4/32U4 [DATASHEET]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 30-4. Active Supply Current vs. Frequency (1 - 16MHz) and T = 25°C

Figure 30-5. Active Supply Current vs. Frequency (1 - 16MHz) and T = 85°C

246810 12 14 16

Frequency (MHz)

ICC (mA)

5.5V

5.0V

4.5V

2.7V

3.6V

4.0V

246810 12 14 16

Frequency (MHz)

ICC (mA)

5.5V

2.7V

3.6V

4.0V

5.0V

4.5V

395

ATmega16U4/32U4 [DATASHEET]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

30.2 Idle Supply Current

Figure 30-6. Idle Supply Current vs. Low Frequency (1MHz) and T = 25°C

Figure 30-7. Idle Supply Current vs. Low Frequency (1MHz) and T = 85°C

5.5V

5.0V

4.5V

4.0V

3.6V

2.7V

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Frequency (MHz)

ICC (mA)

5.5V

5.0V

4.5V

4.0V

3.6V

2.7V

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Frequency (MHz)

ICC (mA)

396

ATmega16U4/32U4 [DATASHEET]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 30-8. Idle Supply Current vs. Frequency (1 - 16MHz) T = 25°C

Figure 30-9. Idle Supply Current vs. Frequency (1 - 16MHz) T = 85°C

246810 12 14 16

Frequency (MHz)

ICC (mA)

5.5V

2.7V

3.6V

4.0V

5.0V

4.5V

3.3V

246810 12 14 16

Frequency (MHz)

ICC (mA)

5.5V

5.0V

4.5V

2.7V

3.6V

4.0V

397

ATmega16U4/32U4 [DATASHEET]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

30.3 Power-down Supply Current

Figure 30-10. Power-Down Supply Current vs. VCC (WDT Disabled)

Figure 30-11. Power-Down Supply Current vs. VCC (WDT Enab led)

85°C

25°C

-40°C

0.5

1.5

2.5

3.5

2.5 2.8 3.1 3.4 3.7 4 4.3 4.6 4.9 5.2 5.5

VCC (V)

ICC (µA)

85°C

25°C

-40°C

2.5 2.8 3.1 3.4 3.7 4 4.3 4.6 4.9 5.2 5.5

VCC (V)

ICC (µA)

398

ATmega16U4/32U4 [DATASHEET]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 30-12. Power-down Supply Current vs. VCC (WDT Enabled, BOD EN)

30.4 Power-save Supply Current

Figure 30-13. Power-save Supply Current vs. VCC (WDT Disabled)

85°C

25°C

-40°C

2.5 2.8 3.1 3.4 3.7 4 4.3 4.6 4.9 5.2 5.5

VCC (V)

ICC (µA)

85°C

25°C

-40°C

110

125

140

155

170

185

200

2.5 2.8 3.1 3.4 3.7 4 4.3 4.6 4.9 5.2 5.5

VCC (V)

ICC (µA)

399

ATmega16U4/32U4 [DATASHEET]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

30.5 Pin Pull-Up

Figure 30-14. I/O Pin Pull-up Resistor Current vs. Input Voltage (VCC = 2.7V)

Figure 30-15. I/O Pin Pull-up Resistor Current vs. Input Voltage (VCC = 5V)

0 0.5 1 1.5 2 2.5 3

VOP (V)

IOP (µA)

85°C

25°C

-40°C

100

120

140

012345

VOP (V)

IOP (µA)

85°C

25°C

-40°C

400

ATmega16U4/32U4 [DATASHEET]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 30-16. Re set Pull-up Resistor Current vs. Reset Pin Voltage (VCC= 5V)

30.6 Pin Driver Strength

Figure 30-17. I/O Pin Output Voltage vs. Sink Current (VCC = 3V)

85°C

25°C

100

120

012345

VRESET (V)

IRESET (µA)

-40°C

85°C

25°C

-40°C

0.5

1.5

2.5

3.5

02468101214161820

(mA)

(V)

401

ATmega16U4/32U4 [DATASHEET]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 30-18. I/O Pin Output Voltage vs. Sink Current (VCC = 5V)

Figure 30-19. I/O Pin Output Voltag e vs. Source Current (VCC = 3V)

85°C

25°C

-40°C

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 2 4 6 8 101214161820

(mA)

(V)

85°C

25°C

-40°C

0.5

1.5

2.5

3.5

02468101214161820

IOH (mA)

VOH (V)

402

ATmega16U4/32U4 [DATASHEET]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 30-20. I/O Pin Output Voltag e vs. Source Current (VCC = 5V)

Figure 30-21. USB DP LO Pull-Up Resistor Current vs. USB Pin Voltage

85°C

25°C

-40°C

3.9

4.1

4.3

4.5

4.7

4.9

5.1

0 2 4 6 8 101214161820

IOH (mA)

VOH (V)

85°C

25°C

-40°C

400

800

1200

1600

2000

2400

2800

0 0.5 1 1.5 2 2.5 3 3.5

VUSB (V)

IUSB (µA)

403

ATmega16U4/32U4 [DATASHEET]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

30.7 Pin Threshold and Hysteresis

Figure 30-22. I/O Pin Inpu t Threshold Voltage vs. VCC (VIH, IO Pin read as ‘1’)

Figure 30-23. I/O Pin Inpu t Threshold Voltage vs. VCC (VIL, IO Pin read as ‘0’)

85°C

25°C

-40°C

0.6

0.8

1.2

1.4

1.6

1.8

2.5 2.8 3.1 3.4 3.7 4 4.3 4.6 4.9 5.2 5.5

(V)

Threshold (V)

85°C

25°C

-40°C

0.6

0.8

1.2

1.4

1.6

1.8

2.5 2.8 3.1 3.4 3.7 4 4.3 4.6 4.9 5.2 5.5

(V)

Threshold (V)

404

ATmega16U4/32U4 [DATASHEET]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 30-24. USB Pin Input Threshold Voltage vs. VCC (VIH, IO Pin read as ‘1’)

Figure 30-25. USB Pin Input Threshold Voltage vs. VCC (VIL, I/O Pin read as ‘0’)

85°C

25°C

-40°C

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2.7 2.8 2.9 3 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8

VCC (V)

Threshold (V)

85°C

25°C

-40°C

0.9

1.1

1.2

1.3

1.4

1.5

1.6

2.7 2.8 2.9 3 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8

VCC (V)

Threshold (V)

405

ATmega16U4/32U4 [DATASHEET]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 30-26. Vbus Pin Input Threshold Voltage vs. VCC (VIH, IO Pin read as ‘1’)

Figure 30-27. Vbus Pin Input Threshold Voltage vs. VCC (VIL, I/O Pin read as ‘0’)

85°C

25°C

-40°C

4.38

4.4

4.42

4.44

4.46

4.48

4.5

4.52

4.54

4.56

4.58

4.6

2.5 2.8 3.1 3.4 3.7 4 4.3 4.6 4.9 5.2 5.5

VCC (V)

Threshold (V)

85°C

25°C

-40°C

1.46

1.47

1.48

1.49

1.5

1.51

1.52

2.5 2.8 3.1 3.4 3.7 4 4.3 4.6 4.9 5.2 5.5

VCC (V)

Threshold (V)

406

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30.8 BOD Threshold

Figure 30-28. BOD Th resholds vs. Temperature (BODLEVEL is 2.6V)

Figure 30-29. BOD Th resholds vs. Temperature (BODLEVEL is 3.5V)

Rising Vcc

Falling Vcc

2.64

2.66

2.68

2.7

2.72

2.74

2.76

2.78

2.8

-50-40-30-20-100 102030405060708090

Temperature (°C)

Threshold (V)

Rising Vcc

Falling Vcc

3.45

3.49

3.53

3.57

3.61

3.65

3.69

3.73

-50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 8090

Temperature (°C)

Threshold (V)

407

ATmega16U4/32U4 [DATASHEET]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 30-30. BOD Th resholds vs. Temperature (BODLEVEL is 4.3V)

Figure 30-31. Bandgap Voltage vs. VCC

Rising Vcc Falling Vcc

4.1

4.15

4.2

4.25

4.3

4.35

4.4

4.45

4.5

4.55

4.6

-50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 8090

Temperature (°C)

Threshold (V)

85°C

25°C

-40°C

1.05

1.06

1.07

1.08

1.09

1.1

1.11

1.5 2 2.5 3 3.5 4 4.5 5 5.5

Vcc (V)

Bandgap Voltage (V)

408

ATmega16U4/32U4 [DATASHEET]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 30-32. Bandgap Voltage vs. Temperature

30.9 Internal Oscillator Sp eed

Figure 30-33. Watchdog Oscillator Frequency vs. Temperature

4.5V

3.0V

1.9V

5.0V

5.5V

1.05

1.06

1.07

1.08

1.09

1.1

1.11

-40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90

Temperature (°C)

Bandgap Voltage (V)

5.5V

4.5V

4.0V

3.0V

1.9V

112

114

116

118

120

122

124

-40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90

Temperature (°C)

FRC (kHz)

409

ATmega16U4/32U4 [DATASHEET]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 30-34. Watchdog Oscillator Frequency vs. VCC

Figure 30-35. Calibrated 8MHz RC Oscillator Frequ en cy vs. Osccal Value

85°C

25°C

-40°C

110

112

114

116

118

120

122

124

1.5 2 2.5 3 3.5 4 4.5 5 5.5

VCC (V)

FRC (kHz)

85°C

25°C

-40°C

0 16 32 48 64 80 96 112 128 144 160 176 192 208 224 240 256

OSCCAL (X1)

FRC (MHz)

410

ATmega16U4/32U4 [DATASHEET]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 30-36. Calibrated 8MHz RC Oscillator Frequ ency vs. Temperature

Figure 30-37. Calibrated 8MHz RC Oscillator Frequency vs. Operating Voltage

5.5V

4.0V

2.7V

7.7

7.8

7.9

8.1

8.2

8.3

8.4

-40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90

Temperature (°C)

FRC (MHz)

85°C

25°C

-40°C

7.6

7.7

7.8

7.9

8.1

8.2

8.3

8.4

2.5 2.8 3.1 3.4 3.7 4 4.3 4.6 4.9 5.2 5.5

(V)

(MHz)

411

ATmega16U4/32U4 [DATASHEET]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 30-38. OSCCAL VALUE STEP SIZE IN% (Base frequency = 0.0MHz)

30.10 Current Consumption of Peripheral Units

Figure 30-39. USB Regulator Level vs. VCC

85°C

25°C

-40°C

0.2

0.4

0.6

0.8

1.2

1.4

0 16 32 48 64 80 96 112 128 144 160 176 192 208 224 240 256

OSCCAL (X1)

FRC change (%)

85°C

25°C

-40°C

2.7

2.8

2.9

3.1

3.2

3.3

3.4

3.5

2.7 2.9 3.1 3.3 3.5 3.7 3.9 4.1 4.3 4.5 4.7 4.9 5.1 5.3 5.5

VCC (V)

ICC (µA)

412

ATmega16U4/32U4 [DATASHEET]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 30-40. USB Reg ulator Level with load 75 vs. VCC

Figure 30-41. ADC Internal VREF vs. VCC

85°C

25°C

-40°C

2.5

2.6

2.7

2.8

2.9

3.1

3.2

3.3

3.4

2.8 3.1 3.4 3.7 4 4.3 4.6 4.9 5.2 5.5

VCC (V)

Current (µA)

85°C

25°C

-40°C

2.45

2.46

2.47

2.48

2.49

2.5

2.51

2.52

2.53

2.54

2.6 2.9 3.2 3.5 3.8 4.1 4.4 4.7 5 5.3 5.6

Voltage (V)

Voltage VIntRef (V)

413

ATmega16U4/32U4 [DATASHEET]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 30-42. Internal Reference Voltage vs. Sink Current

30.11 Current Consumption in Reset and Reset Pulse Width

Figure 30-43. Reset Supply Current vs. Frequency (1 - 20MHz)

85°C

25°C

-40°C

2.43

2.44

2.45

2.46

2.47

2.48

2.49

2.5

2.51

2.52

-7 -6 -5 -4 -3 -2 -1 0

Sink current (mA)

Voltage VIntRef (V)

0.5

1.5

2.5

3.5

246810 12 14 16

Frequency (MHz)

ICC (mA)

5.5V

5.0V

4.5V

2.7V

3.6V

4.0V

414

ATmega16U4/32U4 [DATASHEET SUMMARY]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

31. Register Summary

Address Name Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0 Page

(0xFF) Reserved - - - - - - - -

(0xFE) Reserved - - - - - - - -

(0xFD) Reserved - - - - - - - -

(0xFC) Reserved - - - - - - - -

(0xFB) Reserved - - - - - - - -

(0xFA) Reserved - - - - - - - -

(0xF9) Reserved - - - -

(0xF8) Reserved - - - - - - - -

(0xF7) Reserved - - - - - - - -

(0xF6) Reserved - - - - - - - -

(0xF5) Reserved - - - - - - - -

(0xF4) UEINT - EPINT6:0

(0xF3) UEBCHX - - - - -BYCT10:8

(0xF2) UEBCLX BYCT7:0

(0xF1) UEDATX DAT7:0

(0xF0) UEIENX FLERRE NAKINE - NAKOUTE RXSTPE RXOUTE STALLEDE TXINE

(0xEF) UESTA1X - - - - - CTRLDIR CURRBK1:0

(0xEE) UESTA0X CFGOK OVERFI UNDERFI -DTSEQ1:0 NBUSYBK1:0

(0xED) UECFG1X EPSIZE2:0 EPBK1:0 ALLOC -

(0xEC) UECFG0X EPTYPE1:0 - - - - - EPDIR

(0xEB) UECONX - - STALLRQ STALLRQC RSTDT - - EPEN

(0xEA) UERST -EPRST6:0

(0xE9) UENUM - - - - - EPNUM2:0

(0xE8) UEINTX FIFOCON NAKINI RWAL NAKOUTI RXSTPI RXOUTI STALLEDI TXINI

(0xE7) Reserved - - - -

(0xE6) UDMFN - - - FNCERR - - - -

(0xE5) UDFNUMH - - - - - FNUM10:8

(0xE4) UDFNUML FNUM7:0

(0xE3) UDADDR ADDEN UADD6:0

(0xE2) UDIEN - UPRSME EORSME WAKEUPE EORSTE SOFE MSOFE SUSPE

(0xE1) UDINT - UPRSMI EORSMI WAKEUPI EORSTI SOFI MSOFI SUSPI

(0xE0) UDCON - - - - RSTCPU LSM RMWKUP DETACH

(0xDF) Reserved

(0xDE) Reserved

(0xDD) Reserved

(0xDC) Reserved

(0xDB) Reserved

(0xDA) USBINT - - - - - - - VBUSTI

(0xD9) USBSTA - - - - - -IDVBUS

(0xD8) USBCON USBE -FRZCLK OTGPADE - - -VBUSTE

(0xD7) UHWCON - - - - - - -UVREGE

(0xD6) Reserved

(0xD5) Reserved

(0xD4) DT4 DT4H3 DT4H2 DT4H1 DT4H0 DT4L3 DT4L2 DT4L1 DT4L0

(0xD3) Reserved

(0xD2) OCR4D Timer/Counter4 - Output Compare Register D

(0xD1) OCR4C Timer/Counter4 - Output Compare Register C

(0xD0) OCR4B Timer/Counter4 - Output Compare Register B

(0xCF) OCR4A Timer/Counter4 - Output Compare Register A

(0xCE) UDR1 USART1 I/O Data Register

(0xCD) UBRR1H - - - - USART1 Baud Rate Register High Byte

(0xCC) UBRR1L USART1 Baud Rate Register Low Byte

(0xCB) UCSR1D - - - - - - CTSEN RTSEN

(0xCA) UCSR1C UMSEL11 UMSEL10 UPM11 UPM10 USBS1 UCSZ11 UCSZ10 UCPOL1

(0xC9) UCSR1B RXCIE1 TXCIE1 UDRIE1 RXEN1 TXEN1 UCSZ12 RXB81 TXB81

(0xC8) UCSR1A RXC1 TXC1 UDRE1 FE1 DOR1 PE1 U2X1 MPCM1

(0xC7) CLKSTA - - - - - - RCON EXTON

(0xC6) CLKSEL1 RCCKSEL3 RCCKSEL2 RCCKSEL1 RCCKSEL0 EXCKSEL3 EXCKSEL2 EXCKSEL1 EXCKSEL0

(0xC5) CLKSEL0 RCSUT1 RCSUT0 EXSUT1 EXSUT0 RCE EXTE -CLKS

(0xC4) TCCR4E TLOCK4 ENHC4 OC4OE5 OC4OE4 OC4OE3 OC4OE2 OC4OE1 OC4OE0

(0xC3) TCCR4D FPIE4 FPEN4 FPNC4 FPES4 FPAC4 FPF4 WGM41 WGM40

(0xC2) TCCR4C COM4A1S COM4A0S COM4B1S COM4B0S COM4D1S COM4D0S FOC4D PWM4D

(0xC1) TCCR4B PWM4X PSR4 DTPS41 DTPS40 CS43 CS42 CS41 CS40

(0xC0) TCCR4A COM4A1 COM4A0 COM4B1 COM4B0 FOC4A FOC4B PWM4A PWM4B

(0xBF) TC4H - - - - - Timer/Counter4 High Byte

415

ATmega16U4/32U4 [DATASHEET SUMMARY]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

(0xBE) TCNT4 Timer/Counter4 - Counter Register Low Byte

(0xBD) TWAMR TWAM6 TWAM5 TWAM4 TWAM3 TWAM2 TWAM1 TWAM0 -

(0xBC) TWCR TWINT TWEA TWSTA TWSTO TWWC TWEN -TWIE

(0xBB) TWDR 2-wire Serial Interface Data Register

(0xBA) TWAR TWA6 TWA5 TWA4 TWA3 TWA2 TWA1 TWA0 TWGCE

(0xB9) TWSR TWS7 TWS6 TWS5 TWS4 TWS3 - TWPS1 TWPS0

(0xB8) TWBR 2-wire Serial Interface Bit Rate Register

(0xB7) Reserved - - - - - - - -

(0xB6) Reserved -

(0xB5) Reserved - - - - - - - -

(0xB4) Reserved - - - - - - - -

(0xB3) Reserved - - - - - - - -

(0xB2) Reserved - - - - - - - -

(0xB1) Reserved - - - - - - - -

(0xB0) Reserved - - - - - - - -

(0xAF) Reserved - - - - - - - -

(0xAE) Reserved - - - - - - - -

(0xAD) Reserved - - - - - - - -

(0xAC) Reserved - - - - - - - -

(0xAB) Reserved - - - - - - - -

(0xAA) Reserved - - - - - - - -

(0xA9) Reserved - - - - - - - -

(0xA8) Reserved - - - - - - - -

(0xA7) Reserved - - - - - - - -

(0xA6) Reserved - - - - - - - -

(0xA5) Reserved - - - - - - - -

(0xA4) Reserved - - - - - - - -

(0xA3) Reserved - - - - - - - -

(0xA2) Reserved - - - - - - - -

(0xA1) Reserved - - - - - - - -

(0xA0) Reserved - - - - - - - -

(0x9F) Reserved - - - - - - - -

(0x9E) Reserved - - - - - - - -

(0x9D) OCR3CH Timer/Counter3 - Output Compare Register C High Byte

(0x9C) OCR3CL Timer/Counter3 - Output Compare Register C Low Byte

(0x9B) OCR3BH Timer/Counter3 - Output Compare Register B High Byte

(0x9A) OCR3BL Timer/Counter3 - Output Compare Register B Low Byte

(0x99) OCR3AH Timer/Counter3 - Output Compare Register A High Byte

(0x98) OCR3AL Timer/Counter3 - Output Compare Register A Low Byte

(0x97) ICR3H Timer/Counter3 - Input Capture Register High Byte

(0x96) ICR3L Timer/Counter3 - Input Capture Register Low Byte

(0x95) TCNT3H Timer/Counter3 - Counter Register High Byte

(0x94) TCNT3L Timer/Counter3 - Counter Register Low Byte

(0x93) Reserved - - - - - - - -

(0x92) TCCR3C FOC3A - - - - - - -

(0x91) TCCR3B ICNC3 ICES3 -WGM33 WGM32 CS32 CS31 CS30

(0x90) TCCR3A COM3A1 COM3A0 COM3B1 COM3B0 COM3C1 COM3C0 WGM31 WGM30

(0x8F) Reserved - - - - - - - -

(0x8E) Reserved - - - - - - - -

(0x8D) OCR1CH Timer/Counter1 - Output Compare Register C High Byte

(0x8C) OCR1CL Timer/Counter1 - Output Compare Register C Low Byte

(0x8B) OCR1BH Timer/Counter1 - Output Compare Register B High Byte

(0x8A) OCR1BL Timer/Counter1 - Output Compare Register B Low Byte

(0x89) OCR1AH Timer/Counter1 - Output Compare Register A High Byte

(0x88) OCR1AL Timer/Counter1 - Output Compare Register A Low Byte

(0x87) ICR1H Timer/Counter1 - Input Capture Register High Byte

(0x86) ICR1L Timer/Counter1 - Input Capture Register Low Byte

(0x85) TCNT1H Timer/Counter1 - Counter Register High Byte

(0x84) TCNT1L Timer/Counter1 - Counter Register Low Byte

(0x83) Reserved - - - - - - - -

(0x82) TCCR1C FOC1A FOC1B FOC1C - - - - -

(0x81) TCCR1B ICNC1 ICES1 -WGM13 WGM12 CS12 CS11 CS10

(0x80) TCCR1A COM1A1 COM1A0 COM1B1 COM1B0 COM1C1 COM1C0 WGM11 WGM10

(0x7F) DIDR1 - - - - - - - AIN0D

(0x7E) DIDR0 ADC7D ADC6D ADC5D ADC4D - - ADC1D ADC0D

(0x7D) DIDR2 - - ADC13D ADC12D ADC11D ADC10D ADC9D ADC8D

(0x7C) ADMUX REFS1 REFS0 ADLAR MUX4 MUX3 MUX2 MUX1 MUX0

(0x7B) ADCSRB ADHSM ACME MUX5 - ADTS3 ADTS2 ADTS1 ADTS0

Address Name Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0 Page

416

ATmega16U4/32U4 [DATASHEET SUMMARY]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

(0x7A) ADCSRA ADEN ADSC ADATE ADIF ADIE ADPS2 ADPS1 ADPS0

(0x79) ADCH ADC Data Register High byte

(0x78) ADCL ADC Data Register Low byte

(0x77) Reserved - - - - - - - -

(0x76) Reserved - - - - - - - -

(0x75) Reserved - - - - - - - -

(0x74) Reserved - - - - - - - -

(0x73) Reserved - - - - - - - -

(0x72) TIMSK4 OCIE4D OCIE4A OCIE4B - - TOIE4 - -

(0x71) TIMSK3 - - ICIE3 -OCIE3C OCIE3B OCIE3A TOIE3

(0x70) Reserved - - - - - - - -

(0x6F) TIMSK1 - - ICIE1 -OCIE1C OCIE1B OCIE1A TOIE1

(0x6E) TIMSK0 - - - - - OCIE0B OCIE0A TOIE0

(0x6D) Reserved - - - - - - - -

(0x6C) Reserved - - - - - - - -

(0x6B) PCMSK0 PCINT7 PCINT6 PCINT5 PCINT4 PCINT3 PCINT2 PCINT1 PCINT0

(0x6A) EICRB --ISC61ISC60- - - -

(0x69) EICRA ISC31 ISC30 ISC21 ISC20 ISC11 ISC10 ISC01 ISC00

(0x68) PCICR - - - - - - -PCIE0

(0x67) RCCTRL - - - - - - - RCFREQ

(0x66) OSCCAL RC Oscillator Calibration Register

(0x65) PRR1 PRUSB - - PRTIM4 PRTIM3 -- PRUSART1

(0x64) PRR0 PRTWI -PRTIM0 -PRTIM1 PRSPI - PRADC

(0x63) Reserved - - - - - - - -

(0x62) Reserved - - - - - - - -

(0x61) CLKPR CLKPCE - - - CLKPS3 CLKPS2 CLKPS1 CLKPS0

(0x60) WDTCSR WDIF WDIE WDP3 WDCE WDE WDP2 WDP1 WDP0

0x3F (0x5F) SREG I T H S V N Z C

0x3E (0x5E) SPH SP15 SP14 SP13 SP12 SP11 SP10 SP9 SP8

0x3D (0x5D) SPL SP7 SP6 SP5 SP4 SP3 SP2 SP1 SP0

0x3C (0x5C) Reserved - - - - - - - -

0x3B (0x5B) RAMPZ - - - - - - RAMPZ1 RAMPZ0

0x3A (0x5A) Reserved - - - - - - - -

0x39 (0x59) Reserved - - - - - - - -

0x38 (0x58) Reserved - - - - - - - -

0x37 (0x57) SPMCSR SPMIE RWWSB SIGRD RWWSRE BLBSET PGWRT PGERS SPMEN

0x36 (0x56) Reserved - - - - - - - -

0x35 (0x55) MCUCR JTD - - PUD -- IVSEL IVCE

0x34 (0x54) MCUSR - - USBRF JTRF WDRF BORF EXTRF PORF

0x33 (0x53) SMCR - - - - SM2 SM1 SM0 SE

0x32 (0x52) PLLFRQ PINMUX PLLUSB PLLTM1 PLLTM0 PDIV3 PDIV2 PDIV1 PDIV0

0x31 (0x51) OCDR/

MONDR

OCDR7 OCDR6 OCDR5 OCDR4 OCDR3 OCDR2 OCDR1 OCDR0

Monitor Data Register

0x30 (0x50) ACSR ACD ACBG ACO ACI ACIE ACIC ACIS1 ACIS0

0x2F (0x4F) Reserved - - - - - - - -

0x2E (0x4E) SPDR SPI Data Register

0x2D (0x4D) SPSR SPIF WCOL - - - - -SPI2X

0x2C (0x4C) SPCR SPIE SPE DORD MSTR CPOL CPHA SPR1 SPR0

0x2B (0x4B) GPIOR2 General Purpose I/O Register 2

0x2A (0x4A) GPIOR1 General Purpose I/O Register 1

0x29 (0x49) PLLCSR - - - PINDIV -- PLLE PLOCK

0x28 (0x48) OCR0B Timer/Counter0 Output Compare Register B

0x27 (0x47) OCR0A Timer/Counter0 Output Compare Register A

0x26 (0x46) TCNT0 Timer/Counter0 (8 Bit)

0x25 (0x45) TCCR0B FOC0A FOC0B - - WGM02 CS02 CS01 CS00

0x24 (0x44) TCCR0A COM0A1 COM0A0 COM0B1 COM0B0 --WGM01WGM00

0x23 (0x43) GTCCR TSM - - - - - PSRASY PSRSYNC

0x22 (0x42) EEARH - - - - EEPROM Address Register High Byte

0x21 (0x41) EEARL EEPROM Address Register Low Byte

0x20 (0x40) EEDR EEPROM Data Register

0x1F (0x3F) EECR - - EEPM1 EEPM0 EERIE EEMPE EEPE EERE

0x1E (0x3E) GPIOR0 General Purpose I/O Register 0

0x1D (0x3D) EIMSK -INT6 -- INT3 INT2 INT1 INT0

0x1C (0x3C) EIFR -INTF6 - - INTF3 INTF2 INTF1 INTF0

0x1B (0x3B) PCIFR - - - - - - -PCIF0

0x1A (0x3A) Reserved - - - - - - - -

0x19 (0x39) TIFR4 OCF4D OCF4A OCF4B --TOV4 - -

0x18 (0x38) TIFR3 - - ICF3 -OCF3C OCF3B OCF3A TOV3

Address Name Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0 Page

417

ATmega16U4/32U4 [DATASHEET SUMMARY]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Note: 1. For compatibility with future devices, reserved bits should be written to zero if accessed. Reserved I/O memory

addresses should never be written.

2. I/O registers within the address range $00 - $1F are directly bit-accessible using the SBI and CBI instructions. In these

registers, the value of single bits can be checked by using the SBIS and SBIC instructions.

3. Some of the status flags are cleared by writing a logical one to them. Note that the CBI and SBI instructions will operate

on all bits in the I/O register, writing a one back into any flag read as set, thus clearing the flag. The CBI and SBI instruc-

tions work with registers 0x00 to 0x1F only.

4. When using the I/O specific commands IN and OUT, the I/O addresses $00 - $3F must be used. When addressing I/O

registers as data space using LD and ST instructions, $20 must be added to these addresses. The

ATmega16U4/ATmega32U4 is a complex microcontroller with more peripheral units than can be supported within the 64

location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space from $60 - $1FF in SRAM, only

the ST/STS/STD and LD/LDS/LDD instructions can be used.

0x17 (0x37) Reserved - - - - - - - -

0x16 (0x36) TIFR1 - - ICF1 -OCF1C OCF1B OCF1A TOV1

0x15 (0x35) TIFR0 - - - - - OCF0B OCF0A TOV0

0x14 (0x34) Reserved - - - - - - - -

0x13 (0x33) Reserved - - - - - - - -

0x12 (0x32) Reserved - - - - - - - -

0x11 (0x31) PORTF PORTF7 PORTF6 PORTF5 PORTF4 - - PORTF1 PORTF0

0x10 (0x30) DDRF DDF7 DDF6 DDF5 DDF4 - - DDF1 DDF0

0x0F (0x2F) PINF PINF7 PINF6 PINF5 PINF4 - - PINF1 PINF0

0x0E (0x2E) PORTE -PORTE6 - - - PORTE2 - -

0x0D (0x2D) DDRE -DDE6 - - - DDE2 - -

0x0C (0x2C) PINE -PINE6 - - - PINE2 - -

0x0B (0x2B) PORTD PORTD7 PORTD6 PORTD5 PORTD4 PORTD3 PORTD2 PORTD1 PORTD0

0x0A (0x2A) DDRD DDD7 DDD6 DDD5 DDD4 DDD3 DDD2 DDD1 DDD0

0x09 (0x29) PIND PIND7 PIND6 PIND5 PIND4 PIND3 PIND2 PIND1 PIND0

0x08 (0x28) PORTC PORTC7 PORTC6 - - - - - -

0x07 (0x27) DDRC DDC7 DDC6 - - - - - -

0x06 (0x26) PINC PINC7 PINC6 - - - - - -

0x05 (0x25) PORTB PORTB7 PORTB6 PORTB5 PORTB4 PORTB3 PORTB2 PORTB1 PORTB0

0x04 (0x24) DDRB DDB7 DDB6 DDB5 DDB4 DDB3 DDB2 DDB1 DDB0

0x03 (0x23) PINB PINB7 PINB6 PINB5 PINB4 PINB3 PINB2 PINB1 PINB0

0x02 (0x22) Reserved - - - - - - - -

0x01 (0x21) Reserved - - - - - - - -

0x00 (0x20) Reserved - - - - - - - -

Address Name Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0 Page

418

ATmega16U4/32U4 [DATASHEET SUMMARY]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

32. Instruction Set Summary

Mnemonics Operands Description Operation Flags #Clocks

ARITHMETIC AND LOGIC INSTRUCTIONS

ADD Rd, Rr Add two Registers Rd  Rd + Rr Z,C,N,V,H 1

ADC Rd, Rr Add with Carry two Registers Rd  Rd + Rr + C Z,C,N,V,H 1

ADIW Rdl,K Add Immediate to Word Rdh:Rdl  Rdh:Rdl + K Z,C,N,V,S 2

SUB Rd, Rr Subtract two Registers Rd  Rd - Rr Z,C,N,V,H 1

SUBI Rd, K Subtract Constant from Register Rd  Rd - K Z,C,N,V,H 1

SBC Rd, Rr Subtract with Carry two Registers Rd  Rd - Rr - C Z,C,N,V,H 1

SBCI Rd, K Subtract with Carry Constant from Reg. Rd  Rd - K - C Z,C,N,V,H 1

SBIW Rdl,K Subtract Immediate from Word Rdh:Rdl  Rdh:Rdl - K Z,C,N,V,S 2

AND Rd, Rr Logical AND Registers Rd Rd  Rr Z,N,V 1

ANDI Rd, K Logical AND Register and Constant Rd  Rd K Z,N,V 1

OR Rd, Rr Logical OR Registers Rd  Rd v Rr Z,N,V 1

ORI Rd, K Logical OR Register and Constant Rd Rd v K Z,N,V 1

EOR Rd, Rr Exclusive OR Registers Rd  Rd  Rr Z,N,V 1

COM Rd One’s Complement Rd  0xFF  Rd Z,C,N,V 1

NEG Rd Two’s Complement Rd  0x00  Rd Z,C,N,V,H 1

SBR Rd,K Set Bit(s) in Register Rd  Rd v K Z,N,V 1

CBR Rd,K Clear Bit(s) in Register Rd  Rd  (0xFF - K) Z,N,V 1

INC Rd Increment Rd  Rd + 1 Z,N,V 1

DEC Rd Decrement Rd  Rd  1 Z,N,V 1

TST Rd Test for Zero or Minus Rd  Rd  Rd Z,N,V 1

CLR Rd Clear Register Rd  Rd  Rd Z,N,V 1

SER Rd Set Register Rd  0xFF None 1

MUL Rd, Rr Multiply Unsigned R1:R0  Rd x Rr Z,C 2

MULS Rd, Rr Multiply Signed R1:R0  Rd x Rr Z,C 2

MULSU Rd, Rr Multiply Signed with Unsigned R1:R0  Rd x Rr Z,C 2

FMUL Rd, Rr Fractional Multiply Unsigned R1:R0  (Rd x Rr) << 1 Z,C 2

FMULS Rd, Rr Fractional Multiply Signed R1:R0  (Rd x Rr) << 1 Z,C 2

FMULSU Rd, Rr Fractional Multiply Signed with Unsigned R1:R0  (Rd x Rr) << 1 Z,C 2

BRANCH INSTRUCTIONS

RJMP k Relative Jump PC PC + k + 1 None 2

IJMP Indirect Jump to (Z) PC  Z None 2

EIJMP Extended Indirect Jump to (Z) PC (EIND:Z) None 2

JMP k Direct Jump PC kNone3

RCALL k Relative Subroutine Call PC  PC + k + 1 None 4

ICALL Indirect Call to (Z) PC  ZNone4

EICALL Extended Indirect Call to (Z) PC (EIND:Z) None 4

CALL k Direct Subroutine Call PC  kNone5

RET Subroutine Return PC  STACK None 5

RETI Interrupt Return PC  STACK I 5

CPSE Rd,Rr Compare, Skip if Equal if (Rd = Rr) PC PC + 2 or 3 None 1/2/3

CP Rd,Rr Compare Rd  Rr Z, N,V,C,H 1

CPC Rd,Rr Compare with Carry Rd  Rr  C Z, N,V,C,H 1

CPI Rd,K Compare Register with Immediate Rd  K Z, N,V,C,H 1

SBRC Rr, b Skip if Bit in Register Cleared if (Rr(b)=0) PC  PC + 2 or 3 None 1/2/3

SBRS Rr, b Skip if Bit in Register is Set if (Rr(b)=1) PC  PC + 2 or 3 None 1/2/3

SBIC P, b Skip if Bit in I/O Register Cleared if (P(b)=0) PC  PC + 2 or 3 None 1/2/3

SBIS P, b Skip if Bit in I/O Register is Set if (P(b)=1) PC  PC + 2 or 3 None 1/2/3

BRBS s, k Branch if Status Flag Set if (SREG(s) = 1) then PCPC+k + 1 None 1/2

BRBC s, k Branch if Status Flag Cleared if (SREG(s) = 0) then PCPC+k + 1 None 1/2

BREQ k Branch if Equal if (Z = 1) then PC  PC + k + 1 None 1/2

BRNE k Branch if Not Equal if (Z = 0) then PC  PC + k + 1 None 1/2

BRCS k Branch if Carry Set if (C = 1) then PC  PC + k + 1 None 1/2

BRCC k Branch if Carry Cleared if (C = 0) then PC  PC + k + 1 None 1/2

BRSH k Branch if Same or Higher if (C = 0) then PC  PC + k + 1 None 1/2

BRLO k Branch if Lower if (C = 1) then PC  PC + k + 1 None 1/2

BRMI k Branch if Minus if (N = 1) then PC  PC + k + 1 None 1/2

BRPL k Branch if Plus if (N = 0) then PC  PC + k + 1 None 1/2

BRGE k Branch if Greater or Equal, Signed if (N  V= 0) then PC  PC + k + 1 None 1/2

BRLT k Branch if Less Than Zero, Signed if (N  V= 1) then PC  PC + k + 1 None 1/2

BRHS k Branch if Half Carry Flag Set if (H = 1) then PC  PC + k + 1 None 1/2

BRHC k Branch if Half Carry Flag Cleared if (H = 0) then PC  PC + k + 1 None 1/2

BRTS k Branch if T Flag Set if (T = 1) then PC  PC + k + 1 None 1/2

BRTC k Branch if T Flag Cleared if (T = 0) then PC  PC + k + 1 None 1/2

BRVS k Branch if Overflow Flag is Set if (V = 1) then PC  PC + k + 1 None 1/2

419

ATmega16U4/32U4 [DATASHEET SUMMARY]

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BRVC k Branch if Overflow Flag is Cleared if (V = 0) then PC  PC + k + 1 None 1/2

BRIE k Branch if Interrupt Enabled if ( I = 1) then PC  PC + k + 1 None 1/2

BRID k Branch if Interrupt Disabled if ( I = 0) then PC  PC + k + 1 None 1/2

BIT AND BIT-TEST INSTRUCTIONS

SBI P,b Set Bit in I/O Register I/O(P,b)  1None2

CBI P,b Clear Bit in I/O Register I/O(P,b)  0None2

LSL Rd Logical Shift Left Rd(n+1)  Rd(n), Rd(0)  0 Z,C,N,V 1

LSR Rd Logical Shift Right Rd(n)  Rd(n+1), Rd(7)  0 Z,C,N,V 1

ROL Rd Rotate Left Through Carry Rd(0)C,Rd(n+1) Rd(n),CRd(7) Z,C,N,V 1

ROR Rd Rotate Right Through Carry Rd(7)C,Rd(n) Rd(n+1),CRd(0) Z,C,N,V 1

ASR Rd Arithmetic Shift Right Rd(n)  Rd(n+1), n=0..6 Z,C,N,V 1

SWAP Rd Swap Nibbles Rd(3..0)Rd(7..4),Rd(7..4)Rd(3..0) None 1

BSET s Flag Set SREG(s)  1SREG(s)1

BCLR s Flag Clear SREG(s)  0 SREG(s) 1

BST Rr, b Bit Store from Register to T T  Rr(b) T 1

BLD Rd, b Bit load from T to Register Rd(b)  TNone1

SEC Set Carry C  1C1

CLC Clear Carry C  0 C 1

SEN Set Negative Flag N  1N1

CLN Clear Negative Flag N  0 N 1

SEZ Set Zero Flag Z  1Z1

CLZ Clear Zero Flag Z  0 Z 1

SEI Global Interrupt Enable I  1I1

CLI Global Interrupt Disable I 0 I 1

SES Set Signed Test Flag S  1S1

CLS Clear Signed Test Flag S  0 S 1

SEV Set Twos Complement Overflow. V  1V1

CLV Clear Twos Complement Overflow V  0 V 1

SET Set T in SREG T  1T1

CLT Clear T in SREG T  0 T 1

SEH Set Half Carry Flag in SREG H  1H1

CLH Clear Half Carry Flag in SREG H  0 H 1

DATA TRANSFER INSTRUCTIONS

MOV Rd, Rr Move Between Registers Rd  Rr None 1

MOVW Rd, Rr Copy Register Word Rd+1:Rd  Rr+1:Rr None 1

LDI Rd, K Load Immediate Rd  KNone1

LD Rd, X Load Indirect Rd  (X) None 2

LD Rd, X+ Load Indirect and Post-Inc. Rd  (X), X  X + 1 None 2

LD Rd, - X Load Indirect and Pre-Dec. X  X - 1, Rd  (X) None 2

LD Rd, Y Load Indirect Rd  (Y) None 2

LD Rd, Y+ Load Indirect and Post-Inc. Rd  (Y), Y  Y + 1 None 2

LD Rd, - Y Load Indirect and Pre-Dec. Y  Y - 1, Rd  (Y) None 2

LDD Rd,Y+q Load Indirect with Displacement Rd  (Y + q) None 2

LD Rd, Z Load Indirect Rd  (Z) None 2

LD Rd, Z+ Load Indirect and Post-Inc. Rd  (Z), Z  Z+1 None 2

LD Rd, -Z Load Indirect and Pre-Dec. Z  Z - 1, Rd  (Z) None 2

LDD Rd, Z+q Load Indirect with Displacement Rd  (Z + q) None 2

LDS Rd, k Load Direct from SRAM Rd  (k) None 2

ST X, Rr Store Indirect (X) Rr None 2

ST X+, Rr Store Indirect and Post-Inc. (X) Rr, X  X + 1 None 2

ST - X, Rr Store Indirect and Pre-Dec. X  X - 1, (X)  Rr None 2

ST Y, Rr Store Indirect (Y)  Rr None 2

ST Y+, Rr Store Indirect and Post-Inc. (Y)  Rr, Y  Y + 1 None 2

ST - Y, Rr Store Indirect and Pre-Dec. Y  Y - 1, (Y)  Rr None 2

STD Y+q,Rr Store Indirect with Displacement (Y + q)  Rr None 2

ST Z, Rr Store Indirect (Z)  Rr None 2

ST Z+, Rr Store Indirect and Post-Inc. (Z)  Rr, Z  Z + 1 None 2

ST -Z, Rr Store Indirect and Pre-Dec. Z  Z - 1, (Z)  Rr None 2

STD Z+q,Rr Store Indirect with Displacement (Z + q)  Rr None 2

STS k, Rr Store Direct to SRAM (k)  Rr None 2

LPM Load Program Memory R0  (Z) None 3

LPM Rd, Z Load Program Memory Rd  (Z) None 3

LPM Rd, Z+ Load Program Memory and Post-Inc Rd  (Z), Z  Z+1 None 3

ELPM Extended Load Program Memory R0  (RAMPZ:Z) None 3

ELPM Rd, Z Extended Load Program Memory Rd  (Z) None 3

ELPM Rd, Z+ Extended Load Program Memory Rd  (RAMPZ:Z), RAMPZ:Z RAMPZ:Z+1 None 3

SPM Store Program Memory (Z)  R1:R0 None -

IN Rd, P In Port Rd  PNone1

Mnemonics Operands Description Operation Flags #Clocks

420

ATmega16U4/32U4 [DATASHEET SUMMARY]

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OUT P, Rr Out Port P  Rr None 1

PUSH Rr Push Register on Stack STACK  Rr None 2

POP Rd Pop Register from Stack Rd  STACK None 2

MCU CONTROL INSTRUCTIONS

NOP No Operation None 1

SLEEP Sleep (see specific description for Sleep function) None 1

WDR Watchdog Reset (see specific description for WDR/timer) None 1

BREAK Break For On-chip Debug Only None N/A

Mnemonics Operands Description Operation Flags #Clocks

421

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33. Ordering Information

33.1 ATmega16U4

Notes: 1. For more information on running the USB from internal RC oscillator consult application note AVR291: 8MHz Internal Oscillator Calibration for USB Low

Speed on Atmel ATmega32U4RC.

2. USB operation from internal RC oscillator is only guaranteed for 0°C to 40°C.

3. These parts are shipped with no USB bootloader pre-programmed.

Speed [MHz] Power Supply Ordering Code Default Oscillator Package Operatio n Range

16 2.7 - 5.5V

ATmega16U4-AU External XTAL

44ML

Industrial (-40° to +85°C)

ATmega16U4RC-AU Internal Calib. RC

ATmega16U4-MU

(1)(2)(3) External XTAL

44PW

ATmega16U4RC-MU

(1)(2)(3) Internal Calib. RC

Package Type

44ML ML, 44 - Lead, 10 x 10mm Body Size, 1.0mm Body Thickness

0.8mm Lead Pitch, Thin Profile Plastic Quad Flat Package (TQFP)

44PW PW, 44 - Lead 7.0 x 7.0mm Body, 0.50mm Pitch

Quad Flat No Lead Package (QFN)

422

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33.2 ATmega32U4

Notes: 1. For more information on running the USB from internal RC oscillator consult application note AVR291: 8MHz Internal Oscillator Calibration for USB Low

Speed on Atmel ATmega32U4RC.

2. USB operation from internal RC oscillator is only guaranteed for 0°C to 40°C.

3. These parts are shipped with no USB bootloader pre-programmed.

Speed [MHz] Power Supply Ordering Code Default Oscillator Package Operation Range

16 2.7 - 5.5V

ATmega32U4-AU External XTAL

44ML

Industrial (-40° to +85°C)

ATmega32U4RC-AU Internal Calib. RC

ATmega32U4-MU(1)(2)(3) External XTAL

44PW

ATmega32U4RC-MU(1)

(2) (3) Internal Calib. RC

Package Type

44ML ML, 44 - Lead, 10 x 10mm Body Size, 1.0mm Body Thickness

0.8mm Lead Pitch, Thin Profile Plastic Quad Flat Package (TQFP)

44PW PW, 44 - Lead 7.0 x 7.0mm Body, 0.50mm Pitch

Quad Flat No Lead Package (QFN)

423

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34. Packaging Information

34.1 TQFP44

0.37

0.60

0.17

02/06/2014

424

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Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

34.2 QFN44

425

ATmega16U4/32U4 [DATASHEET]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

426

ATmega16U4/32U4 [DATASHEET]

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35. Errata

The revision letter in this section refers to the revision of the ATmega16U4/ATmega32U4 device.

35.1 ATmega16U4/ATmega32U4 Rev E

•Spike on TW I pin s when TW I is enabled

•High current consumpt ion in sleep mode

•MSB of OCR4A/B/D is write only in 11-bits enhanced PWM mode

1. Spike on TWI pins when TWI is enabled

100 ns negative spike occurs on SDA and SCL pins when TWI is enabled.

Problem Fix/work aro u nd

Enable ATmega16U4/ATmega32U4 TWI before the other nodes of the TWI network.

2. High current consumption in sleep mode

If a pending interrupt cannot wake the part up from the selected mode, the current consumption will

increase during sleep when executing the SLEEP instruction directly after a SEI instruction.

Problem Fix/work aro u nd

Before entering sleep, interrupts not used to wake up the part from the sleep mode should be disabled.

3. MSB of OCR4A/B/D is write only in 11-bits enhanced PWM mode

In the 11-bits enhanced PWM mode the MSB of OCR4A/B/D is write only. A read of OCR4A/B/D will

always return zero in the MSB position.

Problem Fix/work aro u nd

None.

35.2 ATmega16U4/ATmega32U4 Rev D

•Spike on TW I pin s when TW I is enabled

•High current consumpt ion in sleep mode

•Timer 4 11-bits enhanced PWM mode

1. Spike on TWI pins when TWI is enabled

100 ns negative spike occurs on SDA and SCL pins when TWI is enabled.

Problem Fix/work aro u nd

Enable ATmega16U4/ATmega32U4 TWI before the other nodes of the TWI network.

2. High current consumption in sleep mode

If a pending interrupt cannot wake the part up from the selected mode, the current consumption will

increase during sleep when executing the SLEEP instruction directly after a SEI instruction.

Problem Fix/work aro u nd

Before entering sleep, interrupts not used to wake up the part from the sleep mode should be disabled.

3. Timer 4 11-bits enhanced PWM mode

Timer 4 11-bits enhanced mode is not functional.

Problem Fix/work aro u nd

None.

427

ATmega16U4/32U4 [DATASHEET]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

35.3 ATmega16U4/ATmega32U4 Rev C

Not sampled

35.4 ATmega16U4/ATmega32U4 Rev B

•Spike on TW I pin s when TW I is enabled

•High current consumpt ion in sleep mode

•Incorrect execution of VBUSTI interrupt

•Timer 4 11-bits enhanced PWM mode

1. Spike on TWI pins when TWI is enabled

100 ns negative spike occurs on SDA and SCL pins when TWI is enabled.

Problem Fix/work aro u nd

Enable ATmega16U4/ATmega32U4 TWI before the other nodes of the TWI network.

2. High current consumption in sleep mode

If a pending interrupt cannot wake the part up from the selected mode, the current consumption will

increase during sleep when executing the SLEEP instruction directly after a SEI instruction.

Problem Fix/work aro u nd

Before entering sleep, interrupts not used to wake up the part from the sleep mode should be disabled.

3. Incorrect execution of VBUSTI interrupt

The CPU may incorrectly execute the interrupt vector related to the VBUSTI interrupt flag.

Problem fix/work around

Do not enable this interrupt. Firmware must process this USB event by polling VBUSTI.

4. Timer 4 11-bits enhanced PWM mode

Timer 4 11-bits enhanced mode is not functional.

Problem Fix/work aro u nd

None.

35.5 ATmega16U4/ATmega32U4 Rev A

•Spike on TW I pin s when TW I is enabled

•High current consumpt ion in sleep mode

•Increased power consumption in power-down mode

•Internal RC oscillator start up may fail

•Internal RC oscillator calibration

•Incorrect execution of VBUSTI interrupt

•Timer 4 enhanced mode issue

1. Spike on TWI pins when TWI is enabled

100 ns negative spike occurs on SDA and SCL pins when TWI is enabled.

Problem Fix/work aro u nd

Enable ATmega16U4/ATmega32U4 TWI before the other nodes of the TWI network.

2. High current consumption in sleep mode

428

ATmega16U4/32U4 [DATASHEET]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

If a pending interrupt cannot wake the part up from the selected mode, the current consumption will

increase during sleep when executing the SLEEP instruction directly after a SEI instruction.

Problem Fix/work aro u nd

Before entering sleep, interrupts not used to wake up the part from the sleep mode should be disabled.

3. Increased power consumption in power-down mode

The typical power consumption is increased by about 30 µA in power-down mode.

Problem Fix/work aro u nd

None.

4. Internal RC os c ill at or st art up may fail

When the part is configured to start on internal RC oscillator, the oscillator may not start properly after

power-on.

Problem Fix/work aro u nd

Do not configure the part to start on internal RC oscillator.

5. Internal RC os c ill at or calib ra tio n

8 MHz frequency can be impossible to reach with internal RC even when using maximal OSCAL value.

Problem Fix/work aro u nd

None.

6. Incorrect execution of VBUSTI interrupt

The CPU may incorrectly execute the interrupt vector related to the VBUSTI interrupt flag.

Problem fix/work around

Do not enable this interrupt. Firmware must process this USB event by polling VBUSTI.

7. Timer 4 11-bits enhanced PWM mode

Timer 4 11-bits enhanced mode is not functional.

Problem Fix/work aro u nd

None.

429

ATmega16U4/32U4 [DATASHEET]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

36. Datasheet Revision History for ATmega16U4/ATmega32U4

Note that the referring page numbers in this section are referred to this document. The referring revision in this

section are referring to the document revision.

36.1 Rev. 7766J – 04/2016

36.2 Rev. 7766I – 07/2015

36.3 Rev. 7766H – 06/2014

“Memory Programming” on page 353: Updated number of words in a page and number of

pages in the Flash and EEPROM for ATmega16U4 and ATmega32U4. Refer to Table 28-11

and Table 28-12 on page 359.

1. Applied Atmel brands throughout the contents and reorganized the contents.

2. Updated “Power Management and Sleep Modes” on page 43. Part of contents was missing.

1. The first section in “Phase and Frequency Correct PWM Mode” on page 154 has been

corrected.

2. Several corrections are made according to the new template.

3. Trademarks are added to the last page.

4Removed preliminary on the front page

5Updated with new datasheet template from 05-2014

6. Updated description of parts pre-programed with a default USB bootloader in Features on

page 2.

7. Added three footnotes for the RC part numbers in Section 33., “Ordering Information” on page

421.

8. Removed footnote on Frequency range inTable 6-3 on page 30 and Table 6-7 on page 32.

9. Updated values and removed footnote in Table 8-3 on page 55.

10. Removed column VCC=1.5 - 5.5V in Table 29-2 on page 385.

11. Changed footnote for Table 29-2 on page 385.

12. Added max value for Rise/Fall time in Table 29-4 on page 387.

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36.4 Rev. 7766G – 02/2014

36.5 Rev. 7766F – 11/10

36.6 Rev. 7766E – 04/10

1. Updated the “Description” on page 177 of the “Output Compare Modulator (OCM1C0A)” .

Specified when the logical AND and the logical OR will be performed based on the PORTB7.

2. Updated “USART Control and Status Register n D– UCSRnD” on page 213. “Bits 7:2 -

Reserved” are Read only.

3. Updated “Crystal-less Operation” on page 259. The temperature range changed to “within the

0C and +40C.

4. MUX bit in “ADC Control and Status Register B – ADCSRB” on page 294 changed to R/W.

5. Updated Table 24-6 on page 318. Trigger Source: Timer/Counter0 Compare Match updated

to Timer/Counter0 Compare Match A.

6. Updated “DC Characteristics” on page 383. Added Active 16MHz, VCC = 5V, max. 27mA, in

“Icc / Power supply current”.

7. Updated “Register Summary” on page 414. Added UCSRnD at the address CBh.

8. Replaced the “TQFP44” on page 423 and “QFN44” on page 424 by updated package

drawings.

9. Updated the last page according to Atmel new Brand Style Guide (new logo).

1. Replaced the “QFN44” on page 424 by an updated drawing.

2. Updated “ADC Control and Status Register B – ADCSRB” on page 294. Defined the

ADCSRB register as in “ADC Control and Status Register B – ADCSRB” on page 317.

3. Updated the last page according to Atmel new Brand Style Guide.

1. Updated “Features” on page 1.

2. Updated “Features” on page 256.

3. Updated Figure 21-9 on page 261.

4. Updated Section 21.8 on page 263.

5. Updated “Features” on page 297.

6. Updated “Boundary-scan Order” on page 332.

7. Updated “Program And Data Memory Lock Bits” on page 353.

8. Updated Table 28-5 on page 355.

9. Updated “Electrical Characteristics” on page 383.

10. Updated Figure 29-2 on page 386.

431

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36.7 Rev. 7766D – 01/09

36.8 Rev. 7766C – 11/08

36.9 Rev. 7766B – 11/08

36.10 Rev. 7766A – 07/08

11. Added “Typical Characteristics” on page 392.

12. Updated “Ordering Information” on page 421.

13. Updated “Errata” on page 426.

1. Updated Memory section in “Features” on page 1.

2. Added section “Resources” on page 8.

3. Added section “Data Retention” on page 8.

4. Updated “Ordering Information” on page 421.

1. Updated Memory section in “Features” on page 1.

1. Added ATmega16U4 device.

2. Created errata section and added ATmega16U4.

3. Updated High Speed Timer, asynchronous description Section 15. on page 139

1. Initial revision

ATmega16U4/32U4 [DATASHEET]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Table of Contents

1. Pin Configurations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2.1 Block Diagram. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2.2 Pin Descriptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3. About . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3.1 Disclaimer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3.2 Resources. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3.3 Code Examples. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3.4 Data Retention . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

4. AVR CPU Core . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

4.2 Architectural Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

4.3 ALU – Arithmetic Logic Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

4.4 Status Register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

4.5 General Purpose Register File . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

4.6 Stack Pointer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

4.7 Instruction Execution Timing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

4.8 Reset and Interrupt Handling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

5. AVR Memories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

5.1 In-System Reprogrammable Flash Program Memory. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

5.2 SRAM Data Memory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

5.3 EEPROM Data Memory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

5.4 I/O Memory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

6. System Clock and Clock Options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

6.1 Clock Systems and their Distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

6.2 Clock Sources. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

6.3 Low Power Crystal Oscillator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

6.4 Low Frequency Crystal Oscillator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

6.5 Calibrated Internal RC Oscillator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

6.6 External Clock. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

6.7 Clock Switch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

6.8 Clock Output Buffer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

6.9 PLL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

6.10 Clock switch Algorithm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

6.11 Register Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

7. Power Management and Sleep Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

7.1 Idle Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

7.2 ADC Noise Reduction Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

7.3 Power-down Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

7.4 Power-save Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

7.5 Standby Mode. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

7.6 Extended Standby Mode. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

7.7 Power Reduction Register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

7.8 Minimizing Power Consumption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

ATmega16U4/32U4 [DATASHEET]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

7.9 Register Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

8. System Control and Reset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

8.1 Resetting the AVR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

8.2 Reset Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

8.3 Power-on Reset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

8.4 External Reset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

8.5 Brown-out Detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

8.6 Watchdog Reset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

8.7 USB Reset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

8.8 Internal Voltage Reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

8.9 Watchdog Timer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

8.10 Register Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

8.11 MCU Status Register – MCUSR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

9. Interrupts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

9.1 Interrupt Vectors in ATmega16U4/ATmega32U4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

9.2 Register Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

10. I/O-Ports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

10.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

10.2 Ports as General Digital I/O . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

10.3 Alternate Port Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

10.4 Register Description for I/O-Ports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84

11. External Interrupts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88

11.1 Register Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89

12. Timer/Counter0, Timer/Counter1, and Timer/Counter3 Prescalers . . . . . . . . . . . . . . . 92

12.1 Internal Clock Source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92

12.2 Prescaler Reset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92

12.3 External Clock Source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92

12.4 Register Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

13. 8-bit Timer/Counter0 with PWM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94

13.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94

13.2 Timer/Counter Clock Sources. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95

13.3 Counter Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95

13.4 Output Compare Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96

13.5 Compare Match Output Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97

13.6 Modes of Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98

13.7 Timer/Counter Timing Diagrams. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102

13.8 8-bit Timer/Counter Register Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104

14. 16-bit Timers/Counters (Timer/Counter1 and Timer/Counter3) . . . . . . . . . . . . . . . . . . 111

14.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111

14.2 Accessing 16-bit Registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113

14.3 Timer/Counter Clock Sources. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116

14.4 Counter Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116

14.5 Input Capture Unit. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117

14.6 Output Compare Units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119

14.7 Compare Match Output Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121

iii

ATmega16U4/32U4 [DATASHEET]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

14.8 Modes of Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122

14.9 Timer/Counter Timing Diagrams. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129

14.10 16-bit Timer/Counter Register Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131

15. 10-bit High Speed Timer/Counter4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140

15.1 Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140

15.2 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140

15.3 Counter Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144

15.4 Output Compare Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145

15.5 Dead Time Generator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147

15.6 Compare Match Output Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148

15.7 Synchronous update . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151

15.8 Modes of Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151

15.9 Timer/Counter Timing Diagrams. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158

15.10 Fault Protection Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159

15.11 Accessing 10-bit Registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160

15.12 Register Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164

16. Output Compare Modulator (OCM1C0A) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177

16.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177

16.2 Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177

17. Serial Peripheral Interface – SPI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179

17.1 SS Pin Functionality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182

17.2 Register Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185

18. USART . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188

18.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188

18.2 Clock Generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189

18.3 Frame Formats . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192

18.4 USART Initialization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193

18.5 Data Transmission – The USART Transmitter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194

18.6 Data Reception – The USART Receiver. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197

18.7 Asynchronous Data Reception . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201

18.8 Multi-processor Communication Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204

18.9 Hardware Flow Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205

18.10 Examples of Baud Rate Setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 206

18.11 USART Register Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 209

19. USART in SPI Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214

19.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214

19.2 Clock Generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214

19.3 SPI Data Modes and Timing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214

19.4 Frame Formats . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215

19.5 Data Transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 218

19.6 AVR USART MSPIM vs. AVR SPI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220

19.7 USART MSPIM Register Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221

20. 2-wire Serial Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225

20.1 Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225

20.2 2-wire Serial Interface Bus Definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225

20.3 Data Transfer and Frame Format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 226

ATmega16U4/32U4 [DATASHEET]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

20.4 Multi-master Bus Systems, Arbitration and Synchronization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 228

20.5 Overview of the TWI Module. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 230

20.6 Using the TWI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232

20.7 Transmission Modes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 236

20.8 Multi-master Systems and Arbitration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 250

20.9 TWI Register Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 252

21. USB Controller . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 256

21.1 Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 256

21.2 Block Diagram. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 256

21.3 Typical Application Implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257

21.4 Crystal-less Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259

21.5 Design Guidelines. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259

21.6 General Operating Modes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 260

21.7 Power Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 262

21.8 Speed Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263

21.9 Memory Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263

21.10 PAD Suspend . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 264

21.11 Plug-in Detection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265

21.12 USB Software Operating Modes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 266

21.13 Registers Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 267

22. USB Device Operating Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 270

22.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 270

22.2 Power-on and Reset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 270

22.3 Endpoint Reset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 270

22.4 USB Reset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 271

22.5 Endpoint Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 271

22.6 Endpoint Activation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 271

22.7 Address Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 272

22.8 Suspend, Wake-up and Resume . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 272

22.9 Detach. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 272

22.10 Remote Wake-up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 273

22.11 STALL Request. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 273

22.12 CONTROL Endpoint Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 274

22.13 OUT Endpoint Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275

22.14 IN endpoint management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 276

22.15 Isochronous Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 278

22.16 Overflow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 278

22.17 Interrupts. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 278

22.18 Registers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 281

23. Analog Comparator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 293

23.1 Register Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 294

23.2 Analog Comparator Multiplexed Input. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 295

24. Analog to Digital Converter - ADC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 297

24.1 Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 297

24.2 Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 299

24.3 Starting a Conversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 299

24.4 Prescaling and Conversion Timing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 300

24.5 Changing Channel or Reference Selection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303

ATmega16U4/32U4 [DATASHEET]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

24.6 Temperature Sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 304

24.7 ADC Noise Canceler. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 305

24.8 ADC Conversion Result . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 309

24.9 ADC Register Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 313

25. JTAG Interface and On-chip Debug System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 319

25.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 319

25.2 Test Access Port – TAP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 319

25.3 TAP Controller . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 321

25.4 Using the Boundary-scan Chain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 322

25.5 Using the On-chip Debug System. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 322

25.6 On-chip Debug Specific JTAG Instructions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 323

25.7 On-chip Debug Related Register in I/O Memory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 323

25.8 Using the JTAG Programming Capabilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 324

25.9 Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 324

26. IEEE 1149.1 (JTAG) Boundary-scan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 325

26.1 Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 325

26.2 System Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 325

26.3 Data Registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 325

26.4 Boundary-scan Specific JTAG Instructions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 327

26.5 Boundary-scan Related Register in I/O Memory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 328

26.6 Boundary-scan Chain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 329

26.7 Boundary-scan Order . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 332

26.8 Boundary-scan Description Language Files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 335

27. Boot Loader Support – Read-While-Write Self-Programming . . . . . . . . . . . . . . . . . . . 336

27.1 Boot Loader Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 336

27.2 Application and Boot Loader Flash Sections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 336

27.3 Read-While-Write and No Read-While-Write Flash Sections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 336

27.4 Boot Loader Lock Bits. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 339

27.5 Entering the Boot Loader Program . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 340

27.6 Addressing the Flash During Self-Programming. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 343

27.7 Self-Programming the Flash . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 344

28. Memory Programming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 353

28.1 Program And Data Memory Lock Bits. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 353

28.2 Fuse Bits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 354

28.3 Signature Bytes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 356

28.4 Calibration Byte. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 356

28.5 Parallel Programming Parameters, Pin Mapping, and Commands. . . . . . . . . . . . . . . . . . . . . . . . . 356

28.6 Parallel Programming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 359

28.7 Serial Downloading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 367

28.8 Serial Programming Pin Mapping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 368

28.9 Programming via the JTAG Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 371

29. Electrical Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 383

29.1 Absolute Maximum Ratings*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 383

29.2 DC Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 383

29.3 External Clock Drive Waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 385

29.4 External Clock Drive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 385

29.5 System and Reset Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 386

ATmega16U4/32U4 [DATASHEET]

Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

29.6 Maximum speed vs. VCC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 386

29.7 2-wire Serial Interface Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 386

29.8 SPI Timing Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 388

29.9 Hardware Boot Entrance Timing Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 389

30. Typical Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 392

30.1 Active Supply Current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 392

30.2 Idle Supply Current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 395

30.3 Power-down Supply Current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 397

30.4 Power-save Supply Current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 398

30.5 Pin Pull-Up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 399

30.6 Pin Driver Strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 400

30.7 Pin Threshold and Hysteresis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 403

30.8 BOD Threshold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 406

30.9 Internal Oscillator Speed. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 408

30.10 Current Consumption of Peripheral Units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 411

30.11 Current Consumption in Reset and Reset Pulse Width . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 413

31. Register Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 414

32. Instruction Set Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 418

33. Ordering Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 421

33.1 ATmega16U4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 421

33.2 ATmega32U4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 422

34. Packaging Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 423

34.1 TQFP44 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 423

34.2 QFN44 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 424

35. Errata . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 426

35.1 ATmega16U4/ATmega32U4 Rev E . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 426

35.2 ATmega16U4/ATmega32U4 Rev D . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 426

35.3 ATmega16U4/ATmega32U4 Rev C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 427

35.4 ATmega16U4/ATmega32U4 Rev B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 427

35.5 ATmega16U4/ATmega32U4 Rev A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 427

36. Datasheet Revision History for ATmega16U4/ATmega32U4 . . . . . . . . . . . . . . . . . . . . 429

36.1 Rev. 7766J – 04/2016. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 429

36.2 Rev. 7766I – 07/2015 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 429

36.3 Rev. 7766H – 06/2014 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 429

36.4 Rev. 7766G – 02/2014 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 430

36.5 Rev. 7766F – 11/10 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 430

36.6 Rev. 7766E – 04/10 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 430

36.7 Rev. 7766D – 01/09 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 431

36.8 Rev. 7766C – 11/08 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 431

36.9 Rev. 7766B – 11/08 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 431

36.10 Rev. 7766A – 07/08 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 431

XXX

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