ATMEGA16-16MUR - MICROCHIP TECHNOLOGY

Features

•High-perf ormance, Lo w-p ower Atmel® AVR® 8-bit Microcontroller

•Advanced RISC Architecture

– 131 Powerful Instructions – Most Single-clock Cycle Execution

– 32 x 8 General Purpose Working Registers

– Fully Static Operation

– Up to 16 MIPS Throughput at 16 MHz

– On-chip 2-cycle Multiplier

•High Endurance Non-volatile Memory segments

– 16 Kbytes of In-System Self-programmable Flash program memory

– 512 Bytes EEPROM

– 1 Kbyte Internal SRA M

– 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 Independent Lock Bits

In-System Prog ramming by On-chip Boot Program

True Read-While -Wri te Operation

– Programming Lock for Software Security

•JTAG (IEEE std. 1149 .1 Compliant) Interface

– Boundary-scan Capabilities According to the JTAG Standard

– Extensive On-chip Debug Support

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

•Peripheral Features

– Two 8-bit Timer/Counters with Separate Prescalers and Compare Modes

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

Mode

– Real Time Counter with Separate Oscillator

– Four PWM Channels

– 8-channel, 10-bit ADC

8 Single-ended Channels

7 Differential Channels in TQFP Package Only

2 Differential Channels with Programmable Gain at 1x, 10x, or 200x

– Byte-oriented Two-wire Serial Interface

– Programmable Serial USART

– Master/Slave SPI Serial Interface

– Programmable Watchdog Timer with Separa te On-chip Oscillator

– On-chip Analog Comparator

•Special Microcontroller Features

– Power-on Reset and Programmable Brown-out Detection

– Internal Calibrated RC Oscillator

– External and Internal Interru pt Sou rces

– Six Sleep Modes: Idle, ADC Noise Reduction, Power-sa ve, Power-down, Standby

and Extended Standby

•I/O and Packages

– 32 Programmable I/O Lines

– 40-pin PDIP, 44-lead TQFP, and 44-pad QFN/MLF

•Operating Vo ltages

– 2.7V - 5.5V for ATmega16L

– 4.5V - 5.5V for ATmega16

•Speed Grades

– 0 - 8 MHz for ATmega16L

– 0 - 16 MHz for ATmega16

•Power Consumption @ 1 MHz, 3V, and 25°C for ATmega 16L

– Active: 1.1 mA

– Idle Mode: 0.35 mA

– Power-down Mode: < 1 µA

8-bit

Microcontroller

with 16K Bytes

In-System

Programmable

Flash

ATmega16

ATmega16L

Rev. 2466T–AVR–07/10

2466T–AVR–07/10

ATmega16(L)

Configurations Figure 1. Pinout ATmega16

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

other AVR micr ocontr ollers m anufa ct ured o n th e same proce ss tech nolo gy. Min a nd Ma x valu es

will be available after the device is characterized.

(XCK/T0) PB0

(T1) PB1

(INT2/AIN0) PB2

(OC0/AIN1) PB3

(SS) PB4

(MOSI) PB5

(MISO) PB6

(SCK) PB7

RESET

VCC

GND

XTAL2

XTAL1

(RXD) PD0

(TXD) PD1

(INT0) PD2

(INT1) PD3

(OC1B) PD4

(OC1A) PD5

(ICP1) PD6

PA0 (ADC0)

PA1 (ADC1)

PA2 (ADC2)

PA3 (ADC3)

PA4 (ADC4)

PA5 (ADC5)

PA6 (ADC6)

PA7 (ADC7)

AREF

GND

AVCC

PC7 (TOSC2)

PC6 (TOSC1)

PC5 (TDI)

PC4 (TDO)

PC3 (TMS)

PC2 (TCK)

PC1 (SDA)

PC0 (SCL)

PD7 (OC2)

PA4 (ADC4)

PA5 (ADC5)

PA6 (ADC6)

PA7 (ADC7)

AREF

GND

AVCC

PC7 (TOSC2)

PC6 (TOSC1)

PC5 (TDI)

PC4 (TDO)

(MOSI) PB5

(MISO) PB6

(SCK) PB7

RESET

VCC

GND

XTAL2

XTAL1

(RXD) PD0

(TXD) PD1

(INT0) PD2

(INT1) PD3

(OC1B) PD4

(OC1A) PD5

(ICP1) PD6

(OC2) PD7

VCC

GND

(SCL) PC0

(SDA) PC1

(TCK) PC2

(TMS) PC3

PB4 (SS)

PB3 (AIN1/OC0)

PB2 (AIN0/INT2)

PB1 (T1)

PB0 (XCK/T0)

GND

VCC

PA0 (ADC0)

PA1 (ADC1)

PA2 (ADC2)

PA3 (ADC3)

PDIP

TQFP/QFN/MLF

NOTE:

Bottom pad should

be soldered to ground.

2466T–AVR–07/10

ATmega16(L)

Overview The ATmega16 is a low-power CMOS 8-bit microcontroller based on the AVR enha nced RISC

architecture. By executing powerful instructions in a single clock cycle, the ATmega16 achieves

throughputs approa ching 1 MIPS per MH z allowing the syste m designe r to optim ize powe r con-

sumption versus processing speed.

Block Diagram Figure 2. Block Diagram

INTERNAL

OSCILLATOR

WATCHDOG

TIMER

MCU CTRL.

& TIMING

OSCILLATOR

TIMERS/

COUNTERS

INTERRUPT

UNIT

STACK

POINTER

EEPROM

SRAM

STATUS

USART

PROGRAM

COUNTER

PROGRAM

FLASH

INSTRUCTION

DECODER

PROGRAMMING

LOGIC

SPI

ADC

INTERFACE

COMP.

INTERFACE

PORTA DRIVERS/BUFFERS

PORTA DIGITAL INTERFACE

GENERAL

PURPOSE

REGISTERS

ALU

PORTC DRIVERS/BUFFERS

PORTC DIGITAL INTERFACE

PORTB DIGITAL INTERFACE

PORTB DRIVERS/BUFFERS

PORTD DIGITAL INTERFACE

PORTD DRIVERS/BUFFERS

XTAL1

XTAL2

RESET

CONTROL

LINES

VCC

GND

MUX &

ADC

AREF

PA0 - PA7 PC0 - PC7

PD0 - PD7PB0 - PB7

AVR CPU

TWI

AVCC

INTERNAL

CALIBRATED

OSCILLATOR

2466T–AVR–07/10

ATmega16(L)

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 achiev ing throughputs up to ten times faster than con-

ventional CISC microcontrollers.

The ATmega1 6 provides the following fea tures: 16 Kbytes of In-System Programmable Flash

Program memory with Re ad-While-Write capabilities, 512 bytes EEPROM, 1 Kbyte SRAM, 32

general purpose I/ O lines, 32 general p urpose working r egisters, a JTAG interface fo r Boundary-

scan, On-chip Debugging support and programming, three flexible Timer/Counters with com-

pare mode s, Internal and E xternal Interrup ts, a serial programmable USART, a byte oriented

Two-wire Serial Interface, an 8-channel, 10-bit ADC with optional differential input stage with

programmable gain (TQFP package only), a programmable Watchdog Timer with Internal Oscil-

lator, an SPI serial port, and six software selectable power saving modes. The Idle mode stops

the CPU while allowing the USART, Two-wire interface, A/D Converter, SRAM, Timer/Counters,

SPI port, and inte rrupt system to con tinue functio ning. The Powe r-down mod e saves the register

contents but freezes the Oscillator, disabling all other chip functions until the next External Inter-

rupt or Hardware Reset. In Power-save mode, the Asynchronous Timer continues to run,

allowing the user to maintain a timer base while the rest of the device is sleeping. The ADC

Noise Reduction mode stops the CPU and all I/O modules except Asynchronous Timer and

ADC, to minimize switching noise during ADC conversions. In Standby mode, the crystal/reso-

nator Oscillator is running while the rest of the device is sleeping. This allows very fast start-up

combined with low-power consumption. In Extended Standby mode, both the main Oscillator

and the Asynchronous Timer continue to run.

The device is manufactured u sing Atm el’s high den sity nonvolat ile memory te ch nology. The On-

chip ISP Flash allows the program memory to be reprogrammed in-system through an SPI serial

interface, by a conventional nonvolatile memory programm er, or by an On-chip Boot program

running on the AVR core. The boot progra m can use any interface to download the application

program in the App lication Fl ash memory. Sof tware in the Boot Fla sh sect ion wil l continue to run

while the Application Flash section is updated, providing true Rea d-While-Write operation. By

combining an 8-bit RISC CPU with In-System Self-P rogrammable Flash on a monolithic chip,

the Atmel ATmega16 is a powerful microcontroller that provides a highly-fle xible and co st-effec-

tive solution to many embedded control applications.

The ATmega16 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.

Pin Descriptions

VCC Digital supply voltage.

GND Ground.

Port A (PA7..PA0) Port A serves as the analog inputs to the A/D Converter.

Port A also serves as an 8-bit bi-directional I/O port, if the A/D Co nverter is not used. Port pins

can provide inte rnal pull- up r esistors ( selected for each bit) . The Po rt A output buffe rs have sym-

metrical drive characteristics with both high sink and source capability. When pins PA0 to PA7

are used as inputs and are externally pulled low, they will source current if the internal pull-up

resistors are activated. Th e Port A pins are tri-stated when a reset co ndition becomes active,

even if the clock is not running.

2466T–AVR–07/10

ATmega16(L)

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. Th e Port B pins are tri-stated when a reset co ndition becomes active,

even if the clock is not running.

Port B also serves the functions of various special features of the ATmega16 as listed on page

58.

Port C (PC7..PC0) 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 hig h 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 runnin g. If the JTAG interface is enabled , the pull-up resistors on pins

PC5(TDI), PC3(TMS) and PC2(TCK) will be activated even if a reset occurs.

Port C also serves the functions of the JTAG interface and other special features of the

ATmega16 as listed on page 61.

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 hig h 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.

Port D also serves the functions of various special features of the ATmega16 as listed on page

63.

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 15 on page

38. Shorter pulses ar e not guaranteed to generate a reset.

XTAL1 Input to the inverting Oscillator amplifier and input to the internal clock operating circuit.

XTAL2 Output from the inverting Oscillator amplifier.

AVCC AVCC is the supply voltage pin for Port A and the A/D Converter. It should be externally con-

nected to VCC, even if the ADC is not used. If the ADC is used, it should be connected to VCC

through a low-pass filter.

AREF AREF is the analog reference pin for the A/D Converter.

2466T–AVR–07/10

ATmega16(L)

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

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

Note: 1.

Data Retention Reliability Qualification results show that the projected data retention failure rate is much less

than 1 PPM over 20 years at 85°C or 100 years at 25°C.

2466T–AVR–07/10

ATmega16(L)

About Code

Examples This documentatio n contains simple co de examples that br iefly sh ow how to u se various parts of

the device. These code examp les assume that the part specific header file is included b efore

compilation. Be aware that no t all C Compiler vendors include bit definitio ns in the header files

and interrupt ha ndling in C is compiler dependent. Plea se confirm with the C Co mpiler documen-

tation for more details.

2466T–AVR–07/10

ATmega16(L)

AVR CPU Core

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.

Architectural

Overview Figure 3. Block Diagram of the AVR MCU Architecture

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

separate memories and buses for progr am and data. Instructions in the program memory are

executed with a single level pipelining. While one instruction is being executed, the next instruc-

tion 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 × 8-bit general purpose working registers with a single

clock cycle access time. This allows single-cycle Arithmetic Logic Unit (ALU) operation. In a typ-

ical ALU operation, two operands are output from the Register File, the operation is executed,

and the result is stored back in the Register File – in one clock cycle.

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

Space addressing – enablin g efficient address calculations. On e of the these address pointers

can also be used as an address pointer for look up ta bles in Flash Program memory. These

added function registers are the 16-bit X-register, Y-register, and Z-register, described later in

this section.

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

a register. Single regi ster operation s can also be executed in th e ALU. After an ar ithmetic opera-

tion, the Stat us Register is updated to reflect infor mation about the result of the operation.

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

2466T–AVR–07/10

ATmega16(L)

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 for-

mat. Every program memory address contains a 16-bit or 32- bit instruction.

Program Flash me mory space is divided in two sections, the Boot program se ction 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 an d subroutine calls, the retur n address Program Count er (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 eas ily 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 interru pt module has its control register s 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 posi-

tion. The lower the interrupt vector address, the higher the priority.

The I/O memory space contains 64 addresses for CPU peripheral functions as Control Regis-

ters, SPI, and other I/O fun ctions. The I/O Memory can be access ed directly, or as the Data

Space locations following those of the Register File, $20 - $5F.

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 betwee n a re gister an d an immedi ate ar e execut ed . The ALU ope ra tio ns 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 the “Instruction Set” section for a detailed description.

Status Register The Status Regist er conta ins infor mation a bout the result of th e most recently e xecuted ari thme-

tic 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 I nstru ction Se t Re ference. This will in many case s re move the n eed f or using t he

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 inte rr up t. T his mu st be ha nd le d by so ftw ar e.

The AVR Status Register – SREG – is defined as:

• Bit 7 – I: Globa l Interrupt Enable

The Global Interrupt Enable bit must be set for the interrupts to be enabled. The individual inter-

rupt enable control is then performed in separate control registers. If the Global Interrupt Enable

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 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 Value 0 0 0 0 0 0 0 0

2466T–AVR–07/10

ATmega16(L)

• Bit 6 – T: Bit Copy Storage

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

nation 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 t he “Instruction Set Description” for detailed information.

• Bit 4 – S: Sign Bit, S = N ⊕ V

The S-bit is always an exclusive or betwee n the Negative Flag N and the Two’s Complement

Overflow Flag V. See the “Instruction Set Description” for detailed information.

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

The Two’s Complement Overflow Flag V supports two’s complement arithmetics. See the

“Instruction Set Description” for detailed information.

• Bit 2 – N: Negative Flag

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

“Instruction Set Description” for detailed information.

• Bit 1 – Z: Zero Flag

The Zero Flag Z indicates a zero result in an arithmetic or logic operation. See the “Instruction

Set Description” for detailed information.

• Bit 0 – C: Carry Flag

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

Description” for detailed information.

2466T–AVR–07/10

ATmega16(L)

General Purpose

the required performance and flexibility, the following input/output schemes are supported by the

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

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

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

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

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

Figure 4. AVR CPU General Purpose Work ing 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, 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 imple-

mented as SRAM locations, this memo ry organization provides great fle xibility in access of the

registers, as the X-, Y-, and Z-pointer Registers can be set to index any register in the file.

7 0 Addr.

R0 $00

R1 $01

R2 $02

…

R13 $0D

General R14 $0E

Purpose R15 $0F

Working R16 $10

Registers R17 $11

…

R26 $1A X-register Low Byte

R27 $1B X-register High Byte

R28 $1C Y-register Low Byte

R29 $1D Y-register High Byte

R30 $1E Z-register Low Byte

R31 $1F Z-register High Byte

2466T–AVR–07/10

ATmega16(L)

The X-register, Y-

isters 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 5.

Figure 5. The X-register, Y-register, and Z-register

In the differ ent a ddr essing modes the se ad dr ess regist er s have fun cti ons a s fi xed d isp lacement ,

automatic increment, and automatic decrement (see the Instruction Set Reference for details).

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 th e Stack. No te that the Sta ck is implement ed as growing from hig her memor y loca-

tions to lower memor y locat io ns. This im plies t hat a Stack PUSH co mmand decr ease s th e Stack

Pointer. If sof tware r eads the Progr am Counte r from t he Stack af ter a call or an interrupt , unused

bits (15:13) sho uld be masked out.

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 $60. The Stack Pointer is decremented by one when data is pushed onto the Stack

with the PUSH instruction, and it is decremented by two when the return add re ss is pushed on to

the Stack with subroutine call or inte rrupt. The Stack Point er is increme nted by one when data is

popped from the Stack with the POP instruction, an d it is incremented by two 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 implementa-

tions 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 707 0

R27 ($1B) R26 ($1A)

15 YH YL 0

Y - register 707 0

R29 ($1D) R28 ($1C)

15 ZH ZL 0

Z - regis t er 70 7 0

R31 ($1F) R30 ($1E)

Bit 151413121110 9 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 0 0 0 0 0 0

00000000

2466T–AVR–07/10

ATmega16(L)

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 genera te d fro m the select ed clock sour ce for t he

chip. No internal clock division is used.

Figure 6 shows the parallel instruction fetches and instruction executions enabled by the Har-

vard 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 6. The Parallel Instruction Fetches and Instruction Executions

Figure 7 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 destina-

tion register.

Figure 7. Single Cycle ALU Operation

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 wr itten logic one to gether with th e Global Inter rupt

Enable bit in the Status Register in orde r to enable the interrupt. Depending on the Program

Counter value, interrupts ma y be automatically disabled when Boot Lock bits BLB02 or BLB12

are programmed. This feature improves software security. See the section “Memory Program-

ming” on page 259 for details.

The lowest addresses in the progra m memory space are by default defined as the Reset and

Interrupt Vectors. The complete list of vectors is shown in “Interrupts” on page 45. 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

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

Total Execution Time

ALU Operation Execute

Result Write Back

T1 T2 T3 T4

clkCPU

2466T–AVR–07/10

ATmega16(L)

0. The Interru pt Vectors can be moved t o the star t of the Bo ot Flash sect ion by setti ng the IVSEL

bit in the General Interrupt Control Register (GICR). Refer to “Interrupts” on page 45 for more

information. The Reset Vector can also be moved to the start of the boot Flash section by pro-

gramming the BOOTRST Fuse, see “Boot Loader Support – Read-While-Write Self-

Programming” on page 246.

When an interrupt occurs, th e Global Interrupt Enable I-bit is cleared and all interrup ts are dis-

abled. 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 au tomatically 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 Vec-

tor 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. The se

interrupts do not nece ssarily have I nterrupt Fla gs. If t he interr upt condit ion disappear s 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 .

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 f ollowing e x ample sh ows how this can be used to a void interrupts during the

timed EEPROM write sequence.

Assembly Code Example

in r16, SREG ; store SREG value

cli ; disable interrupts during timed sequence

sbi EECR, EEMWE ; start EEPROM write

sbi EECR, EEWE

out SREG, r16 ; restore SREG value (I-bit)

C Code Example

char cSREG;

cSREG = SREG; /* store SREG value */

/* disable interrupts during timed sequence */

_CLI();

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

EECR |= (1<<EEWE);

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

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ATmega16(L)

When using the SEI instruction to enable interrupts, the instruction following SEI will be exe-

cuted before any pending interrupts, as shown in this example.

Interrupt Response

Time The interrupt execution response for all the enabled AVR interrupts is four clock cycles mini-

mum. After four clo ck cycles t he pr og ram vect or add ress fo r t he actua l interr up t ha nd ling rout ine

is executed. During this four 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 in creased by fo ur clock cycles. This incre ase comes in additi on to the

start-up time from the selected sleep mode.

A return from an interrupt handling routine takes four clock cycles. During these four clock

cycles, the Program Counter (two bytes) is popped back from the Stack, the Stack Pointer is

incremented by two, and t he I-bit in SREG is set.

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

_SEI(); /* set global interrupt enable */

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

/* note: will enter sleep before any pending interrupt(s) */

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ATmega16(L)

AVR ATmega16

Memories This section describes the different memories in the ATmega16. The AVR architecture has two

main memory spaces, the Data Memory and the Program Memory space. In addition, the

ATmega16 features an EEPROM Memory for data storage. All three memory spaces are linear

and regular.

In-System

Reprogrammable

Flash Program

Memory

The ATmega16 contains 16 Kbytes On-chip In-System Reprogrammable Flash memory for pro-

gram storage. Since all AVR instructions are 16 or 32 bits wide, the Flash is organized as 8K ×

16. For software security, the Flash Program memory space is divided into two sections, Boot

Program section and Appli cation Program section.

The Flash memory has an endur ance of at least 10 ,0 00 write/ e rase cycles. The ATmega16 Pro-

gram Counter (PC) is 13 bits wide, thus addressing the 8K program memory locations. The

operation of Boot Program section and associa ted Boot Lock bits for software protection are

described in detail in “Boot Loader Support – Rea d-While-Write Self-Programming” on pag e

246. “Memory Programming” on page 259 contains a detailed description on Flash data serial

downloading using the SPI pins or the JTAG interface.

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

– Load Program Memory Instruction Description).

Timing diagrams for instruction fetch and execution are presented in “Instruction Execution Tim-

ing” on page 13.

Figure 8. Program Memory Map

$0000

$1FFF

Application Flash Section

Boot Flash Section

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ATmega16(L)

SRAM Data

Memory Figure 9 shows how the ATmega16 SRAM Memory is organized.

The lower 1120 Data Memory locations ad dress the Registe r File, the I/O Memo ry, and the inter-

nal data SRAM. The firs t 96 locations address the Reg ister File and I/O Memory, and th e next

1024 locations address the internal data SRAM.

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

ment, 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 Displacemen t mode reaches 63 add ress locations from the base address given

by the Y-register or Z-register.

When using register indirect addressing modes with automatic pre-decrement and post-incre-

ment, the address registers X, Y, and Z are decremented or incremented.

The 32 general purpose working registers, 64 I/O Registers, and the 1024 bytes of internal data

SRAM in the ATmega16 are all accessible through all these addressing modes. The Register

File is described in “General Purpose Register File” on page 11.

Figure 9. Data Memory Map

R29

R30

R31

I/O Registers

$00

$01

$02

...

$3D

$3E

$3F

...

$0000

$0001

$0002

$001D

$001E

$001F

$0020

$0021

$0022

...

$005D

$005E

$005F

...

Data Address Space

$0060

$0061

$045E

$045F

...

Internal SRAM

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ATmega16(L)

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 10 .

Figure 10. On-chip Data SRAM Access Cycles

EEPROM Data

Memory The ATmega16 contains 512 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 273, pa ge 278, and page 262, respectively.

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 1. A self-timing function, ho wever, lets

the user software detect when the next byt e can be written . If the user code contain s instructio ns

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 “Prevent-

ing EEPROM Corruption” on page 22 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.

clk

Data

Address Address V alid

T1 T2 T3

Compute Address

Read Write

CPU

Memory Access Instruction Next Instruction

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ATmega16(L)

The EEPR OM Address

EEARL

• Bits 15..9 – Res: Reserved Bits

These bits are reserved bits in the ATmega16 and will always read as zero.

• Bits 8..0 – EEAR8..0: EEPROM Address

The EEPROM Address Registers – EEARH and EEARL – specify the EEPROM address in the

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

511. The initial value of EEAR is undefined. A proper value must be written before the EEPROM

may be accessed.

The EEPROM Data

• Bits 7..0 – EEDR7.0: EEPROM Data

For the EEPROM write oper ation, 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.

The EEPROM Control

• Bits 7..4 – Res: Reserved Bits

These bits are reserved bits in the ATmega16 and will always read as zero.

• Bit 3 – EERIE: EEPROM Ready Interrupt Enable

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

EERIE to zero disables the interrupt. The EEPROM Ready interrupt generates a constant inter-

rupt when EEWE is cleared.

• Bit 2 – EEMWE: EEPROM Master Write Enable

The EEMWE bit determines whether setting EEWE to one causes the EEPROM to be written.

When EEMWE is set, setting EEWE within four clock cycles will write data to the EEPROM at

the selected address If EEMWE is zero, setting EEWE will have no effect. When EEMWE has

been written to one by software, hardware clears the bit to zero after four clock cycles. See the

description of the EEWE bit for an EEPROM write procedure.

Bit 151413121110 9 8

–––––––EEAR8EEARH

EEAR7 EEAR6 EEAR5 EEAR4 EEAR3 EEAR2 EEAR1 EEAR0 EEARL

76543210

Read/WriteRRRRRRRR/W

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

Initial Value0000000X

XXXXXXXX

Bit 76543210

MSB LSB EEDR

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

Initial Value00000000

Bit 76543210

– – – – EERIE EEMWE EEWE EERE EECR

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

Initial Value 0 0 0 0 0 0 X 0

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ATmega16(L)

• Bit 1 – EEWE: EEPROM Write Enable

The EEPROM Write Enable Signal EEWE is the write strobe to the EEPROM. When address

and data are c orrectly set up, the EEWE bit must be written to one to write the value in to the

EEPROM. The EEMWE bit must be written to one before a logical one is written to EEWE, oth-

erwise 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 EEWE becomes zero.

2. Wait until SPMEN in SPMCR becomes zero.

3. Wr ite new EEPROM address to EEAR (optional).

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

5. Write a logical one to the EEMWE bit while writing a zero to EEWE in EECR.

6. Within four clock cycles after setting EEMWE, write a logical one to EEWE.

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 t he F l as h is n e ve r b ein g up da te d by the CPU, step 2 ca n b e om itt ed . Se e “Boot Loader

Support – Read-While-Write Self-Programming” on page 246 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 EEAR or EEDR reGister will be modified, causing the

interrupted EEPROM Access t o fail. It is re commend ed to have the Global Int errupt Fla g cleared

during all the steps to avoid these problems.

When the write access time has elapsed , the EEWE bit is cleared by hardware. The user soft-

ware can poll this bit a nd wait f or a zero bef ore wr iting th e next byte. When EEWE has b een 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 cor-

rect 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 EEWE 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. Table 1 lists the typical pro-

gramming time for EEPROM access from the CPU.

Note: 1. Uses 1 MHz clock, independent of CKSEL Fuse setting.

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

EEPROM. The examples assume that interrupts ar e controlled (for example by disabling inter-

rupts globally) so that no interrupts will occur during execution of these functions. The examples

Table 1. EEPROM Programming Time

Symbol Number of Calibrated RC

Oscillator Cycles(1) Typ Programming Time

EEPROM write (from CPU) 8448 8.5 ms

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ATmega16(L)

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.

Assembly Code Example

EEPROM_write:

; Wait for completion of previous write

sbic EECR,EEWE

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 EEMWE

sbi EECR,EEMWE

; Start eeprom write by setting EEWE

sbi EECR,EEWE

ret

C Code Example

void EEPROM_write(unsigned int uiAddress, unsigned char ucData)

{

/* Wait for completion of previous write */

while(EECR & (1<<EEWE))

;

/* Set up address and data registers */

EEAR = uiAddress;

EEDR = ucData;

/* Write logical one to EEMWE */

EECR |= (1<<EEMWE);

/* Start eeprom write by setting EEWE */

EECR |= (1<<EEWE);

}

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ATmega16(L)

The next code examples show assembly and C functions for reading the EEPROM. The exam-

ples assume that interrupts are controlled so that no interrupts will occur during execution of

these functio ns.

EEPROM Write During

Power-down Sleep

Mode

When entering Power-down Sleep mode while an EEPROM write operation is active, the

EEPROM write operation will continue, and will complete before the Write Access time has

passed. However, when the write operation is completed, the Oscillator continues running, and

as a consequence, the device does not enter Power-down entirely. It is therefore recommended

to verify that the EEPROM write operation is completed before entering Power-down.

Preventing EEPROM

Corruption During periods of low VCC, the EEPROM data can be co rrupted because the supply voltage is

too low for the CPU and the EEPROM to operate properly. These issues a re 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 seque nce to the EEPROM requires a minimum voltage to operate cor rectly. Sec-

ondly, the CPU itself can execute instructions incorrectly, if the su pp ly voltage is too low.

Assembly Code Example

EEPROM_read:

; Wait for completion of previous write

sbic EECR,EEWE

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

unsigned char EEPROM_read(unsigned int uiAddress)

{

/* Wait for completion of previous write */

while(EECR & (1<<EEWE))

;

/* Set up address register */

EEAR = uiAddress;

/* Start eeprom read by writing EERE */

EECR |= (1<<EERE);

/* Return data from data register */

return EEDR;

}

2466T–AVR–07/10

ATmega16(L)

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 Protec-

tion circuit can be used. If a reset occurs while a write operation is in prog ress, the write

operation will be completed provided that the power supply voltag e is sufficient.

I/O Memory The I/O space definition of the ATmega16 is shown in “Register Summary” on page 331.

All ATmega16 I/Os and peripherals are placed in the I/O space. The I/O locations are accessed

by the IN and O UT instru ctions, tran sferri ng data be tween th e 32 gener al purpose wo rking regis-

ters and the I/O space. 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. Refer to the Instruction Set section for more

details. 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.

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 Flag s are cleared b y writing a lo gical one to t hem. Note th at the CBI a nd 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 instructions work with registers $00 to $1F only.

The I/O and Peripherals Control Registers are explained in later sections.

2466T–AVR–07/10

ATmega16(L)

System Clock

and Cloc k

Options

Clock Systems

and their

Distribution

Figure 11 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 red uce power co nsumption, the clo cks to modu les

not being used can be halted by using different sleep modes, as described in “Power Manage-

ment and Sleep Modes” on page 32. The clock systems are detailed Figure 11.

Figure 11. Clock Distribution

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.

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 inter-

rupts are detecte d by asynchronous logic, allowing such interrupts to be detected even if the I/O

clock is halted. Also note that address recognition in the TWI module is carried out asynchro-

nously when clkI/O is halted, enabling TWI address reception in all sleep modes.

Flash Clock – clkFLASH Th e Flash clock controls o peration of the Flash in terface. Th e Flash clock is usually active simul-

taneously with the CPU clock.

Asynchronous Timer

Clock – clkASY

The Asynchronous Timer clock allows the Asynchronous Timer/Counter to be clocked directly

from an external 32 kHz clock crystal. The dedicated clock domain allows using this

Timer/Counter as a real-time counter even when the device is in sleep mode.

General I/O

Modules

Asynchronous

Timer/Counter ADC CPU Core RAM

clkI/O

clkASY

AVR Clock

Control Unit

clkCPU

Flash and

EEPROM

clkFLASH

clkADC

Source Clock

Watchdog Timer

Watchdog

Oscillator

Reset Logic

Clock

Multiplexer

Watchdog Clock

Calibrated RC

Oscillator

Timer/Counter

Oscillator

Crystal

Oscillator

Low-frequency

Crystal Oscillator

External RC

Oscillator

External Clock

2466T–AVR–07/10

ATmega16(L)

ADC Clock – clkADC The ADC is provided with a dedicated clock domain. This allows halting the CPU and I/O clocks

in order to re duce noise ge ner ated by digital cir cuit ry. Th is gives mo re accurat e ADC conversion

results.

Clock Sources The device has the follo wing clock source options, selectable by Flash Fuse bits as shown

below. The cloc k fr om t he se lected so ur ce is i npu t t o th e AVR clo c k gene ra to r, an d rou te d t o the

appropriate modules.

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

The various choices for each clocking option is given in the following sections. When the CPU

wakes up from Power-down or Power-save, the selected clock source is used to time the start-

up, ensuring stable Oscillator operation before instruction execution starts. When the CPU starts

from Reset, there is as an additional delay allowing the power to re ach a stable level before

commencing normal operation. The Watchdog Oscillator is used for timing this real-time part of

the start-up time. The number of WDT Oscillator cycles used for each time-out is shown in Table

3. The frequency of the Watchdog Oscillator is voltage dependent as shown in “ATmega16 Typ-

ical Characteristics” on page 299.

Default Clock

Source The device is shipped with CKSEL = “0001” and SUT = “10”. The default clock source setting is

therefore the 1 MHz Internal RC Oscillator with longest startup time. This default setting ensures

that all users can make their desired clock source setting using an In-System or Parallel

Programmer.

Crystal Oscillator XTAL1 and XTAL2 are input and output , respectively, of an inver ting amplifier which ca n be con-

figured for use as an On-chip Oscillator, as shown in Figure 12. Either a quartz crystal or a

ceramic resonator may be used. The CKOPT Fuse selects between two different Oscillator

amplifier modes. When CKOPT is programmed, the Oscillator output will oscillate will a full rail-

to-rail swing on the output. This mode is suitable when operating in a very noisy environment or

when the output from XTAL2 drives a second clock buffer. This mode has a wide frequency

range. When CKOPT is unprogrammed, the Oscillator has a smaller output swing. This reduces

power consumption considerably. This mode has a limited frequency range and it can not be

used to drive other clock buffers.

For resonators, th e maximum f re quency is 8 MHz with CKOPT un pr ogramm ed an d 16 MHz with

CKOPT programmed. 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

Table 2. Device Clocking Options Select(1)

Device Clocking Option CKSEL3..0

External Cr ystal/Ceramic Resonator 1111 - 1010

External Low-frequency Crystal 1001

Exter nal RC Oscillator 1000 - 0101

Calibrated Internal RC Oscillator 0100 - 0001

Exter nal Clock 0000

Table 3. Number of Watchdog Oscillator Cycles

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

4.1 ms 4.3 ms 4K (4,096)

65 ms 69 ms 64K (65,536)

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ATmega16(L)

choosing capacitors for use with crystals are given in Table 4. For ceramic reso nators, the

capacitor values given by the manufacturer should be used.

Figure 12. Crystal Oscillator Connections

The Oscillator can operate in three different modes, each optimized for a specific frequency

range. The operating mode is selected by the fuses CKSEL3..1 as shown in Table 4.

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

Table 4. Crystal Oscillator Operating Modes

CKOPT CKSEL3..1 Frequency Range

(MHz) Recommended Range for Capacitors

C1 and C2 for Use with Crystals (pF)

1 101(1) 0.4 - 0.9 –

1 110 0.9 - 3.0 12 - 22

1 111 3.0 - 8.0 12 - 22

0 101, 110 , 111 1.0 ≤12 - 22

XTAL2

XTAL1

GND

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ATmega16(L)

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

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 fre-

quency of the device, and if frequency stability at start-up is not important for the application.

Table 5. Start-up Times for the Crystal Oscillator Clock Selection

CKSEL0 SUT1..0

Start-up Time from

Power-down and

Power-save

Additional Delay

from Reset

(VCC = 5.0V) Recommended Usage

0 00 258 CK(1) 4.1 ms Ceramic resonator, fast

rising power

0 01 258 CK(1) 65 ms Cer amic resonator, slowly

rising power

010 1K CK

(2) –Ceramic resonator, BOD

enabled

011 1K CK

(2) 4.1 ms Ceramic resonator, fast

rising power

100 1K CK

(2) 65 ms Ceramic resonator, slowly

rising power

101 16K CK – Cr ystal Oscillator, BOD

enabled

1 10 16K CK 4.1 ms Crystal Oscillator, fast

rising power

1 11 16K CK 65 ms Crystal Oscillator, slowly

rising power

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ATmega16(L)

Low-frequency

Crystal Oscillator To use a 32.7 68 kHz watch crystal a s the clock sour ce for the device, t he Lo w-fre quency Crysta l

Oscillator must be selected by setting the CKSEL Fuses to “1001”. The crystal should be con-

nected as shown in Figure 12. By programming the CKOPT Fuse, the user can enable internal

capacitors on XTAL1 and XTAL2, thereby removing the need for external capacitors. The inter-

nal capacitors have a nominal value of 36 pF.

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

Table 6.

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

application.

External RC

Oscillator For timing insensitive applications, the exte rnal RC configuration shown in Figure 13 can be

used. The frequency is roughly estimated by the equation f = 1/(3RC). C should be at least 22

pF. By progr amm ing t he CKOP T Fu se, t he user can en able a n inte rn al 3 6 pF capacitor b etw een

XTAL1 and GND, thereby removing the need for an external capacitor. For more information on

Oscillator operation and details on how to choose R and C, refer to the External RC Oscillator

application note.

Figure 13. External RC Configuration

The Oscillator can operate in four different modes, each optimized for a specific frequency

range. The operating mode is selected by the fuses CKSEL3..0 as shown in Table 7.

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

SUT1..0

Start-up Time from

Power-down and

Power-save

Additional Delay

from Reset

(VCC = 5.0V) Recommended Usage

00 1K CK(1) 4.1 ms Fast rising power or BOD enabled

01 1K CK(1) 65 ms Slowly rising power

10 32K CK 65 ms Stable frequency at start-up

11 Reserved

Table 7. External RC Oscillator Operating Modes

CKSEL3..0 Frequency Range (MHz)

0101 0.1 ≤ 0.9

0110 0.9 - 3.0

0111 3.0 - 8.0

1000 8.0 - 12.0

XTAL2

XTAL1

GND

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ATmega16(L)

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

Table 8.

Note: 1. This option should not be used when operating close to the maximum frequency of the de vice .

Calibrated Internal

RC Oscillator The Calibrated Internal RC Oscillator provides a fixed 1.0 MHz, 2.0 MHz, 4.0 MHz, or 8.0 MHz

clock. All frequencies are nominal values at 5V and 25°C. This clock may be selected as the

system-clock by programming the CKSEL Fuses as shown in Table 9. If selected, it will operate

with no external components. The CKOPT Fuse should always be unpro-grammed when using

this clock option. During Re set, hardware loads the calib ration byte into the OSCCAL Registe r

and thereby automatically calibrates the RC Oscillator. At 5V, 25°C and 1.0 MHz, 2.0 MHz, 4.0

MHz or 8.0 MHz Oscillator frequency selected, this calibration gives a frequency within ±3% of

the nominal frequency. Using calibration methods as described in application notes available at

www.atmel.com/avr it is possible to achieve ±1% accuracy at any given VCC and Temperature.

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. Fo r more information on the pre-programmed cali-

bration value, see the section “Calibration Byte” on page 261.

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

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

Table 10. XTAL1 and XTAL2 should be left unconnected (NC).

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

Table 8. Start-up Times for the External RC Oscillator Clock Selection

SUT1..0

Start-up Time from

Power-down and

Power-save

Additional Delay

from Reset

(VCC = 5.0V) Recommended Usage

00 18 CK – BOD enabled

01 18 CK 4.1 ms Fast rising power

10 18 CK 65 ms Slowly rising power

11 6 CK(1) 4.1 ms Fast rising power or BOD enabled

Table 9. Internal Calibrated RC Oscillator Operating Modes

CKSEL3..0 Nominal Frequency (MHz)

0001(1) 1.0

0010 2.0

0011 4.0

0100 8.0

Table 10. Start-up Times for the Internal Calibrated RC Oscillator Clock Selection

SUT1..0

Start-up Time from

Power-down and

Power-save

Additional Dela y

from Reset

(VCC = 5.0V) Recommended Usage

00 6 CK – BOD enabled

01 6 CK 4.1 ms Fast rising power

10(1) 6 CK 65 ms Slowly rising power

11 Reserved

2466T–AVR–07/10

ATmega16(L)

Oscillator Calibration

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

Writing the calibration byte to this address will trim the Internal Oscillator to remove process vari-

ations from the Oscillator frequency. This is done automatically during Chip Reset. When

OSCCAL is zero, the lo west avail able fr equency is chosen . Writing non-zer o values to this regis-

ter will increase the frequency of the Internal Oscillator. Writing $FF to the register gives the

highest available frequency. The calibrated Oscillator is used to time EEPROM and Flash

access. If EEPROM or Flash is written, do not calibrate to more than 10% above the nominal fre-

quency. Otherwise, the EEPROM or Flash write may fail. Note that the Oscillator is intended for

calibration to 1.0 MHz, 2.0 MHz, 4.0 MHz, or 8.0 MHz. Tuning to other values is not guaranteed,

as indicated in Table 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

Table 11. Internal RC Oscillator Frequency Range.

OSCCAL Value Min Frequency in Percentage of

Nominal Frequency (%) Max Frequency in P ercentage of

Nominal Frequency (%)

$00 50 100

$7F 75 150

$FF 100 200

2466T–AVR–07/10

ATmega16(L)

External Clock To drive the device from an external clock source, XTAL1 should be driven as shown in Figure

14. To run the device on an external clock, the CKSEL Fuses must be programmed to “0000”.

By programming the CKOPT Fuse, the user can enable an internal 36 pF capacitor between

XTAL1 and GND.

Figure 14. External Clock Drive Configuration

When this clock source is selected, start-up times are de termined by the SUT Fu ses as shown in

Table 12.

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

quency 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. It is required to ensure that the

MCU is kept in reset during such changes in the clock frequency.

Timer/Counter

Oscillator For AVR microcontrollers with Timer/Counter Oscillator pins (TOSC1 and TOSC2), the crystal is

connected directly between the pins. No externa l capacitors are needed. The Oscillator is opti-

mized for use with a 32 .768 kHz watc h crystal. Applying an external clock source to TOSC1 is

not recommended.

Note: The Timer/Counter Oscillator uses the same type of crystal oscillator as Low-Frequency Oscillator

and the internal capacitors have the same nominal value of 36 pF.

Table 12. Start-up Times for the External Clock Selection

SUT1..0

Start-up Time from

Power-down and

Power-save

Additional Delay

from Reset

(VCC = 5.0V) Recommended Usage

00 6 CK – BOD enabled

01 6 CK 4.1 ms Fast rising power

10 6 CK 65 ms Slowly rising power

11 Reserved

EXTERNAL

CLOCK

SIGNAL

2466T–AVR–07/10

ATmega16(L)

Power

Management

and Sleep

Modes

Sleep modes enable the application to shut down unused modules in the MCU, thereby saving

power. The AVR provides vario us sleep modes allowing the user to tailor the power consump-

tion to the application’s requirements.

To enter any of the six sleep modes, the SE bit in MCUCR must be written to logic one and a

SLEEP instruction must be executed. The SM2, SM1, and SM0 bits in the MCUCR Register

select which sleep mode (Idle, ADC Noise Reduction, Power-d own, Power-save, Standb y, or

Extended Standby) will be activated by the SLEEP instruction. See Table 13 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, it 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 11 on page 24 presents the different clock systems in the ATmega16, and their distribu-

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

MCU Control Register

– MCUCR The MCU Control Register contains control bits for power managem ent.

• Bits 7, 5, 4 – SM2..0: Sleep Mode Select Bits 2, 1, and 0

These bits select between the six available sleep modes as shown in Table 13.

Note: 1. Standby mode and Extended Standby mode are only available with external crystals or

resonators.

• Bit 6 – SE: Sleep Enable

The SE bit must be written to lo gic one to make the MCU enter the sle ep mode when the SLEEP

instruction is executed. To avoid the MCU entering the sleep mode unless it is the programmers

purpose, it is recomm ended to write the Sle ep Enable ( SE) bit to o ne just bef ore the e xecution of

the SLEEP instruction and to clear it immediately after waking up.

Bit 76543210

SM2 SE SM1 SM0 ISC11 ISC10 ISC01 ISC00 MCUCR

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

Initial Value00000000

Table 13. Sleep Mode Select

SM2 SM1 SM0 Sleep Mode

000Idle

0 0 1 ADC Noise Reduction

010Power-down

011Power-save

100Reserved

101Reserved

110Standby

(1)

1 1 1 Extended Standby(1)

2466T–AVR–07/10

ATmega16(L)

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 SPI, USART, Analog Comparator, ADC, Two-wire Serial

Interface, Timer /Co unte rs, Wa tchdog, a nd the int erru pt syste m to con tinue o pe ratin g. This sleep

mode basically halts clkCPU and clkFLASH, while allowing the other clocks to run.

Idle mode enables the M CU to wake up from external triggered interrupts as well a s internal

ones like the Timer O verflow and USART Transmit Complete interru pts. If wake-up from the

Analog Comparato r interrupt is not required , the Analog Comparator can b e 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 automati-

cally when this mode is entered.

ADC Noise

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

Noise Reduction mod e, stopping the CPU but allowing the ADC, the External Interrup ts, the

Two-wire Serial Interface address watch, Timer/Counter2 and the Watchdog to continue operat-

ing (if enabled). This sleep m ode basically halts clkI/O, clkCPU, and clkFLASH, while allowing the

other clocks to run.

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

the ADC is enabled, a conve rsion star ts automa tically when this mode is enter ed. Apart form the

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

Reset, a Two-wire Serial Interface Address Match Interrupt, a Timer/Counter2 inte rrupt, an

SPM/EEPROM ready interrupt, an External level interrupt on INT0 or INT1, or an external inter-

rupt on INT2 can wake up the MCU from ADC Noise Reduction mode.

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

Two-wire Serial Interface a ddress watch, and the Watchdog continu e operating (if enabled).

Only an External Reset, a Watchdog Reset, a Brown-out Reset, a Two-wire Serial Interface

address match interrupt, an External level interrupt on INT0 or INT1, or an External interrupt on

INT2 can wake up the MCU. This sleep mo de basically halts a ll gene rated clo cks, allowin g oper-

ation of asynchronous modules only.

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 68

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 25.

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

save mode. This mode is identical to Power-down, with one exception:

If Timer/Counter2 is clocked asynchronously, that is, the AS2 bit in ASSR is set, Timer/Counter2

will run during sleep. The device can wake u p from either Timer Overflow or Output Compare

event from Timer/Counter2 if the corresponding T imer/Counter2 interrupt enable bits are set in

TIMSK, and the Global Interrupt Enable bit in SREG is set.

If the Asynch ronous Tim er is NOT clocked asynchronously, Power-down mode is r ecommended

instead of Power-save mode because the contents of the registers in the Asynchronous Time r

should be considered undefined after wake-up in Power-save mode if AS2 is 0.

This sleep mode basically halts all clocks except clkASY, allowing opera tion only of asynchronous

modules, includin g T ime r/ Co un te r2 if clocke d as yn ch ro no us ly.

2466T–AVR–07/10

ATmega16(L)

Standb y Mode When the SM2..0 bits are 110 and an external crystal/resonator clock o ption 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.

Extended Standb y

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. This mode is identical to

Power-save mode with the exception that the Oscillator is kept running. From Extended Standby

mode, the device wakes up in six clock cycles..

Notes: 1. External Crystal or resonator selected as clock source.

2. If AS2 bit in ASSR is set.

3. Only INT2 or level interrupt INT1 and INT0.

Table 14. Active Clock Domains and Wake Up Sources in the Different Sleep Modes

Active Clock domains Oscillators Wake-up Sources

Sleep

Mode clkCPU clkFLASH clkIO clkADC clkASY

Main Clock

Source Enabled Timer Osc.

Enabled

INT2

INT1

INT0

TWI

Address

Match Timer

SPM /

EEPROM

Ready ADC Other

I/O

Idle X X X X X(2) XXXXXX

ADC

Noise

Redu-

ction

XX X X

(2) X(3) XXXX

Power

Down X(3) X

Power

Save X(2) X(2) X(3) XX

(2)

Standby(1) XX

(3) X

Exten-

ded

Standby(1) X(2) XX

(2) X(3) XX

(2)

2466T–AVR–07/10

ATmega16(L)

Minimizing Po wer

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 possi ble 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.

Analog to Digital

Converter If enabled, the AD C will be enabled in all sleep modes. To save power, the ADC should be dis-

abled 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” on page 204

for details on ADC operation.

Analog Comparator 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 the other sleep

modes, the Analog Comparator is automatically disabled. However , if th e Ana log C omp arat or is

set up to use the Internal Voltage Reference as input, the Analog Comparator should be dis-

abled in all sleep modes. Otherwise, the Internal Voltage Reference will be enabled,

independent of sleep mode. Refer to “Analog Comparator” on page 201 for details on how to

configure the Analo g Comparator.

Brown-out Detector If the Br own-out Detector is not needed in th e application, this module should be tur ned off. If the

Brown-out Detector is enabled by the BODEN Fuse, 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 40 for details on ho w to

configure the Brown-out Detector.

Internal Voltage

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

Analog Comparator or the ADC. If these modules are disable d 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 Volt-

age Reference” on page 42 for details on the start-up time.

Watchdog Timer If the Watchdog Timer is not needed in the app lication, this 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 consump-

tion. Refer to “Watchdog Timer” on pa ge 42 for d et ails on ho w to configu re the Wa tchd og Time r.

Port Pins When entering a sleep mode, all port pins should be configured to use m inimum power. The

most important thing is then to ensure that no pins drive resistive loads. In sleep modes whe re

the both the I/O clock (clkI/O) and the ADC clock (clkADC) are stopped, the inp ut 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-u p conditions, and it will

then be ena bled. Refer to the section “Digital Input Enable and Sleep Modes” on page 54 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.

2466T–AVR–07/10

ATmega16(L)

JTAG Interface and

On-chip Debug

System

If the On-chip debug system is enabled by the OCDEN Fuse and the chip enter Power down or

Power save sleep mode, the main clock source remains enabled. In these sleep modes, this will

contribute significantly to the total current consumption. There are three alternative ways to

avoid this:

• Disable OCDEN Fuse.

• Disable JTAGEN Fuse.

• Write one to the JTD bit in MCUCSR.

The TDO pin is left floating wh en the JTAG interf ace is enabled while th e JTAG TAP controller is

not shifting data. If the hardware connected to the TDO pin does not pull up the logic le vel,

power consumption will increase. Note that the TDI pin for the next device in the scan chain con-

tains a pull-up that avoids this problem. Writing the JTD bit in the MCUCSR register to one or

leaving the JTAG fuse unprogrammed disables the JTAG interface.

2466T–AVR–07/10

ATmega16(L)

System Control

and Reset

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 15 shows the reset

logic. Table 15 defines the electrical parameters of the reset circuitry.

The I/O ports of the AVR are im mediately 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 re ach a stable level befor e normal operat ion starts. The time-out

period of the delay counter is defined by the user through the CKSEL Fuses. The different selec-

tions for the delay period are presented in “Clock Sources” on page 25.

Reset Sources The ATmega16 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 lev el is present on the RESET pin f or longer than the min imum

pulse length.

•Watchd og Reset.

The MCU is reset when the Watchdog Timer period e xpires 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 Registe r, one of the scan

chains of the JTAG system. Refe r to the section “IEEE 1149.1 (JTAG) Boundary-scan” on

page 228 for details.

2466T–AVR–07/10

ATmega16(L)

Figure 15. Reset Logic

Notes: 1. The Power-on Reset will not work unless the supply voltage has been below VPOT (falling).

2. VBOT may be below nominal minimum operating voltage for some devices. For devices where

this is the ca se, the device is tested down to VCC = VBOT during the prod uction test. This gua r-

antees that a Brown-out Reset will occur before VCC drops to a voltage where correct

operation of the microcontroller is no longer guaranteed. The test is performed using

BODLEVEL = 1 for ATmega16L and BODLEVEL = 0 for ATmega16. BODLEVEL = 1 is not

applicable for ATmega16.

Table 15. Reset Characteristics

Symbol Parameter Condition Min Typ Max Units

VPOT

Power-on Reset

Threshold Voltage (ri si ng) 1.4 2.3

Power-on Reset

Threshold Voltage

(falling)(1) 1.3 2.3

VRST RESET Pin Threshold

Voltage 0.1VCC 0.9VCC

tRST Minimum pulse width on

RESET Pin 1.5 µs

VBOT Brown-out Reset

Threshold Voltage(2) BODLEVEL = 1 2.5 2.7 3.2 V

BODLEVEL = 0 3.6 4.0 4.5

tBOD

Minimum low voltage

period for Brown-out

Detection

BODLEVEL = 1 2 µs

BODLEVEL = 0 2

VHYST Brown-out Detector

hysteresis 50 mV

MCU Control and Status

BODEN

BODLEVEL

Delay Counters

CKSEL[3:0]

CK TIMEOUT

WDRF

BORF

EXTRF

PORF

DATA B U S

Clock

Generator

SPIKE

FILTER

Pull-up Resistor

JTRF

JTAG Reset

Watchdog

Oscillator

SUT[1:0]

Watchdog

Timer

Reset Circuit

Brown-out

Reset Circuit

Power-on

Reset Circuit

INTERNAL RESET

COUNTER RESET

2466T–AVR–07/10

ATmega16(L)

Power-on Reset A Power-on Reset (POR) pulse is generated by an On-chip d etection circuit. The detection le vel

is defined in Table 15. 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) cir cuit 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 16. MCU Start-up, RESET Tied to VCC.

Figure 17. MCU Start-up, RESET Extended Externally

RESET

TIME-OUT

NTERNAL

RESET

tTOUT

VPOT

VRST

RESET

TIME-OUT

NTERNAL

RESET

tTOUT

VPOT

VRST

2466T–AVR–07/10

ATmega16(L)

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 15) 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, t he delay counter st arts the MCU after

the Time-out period tTOUT has expired.

Figure 18. External Reset During Operation

Brown-out Detection ATmega16 has an On-chip Brown-out Detection (BOD) circuit for monitoring the VCC level dur-

ing operation by comparing it to a fixed trigger level. The trigger level for the BOD can be

selected by the fuse BODLEVEL to be 2.7V (BODLEVEL unprogrammed), or 4.0V (BODLEVEL

programmed). 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.

The BOD circuit can be enabled/disabled by the fuse BODEN. When the BOD is enabled

(BODEN programmed ), and VCC decreases to a valu e below the trigger level (VBOT- in Figure

19), the Brown-out Reset is immediately activated. When VCC increases above the trigger level

(VBOT+ in Figure 19), 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 lon-

ger than tBOD given in Table 15 .

Figure 19. Brown-out Reset During Operation

RESET

TIME-OUT

INTERNAL

RESET

BOT-

BOT+

tTOUT

2466T–AVR–07/10

ATmega16(L)

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. Refer to

page 42 fo r details on operation of the Watchdog Timer.

Figure 20. Watchdog Reset During Operation

MCU Control and

Status Register –

MCUCSR

The MCU Control and Sta tus Register provides information on which reset source caused an

MCU Reset.

• 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 Powe r-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 occu rs. The bit is reset only by writing a logic zero to the flag.

To make use of the Reset Flags t o identify a reset condition, the user shou ld read and t hen reset

the MCUCSR as early as possible in the program. If the register is cleared before another reset

occurs, the source of th e reset can be found by examining t he Reset Flags.

Bit 76543210

JTD ISC2 – JTRF WDRF BORF EXTRF PORF MCUCSR

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

Initial Value 0 0 0 See Bit Description

2466T–AVR–07/10

ATmega16(L)

Internal Voltage

Reference ATmega16 features an internal bandgap reference. This reference is used for Brown-out Detec-

tion, and it can be used as an input to the Analog Comparator or the ADC. The 2.56V reference

to the ADC is generated from the internal bandgap reference.

Voltage Reference

Enable Signals and

Start-up Time

The voltage reference has a start-up time tha t may influence the way it should be used. T he

start-up time is give n in Tab le 16. To save power, the reference is not always turned on. The ref-

erence is on during the following situations:

1. When the BOD is enabled (by programming the BODEN 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 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.

Watchdog Timer The Watchdog Timer is clocked from a separate On-chip Oscillator which runs at 1 MHz. This is

the typical value at VCC = 5V. See character iza tio n data f or typ ica l values at othe r VCC levels. By

controlling the Watchdog Timer prescaler, the Watchd og Reset interval can be adjusted as

shown in Table 17 on page 43. The WDR – Watchdog Reset – instruction resets the Watchdog

Timer. The Watchdog Timer is also reset when it is disabled and when a Chip Reset occurs.

Eight different clock cycle periods can be selected to determine the reset period. If the reset

period expires without another Watchdog Reset, the ATmega16 resets and executes from the

Reset Vector. For timing details on the Watchdog Reset, refer to page 41.

To prevent unintentional disabling o f the Watchdog, a special turn-off sequence must be fol-

lowed when the Watchdog is disable d. Refer to the description of the Watchdog Timer Control

Figure 21. Watchdog Timer

Table 16. Internal Voltage Reference Characteristics

Symbol Parameter Min Typ Max Units

VBG Bandgap reference voltage 1.15 1.23 1.4 V

tBG Bandgap reference start-up time 40 70 µs

IBG Bandgap reference current consumption 10 µA

WATCHDOG

OSCILLATOR

2466T–AVR–07/10

ATmega16(L)

Watchdog Timer

Control Register –

WDTCR

• Bits 7..5 – Res: Reserved Bits

These bits are reserved bits in the ATmega16 and will always read as zero.

• Bit 4 – WDTOE: Watchdog Turn-off Enable

This bit must be set when the W DE bit is written to logic z ero. Otherwise, the Watchdog will not

be disabled. Once written to one, hardware will clear this bit after four clock cycles. Refer to the

description of t he WDE bit for a Watchdog disable procedure.

• Bit 3 – WDE: Watchdog Enable

When the WDE is written to logic one, the Watchdog Timer is enabled, and if the WDE is written

to logic zero, the Watchdog Timer function is disabled. WDE can only be cleared if the WDTOE

bit has logic level one. To disable an enabled Watchdog Timer, the following procedure must be

followed:

1. In the same operation, write a logic on e to WDTOE and WDE. A logic one must be writ-

ten to WDE even though it is set to on e before the disable operation starts.

2. Within the next four clock cycles, write a logic 0 to WDE. This disables the Watchdog.

• Bits 2..0 – WDP2, WDP1, WDP0: Watchdog Timer Prescaler 2, 1, and 0

The WDP2, WDP1, and WDP0 bits determine the Watchdog Timer prescaling when the Watch-

dog Timer is enabled. The different prescaling values and their corresponding Timeout Periods

are shown in Table 17.

Bit 76543210

– – – WDTOE WDE WDP2 WDP1 WDP0 WDTCR

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

Initial Value 0 0 0 0 0 0 0 0

Table 17. Watchdog Timer Pre scale Select

WDP2 WDP1 WDP0 Number of WDT

Oscillator Cycles Typical Time-out

at VCC = 3.0V T ypical Time-out

at VCC = 5.0V

0 0 0 16K (16,384) 17.1 ms 16.3 ms

0 0 1 32K (32,768) 34.3 ms 32.5 ms

0 1 0 64K (65,536) 68.5 ms 65 ms

0 1 1 128K (131,072) 0.14 s 0.13 s

1 0 0 256K (262,144) 0.27 s 0.26 s

1 0 1 512K (524,288) 0.55 s 0.52 s

1 1 0 1,024K (1,048,576) 1.1 s 1.0 s

1 1 1 2,048K (2,097,152) 2.2 s 2.1 s

2466T–AVR–07/10

ATmega16(L)

The following code example shows one assembly and one C function for turning off the WDT.

The example assumes that interrupts are co ntrolled (f or example by disabling inte rrupts globally)

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

Assembly Code Example

WDT_off:

; Reset WDT

WDR

; Write logical one to WDTOE and WDE

in r16, WDTCR

ori r16, (1<<WDTOE)|(1<<WDE)

out WDTCR, r16

; Turn off WDT

ldi r16, (0<<WDE)

out WDTCR, r16

ret

C Code Example

void WDT_off(void)

{

/* Reset WDT*/

_WDR();

/* Write logical one to WDTOE and WDE */

WDTCR |= (1<<WDTOE) | (1<<WDE);

/* Turn off WDT */

WDTCR = 0x00;

}

2466T–AVR–07/10

ATmega16(L)

Interrupts This section describes the specifics of the interrupt handling as performed in ATmega16. For a

general explanation of the AVR interrupt handling, refer to “Reset and Interrupt Handling” on

page 13.

Interrupt Vectors

in ATmega16

Notes: 1. When the BOOTRST Fuse is programmed, the device will jump to the Boot Loader address at

reset, see “Boot Loader Support – Read-While-Write Self-Programming” on page 246 .

2. When the IVSEL bit in GICR 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.

Table 19 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 progra m code can be placed at th ese locations. This is als o

the case if the Reset Vector is in the Application section while the Interrupt Vectors are in the

Boot section or vice versa.

Table 18. Reset and Interrupt Vectors

Vector No. Program

Address(2) Source Interrupt Definition

1$000

(1) RESET External Pin, Power-on Reset, Brown-out

Reset, Watchdog Reset, and JTAG AVR

Reset

2 $002 INT0 External Interrupt Request 0

3 $004 INT1 External Interrupt Request 1

4 $006 TIMER2 COMP Timer/Counter2 Compare Match

5 $008 TIMER2 OVF Timer/Counter2 Overflow

6 $00A TIMER1 CAPT Timer/Counter1 Capture Event

7 $00C TIMER1 COMPA Timer/Counter1 Compare Match A

8 $00E TIMER1 COMPB Timer/Counter1 Compare Match B

9 $010 TIMER1 OVF Timer/Counter1 Overflow

10 $012 TIMER0 OVF Timer/Counter0 Overflow

11 $014 SPI, STC Serial Transfer Complete

12 $016 USART, RXC USART, Rx Complete

13 $018 USART, UDRE USART Data Register Empty

14 $01A U SART, TXC U SART, Tx Complete

15 $01C ADC ADC Conversion Complete

16 $01E EE_RDY EEPROM Ready

17 $020 ANA_COMP Analog Comparator

18 $022 TWI Two-wire Serial In terface

19 $024 INT2 External Interrupt Request 2

20 $026 TIMER0 COMP Timer/Counter0 Compare Match

21 $028 SPM_RDY Store Program Memory Ready

2466T–AVR–07/10

ATmega16(L)

Note: 1. The Boot Reset Address is shown in Table 100 on page 257. For the BOOTRST Fuse “1”

means unprogrammed while “0” means programmed.

The most typical and general program setup for the Reset and Interrupt Vector Addresses in

ATmega16 is:

Address Labels Code Comments

$000 jmp RESET ; Reset Handler

$002 jmp EXT_INT0 ; IRQ0 Handler

$004 jmp EXT_INT1 ; IRQ1 Handler

$006 jmp TIM2_COMP ; Timer2 Compare Handler

$008 jmp TIM2_OVF ; Timer2 Overflow Handler

$00A jmp TIM1_CAPT ; Timer1 Capture Handler

$00C jmp TIM1_COMPA ; Timer1 CompareA Handler

$00E jmp TIM1_COMPB ; Timer1 CompareB Handler

$010 jmp TIM1_OVF ; Timer1 Overflow Handler

$012 jmp TIM0_OVF ; Timer0 Overflow Handler

$014 jmp SPI_STC ; SPI Transfer Complete Handler

$016 jmp USART_RXC ; USART RX Complete Handler

$018 jmp USART_UDRE ; UDR Empty Handler

$01A jmp USART_TXC ; USART TX Complete Handler

$01C jmp ADC ; ADC Conversion Complete Handler

$01E jmp EE_RDY ; EEPROM Ready Handler

$020 jmp ANA_COMP ; Analog Comparator Handler

$022 jmp TWSI ; Two-wire Serial Interface Handler

$024 jmp EXT_INT2 ; IRQ2 Handler

$026 jmp TIM0_COMP ; Timer0 Compare Handler

$028 jmp SPM_RDY ; Store Program Memory Ready Handler

;

$02A RESET: ldi r16,high(RAMEND) ; Main program start

$02B out SPH,r16 ; Set Stack Pointer to top of RAM

$02C ldi r16,low(RAMEND)

$02D out SPL,r16

$02E sei ; Enable interrupts

$02F <instr> xxx

... ... ...

Table 19. Reset and Interrupt Vectors Placement(1)

BOOTRST IVSEL Reset address Interrupt Vectors Start Address

1 0 $0000 $0002

1 1 $0000 Boot Reset Address + $0002

0 0 Boot Reset Address $0002

0 1 Boot Reset Address Boot Reset Address + $0002

2466T–AVR–07/10

ATmega16(L)

When the BOOTRST Fuse is unprogrammed, the Boot section size set to 2 Kbytes and the

IVSEL bit in the GICR Register is set before any interrupts are enabled, the most typical and

general program setup for the Reset and Interrupt Vector Addresses is:

Address Labels Code Comments

$000 RESET: ldi r16,high(RAMEND) ; Main program start

$001 out SPH,r16 ; Set Stack Pointer to top of RAM

$002 ldi r16,low(RAMEND)

$003 out SPL,r16

$004 sei ; Enable interrupts

$005 <instr> xxx

;

.org $1C02

$1C02 jmp EXT_INT0 ; IRQ0 Handler

$1C04 jmp EXT_INT1 ; IRQ1 Handler

... .... .. ;

$1C28 jmp SPM_RDY ; Store Program Memory Ready Handler

When the BOOTRST Fuse is programmed and the Boot section size set to 2 Kbytes, the most

typical and general program setup for the Reset and Interrupt Vector Addresses is:

Address Labels Code Comments

.org $002

$002 jmp EXT_INT0 ; IRQ0 Handler

$004 jmp EXT_INT1 ; IRQ1 Handler

... .... .. ;

$028 jmp SPM_RDY ; Store Program Memory Ready Handler

;

.org $1C00

$1C00 RESET: ldi r16,high(RAMEND) ; Main program start

$1C01 out SPH,r16 ; Set Stack Pointer to top of RAM

$1C02 ldi r16,low(RAMEND)

$1C03 out SPL,r16

$1C04 sei ; Enable interrupts

$1C05 <instr> xxx

When the BOOTRST Fuse is programmed, the Boot section size set to 2 Kbytes and the IVSEL

bit in the GICR Register is set before any interrupts are enabled, the most typical and general

program setup for the Reset and Interrupt Vector Addresses is:

Address Labels Code Comments

.org $1C00

$1C00 jmp RESET ; Reset handler

$1C02 jmp EXT_INT0 ; IRQ0 Handler

$1C04 jmp EXT_INT1 ; IRQ1 Handler

... .... .. ;

$1C28 jmp SPM_RDY ; Store Program Memory Ready Handler

;

$1C2A RESET: ldi r16,high(RAMEND) ; Main program start

$1C2B out SPH,r16 ; Set Stack Pointer to top of RAM

$1C2C ldi r16,low(RAMEND)

$1C2D out SPL,r16

$1C2E sei ; Enable interrupts

$1C2F <instr> xxx

2466T–AVR–07/10

ATmega16(L)

Moving Interrupts

Between Application

and Boot Space

The General Interrupt Control Register controls the placement of the Interrupt Vector table.

General Interrupt

Control Register –

GICR

• 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 sect ion of the Flash. The actual addres s of the start of the Boot Flash sec tion is deter-

mined by the BOOTSZ Fuses. Refer to the section “Boot Loader Support – Read-While-Write

Self-Programming” on page 246 for details. To av oid unintentional chan ges 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 t o 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 disa bled for four cycles. The I-bit in the Status

Note: If Interrupt Vectors are placed in the Boot Loader section and Boot Lock bit BLB02 is programmed,

interrupts are disabled while e xecuting from the Application section. If Interrupt Vectors are placed

in the Application section and Boot Lock bit BLB12 is programed, interrupts are disabled while

e xecuting from the Boot Loader section. Refer to the section “Boot Loader Support – Read-While-

Write Self-Programming” on page 246 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 explai ned in the IVSEL description above. See Code Example below

Bit 76543210

INT1 INT0 INT2 – – – IVSEL IVCE GICR

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

Initial Value 0 0 0 0 0 0 0 0

2466T–AVR–07/10

ATmega16(L)

Assembly Code Example

Move_interrupts:

; Enable change of interrupt vectors

ldi r16, (1<<IVCE)

out GICR, r16

; Move interrupts to boot Flash section

ldi r16, (1<<IVSEL)

out GICR, r16

ret

C Code Example

void Move_interrupts(void)

{

/* Enable change of interrupt vectors */

GICR = (1<<IVCE);

/* Move interrupts to boot Flash section */

GICR = (1<<IVSEL);

}

2466T–AVR–07/10

ATmega16(L)

I/O Ports

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 chang-

ing drive value (if conf igured a s output) or enablin g/disabling o f pull-up resistors (if con figured as

input). Each output buffer has symmetrical drive characteristics with both hig h sink and source

capability. The pin driver is strong enough to drive LED displays directly. All port pins have indi-

vidually 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 22. Refer to “Electrical Charac-

teristics” on page 291 for a complete list of parameters.

Figure 22. I/O Pin Equivalent Schematic

All registers and bit references in this section are written in general form. A lower case “x” repre-

sents the numbering letter for the p ort, and a lower case “n” represe nts the bit numb er. However,

when using the reg ister or bit defines in a program, the precise for m must be used, that is,

PORTB3 for bit no. 3 in Port B, her e docume nt ed ge ner ally as PO RT xn. The physical I /O Regis-

ters and bit locations are listed in “Register Description for I/O Ports” on page 66.

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.

In addition, the Pull-up Disable – PUD bit in SFIOR disables the pull-up function for all pins in all

ports when set.

Using the I/O port as G eneral Digital I/O is described in “Ports as General Digital I/O” on page

50. Most port pins are m ultiplexed with alternate function s for the peripheral features o n the

device. How each alternate function interferes with the port pin is described in “Alternate Port

Functions” on page 55. Refer to the individual module sections for a full description of the alter-

nate functions.

Note that enabling the alternate function of some of the port pins does not affect the use of the

other pins in the por t as general digital I/O.

Ports as General

Digital I/O The ports are bi-direct iona l I/ O ports with o pti onal in tern al pull-up s. F igur e 23 shows a functiona l

description of one I/O-port pin, here generically called Pxn.

Logic

See Figure 23

"General Digital I/O" for

Details

Pxn

2466T–AVR–07/10

ATmega16(L)

Figure 23. General Digital I/O(1)

Note: 1. WPx, WDx, RRx, RPx, and RDx are common to all pins within the same por t. clkI/O, SLEEP,

and PUD are common to all ports.

Configuring the Pin Each port pin consists of three register bits: DDxn, PORTxn, and PINxn. As shown in “Register

Description for I/O Ports” on pa ge 66 , 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 t he pull-up resist or off, PORTxn h as to be written logic zero or the pi n has to

be configured as an output pin. The port pins are tri-stated when a 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).

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) must occur. Normally, the pull-up enabled state is fully accept-

able, as a high-impedant environment will not notice the differenc e between a strong high driver

and a pull-up. If this is not the case, the PUD bit in the SFIOR Register can be set to disable all

pull-ups in all ports.

clk

RPx

RRx

WPx

RDx

WDx

PUD

SYNCHRONIZER

WDx: WRITE DDRx

WPx: WRITE PORTx

RRx: READ PORTx REGISTER

RPx: READ PORTx PIN

PUD: PULLUP DISABLE

clk

I/O

: I/O CLOCK

RDx: READ DDRx

RESET

CLR

PORTxn

CLR

DDxn

PINxn

DATA BU S

SLEEP

SLEEP: SLEEP CONTROL

Pxn

I/O

2466T–AVR–07/10

ATmega16(L)

Switching between input with pull-up and output low generates the same problem. The user

must use e ither the tri-st ate ({DDxn, PORTxn} = 0b00) or th e output high state ({DDxn, PORTxn}

= 0b11) as an intermediate step.

Table 20 summarizes the control signals for the pin value.

Reading the Pin Value Independent of th e setting of Data Direction bit DDxn, th e port pin can be read through the

PINxn Register bit. As shown in Fi gure 23, the PINxn Registe r bit and the preceding lat ch consti-

tute a synchronizer. This is needed to avoid metastability if the physical pin changes value near

the edge of the internal clock, but it also int roduces a dela y. Figur e 24 shows a timin g 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 24. Synchronization when Reading an Externally Applied Pin Value

Consider the clock period st arting shortly after the first falling edge of the system clock. The latch

is closed when the clock is low, and goes transpa rent 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 Register at the succeeding positive clock edge. As

indicated by the two arrows tpd,max and tpd,min, a single signal transition on the pin will be delayed

between ½ and 1½ system clock pe riod depending upon the time of assertion.

Table 20. Port Pin Configurations

DDxn PORTxn PUD

(in SFIOR) 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 N o Tri-state (Hi-Z)

1 0 X Output No Output Low (Sink)

1 1 X Output No Output High (Source)

SYSTEM CLK

INSTRUCTIONS

SYNC LATCH

PINxn

r17

in r17, PINx

0xFF

0x00

pd, max

XXXXXX

pd, min

2466T–AVR–07/10

ATmega16(L)

When reading back a software assigned pin value, a nop instruction must be inserted as indi-

cated in Figure 25. The out instru ction set s th e “ SYNC LAT CH” sig nal at th e positive edg e o f the

clock. In this case, the delay tpd through the synchronizer is one system clock period.

Figure 25. Synchronization when Reading a Software Assigned Pin Value

nop in r17, PINx

0xFF

0x00 0xFF

tpd

out PORTx, r16

SYSTEM CLK

r16

INSTRUCTIONS

SYNC LATCH

PINxn

r17

2466T–AVR–07/10

ATmega16(L)

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.

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.

Digital Input Enable

and Sleep Modes As shown in Figure 23, the digital input signal can be clamped to ground at the input of the

schmitt-trigger. The signal denoted SLEEP in the figure, is set by the MCU Sleep Controller in

Power-down mode, Power-save mode, Standby mode, and Extended Sta ndby 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 vari-

ous other alternate functions as described in “Alternate Port Functions” on page 55.

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 Inter-

rupt is not enabled, the corresponding External Interrupt Flag will be set when resuming from the

above mentioned sleep modes, as the clamping in these sleep modes produces the requested

logic change.

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(1)

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*/

_NOP();

/* Read port pins */

i = PINB;

...

2466T–AVR–07/10

ATmega16(L)

Unconnected pins If some pins are unused, it is recommended to ensur e that these pins have a def ined level. Even

though most of t he digital inputs are d isabled in the deep sleep modes as descr ibed above, fl oat-

ing inputs should be avoided to 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, sin ce this may ca use excess ive curr ents if the pin is

accidentally configured as an output.

Alternate Port

Functions Most port pins have alternate functions in addition to being General Digital I/Os. Figure 26

shows how the port pin control signals from the simplified Figure 23 can be ov erridden by alter-

nate functions. The overriding sig nals may not be present in all port pins, but the figure serves

as a generic description applica ble to all port pins in the AVR microcontroller family.

Figure 26. Alternate Port Functions(1)

Note: 1. WPx, WDx, RRx, RPx, and RDx are common to all pins within the same por t. clkI/O, SLEEP,

and PUD are common to all ports. All other signals are unique for each pin.

clk

RPx

RRx

WPx

RDx

WDx

PUD

SYNCHRONIZER

WDx: WRITE DDRx

WPx: 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 B U S

DIEOVxn

SLEEP

DIEOExn: Pxn DIGITAL INPUT-ENABLE OVERRIDE ENABLE

DIEOVxn: Pxn DIGITAL INPUT-ENABLE OVERRIDE VALUE

SLEEP: SLEEP CONTROL

Pxn

I/O

2466T–AVR–07/10

ATmega16(L)

Table 21 summarizes the function of the overriding signals. The pin and port indexes from Fig-

ure 26 are not shown in the su cceeding ta bles. The overrid ing sign als are ge nerated inte rnally in

the modules having the alternate function.

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 21. Generic Description of Overriding Signals for Alternate Functions

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 DDO V signal. If this signal is cleared,

the Output driver is enabled b y the DDxn Register bit.

DDOV Data Direction

Override Value If DDOE is set, the Output Driver is enab l ed/ disabled

when DDO V is set/cleared, regardless of the setting of

the DDxn Register bit.

PVOE Port Value Overrid e

Enable If this signal is set and the Output Driver is enabled,

the port value is controlled by the PVOV signal. If

PV OE 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.

DIEOE Digital Input Enable

Override Enable If this bit is set, the Digital Input Enable is controlled b y

the DIEOV signal. If this signal is cleared, the Digital

Input Enable is determined by MCU-state (Normal

Mode, sleep modes).

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 (Nor mal Mode, sleep modes).

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 bef ore the synchroniz er. 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.

2466T–AVR–07/10

ATmega16(L)

Special Function I/O

• Bit 2 – PUD: Pull-up disable

When this bit is written to one, the pull-ups in the I/O ports are disab led even if the DDxn and

PORTxn Registers are configured to enable the pull-ups ({DDxn, PORTxn} = 0b01). See “Con-

figuring the Pin” on page 51 for more details about this feature.

Alternate Functions of

Port A Port A has an alternate function as analog input for the ADC as shown in Table 22. If some Port

A pins are configured as outputs, it is essential that these do not switch when a conversion is in

progress. This might corrupt the result of the conversion.

Table 23 and Table 24 relate the alternate functions of Port A to the overriding signals shown in

Figure 26 on page 55.

Bit 7 6 5 4 3 2 1 0

ADTS2 ADTS1 ADTS0 –ACME PUD PSR2 PSR10 SFIOR

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

Initial Value 0 0 0 0 0 0 0 0

Table 22. Port A Pins Alternate Functions

Port Pin Alternate Function

PA7 ADC7 (ADC input channel 7)

PA6 ADC6 (ADC input channel 6)

PA5 ADC5 (ADC input channel 5)

PA4 ADC4 (ADC input channel 4)

PA3 ADC3 (ADC input channel 3)

PA2 ADC2 (ADC input channel 2)

PA1 ADC1 (ADC input channel 1)

PA0 ADC0 (ADC input channel 0)

Table 23. Overriding Signals for Alternate Functions in PA7..PA4

Signal Name PA7/ADC7 PA6/ADC6 PA5/ADC5 PA4/ADC4

PUOE 0000

PUOV 0000

DDOE 0 0 0 0

DDOV 0 0 0 0

PVOE 0000

PVOV 0000

DIEOE 0000

DIEOV 0000

DI ––––

AIO ADC7 INPUT ADC6 INPUT ADC5 INPUT ADC4 INPUT

2466T–AVR–07/10

ATmega16(L)

Alternate Functions of

Port B The Port B pins with alternat e functions are shown in Table 25.

The alternate pin configuration is as follows:

• SCK – Port B, Bit 7

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 DDB7. When the SPI is

enabled as a Master, the data direction of this pin is controlled by DDB7. When the pin is forced

by the SPI to be an input, the pull-up can still be controlled by the PORTB7 bit.

• MISO – Port B, Bit 6

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 DDB6. When the SPI is

enabled as a Slave, the data direction of this pin is controlled by DDB6. When the pin is forced

by the SPI to be an input, the pull-up can still be controlled by the PORTB6 bit.

Table 24. Overriding Signals for Alternate Functions in PA3..PA0

Signal Name PA3/ADC3 PA2/ADC2 PA1/ADC1 PA0/ADC0

PUOE 0000

PUOV 0000

DDOE 0 0 0 0

DDOV 0 0 0 0

PVOE 0000

PVOV 0000

DIEOE 0000

DIEOV 0000

DI ––––

AIO ADC3 INPUT ADC2 INPUT ADC1 INPUT ADC0 INPUT

Table 25. Port B Pins Alternate Functions

Port Pin Alternate Functions

PB7 SCK (SPI Bus Serial Clock)

PB6 MISO (SPI Bus Master Input/Slave Output)

PB5 MOSI (SPI Bus Master Output/Slave Input)

PB4 SS (SPI Slave Select Input)

PB3 AIN1 (Analog Comparator Negative Input)

OC0 (Timer/Counter0 Output Compare Match Output)

PB2 AIN0 (Analog Comparator Positive Input)

INT2 (External Interrupt 2 Input)

PB1 T1 (Timer/Counter1 External Counter Input)

PB0 T0 (Timer/Counter0 External Counter Input)

XCK (USAR T External Clock Input/Output)

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ATmega16(L)

• MOSI – Port B, Bit 5

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 DDB5. When the SPI is

enabled as a Master, the data direction of this pin is controlled by DDB5. When the pin is forced

by the SPI to be an input, the pull-up can still be controlled by the PORTB5 bit.

•SS

– Port B, Bit 4

SS: Slave Select input. When the SPI is enabled as a Slave, this pin is configured as an input

regardless of the sett ing of DDB4. As a Slave, the SPI is activated when this p in is driven low.

When the SPI is ena bled as a Master, the data direction of this pin is controlled by DDB4. When

the pin is forced by the SPI to be an input, the pull-up can still be controlled by the PORTB4 bit.

• AIN1/OC 0 – Port B, Bit 3

AIN1, Analog Compara tor Negative Inp ut. Configure the port pin as input wit h the internal pull-up

switched off to avoid the digital port function from interfering with the function of the analog

comparator.

OC0, Output Compare Match output: The PB3 pin can serve as an external output for the

Timer/Counter0 Compare Match. The PB3 pin has to be configured as an output (DDB3 set

(one)) to serve this function. The OC0 pin is also the output pin for the PWM mode timer

function.

• AIN0/INT2 – Port B, Bit 2

AIN0, Analog Comparator Positive input. Configure the port pin as input with the internal pull-up

switched off to avoid the digital port function from interfering with the function of the Analog

Comparator.

INT2, External Interrupt Source 2: The PB2 pin can serve as an external interrupt source to the

MCU.

• T1 – Port B, Bit 1

T1, Timer/Counter1 Counter Source.

• T0/XCK – Port B, Bit 0

T0, Timer/Counter0 Counter Source.

XCK, USART External Clock. The Data Direction Register (DDB0) controls whether the clock is

output (DDB0 set) or input (DDB0 cleared). The XCK pin is active only when the USART oper-

ates in Synchronous mode.

Table 26 and Table 27 relate the alternate functions of Port B to the overriding signals shown in

Figure 26 on page 55 . SPI MSTR INPUT and SPI SLAVE OUTPUT constitute the MISO signal,

while MOSI is divided into SPI MSTR OUTPUT and SPI SLAVE INPUT.

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ATmega16(L)

Table 26. Overriding Signals for Alternate Functions in PB7..PB4

Signal

Name PB7/SCK PB6/MISO PB5/MOSI PB4/SS

PUOE SPE • MSTR SPE • MSTR SPE • MSTR SPE • MSTR

PUOV PORTB7 • PUD PORTB6 • PUD PORTB5 • PUD PORTB4 • PUD

DDOE SPE • MSTR SPE • MSTR SPE • MSTR SPE • MSTR

DDOV 0 0 0 0

PVOE SPE • MSTR SPE • MSTR SPE • MSTR 0

PVOV SCK OUTPUT SPI SLAVE OUTPUT SPI MSTR OUTPUT 0

DIEOE 0 0 0 0

DIEOV 0 0 0 0

DI SCK INPUT SPI MSTR INPUT SPI SLAVE INPUT SPI SS

AIO – – – –

Table 27. Overriding Signals for Alternate Functions in PB3..PB0

Signal

Name PB3/OC0/AIN1 PB2/INT2/AIN0 PB1/T1 PB0/T0/XCK

PUOE 0 0 0 0

PUOV 0 0 0 0

DDOE 0 0 0 0

DDOV 0 0 0 0

PVOE OC0 ENABLE 0 0 UMSEL

PVOV OC0 0 0 XCK OUTPUT

DIEOE 0 INT2 ENABLE 0 0

DIEOV 0 1 0 0

DI – INT2 INPUT T1 INPUT XCK INPUT/T0 INPUT

AIO AIN1 INPUT AIN0 INPUT – –

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ATmega16(L)

Alternate Functions of

Port C The Port C pins with alternate functions are shown in Table 28. If the JTAG interface is enabled,

the pull-up resistors on pins PC5(TDI), PC3(TMS) and PC2(TCK) will be activated even if a reset

occurs.

The alternate pin configuration is as follows:

• TOSC2 – Port C, Bit 7

TOSC2, Timer Oscillator pin 2: When the AS2 bit in ASSR is set (one) to enable asynchronous

clocking of Timer/Co unter2, pin PC7 is discon nected from the port, an d becomes the invertin g

output of the Oscillator amplifier. In this mode, a Crystal Oscillator is connected to this pin, and

the pin can not be used as an I/O pin.

• TOSC1 – Port C, Bit 6

TOSC1, Timer Oscillator pin 1: When the AS2 bit in ASSR is set (one) to enable asynchronous

clocking of Timer/Coun ter2, pi n PC6 is disconnect ed fr om the p ort, and b ecomes th e input of the

inverting Oscillator amplifier. In this mode, a Crystal Oscillator is connected to this pin, and the

pin can not be used as an I/O pin.

• TDI – Port C, Bit 5

TDI, JTAG Te st Data I n: Serial input d ata to be shifted in t o the I nstruction Register or Data Re g-

ister (scan chains). When the JTAG interface is enabled, this pin can not be used as an I/O pin.

• TDO – Port C, Bit 4

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 TD0 pin is tri-stated unless TAP states th at shifts out data are entered.

• TMS – Port C, Bit 3

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 – Port C, Bit 2

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.

Table 28. Port C Pins Alternate Functions

Port Pin Alternate Function

PC7 TOSC2 (Timer Oscillator Pin 2)

PC6 TOSC1 (Timer Oscillator Pin 1)

PC5 TDI (JTAG Test Data In)

PC4 TDO (JTAG Test Data Out)

PC3 TMS (JTAG Test Mode Select)

PC2 TCK (JTAG Test Clock)

PC1 SDA (Two-wire Serial Bus Data Input/Output Line)

PC0 SCL (Two-wire Serial Bus Clock Line)

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ATmega16(L)

• SDA – Port C, Bit 1

SDA, Two-wire Serial Interface Data: When the TWEN bit in T WCR is set (one) to enable the

Two-wire Serial Inter face, pin PC1 is disconnected from the port and becomes the Se rial Data

I/O pin for the Two -wire Serial Interface. In this mode, ther e is a spike filter on the pin to sup-

press spikes shorter than 50 ns on the input signal, and the pin is driven by an open drain driver

with slew-rate limitation. When this pin is used by the Two-wire Serial Interface, the pull-up can

still be controlled by the PORTC1 bit.

• SCL – Port C, Bit 0

SCL, Two-wire Serial Interface Clock: When the TWEN bit in TWC R is set (one) to ena ble the

Two-wire Serial Interface, pin PC0 is disconnected from the port and becomes the Serial Clock

I/O pin for the Two -wire Serial Interface. In this mode, ther e is a spike filter on the pin to sup-

press spikes shorter than 50 ns on the input signal, and the pin is driven by an open drain driver

with slew-rate limitation. When this pin is used by the Two-wire Serial Interface, the pull-up can

still be controlled by the PORTC0 bit.

Table 29 and Table 30 relate the alternate functions of Port C to the overriding signals shown in

Figure 26 on page 55.

Table 29. Overriding Signals for Alternate Functions in PC7..PC4

Signal

Name PC7/TOSC2 PC6/TOSC1 PC5/TDI PC4/TDO

PUOE AS2 AS2 JTAGEN JTAGEN

PUOV 0 0 1 0

DDOE AS2 AS2 JTAGEN JTAGEN

DDOV 0 0 0 SHIFT_IR + SHIFT_DR

PVOE 0 0 0 JTAGEN

PVOV 0 0 0 TDO

DIEOE AS2 AS2 JTAGEN JTAGEN

DIEOV 0 0 0 0

DI – – – –

AIO T/C2 OSC OUTPUT T/C2 OSC INPUT TDI –

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ATmega16(L)

Note: 1. When enabled, the Two-wire Serial Interface enables slew-rate controls on the output pins

PC0 and PC1. This is not shown in the figure. In additio n, spi ke filters are conne cted between

the AIO outputs shown in the port figure and the digital logic of the TWI module.

Alternate Functions of

Port D The Port D pins with alternate functions are shown in Table 31.

The alternate pin configuration is as follows:

• OC2 – Port D, Bit 7

OC2, Timer/Counter2 Output Compar e Match outp ut: The PD7 pin can serve as an exte rnal out-

put for the Timer/Counter2 Output Compare. The pin has to be configured as an output (DDD7

set (one)) to serve this function. The OC2 pin is also the output pin for the PWM mode timer

function.

• ICP1 – Port D, Bit 6

ICP1 – Input Capture Pin: The PD6 pin can act as an Input Capture pin for Timer/Counter1.

Table 30. Overriding Signals for Alternate Functions in PC3..PC0(1)

Signal

Name PC3/TMS PC2/TCK PC1/SDA PC0/SCL

PUOE JTAGEN JTAGEN TWEN TWEN

PUOV 1 1 PORTC1 • PUD PORTC0 • PUD

DDOE JTAGEN JTAGEN TWEN TWEN

DDOV 0 0 SDA_OUT SCL_OUT

PVOE 0 0 TWEN TWEN

PVOV 0 0 0 0

DIEOE JTAGEN JTAGEN 0 0

DIEOV 0 0 0 0

DI – – – –

AIO TMS TCK SDA INPUT SCL INPUT

Table 31. Port D Pins Alternate Functions

Port Pin Alternate Function

PD7 OC2 (Timer/Counter2 Output Compare Ma tch Output)

PD6 ICP1 (Timer/Counter1 Input Capture Pin)

PD5 OC1A (Timer/Counter1 Output Compare A Match Output)

PD4 OC1B (Timer/Counter1 Output Compare B Match Output)

PD3 INT1 (External Interrupt 1 Input)

PD2 INT0 (External Interrupt 0 Input)

PD1 TXD (USAR T Output Pin)

PD0 RXD (USART Input Pin)

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ATmega16(L)

• OC1A – Port D, Bit 5

OC1A, Output Compare Match A output: The PD5 pin can serve as an external output for the

Timer/Counter 1 Output Compar e A. The pin has to be conf igure d as an output (DDD5 set (one))

to serve this function. The OC1A pin is also the output pin for the PWM mode timer function.

• OC1B – Port D, Bit 4

OC1B, Output Compare Match B output: The PD4 pin can serve as an external output for the

Timer/Counter 1 Output Compar e B. The pin has to be conf igure d as an output (DDD4 set (one))

to serve this function. The OC1B pin is also the output pin for the PWM mode timer function.

• INT1 – Port D, Bit 3

INT1, External Interrupt Source 1: The PD3 pin can serve as an external interrupt source.

• INT0 – Port D, Bit 2

INT0, External Interrupt Source 0: The PD2 pin can serve as an external interrupt source.

•TXD – Port D, Bit 1

TXD, Transmit Data (Data output pin for the USART). When the USART Transmitter is enabled,

this pin is configured as an output regardless of the value of DDD1.

• RXD – Port D, Bit 0

RXD, Receive Data (Data input pin for the USART). When the USART Receiver is enabled this

pin is configured as an input regardless of the value of DDD0. When the USART forces this pin

to be an input, the pull-up can still be controlled by the PORTD0 bit.

Table 32 and Table 33 relate the alternate functions of Port D to the overriding signals shown in

Figure 26 on page 55.

Table 32. Overriding Signals for Alt ernate Functions PD7..PD4

Signal Name PD7/OC2 PD6/ICP1 PD5/OC1A PD4/OC1B

PUOE 0 0 0 0

PUOV 0 0 0 0

DDOE 0 0 0 0

DDOV 0 0 0 0

PVOE OC2 ENABLE 0 OC1A ENABLE OC1B ENABLE

PVOV OC2 0 OC1A OC1B

DIEOE 0 0 0 0

DIEOV 0 0 0 0

DI – ICP1 INPUT – –

AIO – – – –

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ATmega16(L)

Table 33. Overriding Signals for Alternate Functions in PD3..PD0

Signal Name PD3/INT1 PD2/INT0 PD1/TXD PD0/RXD

PUOE 0 0 TXEN RXEN

PUOV 0 0 0 PORTD0 • PUD

DDOE 0 0 TXEN RXEN

DDOV 0 0 1 0

PVOE 0 0 TXEN 0

PVOV 0 0 TXD 0

DIEOE INT1 ENABLE INT0 ENABLE 0 0

DIEOV1100

DI INT1 INPUT INT0 INPUT – RXD

AIO––––

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ATmega16(L)

Description for I/O

Ports

Port A Data Register –

PORTA

Port A Data Direction

Port A Input Pins

Address – PINA

Port B Data Register –

PORTB

Port B Data Direction

Port B Input Pins

Address – PINB

Bit 76543210

PORTA7 PORTA6 PORTA5 PORTA4 PORTA3 PORTA2 PORTA1 PORTA0 PORTA

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

Initial Value00000000

Bit 76543210

DDA7 DDA6 DDA5 DDA4 DDA3 DDA2 DDA1 DDA0 DDRA

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

Initial Value00000000

Bit 76543210

PINA7 PINA6 PINA5 PINA4 PINA3 PINA2 PINA1 PINA0 PINA

Read/WriteRRRRRRRR

Initial Value N/A N/A N/A N/A N/A N/A N/A N/A

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 Value00000000

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 Value00000000

Bit 76543210

PINB7 PINB6 PINB5 PINB4 PINB3 PINB2 PINB1 PINB0 PINB

Read/WriteRRRRRRRR

Initial Value N/A N/A N/A N/A N/A N/A N/A N/A

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ATmega16(L)

Port C Data Register –

PORTC

Port C Data Direction

Port C Input Pins

Address – PINC

Port D Data Register –

PORTD

Port D Data Direction

Port D Input Pins

Address – PIND

Bit 76543210

PORTC7 PORTC6 PORTC5 PORTC4 PORTC3 PORTC2 PORTC1 PORTC0 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 DDC5 DDC4 DDC3 DDC2 DDC1 DDC0 DDRC

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

Initial Value00000000

Bit 76543210

PINC7 PINC6 PINC5 PINC4 PINC3 PINC2 PINC1 PINC0 PINC

Read/WriteRRRRRRRR

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/WriteRRRRRRRR

Initial Value N/A N/A N/A N/A N/A N/A N/A N/A

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ATmega16(L)

External

Interrupts The External In terr upts ar e trigge red by th e INT0 , INT1, a nd INT2 pin s. Observe that, if enable d,

the interrupts will trigger even if the INT0..2 pins are configured as outputs. This feature provides

a way of generating a software interrupt. The external interrupts can be triggered by a falling or

rising edge or a low level (INT2 is only an edge triggered interrupt). This is set up as indicated in

the specification for the MCU Control Register – MCUCR – and MCU Control and Status Regis-

ter – MCUCSR. When the external interrupt is enabled and is configured as level triggered (only

INT0/INT1), the interrupt will trigger as long as the pin is held low. Note that recognition of falling

or rising edge interrupts on INT0 and INT1 requires the presence of an I/O clock, described in

“Clock Systems and their Distribution” on page 24. Low level interrupts on INT0/INT1 and the

edge interrupt on INT2 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 mod e.

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. This makes the MCU less sensitive to

noise. The changed level is sampled twice by the Wa tchdog Oscillator clock. The period of the

Watchdog Oscillator is 1 µs (nominal) at 5.0V and 25°C. The frequency of the Watchdog Oscilla-

tor is voltage dependent as shown in “Electrical Characteristics” on page 291 . The MCU will

wake up if the input has the required level during this sampling or if it is held until the end of the

start-up tim e. Th e star t-up time is def ine d by th e SUT F uses as des cri bed in “Syste m Clock and

Clock Options” on page 24. If the level is sampled twice by the Watchdog Oscillator clock but

disappears before the end of the start-up time, the MCU will still wake up, but no interrupt will be

generated. The r equired level must be held long enough fo r the MCU to complete the wake up to

trigger the level interrupt.

MCU Control Register

– MCUCR The MCU Control Register contains con trol bits for interrupt sense control and general MCU

functions.

• Bit 3, 2 – ISC11, ISC10: Interrupt Sense Control 1 Bit 1 and Bit 0

The External Interrupt 1 is activated by the external pin INT1 if the SREG I-bit and the corre-

sponding interrupt mask in the GICR are set. The level and edges on the external INT1 pin that

activate the interrupt are de fined in Table 34. The value on the INT1 pin is sampled before

detecting edges. If edge or toggle interrupt is selected, pulses that last longer than one clock

period will generate an interr upt. Shorter pulses are not guaranteed to gen erate an interrupt. If

low level interrupt is selected, the low level must be held until the completion of the currently

executing instruction to gene rate an interrupt.

Bit 76543210

SM2 SE SM1 SM0 ISC11 ISC10 ISC01 ISC00 MCUCR

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

Initial Value00000000

Table 34. Interrupt 1 Sense Control

ISC11 ISC10 Description

0 0 The low level of INT1 generates an interrupt request.

0 1 Any logical change on INT1 generates an interrupt request.

1 0 The falling edge of INT1 generates an interrup t request.

1 1 The rising edge of INT1 generates an interrupt request.

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ATmega16(L)

• Bit 1, 0 – ISC01, ISC00: Interrupt Sense Control 0 Bit 1 and Bit 0

The External Interrupt 0 is activated by the external pin INT0 if the SREG I-flag and the corre-

sponding interrupt mask are set. The level and edges on the external INT0 pin that activate the

interrupt are defined in Table 35. The value on the INT0 pin is sampled before detecting edges.

If edge or toggle interrupt is selected, pulses that last longer than one clock period will generate

an interrupt. Shorter pulses are not guaranteed to g enerate 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 interr upt.

MCU Control and

Status Register –

MCUCSR

• Bit 6 – ISC2: Interrupt Sense Control 2

The Asynchronou s External Inte rrupt 2 is activate d by the exte rnal pin INT2 if the SREG I-bit and

the corresponding interrup t mask in GICR are set. If ISC2 is written to zero, a falling edge on

INT2 activates the interrupt. If ISC2 is written to one, a rising edge on INT2 activates the inter-

rupt. Edges on INT2 are registered asynchronously. Pulses on INT2 wider than the minimum

pulse width given in Table 36 will generate an interrupt. Shorter pulses are not guaranteed to

generate an interrupt. When changing the ISC2 bit, an interrupt can occur. Therefore, it is rec-

ommended to first disable INT2 by clearing its Interrupt Enable bit in the GICR Register. Then,

the ISC2 bit can be changed. Finally, the INT2 Interrupt Flag should be cleared by writing a logi-

cal one to its Interrupt Flag bit (INTF2) in the GIFR Register before the interrupt is re-enabled.

General Interrupt

Control Register –

GICR

• Bit 7 – INT1: External Interrupt Request 1 Enable

When the INT1 bit is set (one) and the I-bit in the Status Register (SREG) is set (one), the exter-

nal pin inter rupt is enab led. The Interrupt Sense Cont rol1 bits 1/0 (ISC1 1 and I SC10) in t he MCU

General Control Register (MCUCR) define whether the External Interrupt is activated on rising

Table 35. Interrupt 0 Sense Control

ISC01 ISC00 Description

0 0 The low level of INT0 generates an interrupt request.

0 1 Any logical change on INT0 generates an interrupt request.

1 0 The falling edge of INT0 generates an interrup t request.

1 1 The rising edge of INT0 generates an interrupt request.

Bit 76543210

JTD ISC2 –JTRF WDRF BORF EXTRF PORF MCUCSR

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

Initial Value 0 0 0 See Bit Description

Table 36. Asynchronous External Interrupt Characteristics

Symbol Parameter Condition Min Typ Max Units

tINT Minimum pulse width for

asynchronous e xternal interrupt 50 ns

Bit 76543210

INT1 INT0 INT2 – – – IVSEL IVCE GICR

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

Initial Value 0 0 0 0 0 0 0 0

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ATmega16(L)

and/or falling edge of the INT1 pin or level sensed. Activity on the pin will cause an interrupt

request even if INT1 is configured as an output. The corresp onding inter rupt o f Externa l Interru pt

Request 1 is executed from the INT1 interrupt Vector.

• Bit 6 – INT0: External Interrupt Request 0 Enable

When the INT0 bit is set (one) and the I-bit in the Status Register (SREG) is set (one), the exter-

nal pin inter rupt is enab led. The Interrupt Sense Cont rol0 bits 1/0 (ISC0 1 and I SC00) in t he MCU

General Control Register (MCUCR) define whether the External Interrupt is activated on rising

and/or falling edge of the INT0 pin or level sensed. Activity on the pin will cause an interrupt

request even if INT0 is configured as an output. The corresp onding inter rupt o f Externa l Interru pt

Request 0 is executed from the INT0 interrupt vector.

• Bit 5 – INT2: External Interrupt Request 2 Enable

When the INT2 bit is set (one) and the I-bit in the Status Register (SREG) is set (one), the exter-

nal pin interrupt is enab led. The Interrupt Sense Control2 bit (ISC2) in the MCU Control and

Status Register (MCUCSR) defines whether the External Interrupt is activated on rising or falling

edge of the INT2 pin. Activity on the pin will cause an interrupt request even if INT2 is configured

as an output. The correspo nding interrupt of External Interrupt Request 2 is executed from the

INT2 Interrupt Vector.

General Interrupt Flag

• Bit 7 – INTF1: External Interrupt Flag 1

When an edge or logic chang e on the INT1 pin trigger s an interr upt requ est, INT F1 becomes set

(one). If the I-bit in SREG and the INT1 bit in GICR are set (one), the MCU will jump to the corre-

sponding 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. This flag is always cleared

when INT1 is configured as a level interrupt.

• Bit 6 – INTF0: External Interrupt Flag 0

When an edge or logic chang e on the INT0 pin trigger s an interr upt requ est, INT F0 becomes set

(one). If the I-bit in SREG and the INT0 bit in GICR are set (one), the MCU will jump to the corre-

sponding 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. This flag is always cleared

when INT0 is configured as a level interrupt.

• Bit 5 – INTF2: External Interrupt Flag 2

When an event on the INT2 pin triggers an interrupt request, INTF2 becomes set (one). If the I-

bit in SREG and the INT2 bit in GICR are set (one), the MCU will jump to the corresponding

Interrupt Vector. The flag is cleared when the interrupt routine is executed . Alt ernat ively, the flag

can be cleared b y writing a logical one to it. Note that when entering some sleep m odes with the

INT2 interrupt disabled, the input buffer on this pin will be disabled. This may cause a logic

change in internal signals which will set the INTF2 Flag. See “Digital Input Enable and Sleep

Modes” on page 54 for more information.

Bit 76543210

INTF1 INTF0 INTF2 – – – – –GIFR

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

Initial Value 0 0 0 0 0 0 0 0

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ATmega16(L)

8-bit

Timer/Counter0

with PWM

Timer/Counter0 is a general purpose, single compare unit, 8-bit Timer/Counter module. The

main features are:

•Single Compare Unit Counter

•Clear Timer on Compare Match (Auto Reload)

•Glitch-free, Phase Correct Pulse Width Modulator (PWM)

•Frequency Generator

•External Event Counter

•10-bit Clock Prescaler

•Overflow and Compare Match Interrupt Sources (TOV0 and OCF0)

Overview A simplified block diagram of the 8 -bit Timer/ Counter is shown in Figure 27. For t he actu al place-

ment of I/O pins, refer to “Pinout ATmega16” on page 2. 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 83.

Figure 27. 8-bit Timer/Counter Block Diagram

Registers The Timer/Counter (TCNT0) and Output Compare Register (OCR0) are 8-bit registers. Interrupt

request (abbreviated to Int.Req. in the figure) signals are all visible in the Timer Interrupt Flag

(TIMSK). TIFR and TIMSK are not shown in the figure since these registers are shared by other

timer units.

The Timer/Counter can be clocked inter nally, via the pre scaler, or by an external clock source on

the T0 pin. T he Clock Se lect logic blo ck controls which clock so urce and edge the Timer/Cou nter

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).

The double buffered Output Compare Register (OCR0) is compared with the Timer/Counter

value at all times. The re sult of the comp are can be us ed by the waveform gen erator to generate

a PWM or variable frequency output on the Output Compare Pin (OC0). See “Output Compare

Timer/Counter

DATA BUS

TCNTn

Waveform

Generation OCn

= 0

Control Logic

0xFF

BOTTOM

count

clear

direction

TOVn

(Int.Req.)

OCRn

TCCRn

Clock Select

Edge

Detector

( From Prescaler )

clk

TOP

OCn

(Int.Req.)

2466T–AVR–07/10

ATmega16(L)

Unit” on page 73. for details. The compare match event will also set the Compare Flag (OCF0)

which can be used to generate an output compare interrupt request.

Definitions Many register and bit references in this document a re written in general form. A lowe r case “n”

replaces the Timer/Counter number, in this case 0. However, when using the register or bit

defines in a program, the precise form must be used, that is, TCNT0 for accessing

Timer/Counter 0 counter value and so on.

The definitions in Table 37 are also used extensively throughout the document.

Timer/Counter

Clock Sources The Timer/Counter can be clocked by an internal or an external clock source. The cloc k source

is selected by the clock select logic which is controlled by the clock select (CS02:0) bits located

in the Timer/Counter Control Register (TCCR0). For details on clock sources and prescaler, see

“Timer/Counter0 and Timer/Counter1 Pr escalers” on page 87.

Counter Unit The main part of the 8-bit Timer/Counter is the programmable bi-directional counter unit. Figure

28 shows a block diagram of the counter and its surroundings.

Figure 28. Counter Unit Block Diagr am

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).

clkTnTimer/Co un te r clo ck, re fe rr ed to as clk T0 in the following.

TOP Signalize that TCNT0 has reached maximum value.

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 inte rnal 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

Table 37. Definitions

BOTTOM The counter reaches the BOTTOM when it becomes 0x00.

MAX The counter r eaches 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 OCR0 Register. Th e

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

2466T–AVR–07/10

ATmega16(L)

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 (TCCR0). There are close connections between how the

counter behaves (counts) and how waveforms a re generated on the Output Compare output

OC0. For more details about advanced counting sequences and waveform generation, see

“Modes of Operation” on page 76.

The Timer/Counter Overflow (TOV0) Flag is set according to the mode of operation selected by

the WGM01:0 bits. TOV0 can be used for generating a CPU interrupt.

Output Compare

Unit The 8-bit comparator continuously compares TCNT0 with the Output Compare Register

(OCR0). Whenever TCNT0 equals OCR0, the comparator signals a match. A match will set the

Output Compare Flag (OCF0) at the next timer clock cycle. If enabled (OCIE0 = 1 and Global

Interrupt Fla g in SREG is set) , the Out p ut Comp ar e F lag gen er ates a n o ut put co mpa re int e rrupt .

The OCF0 Flag is automatically cleared when the interrupt is executed. Alternatively, the OCF0

Flag can be cleared by soft ware by writing a logical one to its I/O bit locatio n. The waveform gen-

erator uses the match sign al to generate an output according to operating mode set by the

WGM01:0 bits and Compare Output mode (COM01:0) bits. The max and bottom signals are

used by the wave form generator fo r handling the special cas es of the extreme va lues in some

modes of operation (See “Modes of Operation” on page 76.).

Figure 29 shows a block diagram of the output compare unit.

Figure 29. Output Compare Unit, Block Diagram

The OCR0 Register is double buffered when using any of the Pulse Width Modulation (PWM)

modes. For the normal and Clear Time r on Compare ( CTC) modes o f operation , the do uble buff-

ering is disabled. The double buffering synchronizes the update of the OCR0 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.

OCFn (Int.Req.)

(8-bit Comparator )

OCRn

OCn

DATA B U S

TCNTn

WGMn1:0

Waveform Generator

top

FOCn

COMn1:0

bottom

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ATmega16(L)

The OCR0 Register access may seem complex, but this is not case. When the double buffering

is enabled, the CPU has access to the OCR0 Bu ffer Register, and if double buff ering is disabled

the CPU will access the OCR0 directly.

For ce Output

Compare In non-PWM waveform generation modes, the match output of the comparator can be forced by

writing a one to the Force O utput Compare (FOC0) bit. Forcing com pare match will not set the

OCF0 Flag or reload/clear the timer, but the OC0 pin will be updated as if a real compare match

had occurred (t he COM01 :0 bits set tings de fin e whet her the OC0 p in is set , clear ed or to gg led) .

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 t imer is st opped. This featur e allows OCR0 to be initialized

to the same value as TCNT0 without triggering an interrupt when the Timer/Counter clock is

enabled.

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, inde-

pendently of whether the Timer/Coun ter is running or not. If the value written to TCNT0 equals

the OCR0 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 downcounting.

The setup of th e OC0 should be pe rformed befo re setting t he Data Direct ion Register fo r the port

pin to output. The easiest way of settin g the OC0 value is to use the Force Output Compa re

(FOC0) strobe bits in Normal mode. The OC0 Register keeps its value even when cha nging

between waveform generation modes.

Be aware that the COM01:0 bits are not double buffered together with the compare value.

Changing the COM01:0 bits will take effect immediately.

Compare Match

Output Unit The Compare Output mode (COM01:0) bits have two functions. The Waveform Generator uses

the COM01:0 bits fo r defining the Output Compare (O C0) state at the next compare match . Also,

the COM01: 0 bits co ntrol th e OC0 p in outpu t sour ce. Figure 30 shows a simplified schematic of

the logic affected by the COM01:0 bit setting. The I/O Registers, I/O bits, and I/O pins in the fig-

ure are shown in bol d. Only t he par ts o f th e ge neral I / O port Con trol Re giste rs (DDR an d PORT)

that are a ffected by th e COM01: 0 bits are shown. Whe n refe rring t o the OC 0 state, t he refer ence

is for the internal OC0 Register, not the OC0 pin. If a System Reset occur, the OC0 Register is

reset to “0”.

2466T–AVR–07/10

ATmega16(L)

Figure 30. Compare Match Output Unit, Schematic

The general I/O port function is overridden by the Output Compare (OC0) from the Waveform

Generator if either of the COM01:0 bits are set. However, the OC0 pin direction (input or output)

is still controlled by the Data Direction Register (DDR) for the port pin. The Data Direction Regis-

ter bit for the OC0 pin (DDR_OC0) must be set as output before the OC0 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 OC0 state before the out-

put is enabled. Note that some COM01:0 bit settings are reserved for certain modes of

operation. See “8-bit Timer/Counter Register Description” on page 83.

Compare Output Mode

and Waveform

Generation

The Waveform Generator uses the COM01:0 bits differently in normal, CTC, and PWM modes.

For all modes, setting the COM01:0 = 0 tells the waveform generator that no action on the OC0

PWM modes refer to Table 39 on page 84. Fo r fast PWM mode, refer to Table 40 on page 84,

and for phase corre ct PWM refer to Table 41 on page 84 .

A change of the COM01:0 bits state will have effect at the first c ompare match after the bits are

written. For non-PWM modes, the action can be forced to have immediate effect by using the

FOC0 strobe bits.

PORT

DDR

OCn

Waveform

Generator

COMn1

COMn0

DATA BU S

FOCn

clkI/O

2466T–AVR–07/10

ATmega16(L)

Modes of

Operation The mode of operation , th at is, the beh avior of the Tim er/Cou nt er an d t he Ou t put Com par e pins,

is defined by the combin ation of the Wavef orm Generat ion mode (WGM 01:0) an d Compare Out-

put mode (COM01:0) bits. The Compare Output mode bits do not affect the counting sequence,

while the Waveform Generation mode bits do. The COM01:0 bits control whether the PWM out-

put generated should be inverted or not (inverted or non-inverted PWM). For non-PWM modes

the COM01:0 bits control whether the output should be set, cleared, or toggled at a compare

match (See “Compare Match Output Unit” on page 74.).

For detailed timing information refer to Figure 34, Figure 35, Figure 36 and Figure 37 in

“Timer/Counter Timing Diagrams” on page 81.

Normal Mode The simplest mode of operation is the normal mode (WGM01:0 = 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 8-bit value (TOP = 0xFF) and then restarts from the bot-

tom (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 overflo w interrupt

that automatically clears the TOV0 Flag, the timer resolution can be increa sed 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 out-

put compare to generate waveforms in Normal mode is not recommended, since this will occupy

too much of the CPU time.

Clear Timer on

Compare Match (CTC)

Mode

In Clear Timer on Compar e or CTC mo de ( WGM01 :0 = 2), the OC R0 Registe r is us ed to man ip-

ulate the counter resolution. In CTC mode the counter is cleared to zero when t he counter value

(TCNT0) matches the OCR0. The OCR0 defines the top value for the counter, hence also its

resolution. This mode allows greater con trol of th e comp are match out put f reque ncy. I t also sim-

plifies the operation of counting external events.

The timing diagram for the CTC mode is shown in Figure 31. The counter value (TCNT0)

increases until a compare match occur s between TCNT0 and OCR0, an d th en count er (TCNT0)

is cleared.

Figure 31. CTC Mode, Timing Diagram

An interrupt can be generated each time the counter value reaches the TOP value by using the

OCF0 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

TCNTn

OCn

(Toggle)

OCn Interrupt Flag Set

1 4

Period

2 3

(COMn1:0 = 1)

2466T–AVR–07/10

ATmega16(L)

the double buffering feature. If the new value written to OCR0 is lower than the current value of

TCNT0, the counter will miss the compare match. The counter will then have to count to its max-

imum value (0xFF) and wrap around starting at 0x00 before the compare match can occur.

For generating a waveform output in CTC mode, the OC0 output can be set to toggle its logical

level on each compare match by setting the Compare Output mode bits to toggle mode

(COM01:0 = 1). The OC0 value will not be visible on the port pin unless the data direction for the

pin is set to output. The wa veform generated will have a m aximum frequency of fOC0 = fclk_I/O/2

when OCR0 is set to zero (0x00). The waveform frequency is defined by the following equation:

The N variable represents the prescale factor (1, 8, 64, 256, or 1024).

As for the Normal mode of op erat ion, the T OV0 Flag is se t in the same tim er clock cycle tha t the

counter counts from MAX to 0x00.

Fast PWM Mode The fast Pulse Width Modulation or fast PWM mode (WGM01:0 = 3) provides a high frequency

PWM waveform generation option. The fast PWM differs from the other PWM option by its sin-

gle-slope operation. The counter counts from BOTTOM to MAX then restarts from BOTTOM. In

non-inverting Compare Output mode, the Outp ut Compare (OC0) is cleared on the compare

match between T CNT0 and OCR0 , and set at BOTTOM. In invertin g Compare Output mo de, the

output is set on compare match and cleared at BOTTOM. Due to the single-slope operation, the

operating fr equency 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 MAX value.

The counter is then cleared at the following timer clock cycle. The timing diagram for the fast

PWM mode is shown in Figure 32. The TCNT0 value is in the timing diagram shown as a histo-

gram 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 betw ee n OC R0 and TCNT0 .

Figure 32. Fast PWM Mode, Timing Diagram

The Timer/Counter O verflow Flag (TOV0) is set each t ime the coun ter reache s MAX. If the inte r-

rupt is enabled, the interrupt handler routine can be used for updating the compare value.

fOCn

fclk_I/O

2N1OCRn+()⋅⋅

-----------------------------------------------=

TCNTn

OCRn Update and

TOVn Interrupt Flag Set

Period

2 3

OCn

(COMn1:0 = 2)

(COMn1:0 = 3)

OCRn Interrupt Flag Set

4 5 6 7

2466T–AVR–07/10

ATmega16(L)

In fast PWM mode, the compar e unit allows gener ation of PWM waveforms on t he OC0 pin. Set-

ting the COM01:0 bits to 2 will produce a non-inverted PWM and an inverted PWM output can

be generated by setting the COM0 1:0 to 3 (See Table 4 0 on page 84). The actual OC0 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 OC0 Register at the compare match between

OCR0 and TCNT0, and clearing (or setting) the OC0 Register at the timer clock cycle the coun-

ter is cleared (changes from MAX to BOTTOM).

The PWM frequency for the output can be calculated by th e following equation:

The N variable represents the prescale factor (1, 8, 64, 256, or 1024).

The extreme values for the OCR0 Register represents special cases when generating a PWM

waveform output in the fast PWM mode. If the OCR0 is set equal to BOTTOM, the output will be

a narrow spike for each MAX+1 timer clock cycle. Setting the OCR0 equal to MAX will result in a

constantly high or low output (depending on the polarity of the output set by the COM01:0 bits.)

fOCnPWM

fclk_I/O

N256⋅

------------------=

2466T–AVR–07/10

ATmega16(L)

Phase Correct PWM

Mode The phase cor rect PWM mode (WG M01:0 = 1) 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 MAX and then from MAX to BOTTOM. In non-

inverting Compare Output mode, the Output Compare (OC0) is cleared on the comp are match

between TCNT0 and OCR0 while upcounting, and set on the compare match while downcount-

ing. 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 symmet-

ric feature of the dual-slope PWM modes, these modes are preferred for motor control

applications.

The PWM resolution for the phase correct PWM mode is fixed to eight bits. In phase correct

PWM mode the counter is incremented until the counter value matches MAX. When the counter

reaches MAX, it changes the count direction. The TCNT0 value will be equal to MAX for one

timer clock cycle. The timing diagram for the phase correct PWM mode is shown on Figure 33.

The TCNT0 value is in the timing diagram shown as a histogram for illustrating the dual-slope

operation. The diagram includes non-in verted and inverted PWM outputs. The small horizontal

line marks on the TC NT 0 slo pe s re pr es en t c omp are matches betwe e n OCR0 and TCNT0.

Figure 33. Phase Correct PWM Mode, Timing Diagram

The Timer/Counter Over flow Flag (TOV0) is set each time the counter reache s 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

OC0 pin. Setting the COM01:0 bits to 2 will produce a non-inverted PWM. An inverted PWM out-

put can be generated by setting the COM01:0 to 3 (see Table 41 on page 84). The actual OC0

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 gen erated by clearing (or setting) the OC0 Register at the compare match

between OCR0 an d TCNT0 when the counter incremen ts, and setting (or clearing) the O C0

TOVn Interrupt Flag Set

OCn Interrupt Flag Set

1 2 3

TCNTn

Period

OCn

(COMn1:0 = 2)

(COMn1:0 = 3)

OCRn Update

2466T–AVR–07/10

ATmega16(L)

PWM frequency for the output when using phase correct PWM can be calculated by the follow-

ing equation :

The N variable represents the prescale factor (1, 8, 64, 256, or 1024).

The extreme values for the OCR0 Register represent special cases when generating a PWM

waveform output in the phase correct PWM mo de. If the OCR0 is set equal to BOTTO M, the out-

put will be continuously low and if set equal to MAX the output will be continuously high for non-

inverted PWM mode. For inve rted PWM the output will have the opposite logic values.

At the very start of Period 2 in Figure 33 OCn has a t ransition from high t o low even though the re

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 33. 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 be correspond to the result of an

up-counting Compare Match.

• The Timer starts counting from a value higher than t he one in OCR0A, and for that reason

misses the Compare Match and hence the OCn change that would ha ve happened on the

way up.

fOCnPCPWM

fclk_I/O

N510⋅

------------------=

2466T–AVR–07/10

ATmega16(L)

Timer/Counter

Timing Diagrams The Timer/Counter is a synchronous design and the timer clock (clkT0) is therefore shown as a

clock enable sign al in the following figures. Th e figures include information on whe n Interrupt

Flags are set. Figure 34 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 34. Timer/Counter Timing Diagram, no Prescaling

Figure 35 shows the same timing data, but with the prescaler enab led.

Figure 35. Timer/Counter Timing Diagram, with Prescaler (fclk_I/O/8)

Figure 36 shows the setting of OCF0 in all modes except CTC mode.

clk

(clk

I/O

/1)

TOVn

clk

I/O

TCNTn MAX - 1 MAX BOTTOM BOTTOM + 1

TOVn

TCNTn

MAX - 1 MAX BOTTOM BOTTOM + 1

clkI/O

clkTn

(clkI/O/8)

2466T–AVR–07/10

ATmega16(L)

Figure 36. Timer/Counter Timing Diagram, Setting of OCF0, with Prescaler (fclk_I/O/8)

Figure 37 shows the setting of OCF0 and the clearing of TCNT0 in CTC mode.

Figure 37. Timer/Counter Timing Diagram, Clear Timer on Compare Match Mode, with Pres-

caler (fclk_I/O/8)

OCFn

OCRn

TCNTn

OCRn Value

OCRn - 1 OCRn OCRn + 1 OCRn + 2

clkI/O

clkTn

(clk

I/O

/8)

OCFn

OCRn

TCNTn

(CTC)

TOP

TOP - 1 TOP BOTTOM BOTTOM + 1

clk

I/O

clk

(clk

I/O

/8)

2466T–AVR–07/10

ATmega16(L)

8-bit

Timer/Counter

Description

Timer/Counter Contr ol

• Bit 7 – FOC0: Force Output Compare

The FOC0 bit is only active when the WGM00 bit specifies a non-PWM mode. However, for

ensuring compatibility with future devices, this bit must be set to zero when TCCR0 is written

when operating in PWM mode. When writing a logical one to the FOC0 bit, an immediate com-

pare match is f orced on the Waveform Gener ation un it. The OC0 output is changed a ccording to

its COM01:0 bits setting. Note that the FOC0 bit is implemented as a strobe. Therefore it is the

value present in the COM01:0 bits that determines t he effect of the forced compare.

A FOC0 strobe will not generate any interrupt, nor will it clear the timer in CTC mode using

OCR0 as TOP.

The FOC0 bit is always read as zero.

• Bit 3, 6 – WGM01:0: Waveform Generation Mode

These bits control the co unting sequence of the counter, the source for the maximum (TOP)

counter value, and what type of Waveform Generation to be used. Modes of operation sup-

ported by the Timer/Counter unit are: Normal mode, Clear Timer on Compare Match (CTC)

mode, and two types of Pulse Width Modulation (PWM) modes. Se e Table 38 and “Modes o f

Operation” on page 76.

Note: 1. The CTC0 and PWM0 bit definition names are now obsolete. Use the WGM01:0 definitions.

However, the functionality and location of these bits are compatible with previous versions of

the timer.

• Bit 5:4 – COM01 :0: Compare Match Output Mode

These bits control the Output Compare pin (OC0) behavior. If one or both of the COM01:0 bits

are set, the OC0 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 OC0 pin must be

set in order to enable the output driver.

Bit 7 6 5 4 3 2 1 0

FOC0 WGM00 COM01 COM00 WGM01 CS02 CS01 CS00 TCCR0

Read/Write 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 38. Waveform Generation Mode Bit Description(1)

Mode WGM01

(CTC0) WGM00

(PWM0) Timer/Counter Mode

of Operation TOP Update of

OCR0 TOV0 Flag

Set-on

0 0 0 Normal 0xFF Immediate MAX

1 0 1 PWM, Phase Correct 0xFF T OP BOTTOM

2 1 0 CTC OCR0 Immediate MAX

3 1 1 F ast PWM 0xFF BOTTOM MAX

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ATmega16(L)

When OC0 is connected to the pin, the function of the COM01:0 bits depends on the WGM01:0

bit setting. Table 39 shows the COM01:0 bit functionality when the WGM01:0 bits are set to a

normal or CTC mode (non-PWM).

Table 40 shows the COM01:0 bit functionality when the WGM01:0 bits are set to fast PWM

mode.

Note: 1. A special case occurs when OCR0 equa ls TOP and COM01 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 77 for

more details.

Table 41 shows the COM01:0 bit functionality when the WGM01:0 bits are set to phase correct

PWM mode.

Note: 1. A special case occurs when OCR0 equa ls TOP and COM01 is set. In this case, the compare

match is ignored, but the set or clear is done at T OP. See “Phase Correct PWM Mode” on page

79 for more details.

Table 39. Compare Output Mode, non-PWM Mode

COM01 COM00 Description

0 0 Normal port operation, OC0 disconnected.

0 1 Toggle OC0 on compare match

1 0 Clear OC0 on compare match

1 1 Set OC0 on compare match

Table 40. Compare Output Mode, Fast PWM Mode(1)

COM01 COM00 Description

0 0 Normal port operation, OC0 disconnected.

01Reserved

1 0 Clear OC0 on compare match, set OC0 at BOTTOM,

(non-inverting mode)

1 1 Set OC0 on compare match, clear OC0 at BOTTOM,

(inverting mode)

Table 41. Compare Output Mode, Phase Correct PWM Mode(1)

COM01 COM00 Description

0 0 Normal port operation, OC0 disconnected.

01Reserved

1 0 Clear OC0 on compare match when up-counting. Set OC0 on compare

match when downcounting.

1 1 Set OC0 on compare match when up-counting. Clear OC0 on compare

match when downcounting.

2466T–AVR–07/10

ATmega16(L)

• Bit 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.

Timer/Counter

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 OCR0 Register.

Output Compare

The Output Compare Register contains an 8-bit value that is continuously compared with the

counter value (TCNT0). A match can be used to generate an output com pare interrupt, or to

generate a waveform output on the OC0 pin.

Timer/Counter

Interrupt Mask

• Bit 1 – OCIE0: Timer/Counter0 Out put Compare Match Interrupt Enable

When the OCIE0 bit is written to one, and the I-bit in the Status Register is set (one), the

Timer/Coun ter0 Compa re Match in terrupt is enabled. The corres ponding in terrupt is executed if

a compare match in Timer/ Counter0 occurs, that is, when the OCF0 bit is set in the Timer/Coun-

ter Interrupt Flag Register – TIFR.

Table 42. Clock Select Bit Description

CS02 CS01 CS00 Description

0 0 0 No clock source (Timer/Counter stopped).

001clkI/O/(No prescaling)

010

clkI/O/8 (From prescaler)

011

clkI/O/64 (From prescaler)

100

clkI/O/256 (From prescaler)

101

clkI/O/1024 (From prescaler)

1 1 0 Exter nal clock source on T0 pin. Clock on falling edge.

1 1 1 External 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

OCR0[7:0] OCR0

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

Initial Value00000000

Bit 76543210

OCIE2 TOIE2 TICIE1 OCIE1A OCIE1B TOIE1 OCIE0 TOIE0 TIMSK

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

2466T–AVR–07/10

ATmega16(L)

• 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 (one), the

Timer/Counter0 Overflow interrupt is enabled. The corresponding interrupt is executed if an

overflow in Timer/Counter0 occurs, that is, when the TOV0 bit is set in the Timer/Counter Inter-

rupt Flag Register – TIFR.

Timer/Counter

Interrupt Flag Register

– TIFR

• Bit 1 – OCF0: Output Compare Flag 0

The OCF0 bit is set (one) when a compare match occurs between the Timer/Counter0 and the

data in OCR0 – Output Compare Register0. OCF0 is cleared by hardware when executing the

corresponding interrupt handling vector. Alternatively, OCF0 is cleared by writing a logic one to

the flag. Whe n the I-bi t in SREG, OCIE0 (Timer/Count er0 Compa re Match Inte rrupt Enab le), and

OCF0 are set (one), the Timer/Counter0 Compare Match Interrupt is executed.

• Bit 0 – TOV0: Timer/Counter0 Overflow Fla g

The bit TOV0 is set (one) when an overflo w occurs in Time r/Coun te r0 . TO V0 is clear ed by har d-

ware when executing the corre spon ding int erru pt han dling vecto r. Alt ernat ively, TOV0 is clear ed

by writing a logic one to the flag. When the SREG I-bit, TOIE0 (Timer/Counter0 Overflow Inter-

rupt Enable), and TOV0 are set (one), the Timer/Counter0 Overflow interrupt is executed. In

phase correct PWM mode, this bit is set when Timer/Counter0 changes counting direction at

$00.

Bit 76543210

OCF2 TOV2 ICF1 OCF1A OCF1B TOV1 OCF0 TOV0 TIFR

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

Initial Value00000000

2466T–AVR–07/10

ATmega16(L)

Timer/Counter0

and

Timer/Counter1

Prescalers

Timer/Counter1 and Timer/Counter0 share the same prescaler module, but the Timer/Counters

can have different prescaler settings. The description below applies to both Timer/Counter1 and

Timer/Counter0.

Internal Clock Source The Timer/Counter can be clocked directly by the syst em clock ( by set ting t he 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 frequency of either fCLK_I/O/8, fCLK_I/O/64, fCLK_I/O/256, or

fCLK_I/O/1024.

Prescaler Reset The prescaler is free running, that is, operates independently of the clock select logic of the

Timer/Counter, and it is shared b y Timer/Counter1 and Time r/Counter0. Since the prescale r 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 prescaling artifacts occurs when

the timer is enabled a nd clo cked by th e prescale r ( 6 > CSn2: 0 > 1). The numb er of syst em 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 execu-

tion. 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.

External Clock Source An external clock source applied to the T1/T0 pin can be used as Timer/Counter clock

(clkT1/clkT0). The T1/T0 pin is sampled once every system clock cycle by the p in syn chro nization

logic. The synchronized (sampled) signal is then passed through the edge detector. Figure 38

shows a functional equivalent block diagram of the T1/T0 synchronization and edge detector

logic. The regist ers are clocked at the p ositive edge of the internal system clock (clkI/O). The latch

is transparent in the hig h period of the internal system clock.

The edge detector generates one clkT1/clkT0 pulse for each positive (CSn2:0 = 7) or negative

(CSn2:0 = 6) edge it detects.

Figure 38. T1/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 app lied to the T1/T0 pin to the counter is updated.

Enabling and disabling of the clock input must be done when T1/T0 has been stable for at least

one system clock cycle, otherwise it is a ri sk that a false Tim er/Counter clo ck pulse is gene rated.

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 sys-

tem clock frequency (fExtClk < fclk_I/O/2) give n a 50/50% duty cycle. Since th e edge detecto r uses

Tn_sync

(To Clock

Select Logic)

Edge DetectorSynchronization

DQDQ

clk

I/O

2466T–AVR–07/10

ATmega16(L)

sampling, the maximum frequency of an external clock it can detect is half the sampling fre-

quency (Nyquist sampling theorem). However, du e to variation of the system clock frequency

and duty cycle caused by Oscillator source (crystal, resonator, and 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 presca le d.

Figure 39. Prescaler for Timer/Counter0 and Timer/Counter 1(1)

Note: 1. The synchronization logic on the input pins (T1/T0) is shown in Figure 38.

Special Function IO

• Bit 0 – PSR10: Prescaler Reset Timer/Counter1 and Timer/Counter0

When this bit is written to one, the Timer/Counter1 and Timer/Counter0 prescaler 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. Note that Timer/Counter1 and Timer/Counter0 share the same prescaler and a

reset of this prescaler will affect both timers. This bit will always be read as zero.

PSR10

Clear

clkT1 clkT0

clkI/O

Synchronization

Bit 7 6 5 4 3 2 1 0

ADTS2 ADTS1 ADTS0 –ACME PUD PSR2 PSR10 SFIOR

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

Initial Value 0 0 0 0 0 0 0 0

2466T–AVR–07/10

ATmega16(L)

16-bit

Timer/Counter1 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 (that is, allows 16-bit PWM)

•Two Independent Output Compare Units

•Double Buffered Output Compare Registers

•One Input Capture Unit

•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

•Four Independent Interrupt Sources (TOV1, OCF1A, OCF1B, and ICF1)

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 ca se “x” replaces the output compa re unit.

However, when using the register or bit defines in a program, the precise form must be used

(that is, TCNT1 for accessing Time r/Counter1 counter value and so on).

A simplified block diagram of the 16-bit Timer/Counter is shown in Figure 40. For the actual

placement of I/O pins, refer to Figure 1 on page 2. 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 Timer/Counter Register Description” on page 110.

2466T–AVR–07/10

ATmega16(L)

Figure 40. 16-bit Timer/Counter Block Diagram(1)

Note: 1. Refer to Figure 1 on page 2, Table 25 on page 58, and Table 31 on page 63 for

Timer/Counter1 pin placement and descrip tion.

Registers The Timer/Counter (TCNT1), Output Compare Registers (OCR1A/B), and Inpu t Capture Regi s-

ter (ICR1) are all 16-bit registers. Special procedures must be followed when accessing the 16-

bit registers. These proced ures are described in the section “Accessing 16-bit Registers” on

page 92. The Timer/ Counter Control Register s (TCCR1A/B) are 8-bit r egisters an d have no CPU

access restrictions. Interrupt requests (abbre viated to Int.Req. in the figur e) signals ar e all visible

in the Timer Interrupt Flag Register (TIFR). All interrupts are individually masked with the Timer

Interrupt Mask Register (TIMSK). TIFR and TIMSK a re not show n in the figur e since these r egis-

ters are shared by other timer units.

The Timer/Counter can be clocked inter nally, via the pre scaler, or by an external clock source on

the T1 pin. T he Clock Se lect logic blo ck controls which clock so urce and edge the Timer/Cou nter

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 (clkT1).

The double buffere d Outp ut Compa re Registers (O CR1A/B) are compared with the Timer /Coun-

ter 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 (OC1A/B). See “Out-

put Compare Units” on page 98. The compare match event will also set the Compare Match

Flag (OCF1A/B) which can be used to generate an output compare interrupt request.

Clock Select

Timer/Counter

DATA BU S

OCRnA

OCRnB

ICRn

TCNTn

Waveform

Generation

Waveform

Generation

OCnA

OCnB

Noise

Canceler

ICPn

Fixed

TOP

Values

Edge

Detector

Control Logic

TOP BOTTOM

Count

Clear

Direction

TOVn

(Int.Req.)

OCnA

(Int.Req.)

OCnB

(Int.Req.)

ICFn (Int.Req.)

TCCRnA TCCRnB

( From Analog

Comparator Ouput )

Edge

Detector

( From Prescaler )

clk

2466T–AVR–07/10

ATmega16(L)

The Input Capture Register can capture the Timer/Counter value at a given external (edge trig-

gered) event on either the Input Capture Pin (ICP1) or on the Analog Comparator pins (See

“Analog Comparator” on page 201.) The Input Capture unit includes a digi tal filtering unit (Nois e

Canceler) for reducing the chance of capturin g noise spikes.

The TOP value, or maximum Timer/Counter value, can in some modes of operation be defined

by either the OCR1A Register, the ICR1 Register, or by a set of fixed values. When using

OCR1A as TOP value in a PWM mode, the OCR1A 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 ru n ti me. I f a fixed TOP valu e is req uired , t he ICR1 Reg i ster ca n be used

as an alternative, freeing the OCR1A to be used as PWM outp ut.

Definitions The following definitions are used extensively throughout the document:

Compatibility The 16-bit Timer/Counter has be en updated and improved from previous versions of the 16-bit

AVR Timer/Counter. This 16-bit Timer/Counter is fully compatible with the earlier version

regarding:

• All 16-bit Timer/Counter related I/O Register a ddress locations, including Timer Interrupt

Registers.

• Bit locations inside all 16-bit Timer/ Co un te r Reg ist er s, including Ti me r In te rrupt Registe rs.

• Interrupt Vectors.

The following control bits have cha nged name, but have same functionalit y and register locatio n:

• PWM10 is changed to WGM10.

• PWM11 is changed to WGM11.

• CTC1 is changed to WGM12.

The following bits are added to the 16-bit Timer/Counter Control Registers:

• FOC1A and FOC1B are added to TCCR1A.

• WGM13 is added to T CCR1B.

The 16-bit Timer/Counter ha s improvements that will affect the compatib ility in some special

cases.

Table 43. Definitions

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 OCR1A or ICR1 Regis-

ter. The assignment is dependent of the mode of operation.

2466T–AVR–07/10

ATmega16(L)

Accessing 16-bit

Registers The TCNT1 , OCR1A/B, and ICR1 are 16 -bit registers t hat ca n be ac cessed by the AVR CPU via

the 8-bit data bu s. Th e 1 6- bit re giste r must be byte accessed u s ing tw o rea d o r write op erat ions.

Each 16-bit tim er has a single 8-bit register fo r temporary storing of the Hig h byte of the 16-bit

access. The same temporary register is shared between all 16-bit registers within each 16-bit

timer. Accessing t he Low byte triggers the 16-bit read or writ e opera tion. When t he 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 t he tem-

porary register in the same clock cycle as the Low byte is read.

Not all 16-bit accesses u ses th e t empor ar y registe r for t h e High byt e. Read ing t he OCR1 A/B 1 6-

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 OCR1A/B and ICR1 Registers. Not e that when using “C”, the compiler hand les the 16-bit

access.

Note: 1. See “About Code Examples” on page 7.

The assembly code examp le returns the TCNT1 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 Regis-

ters, then the result of the access outside the interrupt will be corrupted. Therefore, when both

the main code and the inte rrupt code updat e th e te mpora ry reg iste r, th e main code must di sable

the interrupts during the 16-bit access.

Assembly Code Example(1)

...

; Set TCNT1 to 0x01FF

ldi r17,0x01

ldi r16,0xFF

out TCNT1H,r17

out TCNT1L,r16

; Read TCNT1 into r17:r16

in r16,TCNT1L

in r17,TCNT1H

...

C Code Example(1)

unsigned int i;

...

/* Set TCNT1 to 0x01FF */

TCNT1 = 0x1FF;

/* Read TCNT1 into i */

i = TCNT1;

...

2466T–AVR–07/10

ATmega16(L)

The following code examples show how to do an atomic read of the TCNT1 Register contents.

Reading any of the OCR1A/B or ICR1 Registers can be done by using the same principle.

Note: 1. See “About Code Examples” on page 7.

The assembly code examp le returns the TCNT1 value in the r17:r16 register pair.

Assembly Code Example(1)

TIM16_ReadTCNT1:

; Save global interrupt flag

in r18,SREG

; Disable interrupts

cli

; Read TCNT1 into r17:r16

in r16,TCNT1L

in r17,TCNT1H

; Restore global interrupt flag

out SREG,r18

ret

C Code Example(1)

unsigned int TIM16_ReadTCNT1( void )

{

unsigned char sreg;

unsigned int i;

/* Save global interrupt flag */

sreg = SREG;

/* Disable interrupts */

_CLI();

/* Read TCNT1 into i */

i = TCNT1;

/* Restore global interrupt flag */

SREG = sreg;

return i;

}

2466T–AVR–07/10

ATmega16(L)

The following code examples show how to do an atomic write of the TCNT1 Register contents.

Writing any of the OCR1A/B or ICR1 Registers can be done by using the same principle.

Note: 1. See “About Code Exa mp les” on page 7.

The assembly code example requires that the r17:r16 register pair contains the value to be writ-

ten to TCNT1.

Reusing the

Temporary High Byte

If writing to more than one 16-bit register where the High byte is the same for all reg isters writ-

ten, 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.

Timer/Counter

Clock Sources The Timer/Counter can be clocked by an internal or an external clock source. The cloc k source

is selected by the Clock Select logic which is controlled by the Clock Select (CS12:0) bits

located in the Timer/Counter Control Register B (TCCR1B). For details on clock sources and

prescaler, see “Timer/Counter0 and Timer/Counter1 Prescalers” on page 87.

Assembly Code Example(1)

TIM16_WriteTCNT1:

; Save global interrupt flag

in r18,SREG

; Disable interrupts

cli

; Set TCNT1 to r17:r16

out TCNT1H,r17

out TCNT1L,r16

; Restore global interrupt flag

out SREG,r18

ret

C Code Example(1)

void TIM16_WriteTCNT1 ( unsigned int i )

{

unsigned char sreg;

unsigned int i;

/* Save global interrupt flag */

sreg = SREG;

/* Disable interrupts */

_CLI();

/* Set TCNT1 to i */

TCNT1 = i;

/* Restore global interrupt flag */

SREG = sreg;

}

2466T–AVR–07/10

ATmega16(L)

Counter Unit The main part of the 16-bit Timer/Counter is the programmable 16-bit bi-directional counter unit.

Figure 41 shows a block diagram of the counter and its surroundings.

Figure 41. Counter Unit Block Diagr am

Signal description (internal signals):

Count Increment or decrement TCNT1 by 1.

Direction Select between incr ement and decrement.

Clear Clear TCNT1 (set all bits to zero).

clkT1Timer/Counter clock.

TOP Signalize that TCNT1 has reached maximum value.

BOTTOM Signalize that TCNT1 has reached minimum value (zero) .

The 16-bit counter is mapped into two 8-bit I/O memory locations: Counter High (TCNT 1H) c on-

taining the upper eig ht bits of the counter, and Counter Lo w (TCNT1L) contain ing the lower 8

bits. The TCNT1H Regist er can only be ind irect ly accessed by the CPU. When the CPU d oes an

access to the TCNT1H I/O location, the CPU accesses the High byte temporary register

(TEMP). The temporary register is upd ated with the TCNT1H value when the TCNT1L is read,

and TCNT1H is updated with the temporary register value when TCNT1L 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 TCNT1 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 mo de o f oper at ion us ed, the cou nt er is cleared , in cr eme nted , o r decr em ent ed

at each timer clock (clkT1). The clkT1 can be generated from an external or internal clock source,

selected by the Clock Select bits (CS12:0). When no clock source is selected (CS12:0 = 0) the

timer is stopped. However, the TCNT1 value can be accessed by the CPU, independent of

whether clkT1 is present or not. A CPU write overrides (h as priority over) all counter clear or

count operations.

The counting sequence is determined by the setting of the Waveform Generation Mode bits

(WGM13:0) located in the Timer/Counter Control Registers A and B (TCCR1A and TCCR1B).

There are close connectio ns between how the counter behaves (counts) and how wavefor ms

are generate d on the Out put Compare ou tputs OC1x. For more details abo ut advanced co unting

sequences and waveform generation, see “Modes of Operation” on page 101.

The Timer/Counter Overflow (TOV1) Flag is set according to the mode of operation selected by

the WGM13:0 bits. TOV1 can be used for generating a CPU interrupt.

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 )

clk

2466T–AVR–07/10

ATmega16(L)

Input Capture Unit The Timer/Counter incorporates an Input Capture unit that can capture external events and give

them a time -stamp in dicating t ime o f occurre nce. The externa l signal indica ting an e vent, or mul-

tiple events, can be applied via the ICP1 pin or alternat ively, via the Analog Comparato r 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.

The Input Capture unit is illustrated by the block diagram shown in Figure 42. The elem ents of

the block diagram t hat 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 42. Input Capture Unit Block Diagram

When a change of the lo gic level (a n event) oc curs on the Input Capt ure pin ( ICP1), a lt ernat ively

on the Analog Comparat or 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

(TCNT1) is written to the Input Capture Re gister (ICR1). The Input Capture Flag (ICF1) is set at

the same system clock as the TCNT1 value is copied into ICR1 Register. If enabled (TICIE1 =

1), the Input Capture Flag generates an Input Capture Interrupt. The ICF1 Flag is automatically

cleared when the interrupt is executed. Alternatively the ICF1 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 (ICR1) is done by first reading the Low

byte (ICR1L) and then the High byte (ICR1H). When the Low byte is read the High byte is copied

into the High byte temporary register (TEMP). When the CPU reads the ICR1H I/O location it will

access the TEMP Register.

The ICR1 Register can only be written when using a Waveform Generation mode that utilizes

the ICR1 Register for defining the counter’s TOP value. In these cases the Waveform Genera-

tion mode (WGM13:0) bits must be set before the TOP value can be written to the ICR1

tion before the Low byte is written to ICR1L.

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*

2466T–AVR–07/10

ATmega16(L)

For more information on how to access the 16-bit registers refer to “Accessing 16-bit Registers”

on page 92.

Input Capture Pin

Source The main trigger source for the Input Capture unit is the Input Capture pin (ICP1).

Timer/Counter1 can alternatively use the Analog Comparator output as trigger source for the

Input Capture unit. The An alog Comparator is selected as trigger source by setting the Analog

Comparator In put 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.

Both the Input Capture pin (ICP1) and the Analog Comparator output (ACO) inputs are sampled

using the same technique as for the T1 pin (Figure 38 on page 87). The edge detector is also

identical. However, when the noise canceler is ena bled, 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 wave-

form generation mode that uses ICR1 to define TOP.

An Input Capture can be triggered by software by controlling the port of the ICP1 pin.

Noise Canceler The noise canceler improves noise immunity by using a simple digital filtering scheme. The

noise canceler input is monit ored 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 (ICNC1) bit in

Timer/Counter Control Register B (TCCR1B). When enabled t he noise can celer intr oduces addi-

tional four system clock cycles of delay from a change applied to the input, to the update of the

ICR1 Register. The noise canceler uses the sy stem clock and is therefore not affected by the

prescaler.

Using the Input

Capture Unit The main challenge when using th e Input Capture unit is to a ssign enough process or capacity

for handling the incoming events. The time between two events is critica l. If the processor has

not read the captured value in the ICR1 Register before the next event occurs, the ICR1 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 ICR1 Register should be read as early in the inter-

rupt handler routine as possible. Even though the Input Capture Int errupt has relatively high

priority, the maximum interrupt response time is dep endent on the maximum number of clock

cycles it takes to hand le an y of th e ot he r int er ru pt req ue sts .

Using the Input Capture unit in any mode of operation when the TOP value (resolution) is

actively changed during op eration, is not recommended.

Measurement of an external signal’s duty cycle requires that the trigger edge is changed after

each capture. Changin g the edge sensing must be done as early as po ssible after the ICR1

cleared by software (writing a logical one to the I/O bit location). For measuring frequency only,

the clearing of the ICF1 Flag is not required (if an interrupt handler is used).

2466T–AVR–07/10

ATmega16(L)

Output Compare

Units The 16-bit comparator continuously compares TCNT1 with the Output Compare Register

(OCR1x). If TCNT equals OCR1x the comparator signals a match. A match will set th e Output

Compare Flag (OCF1x) at the next timer clock cycle. If enabled (OCIE1x = 1), the Output Com-

pare Flag generates an output co mpare interrupt. The OCF1x Flag is automatically cleared

when the inter rupt is executed. Alternatively the OCF1x Flag can be cleared by software by writ-

ing 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

(WGM13:0) bits and Compare Output mode (COM1x1:0) 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 101.)

A special feature of output compare unit A allows it to define the Timer/Counter TOP value (that

is, counter resolution). In addition to the counter resolution, the TOP value defines the period

time for waveforms generated by the Waveform Generator.

Figure 43 shows a bloc k diagr am of th e out put compar e unit. The small “n” in the re gister and bit

names indicates t he device number (n = 1 for Timer /Counter 1), a nd the “x” ind icates o utput com-

pare unit (A/B). The elements of the block diagram that are not directly a part of the output

compare unit are gray shaded.

Figure 43. Output Compare Unit, Block Diagram

The OCR1x Register is double buffered when using any of the twelve Pulse Width Modulation

(PWM) modes. Fo r the nor mal and Clear Tim er on Compare ( CTC) modes of operat ion, the dou-

ble buffering is disabled. The double buffering synchronizes the update of the OCR1x Compare

occurrence of odd-length, non-symmetrical PWM pulses, thereby making the output glitch-free.

The OCR1x Register access may seem co mplex, but this is not case. Wh en the double buff ering

is enabled, the CPU has access to the OC R1x Buffer Register, and if double buffering is dis-

abled the CPU will access the OCR1x directly. The content of the OCR1x (Buffer or Compare)

automatically as the TCNT1 and ICR1 Register). Therefore OCR1x is not read via the High byte

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

2466T–AVR–07/10

ATmega16(L)

temporary register (TEMP). However, it is a good practice to read the Low byte first as when

accessing other 16-bit registers. Writing the OCR1x Regi sters must be done via the TEMP Reg-

ister since the compare of all 16 b its is done continuously. The High byte (OCR1xH) ha s to be

written first. When the High byte I/O location is written by the CPU, the TEMP Register will be

updated by the va lue written. Then when the Low by te (OCR1xL) is written to the lower eig ht

bits, the High byte will be copied into the upper 8-bits of either the OCR1x buffer or OCR1x Com-

pare 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 92.

For ce 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 (FOC1x) bit. Forcing compare match will not set the

OCF1x Flag or reload/clear the timer, but the OC1x pin will be updated as if a real compare

match had occurred (the COM1x1:0 bits settings define whether the OC1x pin is set, cleared or

toggled).

Compare Match

Blocking by TCNT1

Write

All CPU writes to the TCNT1 Register will block any compare match that occurs in the next timer

clock cycle, even when the timer is stopped. This feature allows OCR1x to be initialized to the

same value as TCNT1 without triggering an interrupt when the Timer/Counter clock is enabled.

Using the Output

Compare Unit Since writing TCNT1 in any mode of operation will block all compare matches for one timer clock

cycle, there are risks involved when changing TCNT1 when using any of the output compare

units, independent of whether the Timer/Counter is running or not. If the value written to TCNT1

equals the OCR1x value, the compare match will be missed, resulting in incorrect waveform

generation. Do not write the TCNT1 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 TCNT1 value equal to BOTTOM when the counter is downcounting.

The setup of the OC1x should be performed before setting the Data Direction Register for the

port pin to output. The easiest way of setting the OC1x value is to use the force output compare

(FOC1x) strobe bits in Normal mo de. The OC1x Register keeps its value even whe n changing

between waveform generation modes.

Be aware that the COM1x1:0 bits are not double buffered together with the compare value.

Changing the COM1x1:0 bits will take effect immediately.

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ATmega16(L)

Compare Match

Output Unit The Compare Out put mode (CO M1x1:0) bits have two f unctions. Th e Waveform Genera tor uses

the COM1x1:0 bits for defining the Output Compare (OC1x) state at the next compare match.

Secondly the COM1x1:0 bits control the OC1x pin output source. Figure 44 shows a simplified

schematic of th e logic affecte d by the COM1x1: 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 COM1x1:0 bits are shown. When referring to the

OC1x state, the reference is for the internal OC1x Register, not the OC1x pin. If a Syst e m Reset

occur, the OC1x Register is reset to “0”.

Figure 44. Compare Match Output Unit, Schematic

The general I/O port function is overridden by the Output Compare (OC1x) from the Waveform

Generator if either of the COM1x1:0 bits are set. However, the OC1x pin direction (input or out-

put) is still controlled by the Data Direction Register (DDR) for the port pin. The Data Direction

ble on the pin. The port override function is generally independent of the Waveform Generation

mode, but there are some exceptions. Refer to Table 44, Table 45 and Table 46 for details.

The design of the output compare pin logic allows initialization of the OC1x state before the out-

put is enabled. Note that some COM1x1:0 bit settings are reserved for certain modes of

operation. See “16-bit Timer/Counter Register Description” on page 110.

The COM1x1:0 bi ts have no effect on the In put Capture unit.

Compare Output Mode

and Waveform

Generation

The Waveform Generator uses the COM1x1:0 bits differently in normal, CTC, and PWM modes.

For all modes, setting the COM1x1:0 = 0 tells the Waveform Generator that no action on the

OC1x Register is t o be performed on the next com pare match. F or compare output actions in t he

non-PWM modes refer to Table 44 on page 110. For fast PWM mode refe r to Table 45 on page

111, and for phase correct and phase and frequency correct PWM refer to Table 46 on page

111.

A change of the COM1x1:0 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

FOC1x strobe bits.

PORT

DDR

OCnx

Waveform

Generator

COMnx1

COMnx0

DATABU S

FOCnx

clk

I/O

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ATmega16(L)

Modes of

Operation The mode of operation, that is, the behavior of the Timer/Counter and the output compare pins,

is defined by the combination of the Waveform Generat ion mode (WGM13:0) and Compare Out-

put mode (COM1x1: 0) bits. The Co mpare Output mode bits do not aff ect the counti ng sequence,

while the Waveform Gener ation mode bits do . The COM1x1:0 bi ts control whether th e PWM out-

put generated should be inverted or not (inverted or non-inverted PWM). For non-PWM modes

the COM1x1:0 bits control whe ther the output should be set, cleared or to ggle at a compare

match (See “Compare Match Output Unit” on page 100.)

For detailed timing information refer to “Timer/Counter Timing Diagrams” on page 108.

Normal Mode The simplest mode of operation is the Normal mode (WGM13:0 = 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 (TOV1) will be set in

the same timer clock cycle as the TCNT1 becomes zero. The TOV1 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 TOV1 Flag, the timer resolution can be increased by soft-

ware. 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, obse rve that the maximum

interval betwee n the e xtern al event s must not exceed the r esolutio n of t he count er. If the inter val

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 out-

put compare to generate waveforms in Normal mode is not recommended, since this will occupy

too much of the CPU time.

Clear Timer on

Compare Match (CTC)

Mode

In Clear Timer on Compare or CT C mode (WGM13:0 = 4 or 12), the O CR1A or ICR1 Register

are used to manipulat e th e counte r re so lut ion . I n CT C mode t he coun te r is cleare d to zero when

the counter value (TCNT1) matche s either th e OCR1A (WGM13:0 = 4) or the ICR1 (WGM13:0 =

12). The OCR1A or ICR1 define the top value for the counter, hence also its resolution. This

mode allows greate r contr ol of the co mpa re mat ch out put f reque ncy. I t also simpl ifie s the o per a-

tion of counting exter nal events.

The timing diagram for the CTC mode is shown in Figure 45. The counter value (TCNT1)

increases until a compare match occurs with either OCR1A or ICR1, and then counter (TCNT1)

is cleared.

Figure 45. CTC Mode, Timing Diagram

TCNTn

OCnA

(Toggle)

OCnA Interrupt Flag Set

or ICFn Interrupt Flag Set

(Interrupt on TOP)

1 4

Period

2 3

(COMnA1:0 = 1)

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ATmega16(L)

An interrupt can be generated at each time the counter value reach es the TOP value by either

using the OCF1A or ICF1 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. How-

ever, changing th e TOP to a value close to BOTTOM when the cou nter is run ning with no ne or a

low prescaler value must be done with care since the CTC mode does not have the double buff-

ering feature. If the new value written to OCR1A or ICR1 is lower than the current value of

TCNT1, the counter will miss the compare match. The counter will then have to count to its max-

imum value (0xFFFF) and wr ap around startin g at 0x0000 before the co mpare match can occur.

In many cases this feature is not desirable. An alternative will then be to use the fast PWM mode

using OCR1A for defining TOP (WGM13:0 = 15) since the OCR1A then will be double buffered.

For generatin g a wavefor m out put in CT C mod e, t he OC1A ou tput can be se t to toggle it s logica l

level on each compare match by setting the compare output mode bits to toggle mode

(COM1A1:0 = 1). The OC1A value will not be visible on the port pin unless the data direction for

the pin is set to output (DDR_OC1A = 1). The waveform generated will have a maximum fre-

quency of fOC1A = fclk_I/O/2 when OCR1A is set to zero (0x0000). The wavefo rm frequency is

defined by the fo llow i ng equation:

The N variable re pr esents the prescaler factor (1, 8, 64, 25 6, or 10 24 ).

As for the Normal mode of op erat ion, the T OV1 Flag is se t in the same tim er clock cycle tha t the

counter counts from MAX to 0x0000.

Fast PWM Mode The fast Pu lse Width Modulation or fast PWM mode (W GM13:0 = 5,6,7,14, or 15) provides a

high frequency PWM waveform generation optio n. 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 (OC1x) is cleared

on the compare match between TCNT1 and OCR1x, and set at BOTTOM. In inverting Compare

Output mode output is set on compar e match and clear ed at BOTTOM. Due to th e single-slope

operation, t he operating fr equency of the f ast PWM mode can be twice as high as the phase cor-

rect 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, capaci-

tors), hence reduces total system cost.

The PWM resolut ion f or f ast PWM ca n be fixe d to 8- bit, 9 -bit, o r 10-bit , o r def in ed by ei th er I CR1

or OCR1A. The minimum resolution allowed is 2-bit (ICR1 or OCR1A set to 0x0003), and the

maximum resolution is 16-bit (ICR1 or OCR1A 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 (WGM13:0 = 5, 6, or 7), the value in ICR1 (WGM13:0 =

14), or the value in OCR1A (WGM13:0 = 15). The counter is then cleared at the following timer

clock cycle. The timing diagram for the fast PWM mode is shown in Figure 46. The figure shows

fast PWM mode whe n OC R1 A or IC R1 is used to define TOP. The TCNT1 value is in the timing

diagram shown as a histogram for illustrating the single-slope operation. The diagra m includes

non-inverted and inverted PWM outputs. The small h orizontal line marks on the TCNT1 slopes

represent compare matches between OCR1x and TCNT1. The OC1x Interrupt Flag will be set

when a compare match occurs.

fOCnA

fclk_I/O

2N1OCRnA+()⋅⋅

---------------------------------------------------=

RFPWM

TOP 1+()log 2()log

-----------------------------------=

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ATmega16(L)

Figure 46. Fast PWM Mode, Timing Diagram

The Timer/Counter Overflow Flag (TOV1) is set each time the counter reaches TOP. In addition

the OC1A or ICF1 Flag is set at the same timer clock cycle as TOV1 is set when either O CR1A

or ICR1 is used fo r defining t he TOP value. If o ne of the inte rrupts are enab led, the interr upt han-

dler routine can be used for updating the TOP and compare values.

When changing the TOP value the prog ram 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 TCNT1 and the OCR1x.

Note that when using fixed TOP values the unused bits are masked to zero when any of the

OCR1x Registers are written.

The procedure for updating ICR1 differs from updating OCR1A when used for defining the TOP

value. The ICR1 Register is not double buffered. This means that if ICR1 is changed to a low

value when the counter is ru nning wit h none or a low prescaler value, there is a r isk that t he new

ICR1 value written is lower than the current value of TCNT1. 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 OCR1A Register however, is double buffered. This feature allows the OCR1A I/O location

to be written anytime. When the OCR1A I/O location is written the value written will be put into

the OCR1A Buffer Register. The OCR1A Compare Register will then be updated with the value

in the Buffer Register at the next timer clock cycle the TCNT1 matches TOP. The update is done

at the same timer clock cycle as the TCNT1 is cleared and the TOV1 Flag is set.

Using the ICR1 Regis ter for defining TOP work s well when using fixed TOP values. By using

ICR1, the OCR1A Register is free to be used for generating a PWM output on OC1A. However,

if the base PWM frequency is actively changed (by changing the TOP value), using the OCR1A

as TOP is clearly a better choice due to its double buffer feature.

In fast PWM mode, the compare units allow ge neration of PWM waveforms on the OC1x pins.

Setting the COM1x1:0 bits to 2 will produce a non-inverted PWM and an inverted PWM output

can be generated by setting the COM1x1:0 to 3 (See Table 44 on page 110). The actual OC1x

value will only be visible o n the port pin if the data direction for the port pin is set a s output

(DDR_OC1x). The PWM waveform is generated by setting (or clearing) the OC1x Register at

the compare match between OCR1x and TCNT1, and clearing (or setting) the OC1x Register at

the timer clock cycle the counter is cleared (changes from TOP to BOTTOM).

TCNTn

OCRnx / TOP Update and

TOVn Interrupt Flag Set and

OCnA 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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ATmega16(L)

The PWM frequency for the output can be calculated by th e following equation:

The N variable re pr esents the prescaler divider (1, 8, 64 , 25 6, or 10 24 ).

The extreme values for the OCR1x Register represents special cases when generating a PWM

waveform output i n the fast PWM mode. If the OCR1x is set equal to BOTT OM (0x0000) the ou t-

put will be a narrow spike for each TOP+1 timer clock cycle. Setting the OCR1x equal to TOP

will result in a constant high or low output (depending on the polarity of the output set by the

COM1x1:0 bits.)

A frequency (with 50% duty cycle) waveform output in fast PWM mode can be achieved by set-

ting OC1A to toggle its logical level on each compare match (COM1A1:0 = 1). This applies only

if OCR1A is used to define the TOP v alue (WGM13:0 = 15). The waveform generated will have

a maximum frequency of fOC1A = fclk_I/O/2 when OCR1A is set to zero (0x0000). This feature is

similar to the OC1A toggle in CTC mode , except t he double bu ffer f eature of the ou tput co mpare

unit is enabled in the fast PWM mode.

Phase Correct PWM

Mode The phas e correct Pulse Wid th Modulation or phase correct PWM mode (WGM13:0 = 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 rep eatedly from BOTTOM (0 x0000) to TOP and th en from TOP to

BOTTOM. In non-inverting Compare Output mode, the Output Compare (OC1x) is cleared on

the compare match between TCNT1 and OCR1x while upcounting, and set on the compare

match while downcounting. In inverting Output Compare m ode, the operation is inverted. Th e

dual-slope operatio n has lower maximum op erati on frequency tha n single slope ope ration. How-

ever, due to the symmetric feat ure of th e dual-s lope PWM modes, th ese modes a re prefer red for

motor control applications.

The PWM resolution for the phase correct PWM mode can be fixed to 8-bit, 9-bit, or 10-bit, or

defined by either ICR1 or OCR1A. The minimum resolution allowed is 2-bit (ICR1 or OCR1A set

to 0x0003), an d the ma ximu m resolu tio n is 16-b it ( I CR1 or OCR1 A set to MAX). Th e PWM r eso-

lution 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 (WGM13:0 = 1, 2, or 3), the value in ICR1

(WGM13:0 = 10), or the value in OCR1A (WGM13:0 = 11). The counter has then reached the

TOP and changes the count direction. The TCNT1 value will be equal to TOP for one timer clock

cycle. The timing diagram for the phase correct PWM mode is shown on Figure 47. The figure

shows phase correct PWM mode when OCR1A or ICR1 is used to define TOP. The TCNT1

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 TCNT1 slopes repre sent compare matches betwe en OCR1x and TCNT1. The OC1x Inter-

rupt Flag will be set when a compare match occurs.

fOCnxPWM

fclk_I/O

N1TOP+()⋅

-----------------------------------=

RPCPWM

TOP 1+()log 2()log

-----------------------------------=

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ATmega16(L)

Figure 47. Phase Correct PWM Mode, Timing Diagram

The Timer/Count er Ove rfl ow Flag (TOV1) is se t each t ime th e co unte r r ea ches BO TTOM. When

either OCR1A or ICR1 is used for defining the TOP value, the OC1A or ICF1 Flag is set accord-

ingly at the same timer clock cycle as the OCR1x 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 prog ram 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 TCNT1 and the OCR1x.

Note that when using fixed TOP values, the unused bits are masked to zero when any of the

OCR1x Registers are written. As the thir d period shown in Figure 47 illustrates, changing the

TOP actively while the Timer/Counter is running in the phase correct mode can result in an

unsymmetrical out put. T he rea son for this can be fo und in th e time of up date of the OCR1x Re g-

ister. Since the OCR 1x update occurs at TOP, the PWM period starts and ends at TOP. Th is

implies that the length of the falling slo pe 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 changin g the TOP value while the Timer/ Counter is running. When using a static

TOP value there are pr actically no differences between the two modes of operatio n.

In phase correct PWM mode, the compare units allow generation of PWM waveforms on the

OC1x pins. Setting the COM1x1:0 bits to 2 will produce a non-inve rted PWM and an inverted

PWM output can be generated by setting the COM1x1:0 to 3 (See Table 44 on page 110). The

actual OC1x value will only be visible on the port pin if the data direction for the port pin is set as

output (DDR_O C1x) . The PWM wavef orm is gene rate d by set ting (or cle aring ) the OC 1x Regis-

ter at the compare match between OCR1x and TCNT1 when the counter increments, and

clearing (or setting) the OC1 x Register at compare match between OCR1x and TCNT1 when

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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ATmega16(L)

the counter decreme nts. The PWM fre quency for the output when using phase correct PWM can

be calculated by the following equation:

The N variable re pr esents the prescaler divider (1, 8, 64 , 25 6, or 10 24 ).

The extreme values for the OCR1x Register represent special cases when generating a PWM

waveform output in the phas e correct PWM mode. If the OCR1x 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.

Phase and Frequency

Correct PWM Mode The ph ase and freque ncy correct Pu lse Width Mod ulation, or phase and fr equency cor rect PWM

mode (WGM13:0 = 8 or 9) provides a high resolution phase and frequency correct PWM wave-

form generation option. The phase and frequency correct PWM mode is, like the phase correct

PWM mode, based on a dual-slope operation. The counter counts repeatedly from BOTTOM

(0x0000) to TOP and then from TO P to BOTTOM. In non-inverting Compare Output mode, the

Output Compare (OC1x) is cleared on the compare match between TCNT1 and OCR1x while

upcounting, and set on the com pare match while downcounting. In inverting Comp are Output

mode, the operation is inverted. The dual-slope operation gives a lower maximum operation fre-

quency compared to the single-slope operation. However, due to the symmetric feature of the

dual-slope PWM mo de s, th es e mo de s ar e pr eferred for moto r co ntr o l app lica tio ns.

The main difference between the phase correct, and the phase and frequency correct PWM

mode is the time the OCR1x Register is updated by the OCR1x Buffer Register, (see Figure 47

and Figure 48).

The PWM resolution for the phase and frequency correct PWM m ode can be defin ed by either

ICR1 or OCR1A. The minimum resolution allowed is 2-bit (ICR1 or OCR1A set to 0x0003), and

the maximum resolution is 16-bit (ICR1 or OCR1A set t o 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 ICR1 (WGM13:0 = 8), or the value in OCR1A (WGM13:0 = 9). Th e

counter has then reached the TOP and changes the count direction. The TCNT1 value will be

equal to TOP for one timer clock cycle. The timing diagram for the phase correct and frequency

correct PWM mode is sh own on Figu re 48. T he fi gure sho ws phase and fre que ncy corr ect PWM

mode when OCR1A or ICR1 is used to defin e TOP. The TCNT1 value is in the timing diag ram

shown as a histogram for illustrating the dual-slope operation. The diagram includes non-

inverted and in verted PWM o utputs. The small horizonta l line marks on the TCNT1 slopes repre-

sent compare matches between OCR1x and TCNT1. The OC1x Interrupt Flag will be set when a

compare match occurs.

fOCnxPCPWM

fclk_I/O

2NTOP⋅⋅

----------------------------=

RPFCPWM

TOP 1+()log 2()log

-----------------------------------=

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ATmega16(L)

Figure 48. Phase and Frequency Correct PWM Mod e, Timing Diagram

The Timer/Counter Overflow Flag (TOV1) is set at the same timer clock cycle as the OCR1x

Registers are updat ed with the doub le buffer value ( at BOTTO M). Wh en either OCR1A o r ICR1

is used for defining the TO P value, the OC1A or ICF1 Flag set when TCNT1 has reached TOP.

The Interrupt Flags can then be used to generate an int er rup t ea ch ti me t he count er re aches the

TOP or BOTTOM value.

When changing the TOP value the prog ram 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 TCNT1 and the OCR1x.

As Figure 48 shows the output generated is, in contrast to the phase correct mode, symmetrical

in all periods. Since the OCR1x 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 fre-

quency correct.

Using the ICR1 Regis ter for defining TOP work s well when using fixed TOP values. By using

ICR1, the OCR1A Register is free to be used for generating a PWM output on OC1A. However,

if the base PWM frequency is a ctively changed by chang ing the TOP value, using t he OCR1A 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 wave-

forms on the OC1x pins. Setting the COM1x1:0 bits to 2 will produce a non-inverted PWM and

an inverted PWM o utput can be genera ted by setting the CO M1x1:0 to 3 (See Table on page

111). The actual OC1x value will only be visible on the port pin if the data direction for the port

pin is set as output (DDR_OC1x). The PWM waveform is generated by setting (or clearing) the

OC1x Register at the compare match between OCR1x and TCNT1 when the counter incre-

ments, and clearing (or setting) the OC1x Register at compare match between OCR1x and

TCNT1 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 re pr esents the prescaler divider (1, 8, 64 , 25 6, or 10 24 ).

The extreme values for the OCR1x Register represents special cases when generating a PWM

waveform output in the phas e correct PWM mode. If the OCR1x is set equal to BOTTOM the

OCRnx / TOP Update

and

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 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 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.

Timer/Counter

Timing Diagrams The Timer/Counter is a synchronous design and the timer clock (clkT1) is therefore shown as a

clock enable sign al in the following figures. Th e figures include information on whe n Interrupt

Flags are set, and when the OCR1x Register is up dated with the OCR1x buffer value (only for

modes utilizing double buffering). Figure 49 shows a timing diagram for the setting of OCF1x.

Figure 49. Timer/Counter Timing Diagram, Setting of OCF1x, No Prescaling

Figure 50 shows the same timing data, but with the prescaler enab led.

Figure 50. Timer/Count er Timing Diagram, Setting of OCF1x, with Prescaler (fclk_I/O/8)

Figure 51 shows the count sequence clo se to TOP in various modes. When using phase and

frequency correct PWM mode t he OCR1x Register is updated at BOTTOM. The timin g diagrams

clk

(clkI/O/1)

OCFnx

clk

I/O

OCRnx

TCNTn

OCRnx Value

OCRnx - 1 OCRnx OCRnx + 1 OCRnx + 2

OCFnx

OCRnx

TCNTn

OCRnx Value

OCRnx - 1 OCRnx OCRnx + 1 OCRnx + 2

clk

I/O

clk

(clkI/O/8)

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will be the same, but TOP should be replaced by BOTTOM, TOP-1 by BOTTO M+1 and so on.

The same renaming applies for modes that set the TOV1 Flag at BOTTOM.

Figure 51. Timer/Counter Timing Diagram, no Prescaling

Figure 52 shows the same timing data, but with the prescaler enab led.

Figure 52. Timer/Counter Timing Diagram, with Prescaler (fclk_I/O/8)

TOVn

(FPWM)

and ICFn

(if used

as TOP)

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)

clk

I/O

TOVn (FPWM)

and ICFn (if used

as TOP)

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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16-bit

Timer/Counter

Description

Timer/Counter1

Control Register A –

TCCR1A

• Bit 7:6 – COM1A1:0: Compare Output Mode for Channel A

• Bit 5:4 – COM1B1:0: Compare Output Mode for Channel B

The COM1A1:0 and COM1B1:0 control the Output Compare pins (OC1A and OC1B respec-

tively) behavior. If one or both of the COM1A1:0 bits are written to one, the OC1A output

overrides the normal port functionality of the I/O pin it is connected to. If one or both of the

COM1B1:0 bit are written to one, the OC1B output overrides the normal port functionality of the

I/O pin it is connected to. However, note that the Data Direction Register (DDR) bit correspond-

ing to the OC1A or OC 1B pin mu st be set in or de r to ena ble the out put dr ive r.

When the OC1A or OC1B is connected to the pin, the function of the COM1x1:0 bits is depen-

dent of the WGM13:0 bits setting. Table 44 shows the COM1x1:0 bit functionality when the

WGM13:0 bits are set to a normal or a CTC mode (non-PWM).

Table 45 shows the COM1x1:0 bit functionality when the WGM13:0 bits are set to the fast PWM

mode.

Bit 76543210

COM1A1 COM1A0 COM1B1 COM1B0 FOC1A FOC1B WGM11 WGM10 TCCR1A

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

Initial Value00000000

Table 44. Compare Output Mode, non-PWM

COM1A1/COM1B1 COM1A0/COM1B0 Description

0 0 Normal port operation, OC1A/OC1B

disconnected.

0 1 Toggle OC1A/OC1B on compare match

1 0 Clear OC1A/OC1B on compare match (Set

output to low level)

1 1 Set OC1A/OC1B on compare match (Set

output to high level)

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Note: 1. A special case occurs when OCR1A/OCR1B equals TOP and COM1A1/COM1B1 is set. In

this case the compare match is ignored, but the set or clear is done at BOTTOM. See “Fast

PWM Mode” on page 102. for more details.

Table 46 shows the COM1x1:0 bit functionality when the WGM13:0 bits are set to the phase cor-

rect or the phase and frequency correct, PWM mode.

Note: 1. A special case occurs when OCR1A/OCR 1B equals TOP and COM1A1/COM1B1 is set. See

“Phase Correct PWM Mode” on page 104. for more details.

• Bit 3 – FOC1A: Force Output Compare for Channel A

• Bit 2 – FOC1B: Force Output Compare for Channel B

The FOC1A/FOC1B bits are only active when the WGM13:0 bits specifies a non-PWM mode.

However, for ensuring compatibility with future devices, these bits must be set to zero when

TCCR1A is written when operating in a PWM mode. When writing a logical one to the

FOC1A/FOC1B bit, an immediate compare match is forced on the Waveform Generation unit.

The OC1A/OC1B output is changed according to its COM1x1:0 bits setting. Note that the

FOC1A/FOC1B bits are implemented as strobes. Therefore it is the value present in the

COM1x1:0 bits that determine the effect of the forced compare.

Table 45. Compare Output Mode, Fast PWM(1)

COM1A1/COM1B1 COM1A0/COM1B0 Description

0 0 Normal port operation, OC1A/OC1B

disconnected.

0 1 WGM13:0 = 15: Toggle OC1A on Compare

Match, OC1B disconnected (normal port

operation).

For all other WGM13:0 settings, normal port

operation, OCnA/OCnB disconnected.

1 0 Clear OC1A/OC1B on compare match, set

OC1A/OC1B at BOTTO M,

(non-inv erting mode)

1 1 Set OC1A/OC1B on compare match, clear

OC1A/OC1B at BOTTO M,

(inverting mode)

Table 46. Compare Output Mode, Phase Correct and Phase and Frequency Correct PWM (1)

COM1A1/COM1B1 COM1A0/COM1B0 Description

0 0 Normal port operation, OC1A/OC1B

disconnected.

0 1 WGM13:0 = 9 or 14: Toggle OCnA on

Compare Match, OCnB disconnected (normal

port operation).

For all other WGM13:0 settings, normal port

operation, OC1A/OC1B disconnected.

1 0 Clear OC1A/OC1B on compare match when

up-counting. Set OC1A/OC1B on compare

match when downcounting.

1 1 Set OC1A/OC1B on compare match when up-

counting. Clear OC1A/OC1B on compare

match when downcounting.

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A FOC1A/FOC1B strobe will not generate any interrupt nor will it clear the timer in Clear Timer

on Compare match (CTC) mode using OCR1A as TOP.

The FOC1A/FOC1B bits are always read as zero.

• Bit 1:0 – WGM11:0: Waveform Generation Mode

Combined with the WGM13: 2 bit s found in t he TCCR1B Register, these bit s contr ol the counting

sequence of the counter, the source for maximum (TOP) counter value, and what type of wave-

form generation to be used, see Table 47. 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. (See “Modes of Ope ration” on page 101.)

Note: 1. The CTC1 and PWM11:0 bit definition names are obsolete. Use the WGM12:0 definition s. However, the functionality and

location of these bits are compatible with previous versions of the timer.

Table 47. Waveform Genera tion Mode Bit Description(1)

Mode WGM13 WGM12

(CTC1) WGM11

(PWM11) WGM10

(PWM10) Timer/Counter Mode of Opera tion TOP Update of

OCR1x TOV1 Flag Set

00 0 0 0

Normal 0xFFFF Immediate MAX

10 0 0 1

PWM, Phase Correct, 8-bit 0x00FF TOP BOTTOM

20 0 1 0

PWM, Phase Correct, 9-bit 0x01FF TOP BOTTOM

30 0 1 1

PWM, Phase Correct, 10-bit 0x03FF TOP BOTTOM

40 1 0 0

CTC OCR1A Immediate MAX

50 1 0 1

Fast PWM, 8-bit 0x00FF BOTTOM TOP

60 1 1 0

Fast PWM, 9-bit 0x01FF BOTTOM TOP

70 1 1 1

Fast PWM, 10-bit 0x03FF BOTTOM TOP

81 0 0 0

PWM, Phase and Frequency Correct ICR1 BOTTOM BOTTOM

91 0 0 1

PWM, Phase and Frequency Correct OCR1A BOTTOM BOTTOM

10 1 0 1 0 PWM, Ph ase Corre ct ICR1 TOP BOTTOM

11 1 0 1 1 PWM, Phase Correct OCR1A TOP BOTTOM

12 1 1 0 0 CTC ICR1 Immediate MAX

13 1 1 0 1 Reserved – – –

14 1 1 1 0 Fast PWM ICR1 BOTTOM TOP

15 1 1 1 1 Fast PWM OCR1A BOTTOM TOP

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Timer/Counter1

Control Register B –

TCCR1B

• Bit 7 – ICNC1: 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 (ICP1) is filtered. The filter function requires four

successive equal va lued samples of the ICP1 pin for changing its output. The Input Capture is

therefore delayed by four Oscillator cycles when the Noise Canceler is enabled.

• Bit 6 – ICES1: Input Capture Edge Select

This bit selects which edge on the Inp ut Capture Pin (ICP1) that is used to trigger a capture

event. When the ICES1 bit is written to zero, a falling (negative) edge is used as trigger, and

when the ICES1 bit is written to one, a rising (pos itive) edge will trigger the capture.

When a capture is triggered according to the ICES1 setting, the counter value is copied into the

Input Capture Register (ICR1). The event will also set the Input Capture Flag (ICF1), and this

can be used to cause an Input Capture Interrupt, if this interrupt is enabled.

When the ICR1 is used as TOP value (see description of the WGM13:0 bits located in the

TCCR1A and the TCCR1B Re gister), the ICP1 is disconnected and consequen tly the Input Ca p-

ture 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 TCCR1B is written.

• Bit 4:3 – WGM13:2: Waveform Generation Mode

See TCCR1A Register description.

• Bit 2:0 – CS12:0: Clock Select

The three Clock Select bits select the clock source to be used by the Timer/Counter, see Figure

49 and Figure 50.

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

Table 48. Clock Select Bit Description

CS12 CS11 CS10 Description

0 0 0 No clock source (Timer/Counter stopped).

001clk

I/O/1 (No prescaling)

010clk

I/O/8 (From prescaler)

011clk

I/O/64 (From prescaler)

100clk

I/O/256 (From prescaler)

101clk

I/O/1024 (From prescaler)

1 1 0 Exter nal clock source on T1 pin. Clock on falling edge.

1 1 1 External clock source on T1 pin. Clock on rising edge.

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If external pin modes are used for the Timer/Counter1, transitions on the T1 pin will clock the

counter even if the pin is configured as an output. This feature allows software control of the

counting.

Timer/Counter1 –

TCNT1H and TCNT1L

The two Timer/Counter I/O locations (TCNT1H and TCNT1L, combined TCNT1) 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 92.

Modifying the counter (TCNT1) while the counter is running introduces a risk of missing a com-

pare match between TCNT1 and one of the OCR1x Registers.

Writing to the TCNT1 Register blocks ( rem oves) the comp ar e matc h on the followin g timer clock

for all compare units.

Output Compare

OCR1AH and OCR1AL

Output Compare

OCR1BH and OCR1BL

The Output Com pare Registers contain a 16-bit value that is contin uously compared with the

counter value (TCNT1). A match can be used to generate an output com pare interrupt, or to

generate a waveform output on the OC1x pin.

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 tempor ary Hig h Byte Re gister (T EMP). This temp orary reg ister is sh ared by a ll the

other 16-bit registers. See “Accessing 16-bit Registers” on page 92.

Input Capture Register

1 – ICR1H a nd ICR1L

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

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 Value00000000

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

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 Value00000000

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The Input Capture is updated with the counter (TCNT1) value each time an event occurs on the

ICP1 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 Captur e Register is 16-bit in size. To ensure t hat both the high and Low byt es are read

simultaneously when the CPU accesses these registers, the access is performed using an 8-bit

temporary High Byte Register (TEMP). Th is temporary register is shared by all the other 16-bit

registers. See “Accessing 16-bit Registers” on page 92.

Timer/Counter

Interrupt Mask

Note: 1. This register contains interrupt control bits for several Timer/Counters, but only Timer1 bits are

described in this section. Th e remaining bits are describ ed in their respective timer sections.

• Bit 5 – TICIE1: Timer/Counter1, 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/Counter 1 Input Capture Interrupt is enabled. The correspond ing Interrupt

Vector (See “Interrupts” on page 45.) is executed when the ICF1 Flag, located in TIFR, is set.

• Bit 4 – OCIE1A: Timer/Counter1, 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), th e Timer/Counter1 Output Compare A match interrupt is enabled. The corresponding

Interrupt Vector (See “Interrupts” on page 45.) is executed when the OCF1A Flag, located in

TIFR, is set.

• Bit 3 – OCIE1B: Timer/Counter1, 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), th e Timer/Counter1 Output Compare B match interrupt is enabled. The corresponding

Interrupt Vector (See “Interrupts” on page 45.) is executed when the OCF1B Flag, located in

TIFR, is set.

• Bit 2 – TOIE1: Timer/Counter1, 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/Co unter1 O verflow In terrup t is enable d. The corr espond ing Inte rrupt Vecto r

(See “Interrupts” on page 45.) is executed when the TOV1 Flag, located in TIFR, is set.

Timer/Counter

Interrupt Flag Register

– TIFR

Note: This register contains flag bits for several Timer/Counters, but only Timer1 bits are described in

this section. The remaining bits are described in their respective timer sections.

Bit 76543210

OCIE2 TOIE2 TICIE1 OCIE1A OCIE1B TOIE1 OCIE0 TOIE0 TIMSK

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

OCF2 TOV2 ICF1 OCF1A OCF1B TOV1 OCF0 TOV0 TIFR

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

Initial Value00000000

116

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• Bit 5 – ICF1: Timer/Counter1, Input Capture Flag

This flag is set when a capture event occurs on the ICP1 pin. When the Input Capture Register

(ICR1) is set by the WGM1 3:0 t o be used as the TOP value, the ICF1 F lag is set when the coun-

ter reaches the TO P value.

ICF1 is automatically cleared when the Input Capture Interrupt Vector is executed. Alternatively,

ICF1 can be cleared by wr iting a logic one to its bit location.

• Bit 4 – OCF1A: Timer/Counter1, Output Compare A Match Flag

This flag is set in the timer clock cycle after the counter (TCNT1) value matches the Output

Compare Register A (OCR1A).

Note that a Forced Output Compare (FOC1A) strobe will not set the OCF1A Flag.

OCF1A is automatically cleared when the Output Compare Match A Interrupt Vector is exe-

cuted. Alternatively, OCF1A can be cleared by writing a logic one to its bit location.

• Bit 3 – OCF1B: Timer/Counter1, Output Compare B Match Flag

This flag is set in the timer clock cycle after the counter (TCNT1) value matches the Output

Compare Register B (OCR1B).

Note that a forced output compare (FOC1B) strobe will not set the OCF1B Flag.

OCF1B is automatically cleared when the Output Compare Match B Interrupt Vector is exe-

cuted. Alternatively, OCF1B can be cleared by writing a logic one to its bit location.

• Bit 2 – TOV1: Timer/Counter1, Overflow Flag

The setting of th is flag is depen dent of the WGM1 3:0 bits setting . In normal and CTC modes, the

TOV1 Flag is set when the timer overflows. Refer to Table 47 on page 112 for the TOV1 Flag

behavior when using another WGM13:0 bit setting.

TOV1 is automatically cleared whe n the Timer/Counter1 Overflow interrupt vector is executed.

Alternatively, TOV1 can be cleared by writing a logic one to its bit location .

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ATmega16(L)

8-bit

Timer/Counter2

with PWM and

Asynchronous

Operation

Timer/Counter2 is a general purpose, single compare unit, 8-bit Timer/Counter module. The

main features are:

•Single Compare unit Counter

•Clear Timer on Compare Match (Auto Reload)

•Glitch-free, Phase Correct Pulse Width Modulator (PWM)

•Frequency Generator

•10-bit Clock Prescaler

•Overflow and Compare Match Interrupt Sources (TOV2 and OCF2)

•Allows clocking from External 32 kHz Watch Crystal Independent of the I/O Cloc k

Overview A simplified block diagram of the 8 -bit Timer/ Counter is shown in Figure 53. For t he actu al place-

ment of I/O pins, refer to “Pinout ATmega16” on page 2. 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 12 8.

Figure 53. 8-bit Timer/Counter Block Diagram

Registers The Timer/Counter (TCNT2) and Output Compare Register (OCR2) are 8-bit registers. Interrupt

request (shorten as Int.Req.) signals are all visible in the Timer Interrupt Flag Register (TIFR).

All interrupts are individually masked with the Timer Interrupt Mask Register (TIMSK). TIFR and

TIMSK are not shown in the figure since these registers are share d by other timer units.

The Timer/Counter can be clocked internally, via the prescaler, or asynchronously clocked from

the TOSC1/2 pins, as detailed later in this section. The asynchronous operation is controlled by

the Asynchronous Status Register (ASSR). The Clock Select logic block controls which clock

source the Timer/Cou nter uses to incremen t (or decrement) its value. The Timer/ Counter is inac-

tive when no clo ck source is selected. The out put from th e Clock Select logic is referre d to as the

timer clock (clkT2).

Timer/Counter

DATABUS

TCNTn

Waveform

Generation OCn

= 0

Control Logic

0xFF

TOPBOTTOM

count

clear

direction

TOVn

(Int.Req.)

OCn

(Int.Req.)

Synchronization Unit

OCRn

TCCRn

ASSRn

Status flags

clk

I/O

clk

ASY

Synchronized Status flags

asynchronous mode

select (ASn)

TOSC1

T/C

Oscillator

TOSC2

Prescaler

clk

I/O

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The double buffered Output Compare Register (OCR2) is compared with the Timer/Counter

value at all times. The re sult of the comp are can be us ed by the waveform gen erator to generate

a PWM or variable frequency output on the Output Compare Pin (OC2). See “Output Compare

Unit” on page 119. for details. The compare match event will also set the Compare Flag (OCF2)

which can be used to generate an output compare interrupt request.

Definitions Many register and bit references in this document a re written in general form. A lowe r case “n”

replaces the Timer/Counter number, in this case 2. However, when using the register or bit

defines in a program, the precise form must be used (that is, TCNT2 for accessing

Timer/Counter2 counter value and so on). The definitions in Table 49 are also used extensively

throughout the document.

Timer/Counter

Clock Sources The Timer/Counter can be clocked by an internal synchronous or an external asynchronous

clock source. The clock source clkT2 is by default equal to the MCU clock, clkI/O. W hen the A S2

bit in the ASSR Register is written to logic one, th e clock sour ce is taken fr om the Timer /Counter

Oscillator connected to TOS C1 and TOSC2. For details on asy nchronous operation, see “Asyn-

chronous Statu s Register – ASSR” on pa ge 131. For details on clock sources and prescaler , see

“Timer/Counter Prescaler” on page 134.

Counter Unit The main part of the 8-bit Timer/Counter is the programmable bi-directional counter unit. Figure

54 shows a block diagram of the counter and its surrounding environment.

Figure 54. Counter Unit Block Diagr am

Signal description (internal signals):

count Increment or decrement TCNT2 by 1.

direction Selects between increment and decrement.

clear Clear TCNT2 (set all bits to zero).

clkT2Timer/Counter clock.

top Signalizes that TCNT2 has reached maximum value.

Table 49. Definitions

BOTTOM The counter reaches the BOTTOM when it becomes zero (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 OCR2 Register. The

assignment is dependent on the mode of operation.

DATA BUS

TCNTn Control Logic

count

TOVn

(Int.Req.)

topbottom

direction

clear

TOSC1

T/C

Oscillator

TOSC2

Prescaler

clk

I/O

clk Tn

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bottom Signalizes that TCNT2 has reached minimum value (zero).

Depending on the mo de o f oper at ion us ed, the cou nt er is cleared , in cr eme nted , o r decr em ent ed

at each timer clock (clkT2). clkT2 can be generated from an external or inte rnal clock source,

selected by the Clock Select bits (CS22:0). When no clock source is selected (CS22:0 = 0) the

timer is stopped. However, the TCNT2 value can be accessed by the CPU, regardless of

whether clkT2 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 WGM21 and WGM20 bits located in

the Timer/Counter Control Register (TCCR2). There are close connections between how the

counter behaves (counts) and how waveforms a re generated on the Output Compare output

OC2. For more details about advanced counting sequences and waveform generation, see

“Modes of Operation” on page 122.

The Timer/Counter Overflow (TOV2) Flag is set according to the mode of operation selected by

the WGM21:0 bits. TOV2 can be used for generating a CPU interrupt.

Output Compare

Unit The 8-bit comparator continuously compares TCNT2 with the Output Compare Register

(OCR2). Whenever TCNT2 equals OCR2, the comparator signals a match. A match will set the

Output Compare Flag (OCF2) at the next timer clock cycle. If enabled (OCIE2 = 1), the Output

Compare Flag generates an output compare interrupt. The OCF2 Flag is automatically cleared

when the interrupt is executed. Alternatively, the OCF2 Flag can be cleared by software by writ-

ing 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 WGM21:0 bits and Compare Output

mode (COM21:0) bits. The m ax and bottom signals are used b y the wavefo rm generator for han-

dling the special case s of the extreme values in some modes of operation ( “Modes of Opera tion”

on page 122). Figure 55 shows a block diagram of the output compare unit.

Figure 55. Output Compare Unit, Block Diagram

The OCR2 Register is double buffered when using any of the Pulse Width Modulation (PWM)

modes. For the normal and Clear Time r on Compare ( CTC) modes o f operation , the do uble buff-

ering is disabled. The double buffering synchronizes the update of the OCR2 Compare Register

OCFn (Int.Req.)

(8-bit Comparator )

OCRn

OCxy

DATA BU S

TCNTn

WGMn1:0

Waveform Generator

top

FOCn

COMn1:0

bottom

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ATmega16(L)

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 OCR2 Register access may seem complex, but this is not case. When the double buffering

is enabled, the CPU has access to the OCR2 Bu ffer Register, and if double buff ering is disabled

the CPU will access the OCR2 directly.

For ce Output

Compare In non-PWM waveform generation modes, the match output of the comparator can be forced by

writing a one to the Force O utput Compare (FOC2) bit. Forcing com pare match will not set the

OCF2 Flag or reload/clear the timer, but the OC2 pin will be updated as if a real compare match

had occurred (the COM21:0 bits settings def ine whether the OC2 pin is set, cleared or toggled).

Compare Match

Blocking by TCNT2

Write

All CPU write operations to the TCNT2 Register will block any compare match that occurs in the

next timer clock cycle, even when the t imer is st opped. This featur e allows OCR2 to be initialized

to the same value as TCNT2 without triggering an interrupt when the Timer/Counter clock is

enabled.

Using the Output

Compare Unit Since writing TCNT2 in any mode of operation will block all compare matches for one timer clock

cycle, there are risks involved when changing TCNT2 when using the output compare unit, inde-

pendently of whether the Timer/Coun ter is running or not. If the value written to TCNT2 equals

the OCR2 value, the compare match will be missed, resulting in incorrect waveform generation.

Similarly, do not write the TCNT2 value equal to BOTTOM when the counter is downcounting.

The setup of th e OC2 should be pe rformed befo re setting t he Data Direct ion Register fo r the port

pin to output. The easiest way of settin g the OC2 value is to use the Force Output Compa re

(FOC2) strobe bit in Normal mode. The OC2 Register keeps its value even when changing

between Waveform Generation modes.

Be aware that the COM21:0 bits are not double buffered together with the compare value.

Changing the COM21:0 bits will take effect immediately.

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Compare Match

Output Unit The Compare Output mode (COM21:0) bits have two functions. The Waveform Generator uses

the COM21:0 bits fo r defining the Output Compare (O C2) state at the next compare match . Also,

the COM21: 0 bits co ntrol th e OC2 p in outpu t sour ce. Figure 56 shows a simplified schematic of

the logic affected by the COM21:0 bit setting. The I/O Registers, I/O bits, and I/O pins in the fig-

ure are shown in bold. Only the parts of the general I/O Port Control Registers (DDR and PORT)

that are a ffected by th e COM21: 0 bits are shown. Whe n refe rring t o the OC 2 state, t he refer ence

is for the internal OC2 Register, not the OC2 pin.

Figure 56. Compare Match Output Unit, Schematic

The general I/O port function is overridden by the Outpu t Compare (OC2) from the waveform

generator if either of the COM21:0 bits are set. However, the OC2 pin direction (input or output)

is still controlled by the Data Direction Register (DDR) for the port pin. The Data Direction Regis-

ter bit for the OC2 pin (DDR_OC2) must be set as output before the OC2 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 OC2 state before the out-

put is enabled. Note that some COM21:0 bit settings are reserved for certain modes of

operation. See “8-bit Timer/Counter Register Description” on page 128.

Compare Output Mode

and Waveform

Generation

The waveform genera tor uses the COM21:0 b its differently in Normal, CTC, and PWM modes.

For all modes, setting the COM21:0 = 0 tells the Waveform Generator that no action on the OC2

PWM modes refer to Ta ble 51 on page 129. For fast PWM mode, refer to Table 52 on page 129,

and for phase corre ct PWM refer to Table 53 on page 129 .

A change of the COM21:0 bits state will have effect at the first c ompare match after the bits are

written. For non-PWM modes, the action can be forced to have immediate effect by using the

FOC2 strobe bits.

PORT

DDR

OCn

Waveform

Generator

COMn1

COMn0

DATA B U S

FOCn

clk

I/O

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Modes of

Operation The mode of operation, that is, the behavior of the Timer/Counter and the output compare pins,

is defined by the combin ation of the Wavef orm Generat ion mode (WGM 21:0) an d Compare Out-

put mode (COM21:0) bits. The Compare Output mode bits do not affect the counting sequence,

while the Waveform Generation mode bits do. The COM21:0 bits control whether the PWM out-

put generated should be inverted or not (inverted or non-inverted PWM). For non-PWM modes

the COM21:0 bits control whether the output should be set, cleared, or toggled at a compare

match (See “Compare Match Output Unit” on page 121.).

For detailed timing information refer to “Timer/Counter Timing Diagrams” on page 126.

Normal Mode The simplest mode of operation is the Normal mode (WGM21:0 = 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 8-bit value (TOP = 0xFF) and then restarts from the bot-

tom (0x00). In normal operation the Timer/Counter Overflow Flag (TOV2) will be set in the same

timer clock cycle as the TCNT2 becomes zero. The TOV2 Flag in this case behaves like a ninth

bit, except that it is only set, not cleared. However, combined with the timer overflo w interrupt

that automatically clear s the TOV2 Flag, the timer resolution can be incr eased by software. There

are no special cases to consider in the normal mode, a new counter value can be written

anytime.

The Output Compare un it ca n be u s e d to ge ne r ate in te rr up ts at some given t i m e. Usin g the o ut -

put compare to generate waveforms in normal mode is not recommended, since this will occupy

too much of the CPU time.

Clear Timer on

Compare Match (CTC)

Mode

In Clear Timer on Compar e or CTC mo de ( WGM21 :0 = 2), the OC R2 Registe r is us ed to man ip-

ulate the counter resolution. In CTC mode the counter is cleared to zero when t he counter value

(TCNT2) matches the OCR2. The OCR2 defines the top value for the counter, hence also its

resolution. This mode allows greater con trol of th e comp are match out put f reque ncy. I t also sim-

plifies the operation of counting external events.

The timing diagram for the CTC mode is shown in Figure 57. The counter value (TCNT2)

increases until a compare match occur s between TCNT2 and OCR2, an d th en count er (TCNT2)

is cleared.

Figure 57. CTC Mode, Timing Diagram

An interrupt can be generated each time the counter value reaches the TOP value by using the

OCF2 Flag. 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 run-

ning with none or a low pres caler value must be done with care since the CTC mode does not

have the double buffering feature. If the new value written to OCR2 is lower than the current

TCNTn

OCn

(Toggle)

OCn Interrupt Flag Set

1 4

Period

2 3

(COMn1:0 = 1)

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ATmega16(L)

value of TCNT2, 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 OC2 output can be set to toggle its logical

level on each compare match by setting the Compare Output mode bits to toggle mode

(COM21:0 = 1). The OC2 value will not be visible on the port pin unless the data direction for the

pin is set to output. The wa veform generated will have a m aximum frequency of fOC2 = fclk_I/O/2

when OCR2 is set to zero (0x00). The waveform frequency is defined by the following equation:

The N variable represents the prescale factor (1, 8, 32, 64, 128, 256, or 1024).

As for the Normal mode of operation, the TOV2 Flag is set in the same timer clock cycle that the

counter counts from MAX to 0x00.

Fast PWM Mode The fast Pulse Width Modulation or fast PWM mode (WGM21:0 = 3) provides a high frequency

PWM waveform generation option. The fast PWM differs from the other PWM option by its sin-

gle-slope operation. The counter counts from BOTTOM to MAX then restarts from BOTTOM. In

non-inverting Compare Output mode, the Outp ut Compare (OC2) is cleared on the compare

match between T CNT2 and OCR2 , and set at BOTTOM. In invertin g Compare Output mo de, the

output is set on compare match and cleared at BOTTOM. Due to the single-slope operation, the

operating fr equency of the fast PWM mode can be twice as high as the phase correct PWM

mode that uses dual-slop e operation. This hig h 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 MAX value.

The counter is then cleared at the following timer clock cycle. The timing diagram for the fast

PWM mode is shown in Figure 58. The TCNT2 value is in the timing diagram shown as a histo-

gram for illustrating the single-slope operation. The diagram includes non-inverted and inverted

PWM outputs. The small horizontal line marks on the TCNT2 slopes represent compare

matches betw ee n OC R2 and TCNT2 .

Figure 58. Fast PWM Mode, Timing Diagram

The Timer/Counter Overflow Flag (TOV2) is se t each time th e cou nter re ache s MAX. If the int er-

rupt is enabled, the interrupt handler routine can be used for updating the compare value.

fOCn

fclk_I/O

2N1OCRn+()⋅⋅

-----------------------------------------------=

TCNTn

OCRn Update and

TOVn Interrupt Flag Set

Period

2 3

OCn

(COMn1:0 = 2)

(COMn1:0 = 3)

OCRn Interrupt Flag Set

4 5 6 7

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ATmega16(L)

In fast PWM mode, the compar e unit allows gener ation of PWM waveforms on t he OC2 pin. Set-

ting the COM21:0 bits to 2 will produce a non-inverted PWM and an inverted PWM output can

be generated by setting the COM21:0 to 3 (see Table 52 on page 129). The actual OC2 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 OC2 Register at the compare match between

OCR2 and TCNT2, and clearing (or setting) the OC2 Register at the timer clock cycle the coun-

ter is cleared (changes from MAX to BOTTOM).

The PWM frequency for the output can be calculated by th e following equation:

The N variable represents the prescale factor (1, 8, 32, 64, 128, 256, or 1024).

The extreme values for the OCR2 Register represent special cases when generating a PWM

waveform output in the fast PWM mode. If the OCR2 is set equal to BOTTOM, the output will be

a narrow spike for each MAX+1 timer clock cycle. Setting the OCR2 equal to MAX will result in a

constantly high or low output (depending on the polarity of the output set by the COM21:0 bits.)

A frequency (with 50% duty cycle) waveform output in fast PWM mode can be achieved by set-

ting OC2 to toggle its logical level on each compare match (COM21:0 = 1). The waveform

generated will have a maximum frequency of foc2 = fclk_I/O/2 when OCR2 is set to zero. This f ea-

ture is similar to the OC2 toggle in CTC mode, except the double buffer feature of the output

compare unit is enabled in the fast PWM mode.

Phase Correct PWM

Mode The phase cor rect PWM mode (WG M21:0 = 1) 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 MAX and then from MAX to BOTTOM. In non-

inverting Compare Output mode, the Output Compare (OC2) is cleared on the comp are match

between TCNT2 and OCR2 while upcounting, and set on the compare match while downcount-

ing. 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 symmet-

ric feature of the dual-slope PWM modes, these modes are preferred for motor control

applications.

The PWM resolution for the phase correct PWM mode is fixed to 8 bits. In phase correct PWM

mode the counter is incremented until the counter value matches MAX. When the counter

reaches MAX, it changes the count direction. The TCNT2 value will be equal to MAX for one

timer clock cycle. The timing diagram for the phase correct PWM mode is shown on Figure 59.

The TCNT2 value is in the timing diagram shown as a histogram for illustrating the dual-slope

operation. The diagram includes non-in verted and inverted PWM outputs. The small horizontal

line marks on the TC NT 2 slo pe s re pr es en t c omp are matches betwe e n OCR2 and TCNT2.

fOCnPWM

fclk_I/O

N256⋅

------------------=

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Figure 59. Phase Correct PWM Mode, Timing Diagram

The Timer/Counter Overflow Flag (TOV2) 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

OC2 pin. Setting the COM21:0 bits to 2 will produce a non-inverted PWM. An inverted PWM out-

put can be generated by setting the COM21:0 to 3 (see Table 53 on page 129). The actual OC2

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 gen erated by clearing (or setting) the OC2 Register at the compare match

between OCR2 an d TCNT2 when the counter incremen ts, and setting (or clearing) the O C2

PWM frequency for the output when using phase correct PWM can be calculated by the follow-

ing equation :

The N variable represents the prescale factor (1, 8, 32, 64, 128, 256, or 1024).

The extreme values for the OCR2 Register represent special cases when generating a PWM

waveform output in the phase correct PWM mo de. If the OCR2 is set equal to BOTTO M, the out-

put will be continuously low and if set equal to MAX the output will be continuously high for non-

inverted PWM mode. For inve rted PWM the output will have the opposite logic values.

At the very start of Period 2 in Figure 59 OCn has a transition from high to l ow even though

there is no Compare Match. The point of this transition is to guarantee sym metry around BOT-

TOM. There are two cases that will give transition without Compare Match:

• OCR2A changes its value from Max, like in Figure 59. When the OCR 2A 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 be co rrespond the the result of an

up-counting Compare Match.

• The Timer starts counting from a value higher than t he one in OCR2A, and for that reason

misses the Compare Match and hence the OCn that would have happened on the way up.

TOVn Interrupt Flag Set

OCn Interrupt Flag Set

1 2 3

TCNTn

Period

OCn

(COMn1:0 = 2)

(COMn1:0 = 3)

OCRn Update

fOCnPCPWM

fclk_I/O

N510⋅

------------------=

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ATmega16(L)

Timer/Counter

Timing Diagrams The following figures show the Timer/Counter in Synchronous mode, and the timer clock (clkT2)

is therefore shown as a clock enable sign al. In Asynch ronous mode, clkI/O should be replaced by

the Timer/Counter Oscillator clock. The figures include information on when Interrupt Flags are

set. Figure 60 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 60. Timer/Counter Timing Diagram, no Prescaling

Figure 61 shows the same timing data, but with the prescaler enab led.

Figure 61. Timer/Counter Timing Diagram, with Prescaler (fclk_I/O/8)

Figure 62 shows the setting of OCF2 in all modes except CTC mode.

clk

(clkI/O/1)

TOVn

clk

I/O

TCNTn

MAX - 1 MAX BOTTOM BOTTOM + 1

TOVn

TCNTn MAX - 1 MAX BOTTOM BOTTOM + 1

clkI/O

clkTn

(clk

I/O

/8)

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ATmega16(L)

Figure 62. Timer/Counter Timing Diagram, Setting of OCF2, with Prescaler (fclk_I/O/8)

Figure 63 shows the setting of OCF2 and the clearing of TCNT2 in CTC mode.

Figure 63. Timer/Counter Timing Diagram, Clear Timer on Compare Match Mode, with Pres-

caler (fclk_I/O/8)

OCFn

OCRn

TCNTn

OCRn Value

OCRn - 1 OCRn OCRn + 1 OCRn + 2

clkI/O

clkTn

(clk

I/O

/8)

OCFn

OCRn

TCNTn

(CTC)

TOP

TOP - 1 TOP BOTTOM BOTTOM + 1

clk

I/O

clk

(clk

I/O

/8)

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ATmega16(L)

8-bit

Timer/Counter

Description

Timer/Counter Contr ol

• Bit 7 – FOC2: Force Output Compare

The FOC2 bit is only active when the WGM bits specify a non-PWM mode. However, for ensur-

ing compatibility with future devices, this bit must be set to zero when TCCR2 is written when

operating in PWM mode. When writing a logical one to the FOC2 bit, an immediate compare

match is forced on the waveform generation unit. The OC2 output is changed according to its

COM21:0 bits setting. Note that the FOC2 bit is implemented as a strobe. Therefore it is the

value present in the COM21:0 bits that determines t he effect of the forced compare.

A FOC2 strobe will not generate any interrupt, nor will it clear the timer in CTC mode using

OCR2 as TOP.

The FOC2 bit is always read as zero.

• Bit 3, 6 – WGM21:0: Waveform Generation Mode

These bits control the co unting sequence of the counter, the source for the maximum (TOP)

counter value, and what type of waveform generation to be used. Modes of operation supported

by the Timer/Counter unit are: Normal mode, Clear Timer on Compare match (CTC) mode, and

two types of Pulse Width Modulation (PWM) modes. See Table 5 0 and “Modes of Operation” on

page 122.

Note: 1. The CTC2 and PWM2 bit definition names are now obsolete. Use the WGM21:0 definitions.

However, the functionality and location of these bits are compatible with previous versions of

the timer.

• Bit 5:4 – COM21 :0: Compare Match Output Mode

These bits control the Output Compare pin (OC2) behavior. If one or both of the COM21:0 bits

are set, the OC2 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 OC2 pin must be set

in order to enable the output driver.

Bit 7 6 5 4 3 2 1 0

FOC2 WGM20 COM21 COM20 WGM21 CS22 CS21 CS20 TCCR2

Read/Write 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 50. Waveform Generation Mode Bit Description(1)

Mode WGM21

(CTC2) WGM20

(PWM2) Timer/Counter Mode of

Operation TOP Update of

OCR2 TOV2 Flag

Set on

0 0 0 Normal 0xFF Immediate MAX

1 0 1 PWM, Phase Correct 0xFF TOP BOTTOM

2 1 0 CTC OCR2 Immediate MAX

31 1Fast PWM 0xFFBOTTOMMAX

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When OC2 is connected to the pin, the function of the COM21:0 bits depends on the WGM21:0

bit setting. Table 51 shows the COM21:0 bit functionality when the WGM21:0 bits are set to a

normal or CTC mode (non-PWM).

Table 52 shows the COM21:0 bit functionality when the WGM21:0 bits are set to fast PWM

mode.

Note: 1. A special case occurs when OCR2 equa ls TOP and COM21 is set. In this case, the compare

match is ignored, but the set or clear is done at BOTTOM. See “Fast PWM Mode” on page 123

for more details.

Table 53 shows the COM21:0 bit functionality when the WGM21:0 bits are set to phase correct

PWM mode

Note: 1. A special case occurs when OCR2 equa ls TOP and COM21 is set. In this case, the compare

match is ignored, but the set or clear is done at T OP. See “Phase Correct PWM Mode” on page

124 for more details .

Table 51. Compare Output Mode, non-PWM Mode

COM21 COM20 Description

0 0 Normal port operation, OC2 disconnected.

0 1 Toggle OC2 on compare match

1 0 Clear OC2 on compare match

1 1 Set OC2 on compare match

Table 52. Compare Output Mode, Fast PWM Mode(1)

COM21 COM20 Description

0 0 Normal port operation, OC2 disconnected.

01Reserved

1 0 Clear OC2 on compare match, set OC2 at BOTTOM,

(non-inverting mode)

1 1 Set OC2 on compare match, clear OC2 at BOTTOM,

(inverting mode)

Table 53. Compare Output Mode, Phase Correct PWM Mode(1)

COM21 COM20 Description

0 0 Normal port operation, OC2 disconnected.

01Reserved

1 0 Clear OC2 on compare match when up-counting. Set OC2 on compare

match when downcounting.

1 1 Set OC2 on compare match when up-counting. Clear OC2 on compare

match when downcounting.

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• Bit 2:0 – CS22:0: Clock Select

The three Clock Se lect bits select the cl ock source to be use d by the Timer/Co unter, see Table

54.

Timer/Counter

The Timer/Counter Register gives direct access, both for read and write operations, to the

Timer/Counter unit 8-bit counter. Writing to the TCNT2 Register blocks (removes) the compare

match on the following timer clock. Modifying the counter (TCNT2) while the counter is running,

introduces a risk of missing a compare match between TCNT2 and the OCR2 Register.

Output Compare

The Output Compare Register contains an 8-bit value that is continuously compared with the

counter value (TCNT2). A match can be used to generate an output com pare interrupt, or to

generate a waveform output on the OC2 pin.

Table 54. Clock Select Bit Description

CS22 CS21 CS20 Description

0 0 0 No clock source (Timer/Counter stopped).

001

clkT2S/(No prescaling)

010

clkT2S/8 (From prescaler)

011

clkT2S/32 (From prescaler)

100

clkT2S/64 (From prescaler)

101

clkT2S/128 (From prescaler)

110clk

T2S/256 (From prescaler)

111clk

T2S/1024 (From prescaler)

Bit 76543210

TCNT2[7:0] TCNT2

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

Initial Value00000000

Bit 76543210

OCR2[7:0] OCR2

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

Initial Value00000000

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Asynchronous

Operation of the

Timer/Counter

Asynchronous Status

• Bit 3 – AS2: Asynchronous Timer/Counter2

When AS2 is written to zero, Timer/Counter 2 is clocked from the I/O clock, clkI/O. When AS2 is

written to one, Timer/Counter2 is clocked from a Crystal Oscillator connected to the Timer Oscil-

lator 1 (TOSC1) pin. When the value of AS2 is changed, the contents of TCNT2, OCR2, and

TCCR2 might be corr up te d.

• Bit 2 – TCN2UB: Timer/Counter2 Update Busy

When Timer/Counter2 operates asynchrono usly and TCNT2 is written, this bit becomes set.

When TCNT2 has been updated from the temporary storage register, this bit is cleared by hard-

ware. A logical zero in this bit indicates that TCNT2 is ready to be updated with a new value.

• Bit 1 – OCR2UB: Output Compare Register2 Update Busy

When Timer/Counter2 operates asynchronously and OCR2 is written, this bit becomes set.

When OCR2 has been updated from the temporary storage register, this bit is cleared by hard-

ware. A logical zero in this bit indicates that OCR2 is ready to be updated with a new value.

• Bit 0 – TCR2UB: Timer/Counter Control Register2 Update Busy

When Timer/Counter2 operates asynchronously and TCCR2 is written, this bit becomes set.

When TCCR2 has been upda ted fr om the tem por ary sto ra ge r egister , t his bit is cleared by hard-

ware. A logical zero in this bit indicates that TCCR2 is ready to be updated with a new value.

If a write is performed to any of the three Timer/Counter2 Registers while its update busy flag is

set, the updated value might get corrupted and cause an unintentional interrupt to occur.

The mechanisms for reading TCNT2, OCR2, and TCCR2 are different. When reading TCNT2,

the actual timer value is read. When reading OCR2 or TCCR2, the value in the temporary stor-

age register is read.

Asynchronous

Operation of

Timer/Counter2

When Timer/Coun ter2 operates asynchronously, some considerations must be taken .

• Warning: When switching between asynchronous and synchronous clocking of

Timer/Counter2, the Timer Registers TCNT2, OCR2, and TCCR2 might be corrupted. A

safe procedure for switching clock source is:

1. Disable the Timer/Counter2 interrupts by clearing OCIE2 and TOIE2.

2. Select clock source by setting AS2 as appropriate.

3. Write new values to TCNT2, O CR2, and TCCR2.

4. To switch to asynchronous operation: Wait f or TCN2UB, OCR2UB, and TCR2UB.

5. Clear the Timer/Counter2 Interrupt Flags.

6. Enable interrupts, if needed.

Bit 76543 2 1 0

–––– AS2 TCN2UB OCR2UB TCR2UB ASSR

Read/WriteRRRRR/WR R R

Initial Value 0 0 0 0 0 0 0 0

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• The Oscillator is optimized for use with a 32.768 kHz watch crystal. Applying an external

clock to the T OSC1 pin may result in incorrect Timer/Cou nter2 operation. The CPU main

clock frequency must be more than four times the Oscillator frequency.

• When writing to one of the registers TCNT2, OCR2, or TCCR2, the value is transferred to a

temporary register, and latched after two positive edges on TOSC1. The user should not

write a new value before the content s of the temporary register have been transferred to its

destination. Each of the three mentioned registers have their individual temporary register,

which means f or e xample that writing to TCNT2 does not disturb an OCR2 write in prog ress.

To detect that a transfer to the destination register has taken place, the Asynchronous Status

• When entering Power-save or Extended Standby mode after having written to TCNT2,

OCR2, or TCCR2, the user must wait until the written reg ister has been updated if

Timer/Counter2 is used to wake up the device. Otherwise, the MCU will enter sleep mode

before the changes are effective. This is particularly important if the Output Compar e2

interrupt is used to wak e up th e de vice, since t he output compa re function is disab led during

writing to OCR2 or TCNT2. If the write cycle is not finished, and the M CU enters sleep mode

bef ore the OCR2UB bit returns to zero, the device will never receive a compare match

interrupt, and the MCU will not wak e up.

• If Timer/Counter2 is used to wake the device up from Power-save or Extended Standby

mode, precautions must be taken if the user wants to re-enter one of these modes: The

interrupt logic needs one TOSC1 cycle to be reset. If the time between wake-up and re-

entering sleep mode is less than one TOSC1 cycle, the interrupt will not occur, and the

device will fail to wake up. If the user is in doubt whether the time bef ore re-entering Power-

save or Extended Standby mode is sufficient, the following algorithm can be used to ensure

that one TOSC1 cycle has elapsed:

1. Write a value to TCCR2, TCNT2, or OCR2.

2. Wait until the corresponding Update Busy Flag in ASSR retu rns to z ero.

3. Enter Power-save or Extended Standby mode.

• When the asynchronous oper ation is selected, the 32 .768 kHz Oscillator f or Timer/Count er2

is always running, except in Power-down and Standby modes. After a Power-up Reset or

wake-up from Power-down or Standby mode, the user should be awa re of the fact that this

Oscillator might take as long as one second to stabilize. The user is advised to wait for at

least one seco nd before using Timer/Coun ter2 after po w e r-up or w ake-up fr om Pow er-down

or Standby mode. The contents of all Timer/Counter2 Registers must be considered lost

after a wake-up from Power-down or Standby mo de due to unstable clock signal upon start-

up, no matter whether the Oscillator is in use or a clock signal is applied to the TOSC1 pin.

• Description of wake up from Power-save or Extended Standby mode when the timer is

clock ed asynch ronously: When the interrupt condition is met , the w ake up process is sta rted

on the follo wing cycle o f the timer clock, that is , the timer is alw ays advanced by at least one

before the processor can read th e cou n te r value. After wake-up, the MCU is halte d for fo ur

cycles, it executes the interrupt routine, and resumes execut ion from the instruction

following SLEEP.

• Reading of the TCNT2 Register shortly after wake-up from Power-save may give an

incorrect result. Since TCNT2 is clocked on the asynchronous TOSC clock, reading TCNT2

must be done t hrough a register synchronized to the internal I/O clock domain.

Synchronization takes place for every rising TOSC1 edge. When waking up from Power-

sav e mode , and the I/O cloc k (clkI/O) again becomes activ e, TCNT2 will read as the pre vious

value (before entering sleep) until the next rising TOSC1 edge. The phase of the TOSC

clock after waking up from Power-save mode is essentially unpredictable, as it depends on

the wa ke-up time. The recommended procedure for reading TCNT2 is thus as follows:

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1. Write any value to either of the registers OCR2 or TCCR2.

2. Wait f or the corresponding Update Busy Flag to be cleared.

3. Read TCNT2.

• During asynchronous operation, the synchronization of the Interrupt Flags for the

asynchronous timer tak es three pr ocessor cycles plus one timer cycle . The timer is there fore

advanced by at least one before the processor can read the timer value causing th e setting

of the Interrupt Flag. The output compare pin is changed on the timer clock and is not

synchronized to the processor clock.

Timer/Counter

Interrupt Mask

• Bit 7 – OCIE2: Timer/Counter2 Out put Compare Match Interrupt Enable

When the OCIE2 bit is written to one and the I-bit in the Status Register is set (one), the

Timer/Coun ter2 Compa re Match in terrupt is enabled. The corres ponding in terrupt is executed if

a compare match in Timer/ Counter2 occurs, that is, when the OCF2 bit is set in the Timer/Coun-

ter Interrupt Flag Register – TIFR.

• Bit 6 – TOIE2: Timer/Counter2 Overflow Interrupt Enable

When the TOIE2 bit is written to one and the I-bit in the Status Register is set (one), the

Timer/Counter2 Overflow interrupt is enabled. The corresponding interrupt is executed if an

overflow in Timer/Counter2 occurs, that is, when the TOV2 bit is set in the Timer/Counter Inter-

rupt Flag Register – TIFR.

Timer/Counter

Interrupt Flag Register

– TIFR

• Bit 7 – OCF2: Output Compare Flag 2

The OCF2 bit is set (one) when a compare match occurs between the Timer/Counter2 and the

data in OCR2 – Output Compare Register2. OCF2 is cleared by hardware when executing the

corresponding interrupt handling vector. Alternatively, OCF2 is cleared by writing a logic one to

the flag. Whe n the I-bi t in SREG, OCIE2 (Timer/Count er2 Compa re match Inte rrupt Enab le), and

OCF2 are set (one), the Timer/Counter2 Compare match Interrupt is executed.

• Bit 6 – TOV2: Timer/Counter2 Overflow Fla g

The TOV2 bit is set (one) when an overflo w occurs in Time r/Coun te r2 . TO V2 is clear ed by har d-

ware when executing the corre spon ding int erru pt han dling vecto r. Alt ernat ively, TOV2 is clear ed

by writing a logic one to the flag. When the SREG I-bit, TOIE2 (Timer/Counter2 Overflow Inter-

rupt Enable), and TOV2 are set (one), the Timer/Counter2 Overflow interrupt is executed. In

PWM mode, this bit is set when Timer/Counter2 changes counting dir ection at $00.

Bit 76543210

OCIE2 TOIE2 TICIE1 OCIE1A OCIE1B TOIE1 OCIE0 TOIE0 TIMSK

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

OCF2 TOV2 ICF1 OCF1A OCF1B TOV1 OCF0 TOV0 TIFR

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

Initial Value00000000

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Timer/Counter

Prescaler Figure 64. Prescaler for Timer/Counter2

The clock source for Timer/Counter2 is named clkT2S. clkT2S is by default connected to the main

system I/O clock clkIO. By setting the AS2 bit in ASSR, Timer/Counter2 is asynchronously

clocked from the TOSC1 pin. This enables use of Timer/Counter2 as a Real Time Counter

(RTC). When AS2 is set, pins TOSC1 and TOSC2 are disconnected from Port C. A crystal can

then be conne cted between the TOSC1 and TOSC2 pins to serve as an independent clock

source for Timer/Counter2. The Oscillator is optimized for use with a 32.768 kHz crystal. Apply-

ing an external clock source to TOSC1 is not recommended.

For Timer/Counter2, the possible prescaled selections are: clkT2S/8, clkT2S/32, clkT2S/64,

clkT2S/128, clkT2S/256, and clkT2S/1024. Additionally, clkT2S as well as 0 (stop) may be selected.

Setting the PSR2 bit in SFIOR resets the prescaler. This allows the user to operate with a pre-

dictable prescaler.

Special Function IO

• Bit 1 – PSR2: Prescaler Reset Timer/Counter2

When this bit is written to one, the Timer/Counter2 prescaler 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 be read as zero if Timer/Counter2 is clocked by the internal CPU clock. If this bit is

written when Timer/Counter2 is operating in asynchronous mode, the bit will remain one until the

prescaler has been reset.

10-BIT T/C PRESCALER

TIMER/COUNTER2 CLOCK SOURCE

clk

I/O

clk

T2S

TOSC1

AS2

CS20

CS21

CS22

clk

T2S

clk

T2S

/64

clk

T2S

/128

clk

T2S

/1024

clk

T2S

/256

clk

T2S

/32

PSR2

Clear

clk

Bit 7 6 5 4 3 2 1 0

ADTS2 ADTS1 ADTS0 –ACME PUD PSR2 PSR10 SFIOR

Read/Write R/W R/W 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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Serial

Peripheral

Interface – SPI

The Serial Peripheral Interface (SPI) allows high-speed synchronous data transfer between the

ATmega16 and peripheral devices or between several AVR devices. The ATmega16 SPI

includes the following feat ures:

•Full-duplex, Three-wire Synchronous Data Transfer

•Master or Slave Operation

•LSB First or MSB First Data Transfer

•Seven Pr ogrammable Bit Rates

•End of Transmission Interrupt Flag

•Write Collision Flag Protection

•Wake-up from Idle Mode

•Double Speed (CK/2) Master SPI Mode

Figure 65. SPI Block Diagram(1)

Note: 1. Refer to Figure 1 on page 2, and Table 25 on page 58 for SPI pin placement.

The interconnection between Master and Slave CPUs with SPI is shown in Figure 66. The sys-

tem consists of two Shift Registers, and a Master clock generator. T he 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 SCK line to interchange data. Data is always shifted from Mas-

ter to Slave on t he Ma ster Out – Slave In, M OSI, line , and f rom Slave t o Maste r on th e M aste r 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

SPI2X

DIVIDER

/2/4/8/16/32/64/128

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byte to the SPI Data Register starts the SPI clock generator, an d 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 sta te, software may update the contents of the SPI Data

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 Inte rrupt Enable bit, SPIE , in the SPCR Register is set, an interrupt

is requested. The Slave may continue to place new data to be sent into SPDR before reading

the incoming data. The last incoming byte will be kept in the Buffer Register for later use.

Figure 66. SPI Master-Slave Interconnection

The system is single buffered in the transmit direction and double buffered in the receive direc-

tion. 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. Oth-

erwise, 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 minimum low and hi gh periods should be:

Low periods: Longer than 2 CPU clock cycles.

High periods: Longer than 2 CPU clock cycles.

When the SPI is enab led, th e data di rection of the MO SI, MISO , SCK, and SS pins is overrid den

according to Table 55 on page 136. For more details on automatic port overrides, refer to “Alter-

nate Port Functions” on page 55.

Table 55. SPI Pin Overrides

Pin Direction, Master SPI Direction, Slave SPI

MOSI User Defined Input

MISO Input User Defined

SCK User Defined I nput

SS User Defined Input

MSB MASTER LSB

8 BIT SHIFT REGISTER

MSB SLAVE LSB

8 BIT SHIFT REGISTER

MISO

MOSI

SPI

CLOCK GENERATOR

SCK

MISO

MOSI

SCK

SHIFT

ENABLE

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Note: See “Alternate Functions of Port B” on page 58 for a detailed description of how to define the

direction of the user defined SPI pins.

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

actual data direction bits for these pins. For example if MOSI is placed on pin PB5, replace

DD_MOSI with DDB5 and DDR_SPI with DDRB.

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Note: 1. See “About Code Examples” on page 7.

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 “About Code Examples” on page 7.

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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SS Pin

Functionality

Slave Mode When the SPI is configured as a Slave, th e 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 inp uts. When SS is driven high , all pins are inputs except MISO which can be user

configured as an output, 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.

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 w ill immediately

reset the send and receive logic, and drop any partially received data in the Shift Register.

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 a s an inpu t, it m ust be held h igh to ensu re Mast er SPI operat ion. 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 da ta to it. To avoid bus cont ention, 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 becoming 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 possi-

bility that SS is driven low, the interrupt should always check that the MSTR bit is still set. If the

MSTR bit has been clear ed by a Slav e Select, it must be set by the user t o re-enable SPI Master

mode.

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 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 Value00000000

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• Bit 4 – MSTR: Master/Slave Select

This bit selects Master SPI mode when written to on e, 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 S PI Mas-

ter 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 67 and Figure 68 for an example. The CPOL functionality is summa-

rized 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 67 and Figure 68 for an example. The CPHA func-

tionality is summarized below:

• Bits 1, 0 – SPR1, SPR0: SPI Clock Rate Select 1 and 0

These two bits contr ol the SCK ra te of t he dev ice configur ed as a Mast er. SPR1 and SPR0 ha ve

no effect on the Slave. The relationship between SCK and the Oscillator Clock frequency fosc is

shown in the following table:

Table 56. CPOL Functionality

CPOL Leading Edge Trailing Edge

0 Rising Falling

1 Falling Rising

Table 57. CPHA Functionality

CPHA Leading Edge Trailing Edge

0 Sample Setup

1Setup Sample

Table 58. Relationship Between SCK and the Oscillator Frequency

SPI2X SPR1 SPR0 SCK Frequency

000

fosc/4

001

fosc/16

010

fosc/64

011

fosc/128

100

fosc/2

101

fosc/8

110

fosc/32

111

fosc/64

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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 interru pts 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 t he SPI F b it) ar e cleare d b y fi r st r eading t h e SPI Stat u s Regi ste r wit h WCOL set ,

and then accessing the SPI Data Register.

• Bit 5..1 – Res: Reserved Bits

These bits are reserved bits in the ATmega16 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 58). 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 ATmega16 is also used for program memory and EEPROM download-

ing or uploading. See page 273 for SPI Serial Programming and Verification.

SPI Data Register –

SPDR

The SPI Data Register is a read/write register used for data transfer between the Register File

and the SPI Shift Register. Writing to the register initiates data transmission. Reading the regis-

ter causes the Shif t Reg i ste r Rece ive bu ffer to be read.

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 Value X X X X X X X X Undefi ned

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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

67 and Figure 68. 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

56 and Table 57, as done below:

Figure 67. SPI Transfer Format with CPHA = 0

Figure 68. SPI Transfer Format with CPHA = 1

Table 59. CPOL and CPHA Functionality

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

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 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 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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USART The Universa l Synchronous and Asynch ronous serial Receiver and Transmitter (USART) is a

highly flexible serial co mmunication device. The main features are:

•Full Duplex Operation (Independent Serial Receive and Transmit Registers)

•Asynchronous or Synchronous Operation

•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 Suppor ted by Hardware

•Data OverRun Detection

•Framing Error Detection

•Noise Filtering Includes False Start Bit Detection and Digital Low Pass Filter

•Three Separate Interrupts on TX Complete, TX Data Register Empty, and RX Complete

•Multi-processor Communication Mode

•Double Speed Asynchronous Communication Mode

Overview A simplified block diagram of the USART tran smitter is shown in Figure 69. CPU accessible I/O

Registers and I/O pins are shown in bold.

Figure 69. USART Block Diagram(1)

Note: 1. Refer to Figure 1 on page 2, Table 33 on page 65, and Table 27 on page 60 for USART pin

placement.

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

DATABUS

OSC

SYNC LOGIC

Clock Generator

Transmitter

Receiver

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The dashed boxe s in the blo ck diagr am se parat e th e thr ee main pa rts of the USART ( list ed fro m

the top): Clock Generator, Transmitter and Receiver. Control Registers are shared by all units.

The clock generation logic co nsists of synchroniza ti on logic for external clock input used by syn-

chronous Slave operation, and the baud rate generator. The XCK (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 (UDR). Th e receiver supports the same frame formats as the transmitter, and can

detect frame error, data overrun and parity errors.

AVR USART vs. AVR

UART – Compatibility The USART is fully compatible with the AVR UART regarding:

• Bit locations inside all USART Registers

• Baud Rate Generation

• Transmitter Operation

• Transmit Buffer Functionality

• Receiver Operation

However, the receive buffering has two improvements that will affect the compatibility in some

special cases:

• A second Buffer Register has been added. The two Buffer Registers operate as a circular

FIFO buffer. Therefore the UDR must only be read once for each incoming data! More

import an t is the fact that the Error Flags (FE an d DO R) an d th e 9t h da ta bit (RXB8 ) ar e

buffered with the data in the receive buffer. Therefore the status bits must always be read

bef ore the UDR Register is read. Otherwise the error status will be lost since the buff er state

is lost.

• The receiver Shift Register can now act a s a third buffer level. This is done by allowing the

received data to remain in the serial Shift Register (see Figure 69) if the Buff er Registers are

full, until a ne w start bit is detected. The USAR T is theref ore more resist ant to Data Ov erRun

(DOR) erro r con d i tio ns.

The following control bits have cha nged name, but have same functionalit y and register locatio n:

• CHR9 is changed to UCSZ2

• OR is changed to DOR

Clock Generation The clock generation logic generates the base clock for the Transmitte r and Receiver. The

USART supports four modes of clock operation: Normal Asynchronous, Double Speed Asyn-

chronous, Master Synchronous and Slave Synchronous mode. The UMSEL bit in USART

Control and Status Register C (UCSRC) selects between asynchronous and synchronous oper-

ation. Double Speed (Asynchronous mode only) is controlled by the U2X found in the UCSRA

pin (DDR_XCK) controls whether the clock source is internal (Master mode) or external (Slave

mode). The XCK pin is only active when using Synchronous mode.

Figure 70 shows a block diagram of the clock generation logic.

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Figure 70. Clock Generation Logic, Block Diagram

Signal description:

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).

Internal Clock

Generation – The

Baud Rate Generator

Internal clock generation is used for the asynchronous and the synchronous Master modes of

operation. The descrip tion in this section refers to Figure 70.

The USART Baud Rate Register (UBRR) and the down-counter connected to it function as a

programmable prescaler or bau d rate generator. The down-counter, running at system clock

(fosc), is loaded with the UBRR value each time the counter has counted down to zero or when

the UBRRL Register is written. A clock is generated each time the counter reaches zero. This

clock is the baud rate generator clock output (= fosc/(UBRR+1)). The Transm itter divides the

baud rate g enerato r clock outpu t by 2, 8 or 16 d epending on mode. Th e baud r ate genera tor ou t-

put 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

UMSEL, U2X and DDR_XCK bits.

Table 60 contains equations for calculating the baud rate (in bits per second) and for calc ula tin g

the UBRR value for each mode of operation using an internally generated clock source.

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).

BAUD Baud rate (in bits per second, bps)

fOSC System Oscillator clock frequency

UBRR Contents of the UBRRH and UBRRL Registers, (0 - 4095)

Some examples of UBRR values for some system clock frequencies are found in Table 68 (see

page 168).

Double Speed

Operation (U2X) The transfer rate can be doubled b y setting the U2X b it in UCSRA. Set ting this bit only has e ffect

for the asynchronous operation. Set this bit to zero when using synchronous op eration.

Setting this bit will red uce 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.

External Clock External clocking is used by the synchronous Slave modes of operation. The description in this

section refers to Figure 70 for details.

External clock input from the XCK 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 XCK clock frequency is lim-

ited 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.

Table 60. Equations for Calculating Baud Rate Register Setting

Operating Mode Equation for Calculating

Baud Rate(1)

Equation for

Calculating UBRR

Value

Asynchronous Normal Mode

(U2X = 0)

Asynchronous Double Speed Mode

(U2X = 1)

Synchronous Master Mode

BAUD fOSC

16 UBRR 1+()

---------------------------------------=

UBRR fOSC

16BAUD

------------------------1–=

BAUD fOSC

8UBRR 1+()

-----------------------------------=

UBRR fOSC

8BAUD

-------------------- 1–=

BAUD fOSC

2UBRR 1+()

-----------------------------------=

UBRR fOSC

2BAUD

-------------------- 1–=

fXCK

fOSC

-----------

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Synchronous Clock

Operation When Synchronous mode is used (UMSEL = 1), the XCK 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 basic principle is that data input (on RxD) is sampled at the

opposite XCK clock edge of the edge the data output (TxD) is changed.

Figure 71. Synchronous Mode XCK Timing.

The UCPOL bit UCRSC selects which XCK clock edge is used for data sampling and which is

used for data change. As Figure 71 shows, when UCPOL is zero the data will be changed at ris-

ing XCK edge and sampled at falling XCK edge. If UCPOL is set, the data will be changed at

falling XCK edge and sampled at rising XCK edge.

Frame Formats A serial frame is define d to be one ch aract er of da ta 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 va lid 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 w ith the s tart bit f ollowe d by t he leas t signif icant da ta b it. Then the next da ta bit s,

up to a total of nin e, are succeed ing, endin g with t he most sig nificant bit. If enab led, the pa rity 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 id le (high) state.

Figure 72 illustrates the possible combinations of the frame formats. Bits inside brackets are

optional.

Figure 72. Frame Formats

St Start bit, always low.

(n) Data bits (0 to 8).

PParity bit. Can be odd or even.

Sp Stop bit, always high.

RxD / TxD

XCK

RxD / TxD

XCK

UCPOL = 0

UCPOL = 1

Sample

10 2 3 4 [5] [6] [7] [8] [P]St Sp1 [Sp2] (St / IDLE)(IDLE)

FRAME

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IDLE No transfers on the communication line (RxD or TxD). An IDLE line must be

high.

The frame format used by the USART is set by the UCSZ2:0, UPM1:0, and USBS bits in

UCSRB and UCSRC. 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 (UCSZ2:0) bits select the number of data bits in the frame. The

USART Parity mode (UPM1:0) 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 (USBS) 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.

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::

Peven Parity bit using even parity

Podd Parity bit using odd parity

dnData bit n of the ch aracter

If used, the parity bit is located between the last data bit and first stop bit of a serial frame.

USAR T

Initialization The USART has to be initialized befor e any communication can t ake place. The initializatio n pro-

cess 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 -in itia liza tio n with cha ng ed bau d rat e o r fr ame form at , be sure th at ther e are no

ongoing transmissions during the period the registers are changed. The TXC 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 TXC Flag must be

cleared before each transmission (before UDR is written) if it is used for this purpose.

The following simple USART initializatio n code examples show one assembly and one C func-

tion that are equal in functionality. The examples assume asynchronous operation using polling

(no interru pts enable d) and a fixed fr ame format . The bau d rate is given as a funct ion paramet er.

For the assembly code, the baud rate parameter is assumed to be stored in the r17:r16 regis-

ters. When the function writes to the UCSRC Register, the URSEL bit (MSB) must be set due to

the sharing of I/O location by UBRRH and UCSRC.

Peven dn1–…d3d2d1d00

Podd

⊕⊕⊕⊕⊕⊕

dn1–…d3d2d1d01⊕⊕⊕⊕⊕⊕

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Note: 1. See “About Code Exa mp les” on page 7.

More advanced initialization ro ut ines can be mad e th at include fr ame forma t a s para met ers , dis-

able 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.

Data Transmission – The USART Transmitter

The USART Transmitter is enabled by setting the Transmit Enable (TXEN) bit in the UCSRB

den by the USART and given t he functi on as th e tran smitter’s ser ial outpu t. The b aud rate, mode

of operation and frame format must be set up once before doing any transmissions. If synchro-

nous operation is used, the clock on the XCK pin will be overridden and used as transmission

clock.

Sending Frames with

5 to 8 Data Bit A data transmission is initiated by loadin g the transmit buf fer with t he data to be tra nsmitted. The

CPU can load th e transmit buffer by writing to the UDR I/O loca tion. The buffered d ata in the

transmit buffer will be moved to the Shift Register when the Shift Register is ready to send a new

Assembly Code Example(1)

USART_Init:

; Set baud rate

out UBRRH, r17

out UBRRL, r16

; Enable receiver and transmitter

ldi r16, (1<<RXEN)|(1<<TXEN)

out UCSRB,r16

; Set frame format: 8data, 2stop bit

ldi r16, (1<<URSEL)|(1<<USBS)|(3<<UCSZ0)

out UCSRC,r16

ret

C Code Example(1)

#define FOSC 1843200// Clock Speed

#define BAUD 9600

#define MYUBRR FOSC/16/BAUD-1

void main( void )

{

...

USART_Init ( MYUBRR );

...

}

void USART_Init( unsigned int ubrr)

{

/* Set baud rate */

UBRRH = (unsigned char)(ubrr>>8);

UBRRL = (unsigned char)ubrr;

/* Enable receiver and transmitter */

UCSRB = (1<<RXEN)|(1<<TXEN);

/* Set frame format: 8data, 2stop bit */

UCSRC = (1<<URSEL)|(1<<USBS)|(3<<UCSZ0);

}

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frame. The Shif t Regi ster is loaded wit h new dat a if it is in id le stat e ( no ongoin g tr ansmission ) or

immediately after the last sto p bit of the pr evious frame is transmitte d. When th e Shift Register is

loaded with new data, it will transfer one complete frame at the rate given by the Baud Register,

U2X bit or by XCK depending on mode of operation.

The following code examples show a simple USART transmit function based on polling of the

Data Register Empty (UDRE) Flag. When using f rames with less than eight bits, the most signif-

icant bits written to the UDR are ignored. The USART has to be initialized before the function

can be used. For the assembly code, the data to be sen t is assumed to be store d in Register

R16

Note: 1. See “About Code Exa mp les” on page 7.

The function simply waits for th e tr ansm it bu ffer to be empty by checking the UDRE 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.

Assembly Code Example(1)

USART_Transmit:

; Wait for empty transmit buffer

sbis UCSRA,UDRE

rjmp USART_Transmit

; Put data (r16) into buffer, sends the data

out UDR,r16

ret

C Code Example(1)

void USART_Transmit( unsigned char data )

{

/* Wait for empty transmit buffer */

while ( !( UCSRA & (1<<UDRE)) )

;

/* Put data into buffer, sends the data */

UDR = data;

}

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Sending Frames with

9 Data Bit If 9-bit characters are used (UCSZ = 7), the ninth bit must be written to th e TXB8 bit in UCSRB

before the Low byte of the character is written to UDR. 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.

Note: 1. These transmit functions are written to be general functions. They can be optimized if the con-

tents of the UCSRB is static. (that is, only the TXB8 bit of the UCSRB Register is used after

initialization).

The ninth bit can be used for indicating an address frame when using multi processor communi-

cation mode or for ot he r pr ot oc ol ha nd lin g as fo r ex a m ple syn chr o niza tio n.

Transmitter Flags and

Interrupts The USART transmitter has two flags that indicate its state: USART Data Register Empty

(UDRE) and Transmit Complete (TXC). Both flags can be used for generating interrupts.

The Data Register Empty (UDRE) 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 bee n moved into the Shift Register. For compat-

ibility with future devices, always write this bit to zero when writing the UCSRA Register.

When the Data Register empty Interrupt Enable (UDRIE) bit in UCSRB is written to one, the

USART Data Register Empty Interrupt will be executed as long as UDRE is set (provided that

global interrupts are enabled). UDRE is cleared by writing UDR. When interrupt-driven data

transmission is used, the Data Register Empty Interrupt routine must either write new data to

UDR in order to clear UDRE or disable the Da ta Re gister empt y Inter rupt, ot herwise a new inte r-

rupt will occur once the interrupt routine terminates.

Assembly Code Example(1)

USART_Transmit:

; Wait for empty transmit buffer

sbis UCSRA,UDRE

rjmp USART_Transmit

; Copy 9th bit from r17 to TXB8

cbi UCSRB,TXB8

sbrc r17,0

sbi UCSRB,TXB8

; Put LSB data (r16) into buffer, sends the data

out UDR,r16

ret

C Code Example(1)

void USART_Transmit( unsigned int data )

{

/* Wait for empty transmit buffer */

while ( !( UCSRA & (1<<UDRE))) )

;

/* Copy 9th bit to TXB8 */

UCSRB &= ~(1<<TXB8);

if ( data & 0x0100 )

UCSRB |= (1<<TXB8);

/* Put data into buffer, sends the data */

UDR = data;

}

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The Transmit Complete (TXC) Flag bit is set one when the entire frame in the transmit Shift

The TXC 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 TXC Flag is useful in half-duplex

communication interface s (like the RS485 standard), where a transmitting application mu st enter

receive mode and free the communication bus immediat ely after completing the transmission.

When the Transmit Com pete In terrupt Enable (TXCIE) bit i n UCSRB is set, the USART Transmit

Complete Interrupt will be executed when the TXC Flag becomes set (provided that global inter-

rupts are enabled). When the transmit complete interrupt is used, the interrupt handling routine

does not have to clear the TXC Flag, this is done automatically when the interrupt is executed.

Parity Generator The parity generator calculates the parity b it for the serial frame data. When parity bit is enabled

(UPM1 = 1), the transmitte r control logic inserts the parity bit betwee n the last data bit and the

first stop bit of the frame that is sent.

Disabling the

Transmitter The disabling of the transmitter (setting the TXEN to zero) will not become effective until ongoing

and pending transmissions are completed, that is, when the transmit Shift Register and transmit

Buffer Register do not contain data to be transmitted. When disabled, the transmitter will no lon-

ger override the TxD pin.

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Data Reception –

The USART

Receiver

The USART Receive r is enabled by writing the Receive Enable (RXEN) bit in the UCSRB Regis-

ter to one. When the receiver is enabled, the normal pin operation of the RxD pin is overridden

by the USART and gi ven the f unction as the receiver’s serial input. The baud rate, mode of oper-

ation and frame format must be set up once before any serial reception can be done. If

synchronous operation is used, the clock on the XCK pin will be used as transfer clock.

Receiving Frames with

5 to 8 Data Bits T he receiver starts data reception when it d etects a valid start bit. Ea ch bit th at follows t he start

bit will be sampled at the baud rate or XCK clock, and shifted into the receive Shift Register until

the first stop bit of a frame is re ceived. A second stop bit will be ignored by the receiver. Wh en

the first stop bit is rece ive d, tha t is, a com ple te serial frame is present in the receive Shift Regis-

ter, the contents of the Shift Register will be moved into the receive buffer. The receive buffer

can then be read by reading the UDR I/O location.

The following code example shows a simple USART receive function based on polling of the

Receive Complete (RXC) Flag. When using frames with less than eight bits the most significant

bits of the data read from the UDR will be masked to zero. The USART has to be initialized

before the function can be used.

Note: 1. See “About Code Exa mp les” on page 7.

The function simply waits for data to be present in the receive buffer by checking the RXC Flag,

before readin g the buffer and returnin g the value.

Assembly Code Example(1)

USART_Receive:

; Wait for data to be received

sbis UCSRA, RXC

rjmp USART_Receive

; Get and return received data from buffer

in r16, UDR

ret

C Code Example(1)

unsigned char USART_Receive( void )

{

/* Wait for data to be received */

while ( !(UCSRA & (1<<RXC)) )

;

/* Get and return received data from buffer */

return UDR;

}

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Receiving Frames with

9 Databits If 9 bit characters are used (UCSZ=7) the ninth bit must be read from the RXB8 bit in UCSRB

before reading the low bits from the UDR. This rule applies to the FE, DOR and PE status Flags

as well. Read status from UCSRA, then data from UDR. Reading the UDR I/O location will

change the state of the receive buffer FIFO and consequently the TXB8, FE, DOR and PE bits,

which all are stored in the FIFO, will change.

The following code example sho ws a simple USART receive function that handles both 9-bit

characters and the status bits.

Note: 1. See “About Code Exa mp les” on page 7.

Assembly Code Example(1)

USART_Receive:

; Wait for data to be received

sbis UCSRA, RXC

rjmp USART_Receive

; Get status and 9th bit, then data from buffer

in r18, UCSRA

in r17, UCSRB

in r16, UDR

; If error, return -1

andi r18,(1<<FE)|(1<<DOR)|(1<<PE)

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 ( !(UCSRA & (1<<RXC)) )

;

/* Get status and 9th bit, then data */

/* from buffer */

status = UCSRA;

resh = UCSRB;

resl = UDR;

/* If error, return -1 */

if ( status & (1<<FE)|(1<<DOR)|(1<<PE) )

return -1;

/* Filter the 9th bit, then return */

resh = (resh >> 1) & 0x01;

return ((resh << 8) | resl);

}

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The rece ive function example r eads all th e I/O Registers into the Register File before any com-

putation 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.

Receive Compete Fla g

and Interrupt The USART Receiver has one flag that indicates the receiver state.

The Receive Complete (RXC) Flag indicates if there are unread data present in the receive buf-

fer. This flag is one when unread data exist in the receive buffer, and zero when the receive

buffer is empty (t hat is, does not co ntain any unread d ata). If the receiver is disabled (RXEN =

0), the receive buffer will be flushed and consequently the RXC bit will become zero.

When the Receive Comple te Interrupt Enable (RXCIE) in UCSRB is set, the USART Receive

Complete Interrupt will be executed as long as the RXC Flag is set (provided that global inter-

rupts are enabled). When interrupt-driven data reception is used, the receive complete routine

must read th e received data from UDR in order to clear the RXC Flag, otherwise a new interrupt

will occur once the interrupt routine terminates.

Receiver Error Flags The USART Receiver has three Error Flags: Frame Error (FE), Data OverRun (DOR) and Parity

Error (PE). All can be accessed by reading UCSRA. Common for the Error Flags is that they are

located in the receive buffer together with the frame for which they indicate the error statu s. Due

to the buffering of the Error Flags, the UCSRA must be read before the receive buffer (UDR),

since reading the UDR I/O location changes the buffer read location. Another equality for the

Error Flags is that they can not be altered b y software doing a write to the flag location. How-

ever, all flags must be set to zero when the UCSRA is written for upward compatibility of future

USART implementations. None of the Error Flags can generate interrupts.

The Frame Error (FE) Flag indicates the state of the first stop bit of the next readable fram e

stored in the receive buffer. The FE Flag is zero whe n the stop bit was correctly read (as one),

and the FE 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 FE Flag

is not affected by the setting of the USBS bit in UCSRC 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

UCSRA.

The Data OverRun (DOR) 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 Shif t Register, a nd a new start bit is detected. I f the DOR Flag i s set there was one or

more serial frame lost between the frame last read from UDR, and the next frame read from

UDR. For compatibility with future devices, always write this bit to zero when w riting to UCSRA.

The DOR Flag is cleared wh en the frame r eceived was successfully moved from the Shift Regis-

ter to the receive buffer .

The Parity Error (PE) Flag indicates that the next frame in the receive buffer had a parity error

when received. If parity check is not enabled the PE bit will always be read zero. For compatibil-

ity with future devices, always set this bit to zero when writing to UCSRA. For more details see

“Parity Bit Calculation” on page 149 and “Parity Checker” on page 156 .

Parity Checker The Parity Checker is active when the high USART Parity m ode ( U PM1) b it is set . Type o f pa rit y

check to be performed (odd or even) is selected by the UPM0 bit. When enabled, the parity

checker calculates the parity of the data bits in incoming frames and compares the result with

the parity bit f rom the serial fr ame. The resu lt of the ch eck is stored in the rece ive buffe r together

with the received data and stop bits. The Parity Error (PE) Flag can then be read by software to

check if the frame had a parit y error.

The PE 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 (UPM1 = 1). This bit is valid

until the receive buffer (UDR) is read.

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Disabling the Receiver In contrast to the Transmitter, disabling of the Receiver will be immediate. Data from ongoin g

receptions will therefore be lost. When disabled (that is, the RXEN is set to zero) the Receiver

will no longer override the normal function of the RxD port pin. The receiver buffer FIFO will be

flushed when the receiver is disabled. Remaining data in the buffer will be lost

Flushing the Receive

Buffer The receiver buffer FIFO will be flushed when the Receiver is disabled, that is, the buffer will be

emptied of its contents. Unrea d data will be lost. If the buffer has to be flushed during normal

operation, due to f or inst an ce an err or condition , r ead the UDR I/O loca tio n until th e RXC Flag is

cleared. The following code example shows how to flush the re ceive buffer.

Note: 1. See “About Code Exa mp les” on page 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 RxD pin. The data recovery logic sam-

ples and low pass filters each incoming b it, 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 fram e size in number of bits.

Asynchronous Clock

Recovery The clock recovery logic synchronizes internal clock to the incoming serial frames. Figure 73

illustrates the sampling process of the start bit of an incoming frame. The sample rate is 16 times

the baud rate f or Normal mode, and 8 time s the baud rat e for Double Spe ed mode . The hor izon-

tal arrows illustrate the synchronization variation due to the sampling process. Note the larger

time variation when using the doub le speed mode (U2X = 1) of operatio n. Samples denoted zero

are samples done when the RxD line is idle (that is, no communication activity).

Figure 73. Start Bit Sampling

When the clock reco very logic detects a h igh (idle) to low (start) transition on the RxD line, the

start bit detection sequence is initiated. Let sample 1 denote the first zero-sampl e as shown in

the figure. T he clock re covery logic then uses sam ples 8 , 9, and 1 0 for Nor mal mod e, and sam -

ples 4, 5, and 6 for Double Speed mode (indicated with sample numbers inside boxes on the

Assembly Code Example(1)

USART_Flush:

sbis UCSRA, RXC

ret

in r16, UDR

rjmp USART_Flush

C Code Example(1)

void USART_Flush( void )

{

unsigned char dummy;

while ( UCSRA & (1<<RXC) ) dummy = UDR;

}

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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figure), t o decide if a valid start bit is re ceive d. If two or m ore o f these thre e samples have log ical

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 recov-

ery logic is synchronized and the data recovery can begin. The synchronization process is

repeated for each start bit.

Asynchronous Data

Recovery When the rece iver clock is synchronized to the start bit, the data recovery can begin. The d ata

recovery unit uses a state machine that has 16 states for each bit in normal mode and 8 states

for each bit in Double Speed mode. Figure 74 shows the sampling of the data bits and the parity

bit. Each of the sample s is given a number that is equal to the state of the recovery unit.

Figure 74. 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 ha ving th e samp le num ber inside boxes. The majori ty vot i ng 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 RxD 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 75 shows the sampling of the stop bit and the earliest possib le beginning of t he start bit of

the next frame.

Figure 75. Stop Bit Sampling and Next Start Bit Sampling

The same majority voting is done to the stop bit as done fo r the other bits in the fr ame. If the stop

bit is registered to have a logic 0 value, the Frame Error (FE) Flag will be set.

A new high to low tr ansit ion ind i cating the start bit of a n ew fra me can com e r igh t aft er th e 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 75 . For Double Speed mode th e first low level must be delayed t o (B).

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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Asynchronous

Operational Range The operational range of the receiver is dependent on the mismatch between the received bit

rate and the inter nally gene rated baud ra te. If the Tra nsmitter is sending f rames at too fast or too

slow bit rates, or the internally generated baud rate of the receiver does not have a similar (see

Table 61) base fr equency, 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.

D Sum of character size and parity size (D = 5 to 10 bit)

S Samples per bit. S = 16 for Normal Spe ed 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 61 and Table 62 list the maximum receiver baud rate error that can be to lerated. Note th at

Normal Speed mode has highe r toleration of baud rate vari ations.

Table 61. Recommended Maximum Receiver Baud Rate Error for Normal Speed Mode (U2X =

# (Data+Parity Bit) Rslow (%) Rfast(%) Max To tal

Error (%) Recommended Ma x

Receiver Error (%)

5 93.20 106.67 +6.67/-6.8 ±3.0

6 94.12 105.79 +5.79/-5.88 ±2.5

7 94.81 105.11 +5.11/-5.19 ±2.0

8 95.36 104.58 +4.58/-4.54 ±2.0

9 95.81 104.14 +4.14/-4.19 ±1.5

10 96.17 103.78 +3.78/-3.83 ±1.5

Table 62. Recommended Maximum Receiver Baud Rate Error for Double Spee d Mode (U 2X =

# (Data+Parity Bit) Rslow (%) Rfast (%) Max Total

Error (%) Recommended Max

Receiver Error (%)

5 94.12 105.66 +5.66/-5.88 ±2.5

6 94.92 104.92 +4.92/-5.08 ±2.0

7 95.52 104.35 +4.35/-4.48 ±1.5

Rslow

D1+()S

S1–DS⋅SF

-------------------------------------------=

Rfast

D2+()S

D1+()SS

-----------------------------------=

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The recommendations of the maximum receiver baud rate error was made under the assump-

tion that the receiver and transmitter equally divides the maximum total error.

There are two possible sources fo r the receivers baud rate error. The rece iver’s system clock

(XTAL) will always have some minor instability over the supply voltage range and the tempera-

ture range. When using a crystal to generate the system clock, this is rarely a problem, but for a

resonator th e syste m clock ma y diff er more t han 2% dep en ding o f th e reson ator s t oler ance . 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.

8 96.00 103.90 +3.90/-4.00 ±1.5

9 96.39 103.53 +3.53/-3.61 ±1.5

10 96.70 103.23 +3.23/-3.30 ±1.0

Table 62. Recommended Maximum Receiver Baud Rate Error for Double Spee d Mode (U 2X =

# (Data+Parity Bit) Rslow (%) Rfast (%) Max Total

Error (%) Recommended Max

Receiver Error (%)

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ATmega16(L)

Multi-processor

Communication

Mode

Setting the Multi-processor Communication mode (MPCM) bit in UCSRA enables a filtering

function of incoming frames received b y 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 MPCM

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 se t up to rec eive frame s that contain 5 to 8 data bits, then the first stop bit indi-

cates if the frame contains data or address information. If the receiver is set up for frames with

nine data bits, then the ninth bit (RXB8) is used for identifying address and data frames. When

the frame type bit (the first sto p or t he nin th b it) is o ne, th e f r ame con ta ins a n add ress. When t he

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.

Using MPCM For an MCU to act as a Master MCU, it can use a 9-bit character frame format (UCSZ = 7). The

ninth bit (TXB8) must be set when an add ress frame (TXB8 = 1) or cle ared when a data frame

(TXB = 0) is bein g transmit ted. Th e Slave MCUs mu st in th is ca se be set to use a 9- bit charact er

frame format.

The following proce dure should be used to exchang e data in Multi-processor Communicatio n

mode:

1. All Slave MCUs are in Multi-processor Communication mode (MPCM in UCSRA is set).

2. The Master MCU sends an address frame , and all Sla v es rece iv e and read this frame . In

the Slave MCUs, the RXC Flag in UCSRA will be set as normal.

3. Each Sla ve MCU reads the UDR Register and determines if it has been selected. If so, it

clears the MPCM bit in UCSRA, otherwise it waits for the next address byt e and keeps

the MPCM 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 MPCM bit set, will ignore the data frames.

5. When the last data frame is received by the addressed MCU, the addressed MCU sets

the MPCM bit and w aits for a ne w ad dress fr ame f rom Master. The process then repeats

from 2.

Using any of the 5-bit to 8-bit character frame formats is possible, but impractical since the

receiver must change bet ween usin g n and n+1 chara cte r fra me fo rmats. Thi s makes f ull-dup lex

operation diff icu lt since the tra nsmitte r and r eceiver use s the same char acter size se tting. If 5-bit

to 8-bit character frames are used, the transmitter must be set to use two stop bit (USBS = 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 MPCM bit. The

MPCM bit shares the same I/O location as the TXC Flag and this might accidentally be cleared

when using SBI or CBI instructions.

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Accessing

UBRRH/ UCSRC

Registers

The UBRRH Register shares the same I/O location as the UCSRC Register. Therefore some

special consideration must be taken when accessing this I/O location.

Write Access When doing a write access of this I/O location, the high bit of the value written, the USART Reg-

ister Select (URSEL) bit, controls which one of the two registers that will be written. If URSEL is

zero during a write operation, the UBRRH value will be updated. If URSEL is one, the UCSRC

setting will be updated.

The following code examples show how to access the two registers.

Note: 1. See “About Code Exa mp les” on page 7.

As the code examples illustrate, write accesses of the two registers are relatively unaffected of

the sharing of I/O location.

Read Access Doing a read access to the UBRRH or the UCSRC Regis ter is a more complex operation. How-

ever, in most applications, it is rarely necessary to read any of these re gisters.

The read access is controlled by a timed sequence. Reading the I/O location once returns the

UBRRH Register contents. If the register location was r ead in previous system clock cycle, read-

ing the register in the current clock cycle will return the UCSRC contents. Note that the timed

sequence for r ea ding the UCSRC is an atom ic op erat ion. In te rr upts m ust th eref or e be cont r olled

(for example by disabling interrupts globally) during the read operation.

Assembly Code Example (1)

...

; Set UBRRH to 2

ldi r16,0x02

out UBRRH,r16

...

; Set the USBS and the UCSZ1 bit to one, and

; the remaining bits to zero.

ldi r16,(1<<URSEL)|(1<<USBS)|(1<<UCSZ1)

out UCSRC,r16

...

C Code Example(1)

...

/* Set UBRRH to 2 */

UBRRH = 0x02;

...

/* Set the USBS and the UCSZ1 bit to one, and */

/* the remaining bits to zero. */

UCSRC = (1<<URSEL)|(1<<USBS)|(1<<UCSZ1);

...

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The following code example shows how to read the UCSRC Register contents.

Note: 1. See “About Code Exa mp les” on page 7.

The assembly code examp le returns the UCSRC value in r16.

Reading the UBRRH content s is not an a tomic oper ation and theref ore it can be re ad as an or di-

nary register , as lon g as the pr evious instruction did not access the register location.

USART Register

Description

USART I/O Data

The USART Transmit Data Buffer Regist er and USART Receive Data Buffer Regist ers share t he

same I/O address referred to as USART Data Register or UDR. The Transmit Data Buffer Reg-

ister (TXB) will be the destination for data written to the UDR Register location. Reading the

UDR Register location will return the contents of the Receive Data Buffer Register (RXB).

For 5-bit, 6-bit, 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 UDRE Flag in the UCSRA Register is set. Data

written to UDR when the UDRE 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 seri-

ally transmitted on the TxD pin.

The receive buffer consists of a two level FIFO. The FIFO w ill change its state whenever the

receive buffer is accesse d. Due to this behavior of the rece ive buffer, do not use read mo dify

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.

Assembly Code Example(1)

USART_ReadUCSRC:

; Read UCSRC

in r16,UBRRH

in r16,UCSRC

ret

C Code Example(1)

unsigned char USART_ReadUCSRC( void )

{

unsigned char ucsrc;

/* Read UCSRC */

ucsrc = UBRRH;

ucsrc = UCSRC;

return ucsrc;

}

Bit 76543210

RXB[7:0] UDR (Read)

TXB[7:0] UDR (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

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ATmega16(L)

USART Control and

Status Register A –

UCSRA

• Bit 7 – RXC: USART Receive Complete

This flag bit is set when th ere are unr ead data in the rece ive buffer a nd cleared whe n the rece ive

buffer is e mpty (t hat is, do es not cont ain any u nread data) . If the rece iver is disable d, th e re ceive

buffer will be flushed and consequently the RXC bit will become zero. The RXC Flag can be

used to generate a Receive Complete interrupt (see descr iption of the RXCIE bit).

• Bit 6 – TXC: 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 (UDR). The TXC Flag bit is auto-

matically cleared when a transmit comple te interrup t is execut ed, or it ca n be clea red by writin g

a one to its bit location. The TXC Fla g can gen er ate a Tra nsmit Complete int err upt (see descr ip-

tion of the TXCIE bit).

• Bit 5 – UDRE: USART Data Register Empty

The UDRE Flag indicates if the transmit buffer (UDR) is ready to receive new data. If UDRE is

one, the buffer is empty, and therefore ready to be written. The UDRE Flag can generate a Data

UDRE is set after a reset to indicate that the transmitter is ready.

• Bit 4 – FE: Frame Error

This bit is set if the n ext charact er in th e rece ive bu ffer h ad a Fra me Error when received . th at is,

when the first stop bit of the next character in the receive buffer is zero. This bit is valid until the

receive buffer (UDR) is read. The FE bit is zero when the stop bit of received data is one. Always

set this bit to zero when writing to UCSRA.

• Bit 3 – DOR: 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 (UDR) is read. Always set this bit

to zero when writing to UCSRA.

• Bit 2 – PE: Parity Error

This bit is set if the next character in the receive buffer had a Parity Error when received and t he

parity checking was enabled at that point (UPM1 = 1). This bit is valid until the receive buffer

(UDR) is read. Always set this bit to zero when writing to UCSRA.

• Bit 1 – U2X: Double the USART Transmission Speed

This bit only has effect for the asynchronous ope ration. Write this bit to zero when using syn-

chronous operatio n.

Writing this bit to one will reduce the divisor of the baud rate divider from 16 to 8 effectively dou-

bling the transfer r ate for asynchronous communication.

Bit 76543210

RXC TXC UDRE FE DOR PE U2X MPCM UCSRA

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

Initial Value00100000

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ATmega16(L)

• Bit 0 – MPCM: Multi- processor Communication Mode

This bit enables the Multi-processor Com munication mode. When the MPCM bit is written to

one, all the incoming frames received by the USART receiver that do not contain address infor-

mation will be ignored. The transmitter is unaffected by the MPCM setting. For more detailed

information see “Multi-processor Communicati on Mode” on page 161.

USART Control and

Status Register B –

UCSRB

• Bit 7 – RXCIE: RX Complete Interrupt Enable

Writing this bit to one en ables inte rrupt o n the RXC Flag. A USART Receive Complete Interrupt

will be generated only if the RXCIE bit is written to one, the Global Interrupt Flag in SREG is writ-

ten to one and the RXC bit in UCSRA is set.

• Bit 6 – TXCIE: TX Complete Interrupt Enable

Writing this bit to o n e en ab les in te rr upt o n th e TXC Flag . A USART Transmit Com ple te Inte rr up t

will be generated only if the TXCIE bit is written to one, the Global Interrupt Flag in SREG is writ-

ten to one and the TXC bit in UCSRA is set.

• Bit 5 – UDRIE: USART Data Register Empty Interrupt Enable

Writing this bit to one enables interrupt on the UDRE 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 UDRE bit in UCSRA is set.

• Bit 4 – RXEN: Receiver Enable

Writing this bit to one enables the USART Receiver. The Receiver will override normal port oper-

ation for the RxD pin when enabled. Disabling the Receiver will flush the receive buffer

invalidating the FE, DOR, and PE Flags.

• Bit 3 – TXEN: Transmitter Enable

Writing this bit to one enables the USART Transmitter. The Transmitter will override normal port

operation for the TxD pin when enabled. The disabling of the Transmitter (writing TXEN to zero)

will not become effective until ongoing and pending transmissions are completed, that is, 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 TxD port.

• Bit 2 – UCSZ2: Character Size

The UCSZ2 bits combined with the UCSZ1:0 bit in UCSRC sets the number of data bits (Char-

acter Size) in a frame the receiver and transmitter use.

• Bit 1 – RXB8: Receive Data Bit 8

RXB8 is the nint h data bit of t he received charac ter when operating with serial frames with nine

data bits. Must be read before reading the low bits from UDR.

Bit 76543210

RXCIE TXCIE UDRIE RXEN TXEN UCSZ2 RXB8 TXB8 UCSRB

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

Initial Value00000000

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ATmega16(L)

• Bit 0 – TXB8: Transmit Data Bit 8

TXB8 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 UDR.

USART Control and

Status Register C –

UCSRC

The UCSRC Register shares t he same I/O locat ion as the UBRRH Reg ister. See the “Accessing

UBRRH/ UCSRC Registers” on page 162 section which describes how to access this register.

• Bit 7 – URSEL: Register Select

This bit selects between accessing the UCSRC or the UBRRH Register. It is read as one when

reading UCSRC. The URSEL must be one when writing the UCSRC.

• Bit 6 – UMSEL: USART Mode Select

This bit selects between Asynchronous and Synchronous mode of operation.

• Bit 5:4 – UPM1:0: Parity Mode

These bits ena ble and set type of parity gen eration and check. If enab led, the transmit ter 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 UPM0 setting.

If a mismatch is detected, the PE Flag in UCSRA will be set.

• Bit 3 – USBS: Stop Bit Select

This bit selects the number of Stop Bits to be inserted by the Transmitter. The Receiver ignores

this setting.

Bit 76543210

URSEL UMSEL UPM1 UPM0 USBS UCSZ1 UCSZ0 UCPOL UCSRC

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

Initial Value10000110

Table 63. UMSEL Bit Settings

UMSEL Mode

0 Asynchronous Operation

1 Synchronous Operation

Table 64. UPM Bits Settings

UPM1 UPM0 Parity Mode

0 0 Disabled

01Reserved

1 0 Enabled, Even Parity

1 1 Enabled, Odd Parity

Table 65. USBS Bit Settings

USBS Stop Bit(s)

01-bit

12-bit

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ATmega16(L)

• Bit 2:1 – UCSZ1:0: Character Size

The UCSZ1:0 bits combined with the UCSZ2 bit in UCSRB sets the number of data bits (Char-

acter Size) in a frame the Receiver and Transmitter use.

• Bit 0 – UCPOL: Clock Polarity

This bit is used for Synchronous mode only. Write this bit to zero when Asynchronous mo de is

used. The UCPOL bit sets the relationship between data output change and data input sample,

and the synchronous clock (XCK).

USART Baud Rate

Registers – UBRRL

and UBRRH

The UBRRH Register shares t he same I/O locat ion as the UCSRC Reg ister. See the “Accessing

UBRRH/ UCSRC Registers” on page 162 section which describes how to access this register.

• Bit 15 – URSEL: Register Select

This bit selects between accessing the UBRRH or the UCSRC Regis ter. It is read as zero wh en

reading UBRRH. The URSEL must be zero when writing the UBRRH.

• Bit 14:12 – 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.

Table 66. UCSZ Bits Settings

UCSZ2 UCSZ1 UCSZ0 Character Siz e

000 5-bit

001 6-bit

010 7-bit

011 8-bit

100 Reserved

101 Reserved

110 Reserved

111 9-bit

Table 67. UCPOL Bit Settings

UCPOL Transmitted Data Changed (Output of

TxD Pin) Received Data Sampled (Input on

RxD Pin)

0 Rising XCK Edge Falling XCK Edge

1 Falling XCK Edge Rising XCK Edge

Bit 151413121110 9 8

URSEL – – – UBRR[11:8] UBRRH

UBRR[7:0] UBRRL

76543210

Read/Write R/W R R R 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 Value00000000

00000000

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• 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 8 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.

Examples of Baud

Rate Setting For standard crystal and resonator fre quencies, the most commonly used ba ud rates for asyn-

chronous op eration can be genera ted by using the UBRR sett ings in Table 68. UBRR values

which yield an actual baud ra te dif fering le ss than 0.5 % from t he tar get ba ud rat e, are bold in t he

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 159). The error values are calculated using the following equation:

Error[%] BaudRateClosest Match

BaudRate

-------------------------------------------------------- 1–

⎝⎠

⎛⎞

100%•=

Table 68. Examples of UBRR Settings for Commonly Used Oscillator Frequencies

Baud

Rate

(bps)

fosc = 1.0000 MHz fosc = 1.8432 MHz fosc = 2.0000 MHz

U2X = 0 U2X = 1 U2X = 0 U2X = 1 U2X = 0 U2X = 1

UBRR Error UBRR Error UBRR Error UBRR Error UBRR Error UBRR Error

2400 250.2%510.2%470.0%950.0%510.2%1030.2%

4800 120.2%250.2%230.0%470.0%250.2%510.2%

9600 6 -7.0% 12 0.2% 11 0.0% 23 0.0% 12 0.2% 25 0.2%

14.4k 3 8.5% 8 -3.5% 7 0.0% 15 0.0% 8 -3.5% 16 2.1%

19.2k 2 8.5% 6 -7.0% 5 0.0% 11 0.0% 6 -7.0% 12 0.2%

28.8k 1 8.5% 3 8.5% 3 0.0% 7 0.0% 3 8.5% 8 -3.5%

38.4k 1 -18.6% 2 8.5% 2 0.0% 5 0.0% 2 8.5% 6 -7.0%

57.6k 0 8.5% 1 8.5% 1 0.0% 3 0.0% 1 8.5% 3 8.5%

76.8k – – 1 -18.6% 1 -25.0% 2 0.0% 1 -18.6% 2 8.5%

115.2k – – 0 8.5% 0 0.0% 1 0.0% 0 8.5% 1 8.5%

230.4k––––––00.0%––––

250k––––––––––00.0%

Max (1) 62.5 Kbps 125 Kbps 115.2 Kbps 230.4 Kbps 125 Kbps 250 Kbps

1. UBRR = 0, Error = 0.0%

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Table 69. Examples of UBRR Settings for Commonly Used Oscillator Frequencies (Continued)

Baud

Rate

(bps)

fosc = 3.6864 MHz fosc = 4.0000 MHz fosc = 7.3728 MHz

U2X = 0 U2X = 1 U2X = 0 U2X = 1 U2X = 0 U2X = 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 230.0%470.0%250.2%510.2%470.0%950.0%

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 7 0.0% 15 0.0% 8 -3.5% 16 2.1% 15 0.0% 31 0.0%

38.4k 5 0.0% 11 0.0% 6 -7.0% 12 0.2% 11 0.0% 23 0.0%

57.6k 3 0.0% 7 0.0% 3 8.5% 8 -3.5% 7 0.0% 15 0.0%

76.8k 2 0.0% 5 0.0% 2 8.5% 6 -7.0% 5 0.0% 11 0.0%

115.2k 1 0.0% 3 0.0% 1 8.5% 3 8.5% 3 0.0% 7 0.0%

230.4k 0 0.0% 1 0.0% 0 8.5% 1 8.5% 1 0.0% 3 0.0%

250k 0 -7.8% 1 -7.8% 0 0.0% 1 0.0% 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) 230.4 Kbps 460.8 Kbps 250 Kbps 0.5 Mbps 4 60.8 Kbps 921.6 Kbps

1. UBRR = 0, Error = 0.0%

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Table 70. Examples of UBRR Settings for Commonly Used Oscillator Frequencies (Continued)

Baud

Rate

(bps)

fosc = 8.0000 MHz fosc = 11.0592 MHz fosc = 14.7456 MHz

U2X = 0 U2X = 1 U2X = 0 U2X = 1 U2X = 0 U2X = 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% 8 0.0% 17 0.0% 11 0.0% 23 0.0%

115.2k 3 8.5% 8 -3.5% 5 0.0% 11 0.0% 7 0.0% 15 0.0%

230.4k 1 8.5% 3 8.5% 2 0.0% 5 0.0% 3 0.0% 7 0.0%

250k 1 0.0% 3 0.0% 2 -7.8% 5 -7.8% 3 -7.8% 6 5.3%

0.5M 0 0.0% 1 0.0% – – 2 -7.8% 1 -7.8% 3 -7.8%

1M – – 0 0.0% – – – – 0 -7.8% 1 -7.8%

Max (1) 0.5 Mbps 1 Mbps 691.2 Kbps 1.3824 Mbps 921.6 Kbps 1.8432 Mbps

1. UBRR = 0, Error = 0.0%

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Table 71. Examples of UBRR Settings for Commonly Used Oscillator Frequencies (Continued)

Baud

Rate

(bps)

fosc = 16.0000 MHz fosc = 18.4320 MHz fosc = 20.0000 MHz

U2X = 0 U2X = 1 U2X = 0 U2X = 1 U2X = 0 U2X = 1

UBRR Error UBRR Error UBRR Error UBRR Error UBRR Error UBRR Error

2400 416 -0.1% 832 0.0% 479 0.0% 959 0.0% 520 0.0% 1041 0.0%

4800 207 0.2% 416 -0.1% 239 0.0% 479 0.0% 259 0.2% 520 0.0%

9600 103 0.2% 207 0.2% 119 0.0% 239 0.0% 129 0.2% 259 0.2%

14.4k 68 0.6% 138 -0.1% 79 0.0% 159 0.0% 86 -0.2% 173 -0.2%

19.2k 51 0.2% 103 0.2% 59 0.0% 119 0.0% 64 0.2% 129 0.2%

28.8k 34 -0.8% 68 0.6% 39 0.0% 79 0.0% 42 0.9% 86 -0.2%

38.4k 25 0.2% 51 0.2% 29 0.0% 59 0.0% 32 -1.4% 64 0.2%

57.6k 16 2.1% 34 -0.8% 19 0.0% 39 0.0% 21 -1.4% 42 0.9%

76.8k 12 0.2% 25 0.2% 14 0.0% 29 0.0% 15 1.7% 32 -1.4%

115.2k 8 -3.5% 16 2.1% 9 0.0% 19 0.0% 10 -1.4% 21 -1.4%

230.4k 3 8.5% 8 -3.5% 4 0.0% 9 0.0% 4 8.5% 10 -1.4%

250k 3 0.0% 7 0.0% 4 -7.8% 8 2.4% 4 0.0% 9 0.0%

0.5M 1 0.0% 3 0.0% – – 4 -7.8% – – 4 0.0%

1M 0 0.0% 1 0.0% – – – – – – – –

Max (1) 1 Mbps 2 Mbps 1.152 Mbps 2.304 Mbps 1.25 Mbps 2.5 Mbps

1. UBRR = 0, Error = 0.0%

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Two-wire Serial

Interface

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 Addresses

•Multi-master Arbitration Support

•Up to 400 kHz Data Transfer Speed

•Slew-rate Limited Output Drivers

•Noise Suppr e ssion Cir cu itry Rejects Spikes on Bus Lines

•Fully Programmable Slave Address with General Call Support

•Address Recognition causes Wake-up when AVR is in Sleep Mode

Two-wire Serial

Interface Bus

Definition

The Two-wire Serial Interface (TWI) is ideally suited for typical microcontroller applications. The

TWI protoc ol allows t he syste ms desig ner 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 hard-

ware needed to implement the bus is a single pull-up resistor for each of the TWI bus lines. All

devices connec ted to the bus have individual add resses, and mechanisms for reso lving bus

contention are inh erent in the TWI protocol.

Figure 76. TWI Bus Interconnection

TWI Terminology The following definitions are frequently encountered in this section.

Device 1 Device 2 Device 3 Device n

SDA

SCL

........

R1 R2

VCC

Table 72. TWI Terminology

Term Description

Master The device that initiates and termi nates 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 Th e device reading data from the bus.

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Electrical

Interconnection As depicted in Figure 76, 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 tri-state their outputs, allowing the pull-up resistors to pull the line

high. Note that all AVR devices conne cted to the TWI bus must be powered in orde r 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 400 pF and the 7-bit Slave address space. A detailed specification of the electrical char-

acteristics of the TWI is given in “Two-wire Serial Interface Characteristics” on page 294. Two

different sets of specifications are presented there, one relevant for bus speeds below 100 kHz,

and one valid for bus speeds up to 400 kHz.

Data Transfer and

Frame Format

Transferring Bits Each da ta bi t transf er red on the T WI bu s is acco mpa nied by a pu lse on th e cloc k 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 st art and stop conditions.

Figure 77. Data Validity

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 releas-

ing 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 r emainde r of t his datashe et, un less oth erwise note d. As

depicted below, START an d STOP conditions are signalled by cha nging 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 78. START, REPEATED START, and STOP Conditions

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 Mas-

ter’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 add ress b yte is tra nsm it ted firs t. 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. Wh en th e genera l call addr ess followed by a Write bit is transmit ted on t he

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 transmittin g different data.

All addresses of the format 1111 xxx should be reserved for future purposes.

Figure 79. Address Packet Format

SDA

SCL

START STOPREPEATED START

STOP START

SDA

SCL

START

12 789

Addr MSB Addr LSB R/W ACK

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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 receive r has

received the last byte, or for some reason cannot receive any more bytes, it should inform the

transmitter by sending a NACK after the final byte. The MSB of the data byte is transmitted first.

Figure 80. Data Packet Format

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 emp ty message, consisting of a START followed by a STOP cond i-

tion, 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 conseq uence , t he Slave can red uce t he TWI dat a tr an sfe r sp ee d by prol ong ing the

SCL duty cycle.

Figure 81 shows a typical da ta transmission. Note that several data byte s can be transmitted

between the SLA+R/W and the STOP con dition, depending on the software protocol imple-

mented by the application software.

Figure 81. Typical Data Transmission

12 789

Data MSB Data LSB ACK

Aggregate

SDA

SDA from

Transmitter

SDA from

receiverR

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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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 impleme nte d allowing only one of th e Ma st er s to com p let e th e

transmission. All ot her 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 arbitr ation process, it should immediately

s witch to Slav e mode to check whet her it is being addressed b y the winning Maste r . The f act

that multiple Masters ha ve sta rted transmission at t he same time should not be dete ctable to

the Slav es, that is, the data being transferred on the bus must not be corrupted.

• Different Masters may use different SCL frequen cie s. 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 us ed to solve both these problems. The serial clocks from

all Masters will be wired-ANDed, yielding a co mbined 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 82. SCL Synchronization betwee n 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 arbitra tion. Not e that a Ma ster can only lo se arbitr ation wh en 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 Mas ter 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 83. 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 cond itions never

occur. This implies that in multi-master systems, all data transfers must use the same composi-

tion 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.

SDA from

Master A

SDA from

Master B

SDA Line

Synchronized

SCL Line

START Master A Loses

Arbitration, SDA

SDA

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Overview of the

TWI Module The TWI mo dule is comprised of several submodu les, as shown in Fig ure 84. All registers drawn

in a thick line are accessible through the AVR data bus.

Figure 84. Overview of the TWI Module

SCL and SDA Pins These pins interface the AVR TWI with the rest of the MCU system. The out put drivers cont ain a

slew-rate limiter in order to conform to the TWI specification. The input stages contain a spike

suppression unit removing spikes shorter than 50 ns. 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 on es.

Bit Rate Generat or

Unit This unit controls the period of SCL when operating in a Master mode. The SCL period is con-

trolled by settings in the TWI Bit Rate Register (TWBR) and the Prescaler bits in the TWI Status

CPU clock frequency in the Slave must be at least 16 time s higher than the SCL frequen cy. 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: Note: Pull-up resistor values should be selected according to the SCL frequency and the capaci-

tive bus line load. See Table 120 on page 294 for value of pull-up resistor.

Bus Interface Unit This unit contains the Data and Address Shift Register (TWDR), a START/STOP Controller and

Arbitration det ection ha rdwa re . The TWDR con ta ins the a ddress or dat a byt es to be t ransmit ted,

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 Cloc k frequency

16 2(TWBR) 4TWPS

⋅+

-----------------------------------------------------------=

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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 Regis-

ter is not directly accessible by the application software. Howeve r, whe n receiv ing, 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 sle ep mode s, e nabling th e MCU to wa ke up

if addressed by a Mast er .

If the TWI has initiated a transmission as Master, the Arbitration Detection hardware continu-

ously monitor s the transmissio n trying to dete rmine if arbitrat ion is in pr ocess. If the TWI ha s lost

an arbitration, the Control Unit is informed. Correct action can then be taken and appropriate

status codes generated.

Address Match Unit The Address Match unit checks if received address bytes match the 7-bit address in the TWI

Address Register (TWAR). If the TWI General Call Recognition Enable (TWGCE) bit in the

TWAR is written to one, all 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.

Control Unit The Control unit monito rs the TWI bus an d generat es responses cor responding to settings in t he

TWI Control Register (TWCR). When an event requirin g the attention of the application occurs

on the TWI bus, t he TWI Interr upt Flag (TWINT) is asse rted. In the ne xt clock cycle, the TWI Sta-

tus 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 avail-

able. As long as the TW INT Flag is set, th e 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 b y own Slave ad dress or genera l call

• After the TWI has received a data byte

• After a STOP or REPEATED STAR T has been received while still addressed as a Slave.

• When a bus error has occurred due to an illegal START or STOP condition

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TWI Register

Description

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 178 fo r calculating bit rates.

TWI Control Register –

TWCR

The TWCR is used to contr ol the operat ion of the TWI . It is us ed to enable the T WI, to in itiate 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 V ector . While the TWI NT Flag is se t, the SCL lo w perio d is stretch ed. The TWINT

Flag must be cleared by software by writing a logic one to it. Note that this flag is not automati-

cally cleared by hardware when executing the interr upt routine. Also note that clearing this flag

starts the operation of the TWI, so all accesses to the TWI Address Register (TWAR), TWI Sta-

tus 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 followin g conditions are met:

1. The device’s own Slav e 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 disconne cted from the Two-wire

Serial Bus temporarily. Ad dress recognition ca n 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 Two-

wire Serial Bus. The TWI hardware checks if the bus is available, and generates a START con-

dition on the bus if i t is f r ee. However, if the bus is not free, the TWI waits until a STOP condition

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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is detected, and then generates a new START condition to claim the bus Master status. TWSTA

must be cleared by soft ware when the START condition has been transmitted.

• Bit 4 – TWSTO: TWI STOP Condition Bit

Writing the TWSTO bit to one in M aster mode will generate a STOP condition on the Two -wire

Serial Bus. When the STOP condition is executed on the bus, the TWSTO bit is cleared auto-

matically. In Slave mode, setting the TWST O 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 whe n attempting to write to the TWI Dat a Register – TWDR when TWINT is

low. This flag is cleared by writing the TWDR Register when TWINT is high.

• Bit 2 – TWEN: TWI Enab le Bit

The TWEN bit enables TWI operat ion an d acti va te s t he TWI interf ace. When TWEN is writte n 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 acti-

vated for as long as the TWINT Flag is high.

TWI Status Register –

TWSR

• Bits 7..3 – TWS: TWI Status

These five bits reflect the status of the TWI logic and the Two-wire Serial Bus. The different sta-

tus 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 whe n checking the St atus bits. T his makes status checking inde pende nt 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.

Bit 76543210

TWS7 TWS6 TWS5 TWS4 TWS3 – TWPS1 TWPS0 TWSR

Read/WriteRRRRRRR/WR/W

Initial Value11111000

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• Bits 1..0 – TWPS: TWI Prescaler Bits

These bits can be read and writ ten, and control the bit rate prescaler.

To calculate bit rates, se e “Bit Rate Generator Unit” on page 178. The value of TWPS1..0 is

used in the equation.

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 t he process of shifting a byte.

This occurs when the TWI Interrupt Flag (TWINT) is set by hardware. Note that the Data Regis-

ter cannot be initialized b y the user before the first interrup t occurs. Th e data in TWD R 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 automatica lly by the TWI lo gic, the CPU cannot access the ACK bit dir ectly.

• Bits 7..0 – TWD: TWI Data Register

These eight bits contain the next data byte to be transmitted, or the latest data byte received on

the Two-wire Serial Bus.

TWI (Slave) Address

The TWAR should be loaded with the 7-bit Slave address (in the seven most significan t bits of

TWAR) to which the TWI will respond when programmed as a Slave Transmitter or receiver. In

multi-master systems, TWAR must be set in Masters which can be addressed as Slaves by

other Masters.

The LSB of TWAR is use d to enable recognition o f the general call addr ess ($00). 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 Regist er

These seven bits constitute the Slave address of the TWI unit.

Table 73. TWI Bit Rate Prescaler

TWPS1 TWPS0 Prescaler Value

00 1

01 4

10 16

11 64

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 Two-wire Serial Bus.

Using the TWI The AVR TWI is byte-oriented and interrupt based. Interrupts are issued after all bus events, like

reception of a b yte or transmission of a ST ART condition. Because the TWI is interrupt-based,

the application soft war e is fre e to car ry o n oth er oper at ions dur ing a TWI byte t ransf er. Not e that

the TWI Interrupt Enable (TWIE) bit in TWCR together with the Global Interrupt Enable bit in

SREG allow the applica tio n to de cide wh et he r or no t as se rtio n of th e T WINT F lag sho u ld ge n er -

ate an interrupt request. If the TWIE bit is cleared, the application must poll the TWINT Flag in

order to detect actio ns 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 85 is a simple example of how the application can interface to the TWI hardware. In this

example, a Master wishes to tran smit 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 imple-

menting the desired be havior is also presented.

Figure 85. Interfacing the Application to the TWI in a Typical Transmission

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 success-

fully been sent.

3. The application software should now examine the value of TWSR, to make sure that the

START co nditio n was successfully transmitted. If TWSR indicates ot herwise, the applica-

tion software might take some special action, like calling an error routine. Assuming that

the status code is as e xpected, the applicatio n must load SLA+W into TWDR. Remember

that TWDR is used both for address and data. After TWDR has been loaded with the

START SLA+W A DataA 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 signalsinto

TWCR, making 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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desired SLA+W, a specific value m ust be written to TWCR, instructing the TWI hardware

to transmit the SLA+W present in TWDR. Which value to write is descr ib e d late r 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 t ransmitted, the TWINT Flag in TWCR is set, and

TWSR is updated with a status code indicating that t he address packet has successfully

been sent. The stat us cod e will also re fle ct whet he r a Slave ackn owledged the packet or

not.

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

e xpected. If TWSR indicates ot herwise, the application softwa re might tak e 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 st atus code ind ica ting t ha t th e d ata p acket h as successfully be en se nt.

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

e xpected. If TWSR indicates ot herwise, the application softwa re might tak e 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 T WI hardware to transm it

a ST OP condition. Which v alue to write is described later on. How ev er , 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 condi-

tion. Note that TWINT is NOT set after a STOP condition has been sent.

Even though this example is sim ple, it shows the principles involved in all TWI transmissions.

These can be summarized as follows:

• When the TWI has f inished a n oper ation and e xpe cts application r esponse , the TWI NT Flag

is set. The SCL line is pulled low until TWINT is cleared.

• When the TWINT Flag is set, th e user must update all TWI Regist ers with the value relev ant

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 softw are 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 bee n made, for example by using include-files.

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Assembly code example C e xample Comments

1ldi r16, (1<<TWINT)|(1<<TWSTA)|

(1<<TWEN)

out TWCR, r16

TWCR = (1<<TWINT)|(1<<TWSTA)|

(1<<TWEN)

Send START condition

2wait1:

in r16,TWCR

sbrs r16,TWINT

rjmp wait1

while (!(TWCR & (1<<TWINT)))

;

Wait for TW INT Flag set. This indicates

that the START condition has been

transmitted

3in r16,TWSR

andi r16, 0xF8

cpi r16, START

brne ERROR

if ((TWSR & 0xF8) != START)

ERROR();

Check v alue of TWI Status Register . Mask

prescaler bits. If 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 Register. Clear

TWINT bit in TWCR to start transmission

of address

4wait2:

in r16,TWCR

sbrs r16,TWINT

rjmp wait2

while (!(TWCR & (1<<TWINT)))

;

Wait for TW INT Flag set. This indicates

that the SLA+W has been transmitted,

and ACK/NAC K ha s been received.

5in r16,TWSR

andi r16, 0xF8

cpi r16, MT_SLA_ACK

brne ERROR

if ((TWSR & 0xF8) != MT_SLA_ACK)

ERROR();

Check v alue of TWI Status Register . Mask

prescaler bits. If 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

6wait3:

in r16,TWCR

sbrs r16,TWINT

rjmp wait3

while (!(TWCR & (1<<TWINT)))

;

Wait for TW INT Flag set. This indicates

that the DATA has been transmitted, and

ACK/NACK has been received.

7in r16,TWSR

andi r16, 0xF8

cpi r16, MT_DATA_ACK

brne ERROR

if ((TWSR & 0xF8) != MT_DATA_ACK)

ERROR();

Check v alue of TWI Status Register . Mask

prescaler bits. If 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

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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 us ed in th e s am e ap p lication. 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 mo des 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 abbrevia tions:

S: START condition

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 Figur e 87 to Figure 93, 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, action s must be taken by the app lication to continue o r complete the TWI tra nsfer. 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 us ed to de termine the approp riate sof t-

ware action. F or each stat us code, the required software a ction and d etails of the following se rial

transfer are given in Table 74 to Table 77. Note that the prescaler bits are masked to zero in

these tables.

Master Transmitter

Mode In the Master Transmitter mode, a number of data bytes are transmitted to a Slave Receiver

(see Figure 86). 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 enter ed. If SLA+W is tra nsmitted, MT mode is enter ed, if SLA+R is trans-

mitted, 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 86. Data Transfer in Master Transmitter Mode

Device 1

MASTER

TRANSMITTER

Device 2

SLAVE

RECEIVER

Device 3 Device n

SDA

SCL

........

R1 R2

VCC

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A START condition is sent by writing the following value to TWCR:

TWEN must be set to enable the T wo-wire Serial Interface, TWSTA must be written to one to

transmit a START con dition and TWINT must be written to one to clear the TWINT Flag . The

TWI will then test the Two-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 hard-

ware, and the status code in TWSR will be $08 (See Table 74). In order to enter MT mode,

SLA+W must be transmitt ed. This is done by writin g SLA+W to TWDR. There after th e 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 transmitte d and an acknowledgemen t bit has been received, T WINT is

set again and a number of status codes in TWSR are possible. Possible status codes in Master

mode are $18, $20, or $38. The appropriate action to be taken for each of these status codes is

detailed in Table 74.

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 Regis-

ter. 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 generat-

ing 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 $10) the Two-wire Serial Interface can access the

same Slave again, or a new Slave withou t transm itting 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 X10X10 X

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 74. Status Codes for Master Tran smitter Mode

Status Code

(TWSR)

Prescaler Bits

are 0

Status of the Two-wire Serial

Bus and Two-wire Serial Inter-

face Hardware

Application Software Response

Next Action Taken by TWI Hardware

To/from TWDR To TWCR

STA STO TWINT TWEA

$08 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

$10 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

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$18 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

$20 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

$28 Data byte 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

$30 Data byte 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

$38 Arbitration lost in SLA+W or data

bytes No TWDR action or

No TWDR action

Two-wire Serial Bus will be released and not addressed

Slave mode entered

A START condition will be transmitted when the bus be-

comes free

Table 74. Status Codes for Master Tran smitter Mode

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Figure 87. Formats and States in the Master Transmitter Mode

Master Receiver Mode In the Ma ster Receiver mode, a number of data bytes are received from a Slave Transmitter

(see Figure 88). 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 enter ed. If SLA+W is tra nsmitted, MT mode is enter ed, if SLA+R is trans-

mitted, 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 88. Data Transfer in Master Receiver Mode

A START condition is sent by writing the following value to TWCR:

TWEN must be wr itten to one to en able the Two -wire Serial Interfa ce, TWSTA must be written to

one to transmit a START co ndition and TWINT must b e set to clear the TWINT Flag . The TWI

will then test the Two-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 hard-

ware, and the status code in TWSR will be $08 (See Table 74). In order to enter MR mode,

SLA+R must be transmitted. This is done by writing SLA+R to TWDR. Th erea fter the T WIN T bi t

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 tr ansmitt ed and an acknowle dgemen t bi t has b een re ceived, TWI NT is

set again and a number of status codes in TWSR are possible. Possible status codes in Master

mode are $38, $40, or $48. The appropriate action to be taken for each of these status codes is

detailed in Table 75. Received d ata can be read f r om the TWDR Re gister wh en th e TWI NT 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 tr an sfe r is en de d by genera t ing a STOP con dition or a re pea te d 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 $10) the Two-wire Serial Interface can access the

same Slave again, or a new Slave withou t transm itting 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

VCC

Table 75. Status Codes for Master Receiver Mode

Status Code

(TWSR)

Prescaler Bits

are 0

Status of the Two-wire Serial

Bus and Two-wire Serial Inter-

face Hardware

Application Software Response

Next Action Taken by TWI Hardware

To/from TWDR To TWCR

STA STO TWINT TWEA

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Figure 89. Formats and States in th e Master Receiver Mode

$08 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

$10 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

$38 Arbitration lost in SLA+R or NOT

ACK bit No TWDR action or

No TWDR action

Two-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

$40 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

$48 SLA+R has been transmitted;

NOT ACK has been received No TWDR action or

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

$50 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

$58 Data byte has been received;

NOT ACK has been returned Read data byte or

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

Table 75. Status Codes for Master Receiver Mode (Continued)

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

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Slave Receiver Mode In the Slave Receiver mode, a number of data bytes are received from a Master Transmitter

(see Figure 90). All t he statu s codes mentioned in t his section assume that the pr escaler bits are

zero or are masked to zero.

Figure 90. Data Transfer in Slave Receiver Mode

To initiate the Slave Receiver mode, TWAR and TWCR must be initialized as follows:

The upper seven bits are the address to which the Two-wire Serial Interface will respond when

addressed by a Master. If the LSB is set, the TWI will respond to the general call address ($00),

otherwise it will ignore the general call address.

TWEN must be wr itte n t o one t o en able t he TWI . The TWEA b it mu st be wr itt e n to on e to en able

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 addre ssed by its own

Slave address (or the general call address if enabled) followed by the data directio n bit. If the

direction bit is “0” (write), the TWI will operate in SR mode, otherwise ST mode is entered. A fter

its own Slave address and the write bit have been rec eived, the TWINT Flag is set and a valid

status code can be read from TWSR. The status code is used to determine the appropriate soft-

ware action. T he appro pr iat e acti on to be ta ken for ea ch sta tus co de is detaile d in Table 76. The

Slave Receiver mode may also be entered if arbitration is lost while the TWI is in the Master

mode (see states $68 and $78).

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 Two-wire Serial Bus is still monitored and address recognition may

resume at any ti me by se tt ing TWEA. This imp lies t hat the TWEA bit ma y be us ed to te mpo rarily

isolate the TW I fro m th e Tw o- wire Ser i al Bus.

In all sleep mo des other than Id le Mode, the clo ck system to the TWI is t urned off. If the TWEA

bit is set, the interface can still acknowledge its own Slave address or the general call address

by using the Two-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 c locks

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.

TWAR TWA6 TWA5 TWA4 TWA3 TWA2 TWA1 TWA0 TWGCE

Value Device’s Own Slave Address

TWCR TWINT TWEA TWSTA TWSTO TWWC TWEN –TWIE

Value 0100010 X

Device 3 Device n

SDA

SCL

........

R1 R2

VCC

Device 2

MASTER

TRANSMITTER

Device 1

SLAVE

RECEIVER

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Note that the Two-wire Serial Interface Data Register – TWDR does not reflect the last byte

present on the bus when waking up from these sleep modes.

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Table 76. Status Codes for Slave Receiver Mode

Status Code

(TWSR)

Prescaler Bits

are 0

Status of the Two-wire Serial Bus

and Two-wire Serial Interface

Hardware

Application Software Response

Next Action Taken by TWI Hardware

To/from TWDR To TWCR

STA STO TWINT TWEA

$60 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

$68 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

$70 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

$78 Arbitration lost in SLA+R/W as

Master; 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

$80 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

$88 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

$90 Previously addressed with

general call; data has been re-

ceived; 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

$98 Previously addressed with

general 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

$A0 A STOP condition or repeated

START condition has been

received 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 91. Formats and States in the Slave Receiver Mode

Slave Transmitter

Mode In the Slave Transmitter mode, a number of data bytes are transmitted to a Master Receiver

(see Figure 92). All t he statu s codes mentioned in t his section assume that the pr escaler bits are

zero or are masked to zero.

Figure 92. Data Transfer in Slave Transmitter Mode

To initiate the Slave Transmit ter 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

A DATA A

$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

VCC

Device 2

MASTER

RECEIVER

Device 1

SLAVE

TRANSMITTER

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The upper seven bits are the address to which the Two-wire Serial Interface will respond when

addressed by a Master. If the LSB is set, the TWI will respond to the general call address ($00),

otherwise it will ignore the general call address.

TWEN must be wr itte n t o one t o en able t he TWI . The TWEA b it mu st be wr itt e n to on e to en able

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 addre ssed by its own

Slave address (or the general call address if enabled) followed by the data directio n bit. If the

direction bit is “1” (read), the TWI will operate in ST mode, otherwise SR mode is entered. A fter

its own Slave address and the write bit have been rec eived, the TWINT Flag is set and a valid

status code can be read from TWSR. The status code is used to determine the appropriate soft-

ware action. T he appro pr iat e acti on to be ta ken for ea ch sta tus co de is detaile d in Table 77. The

Slave Transmitter mode may also be entered if arbitration is lost while the TWI is in the Master

mode (see state $B0).

If the TWEA bit is written to zero during a transfer, the TWI will transmit the last byte of the trans-

fer. State $C0 or state $C8 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 con tinues the transfer. Thus the Master Receiver receives all “1” as

serial data. State $C8 is e ntered if the Master deman ds additional data bytes (by transmittin g

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 Two-

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 t o temporar ily isolat e the TWI f rom the Two-

wire Serial Bus.

In all sleep mo des other than Id le mode, the clo ck system to the TWI is t urned off. If the TWEA

bit is set, the interface can still acknowledge its own Slave address or the general call address

by using the Two-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 Two-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 77. Status Codes for Slave Transmitter Mode

Status Code

(TWSR)

Prescaler Bits

are 0

Status of the Two-wire Serial Bus

and Two-wire Serial Interface

Hardware

Application Software Response

Next Action Taken by TWI Hardware

To/from TWDR To TWCR

STA STO TWINT TWEA

$A8 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

$B0 Arbitration lost in SLA+R/W as

Master; 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

$B8 Data byte in TWDR has been

transmitted; ACK has been

received

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

$C0 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

$C8 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

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Figure 93. Formats and States in the Slave Transmitter Mode

Miscellaneous States There are two status codes that do not correspond to a defined TWI state, see Table 78.

Status $F8 indica tes that no re levant inform ation is a vailable because the TWINT Flag is not set.

This occurs between other states, and when the TWI is not involved in a serial transfer.

Status $00 indicates that a bus error has occurred during a Two-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.

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

Table 78. Miscellaneous States

Status Code

(TWSR)

Prescaler Bits

are 0

Status of the Two-wire Serial

Bus and Two-wire Serial Inter-

face Hardware

Application Software Response

Next Action Taken by TWI Hardware

To/from TWDR To TWCR

STA STO TWINT TWEA

$F8 No relevant state information

available; TWINT = “0” No TWDR action No TWCR action Wait or proceed current transfer

$00 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.

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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 tran smitted both fro m Master to Slave and vice versa. The Maste r must instr uct

the Slave what location it wants to read, requiring the use of the MT mode. Subsequently, data

must be read fr om the Slave, impl ying the use of the MR mode. Thus, the transf er 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 sys-

tem, 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 owne rship of the bus. The following

figure shows the flow in this transfer.

Figure 94. Combining Several TWI Modes to Access a Serial EEPROM

Multi-master

Systems and

Arbitration

If multiple Master s are connected to the same bu s, transmissions may be initiated sim ultane-

ously 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 95. An Arbitration Example

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 RsSLA+R A DATA A P

Device 1

MASTER

TRANSMITTER

Device 2

MASTER

TRANSMITTER

Device 3

SLAVE

RECEIVER

Device n

SDA

SCL

........

R1 R2

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ATmega16(L)

Several different scenarios may arise during arbitration, as described below:

• Two or more Masters are per forming 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 Mast ers are accessing the same Slav e wit h diff erent data or direction b it. 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 soft ware action.

• Two or more Master s are accessing different Slav es . In this case , arbit ration will occu r 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, d epending on the v alue of the READ/WRITE bit. If the y are not being addre ssed, they

will s witch to not addressed Slave mode or wait until the bus is free and transmit a new

START condit ion, depending on application software action.

This is summarized in Figure 96. Possible status values are given in circles.

Figure 96. 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 DataSTOP

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ATmega16(L)

Analog

Comparator The Analog Comparat or compares the input values on the positive pin AIN0 and negative pin

AIN1. When the voltage on the positive pin AIN0 is higher than the voltage on the negative pin

AIN1, 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. Th e user can select Interrupt triggering on com-

parator output rise, fall or toggle. A block diagram of the comparator and its surrounding logic is

shown in Figure 97.

Figure 97. Analog Comparator Block Diagram(2)

Notes: 1. See Table 80 on pa ge 20 3.

2. Refer to Figure 1 on page 2 and Table 25 on page 58 for Analog Comparator pin placement.

Special Function IO

• Bit 3 – 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 selects the negative input to the Analog Comparator. When this bit is written

logic zero, AIN1 is applied to the negative input of the Analog Comparator. For a detailed

description of this bit, see “Analog Comparator Multiplexed Input” on page 203.

ACBG

BANDGAP

REFERENCE

ADC MULTIPLEXER

OUTPUT

ACME

ADEN

(1)

Bit 7 6 5 4 3 2 1 0

ADTS2 ADTS1 ADTS0 – ACME PUD PSR2 PSR10 SFIOR

Read/Write R/W R/W 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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Analog Comparator

Control and Status

• Bit 7 – ACD: Analog Comparator Disable

When this bit is written logic one, the power to the Ana log Comparator is swit ched 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 Select

When this bit is set, a fixed bandgap reference voltage replaces the positive input to the Analog

Comparator. When this bit is cleared, AI N0 is ap plied to the positive input of the Analog Compar-

ator. See “Internal Voltage Reference” on page 42.

• 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 an d ACIS0 . Th e Ana lo g Comparator In te rrupt 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 inter-

rupt handling ve ctor. Alternatively, ACI is cleared by writing a logic one to the flag.

• Bit 3 – ACIE: Analog Comparator Inter r upt Enable

When the ACIE bit is writte n logic one and t he I-bit in the Status Register is set, the Analog Com-

parator Interrupt is activated. When written logic zero, the interrupt is disabled.

• Bit 2 – ACIC: Analog Comparator Input Capture Enable

When written logic one, this bit enables the Input Capture function in Timer/Counter1 to be trig-

gered by the Ana log Comp arat or. The compar ator o utput is in this case directly connecte d to the

Input Capture front-end logic, making the comparator utilize the noise canceler and edge select

features of the Timer/Cou nter1 Input Capture interru pt. When written logic zero, no connection

between the Analog Compa ra to r and t he I np ut Ca pt ure f unct ion e xists. To make the comp ar ator

trigger the Time r/Counter1 Input Captur e interrupt, the TICIE1 bit in the Timer Inte rrupt Mask

• Bits 1, 0 – ACIS1, ACIS0: Analog Comparator Interrupt Mode Select

These bits determin e which comparator even ts that trigger the Analog Compa rator interr upt. The

different settings are shown in Table 79.

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 Value00N/A00000

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ATmega16(L)

When changing the ACIS1/ACIS0 bits, the Analog Comparator Interrupt must be disabled by

clearing its Inte rrupt Enable bit in the ACSR Reg ister. Ot herwise an inter rupt can occu r when the

bits are changed.

Analog

Comparator

Multiplexed Input

It is possible to select any of the ADC7..0 pins to replace the negative input to the Analog Com-

parator. 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

SFIOR) is set and the ADC is switched off (ADEN in ADCSRA is zero), MUX2..0 in ADMUX

select the input pin to replace the negative input to the Analog Comparator, as shown in Table

80. If ACME is cleared or ADEN is set, AIN1 is applied to the negative input to the Analog

Comparator.

Table 79. ACIS1/ACIS0 Settings

ACIS1 ACIS0 Interrupt Mode

0 0 Comparator Interrupt on Output Toggle

01Reserved

1 0 Comparator Interrupt on Falling Output Edge

1 1 Comparator Interrupt on Risi ng Output Edge

Table 80. Analog Comparator Multiplexed Input

ACME ADEN MUX2..0 Analog Comparator Negative Input

0 x xxx AIN1

1 1 xxx AIN1

1 0 000 ADC0

1 0 001 ADC1

1 0 010 ADC2

1 0 011 ADC3

1 0 100 ADC4

1 0 101 ADC5

1 0 110 ADC6

1 0 111 ADC7

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ATmega16(L)

Analog to

Digital

Converter

Features •10-bit Resolution

•0.5 LSB Integral Non-linearity

•±2 LSB Absolute Accuracy

•13 µs- 260 µs Conversion Time

•Up to 15 kSPS at Maximum Resolution

•8 Multiplexed Single Ended Input Channels

•7 Differential Input Channels

•2 Differential Input Channels with Optional Gain of 10x and 200x

•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 ATmega16 featur es a 10-bit successive approximation ADC. The ADC is connected to an

8-channel Analog Multiplexer which allows 8 single-ended voltage inputs constructed from the

pins of Port A. The single-ended voltage inputs refer to 0V (G ND).

The device also suppor ts 16 differenti al voltage input co mbinations. Two of the differe ntial inputs

(ADC1, ADC0 and ADC3, ADC2) are equipped with a programmable gain stage, providing

amplification steps of 0 dB (1x), 20 dB (10x), or 46 dB (200x) on the differential input voltage

before the A/D conversion. Seven differential analog in put channels share a common negative

terminal (ADC1), while any other ADC input can be selected as the positive input terminal. If 1x

or 10x gain is used, 8-bit resolu tion can be expec ted. If 200x gain is used, 7 -bit resol ution 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 98.

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 211 on how to connect this

pin.

Internal reference volt ages of nominally 2.56V or AVCC are provided On-chip. The voltage re fer-

ence may be externally decoupled at the AREF pin by a capacitor for better noise performance.

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ATmega16(L)

Figure 98. Analog to Digital Converter Block Schem atic

Operation The ADC converts an analog input voltage to a 10-bit digital value through successive approxi-

mation. 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 deco upled by an external capacitor at the AREF pin to im prove

noise immunity.

The analog input channel and differential gain are selected by writing to the MUX bits in

ADMUX. Any of the ADC inp ut pins, a s well as GND and a fixe d band gap voltag e refer ence, can

be selected as single ended inputs t o the ADC. A selection of ADC input pins can be selected as

positive and negative inputs to the differential gain amplifier.

If differential channels are selected, the differential gain stage amplifies the voltage difference

between the selected input channel pair by the selected gain factor. This amplified value then

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 2.56V

REFERENCE

MUX DECODER

MUX4

AVCC

ADC7

ADC6

ADC5

ADC4

ADC3

ADC2

ADC1

ADC0

REFS0

REFS1

ADLAR

CHANNEL SELECTION

GAIN SELECTION

ADC[9:0]

ADC MULTIPLEXER

OUTPUT

GAIN

AMPLIFIER

AREF

BANDGAP

REFERENCE

PRESCALER

SINGLE ENDED / DIFFERENTIAL SELECTION

GND

POS.

INPUT

MUX

NEG.

INPUT

MUX

TRIGGER

SELECT

ADTS[2:0]

INTERRUPT

FLAGS

START

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becomes the analog input to the ADC. If single ended channels are used, the gain amplifier is

bypassed altogether .

The ADC is enabled by setting the ADC Enable bit, ADEN in ADCSRA. Voltage refe rence 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 recommende d to switch off the ADC befor e 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 ad justed, but can optionally be presented left

adjusted by setting the ADLAR bit in ADMUX.

If the result is left ad justed and no more than 8-bit precision is requ ired, 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, an d a conversion completes b efore 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 interr upt which can be trigge red when a conversion co mpletes. When ADC

access to the Data Registers is prohibited be tween reading of ADCH and ADCL, the interrupt

will trigger even if the result is lost.

Star ting 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 w ill be cleared by hardware

when the conversio n is com pleted . If a dif fere nt data channel is selected wh ile a co nver sion 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 SFIOR (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 con-

versions at fixed intervals. If the trigger signal still is set when the conversion completes, a new

conversion will not be started. If another positive edge occurs on the trigger signal during con-

version, 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 with out causing an interr upt. However, the Interrup t Flag must be cleare d in order to

trigger a new conversion at the next interrupt event.

Figure 99. ADC Auto Trigger Logic

ADSC

ADIF

SOURCE 1

SOURCE n

ADTS[2:0]

CONVERSION

LOGIC

PRESCALER

START CLKADC

.EDGE

DETECTOR

ADATE

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ATmega16(L)

Using the ADC Interrupt Flag as a trigger source make s the ADC start a new conver sion as soon

as the ongoing conversion has finished. The ADC then operates in Free Running mode, con-

stantly sampling and up dating 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 us ed to determine if a conversion is in pr ogress. The ADSC bit will be

read as one during a conversion, independently of how the conversion was started.

Prescaling and

Conversion Timing Figure 100. ADC Prescaler

By default, the successive approximation circuitry requires an input clock frequency between 50

kHz and 200 kHz to get maximum resolution. If a lower resolution than 10 bits is needed, the

input clock frequency to the ADC can be higher than 200 kHz to get a higher sample rate.

The ADC module contains a prescaler, which generates an acceptable ADC clock frequency

from any CPU frequency above 100 kHz. 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 runnin g 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 Gain Channels” on

page 209 for details on differential conversion timing.

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-h old ta kes pla ce 1. 5 ADC clock cycles aft er t he sta rt of a norma l con ver-

sion and 13.5 ADC clock cycles after the start of a 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 2 ADC clock cycles after the rising edge on the trigger source sig nal. Three addi-

tional CPU cloc k c ycle s ar e used for sync hr on iza tion logic. When u sin g Dif fer e ntia l mo de , alo n g

with Auto triggering from a so urce other than the ADC Conversion Com plete, each conversion

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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will require 25 ADC clocks. This is because the ADC must be disabled and re-enabled after

every conversion.

In Free Running mode, a new conversion will be started immediately after the conversion com-

pletes, while ADSC remains high . For a summary of conversion times, see Table 81.

Figure 101. ADC Timing Diagram, First Conversion (Single Conversion Mode)

Figure 102. ADC Timing Diagram, Single Conversion

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

12345678910 11 12 13

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 103. ADC Timing Diagram, Auto Triggered Conversion

Figure 104. ADC Timing Diagram, Free Running Conversion

Differential Gain

Channels When using differential gain channels, certain aspects of the conversion need to be taken into

consideration.

Differential conversions are synchronized to the internal clock CKADC2 equal to ha lf the ADC

clock. This synchronization is done automatically by the ADC interface in such a way that the

sample-and-hold occu rs at a specific phase of CKADC2. A conversion initiated by the user (that is,

Table 81. ADC Conversion Time

Condition

Sample & Hold (Cycles

from Start o f

Conversion) Conversion Time (Cycles)

First conversion 13.5 25

Normal conversions, single ended 1.5 13

A uto Triggered conv ersions 2 13.5

Nor m al conversions, differential 1.5/2.5 13/14

12 3 4 5 6 7 8 910 11 12 13

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

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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all single conversions, and the first free running conversion) when CKADC2 is low will take the

same amount of time as a sin gle ended conversion (13 ADC clock cycles from the ne xt pres-

caled clock cycle). A conversion init iated by the user when CK ADC2 is high will take 14 ADC clock

cycles due to the synchronization mechanism. In Free Running mode, a new conversion is initi-

ated immediately after the previous conversion completes, and since CKADC2 is high at this time,

all automatically started (that is, all but the first) free running conversions will take 14 ADC clock

cycles.

The gain stage is optimized f or a bandwidth of 4 kHz at all gain sett ings. Highe r 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. For example, the ADC

clock period may be 6 µs, allowing a channel to be sampled at 12 kSPS, regardless of the band-

width of this channel.

If differential gain chan nels are used and conversion s are started by Auto Triggering, th e ADC

must be swit ch ed o ff be tw ee n conversions. Wh e n Auto Trigge ring is used, th e ADC prescaler is

reset before the conversion is started. Since the gain 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” th en to “1”) , only extended con-

versions are performed. The result from the extended conversions will be valid. S ee “Prescaling

and Conversion Timing” on page 207 for timing details.

Changing Channel

or Reference

Selection

The MUXn and REFS1:0 bits in the ADMUX Register are single buffered through a temporary

selection only takes place at a safe point during the conversion. The channel and reference

selection is continu ously updated until a conve rsion is started. Once the conver sion starts, the

channel and reference se lection is locked to e nsure a sufficient sampling t ime for the ADC. Co n-

tinuous updating resumes in the last ADC clock cycle before the conversion completes (ADIF in

ADCSRA is set). Note tha t 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 refe rence select ion values

to ADMUX until one ADC clock cycle after ADSC is written.

If Auto Triggering is used, the exact tim e 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 te ll if the next conversion is based

on the old or the new settings. ADMUX can be safely updated in the following ways:

1. When ADATE or ADEN is cleared.

2. During conversion, mi nimum one ADC clock cycle after the trigger event.

3. 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 gain stage may take as much as 125 µs to stabilize to the ne w value.

Thus conversions should not be started within the first 125 µs after selecting a new differentia l

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).

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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 chann el before sta rting the conversion. The chan-

nel selection may be changed one ADC clock cycle after writing one to ADSC. However, the

simplest method is t o wa it for the co nversio n to complete b efore cha nging the ch annel sele ction.

In Free Running mode, always select the channel before starting the first conversion. The chan-

nel 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 cha nnel

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.

When switching to a differential gain channel, the first conversion result may have a poor accu-

racy due to the required settling time for the automatic offset cance llation circuitry. The user

should preferably disregard the first conversion result.

ADC Voltage

Reference The reference volt age 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 t he ADC th ro ugh a passive switch. T he inter nal 2.56V ref e renc e is g ener-

ated 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 gro und. VREF can

also be measured at the AREF pin with a high impedant voltmeter. Note that VREF is a high

impedant sour ce, and only a capacitive load should be connected in a system.

If the user has a fixed volt age source connect ed to the AREF pin, the user ma y not use the oth er

reference voltage options in the application, as they will be shorted to the external voltage. If no

external voltage is app lied 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 122 on page 297.

ADC Noise

Canceler The ADC features a noise canceler that enables conversion during sleep mode to reduce noise

induced from the CPU core and o ther I/O pe ripherals. The noise ca nceler can be used with ADC

Noise Reduc tion an d Id le m ode. To m ake use of th is fe atur e, th e fo llowing proce dur e sh ould b e

used:

1. Make sure that the ADC is enabled and is not busy converting. Single Conversion

Mode must be select ed and the ADC conversion complete interrupt must be

enabled.

2. Enter ADC Noise Reduction mode (or Idle mode). The ADC will start a conversion

once the CPU has been halted.

3. If no other interrupts occur before the ADC conversion completes, the ADC interrupt

will wak e up the CPU and e x ecute the ADC Conv ersion Complete interrupt routine. If

another interrupt wakes up the CPU before the ADC conversion is complete, that

interrupt will be ex ecuted, and an ADC Conv ersion Complete interrupt request will be

generated when the ADC con version 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 enter-

ing such sleep modes to avoid excessive power consumption. If the ADC is en abled in such

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sleep modes and the user wants to perform differential conversions, the user is advised to

switch the ADC off an d on aft er wa king up f rom sleep to prom pt an ex tende d conversion to ge t a

valid result.

Analog Input Circuitry The Analog Input Circuitry for single ended chan nels is illustrated in Figure 105. An analog

source applied to ADCn is subjecte d to the pin ca pacitance and inp ut leakage of that pin, re gard-

less of whether that channel is selected as input for the ADC. When the channel is selected, the

source must dr ive the S/H capacitor th rough the series re sistance (combined resistance in the

input path).

The ADC is optimized for analog signals with an output impedance of approximately 10 kΩ or

less. If such a source is used, the samp ling time wi ll be n egligible . I f a sour ce with higher impe d-

ance 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 impedant 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 kΩ 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 105. Analog Input Circuitry

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:

1. Keep analog signal paths as short as possible. Keep them well away from high-

speed switching digital tracks.

2. The AVCC pin on the device should be connected to the digital VCC supply v o ltage

via an LC network as shown in Figure 106.

3. Use the ADC noise canceler function to reduce induced noise from the CPU.

4. 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

IIH

1..100 kΩ

CS/H= 14 pF

VCC/2

IIL

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Figure 106. ADC Power Connections

Offset Compensation

Schemes The gain stage has a built-in offset cancellation circuitry that nulls the offset of differential mea-

surements as much as possible. The remaining offset in the analog path can be measured

directly by selecting the same channe l for both d ifferent ial inputs. T his offset resid ue 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.

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 2 n-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). Idea l value: 0 LSB.

GND

VCC

PA0 (ADC0)

PA1 (ADC1)

PA2 (ADC2)

PA3 (ADC3)

PA4 (ADC4)

PA5 (ADC5)

PA6 (ADC6)

PA7 (ADC7)

AREF

AVCC

GND

PC7

10 µH

100 nF

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Figure 107. 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 108. Gain Err or

• 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 109. Integral Non-linearity (INL)

• Differential Non-linearity (DNL): The maximum deviation of the actual code width (the

interval betw een two adjacent transitions) from the ideal code width (1 LSB). Ideal value: 0

LSB.

Figure 110. Differential Non-linearity (DNL)

• Quantization Error: Due to the qu antization of th e input v oltage int o a finite n umber of co des,

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.

Output Code

REF

Input Voltage

Ideal ADC

Actual ADC

INL

Output Code

0x3FF

0x000

REF

Input Voltage

DNL

1 LSB

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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

where VIN is the voltage on the selected input pin and VREF the selected voltage reference (see

Table 83 on page 217 and Table 84 on page 218). 0x000 represents ground, and 0x3FF repre-

sents 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 inp ut pin,

GAIN the select ed gain fact or, 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 qu ick polarit y check of the r esults, it is sufficient to read the MSB of the r esult

(ADC9 in ADCH). If this bit is one, the result is negative, and if this bit is zero, the res ult is posi-

tive. Figure 111 shows the decoding of the differential input range.

Table 82 shows the result ing output codes if the differe ntial input channel pa ir (ADCn - ADCm) is

selected with a gain of GAIN and a reference voltage of VREF.

Figure 111. Differential Measurement Range

ADC VIN 1024⋅

VREF

--------------------------=

ADC VPOS VNEG

–()GAIN 512⋅⋅

VREF

------------------------------------------------------------------------=

Output Code

0x1FF

0x000

REF

/GAIN Differential Input

Voltage (Volts)

0x3FF

0x200

- V

REF

/GAIN

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Example:

ADMUX = 0xED (ADC3 - ADC2, 10x gain, 2.56V reference, left adjusted result)

Voltage on ADC3 is 300 mV, voltage on ADC2 is 500 mV.

ADCR = 512 × 10 × (300 - 500) / 2560 = -400 = 0x270

ADCL will thus read 0x00, and ADCH will read 0x9C. Writing zero to ADLAR right adjusts

the result: ADCL = 0x70, ADCH = 0x02.

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 Table 83. 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 ADL AR to left adjust the resu lt. Oth erwise, the r esult is ri ght adju sted. Changing t he

ADLAR bit will affect the ADC Data Register immediately, regardless of any ongoing conver-

Table 82. Correlation between Input Voltage and Output Codes

VADCn Read code Corresponding Decimal Value

VADCm + VREF/GAIN 0x1FF 511

VADCm + 511/512 VREF/GAIN 0x1FF 511

VADCm + 510/512 VREF/GAIN 0x1FE 510

... ... ...

VADCm + 1/512 VREF/GAIN 0x001 1

VADCm 0x000 0

VADCm - 1/512 VREF/GAIN 0x3FF -1

... ... ...

VADCm - 511/512 VREF/GAIN 0x201 -511

VADCm - VREF/GAIN 0x200 -512

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 83. Voltage Refere nc e Sele ctio n s for ADC

REFS1 REFS0 Voltage Reference Selection

0 0 AREF, Internal Vref turned off

0 1 AVCC with external capacitor at AREF pin

10Reserved

1 1 Internal 2.56V Voltage Reference with external capacitor at AREF pin

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sions. For a complet e description of this bit, see “The ADC Data Register – ADCL and ADCH” on

page 220.

• Bits 4:0 – MUX4:0: Analog Channel and Gain Selection Bits

The value of these bits selects which combination of analog inputs are connected to the ADC.

These bits also select th e gain f or t he dif fer ential chan nels. Se e Ta ble 84 for details. 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).

Table 84. Input Channel and Gain Selections

MUX4..0 Single Ended

Input Positive Differential

Input Negative Differential

Input Gain

00000 ADC0

00001 ADC1

00010 ADC2

00011 ADC3 N/A

00100 ADC4

00101 ADC5

00110 ADC6

00111 ADC7

01000 ADC0 ADC0 10x

01001 ADC1 ADC0 10x

01010 ADC0 ADC0 200x

01011 ADC1 ADC0 200x

01100 ADC2 ADC2 10x

01101 ADC3 ADC2 10x

01110 ADC2 ADC2 200x

01111 ADC3 ADC2 200x

10000 ADC0 ADC1 1x

10001 ADC1 ADC1 1x

10010 N/A ADC2 ADC1 1x

10011 ADC3 ADC1 1x

10100 ADC4 ADC1 1x

10101 ADC5 ADC1 1x

10110 ADC6 ADC1 1x

10111 ADC7 ADC1 1x

11000 ADC0 ADC2 1x

11001 ADC1 ADC2 1x

11010 ADC2 ADC2 1x

11011 ADC3 ADC2 1x

11100 ADC4 ADC2 1x

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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.

• Bit 6 – ADSC: ADC Start Conversion

In Single Conversion mod e, write this bit t o one to start ea ch conversion. In Free Running Mode,

write this bit to one to st art th e first conversion. Th e first conversion af ter ADSC ha s been writt en

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 initializa-

tion 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 con-

version on a positive e dge of the select ed trigger signal. The trigger sou rce is selected by setting

the ADC Trigger Select bits, ADTS in SFIOR.

• Bit 4 – ADIF: ADC Interrupt Flag

This bit is set when an ADC conversion completes and the Data Re gisters 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. Alter-

natively, 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 ar e used.

• Bit 3 – ADIE: ADC Interrupt Enable

When this bit is wr itte n to on e a n d t he I- bit in SREG is set, the ADC Conversion Complete Inter-

rupt 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.

11101 ADC5 ADC2 1x

11110 1.22V (VBG)N/A

11111 0 V (GND)

Table 84. Input Channel and Gain Selections (Continued)

MUX4..0 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 Value00000000

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The ADC Data

ADCH

ADLAR = 0

ADLAR = 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 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 ADLA R is set, the result is left adjusted. If ADLAR is cleared (default), th e 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 216.

Table 85. ADC Prescaler Selections

ADPS2 ADPS1 ADPS0 Division Factor

000 2

001 2

010 4

011 8

100 16

101 32

110 64

1 1 1 128

Bit 151413121110 9 8

– – – – – – ADC9 ADC8 ADCH

ADC7 ADC6 ADC5 ADC4 ADC3 ADC2 ADC1 ADC0 ADCL

76543210

Read/WriteRRRRRRRR

RRRRRRRR

Initial Value00000000

00000000

Bit 151413121110 9 8

ADC9 ADC8 ADC7 ADC6 ADC5 ADC4 ADC3 ADC2 ADCH

ADC1 ADC0 – –––––ADCL

76543210

Read/WriteRRRRRRRR

RRRRRRRR

Initial Value00000000

00000000

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Special FunctionIO

• Bit 7:5 – ADTS2:0: 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 ADTS2:0 settings will have no effect. A conversion

will be triggered by the rising edge of the selected Interrupt Flag. Note that switching from a trig-

ger 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[2:0]=0) will not cause a trigger event, even if the ADC Interrupt Flag is set.

• Bit 4 – Res: Reserved Bit

This bit is reserved for future use. To ensure compatibility with future devices, this bit must be

written to zero when SFIOR is written.

Bit 76543210

ADTS2 ADTS1 ADTS0 – ACME PUD PSR2 PSR10 SFIOR

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

Initial Value 0 0 0 0 0 0 0 0

Table 86. ADC Auto Trigger Source Selections

ADTS2 ADTS1 ADTS0 Trigger Source

0 0 0 Free Running mode

0 0 1 Analog Compa rator

0 1 0 External Interrupt Request 0

0 1 1 Timer/Counter0 Compare Match

1 0 0 Timer/Counter0 Overflow

1 0 1 Timer/Counter1 Compare Match B

1 1 0 Timer/Counter1 Overflow

1 1 1 Timer/Counter1 Capture Event

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JTAG Interface

and On-chip

Debug System

Features •JTAG (IEEE std. 1149.1 Compliant) Interface

•Boundary-scan Capabilities According to the IEEE std. 1149.1 (JTAG) Standard

•Debugger Access to:

– All Internal Peripheral Units

– Internal and External RAM

– The Internal Register File

– Program Counter

– EEPROM and Flash Memories

– Extensive On-chip Debug Support for Break Conditions, Including

–AVR Break Instruction

– Break on Change of Program Memory Flow

– Single Step Break

– Program Memory Breakpoints on Single Address or Address Range

– Data Memory Breakpoints on Single Address or Address Rang e

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

•On-chip Debugging Supported by AVR Studio®

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 an d Lock bits

• On-chip Debugging

A brief description is given in the following sections. Detailed descriptions for Pro gramming via

the JTAG interface, and using the Boundary-scan Chain can be found in the sections “Program-

ming via the JTAG Interface” on page 278 and “IEEE 1149.1 (JTAG) Boundary-scan” on page

228, r espectively. The On-chip Debug support is considered being private JTAG instr uctions,

and distributed within ATMEL and to select ed third party vendors only.

Figure 112 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 Register or one of several Data Registers as the scan chain

(Shift Register) between the TDI input and TDO 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-leve l te sting . The JTAG Pr og ram ming In te rface ( actually consisting of several physical

and virtual Data Registers) is used for JTAG Serial Progra mming via the JTAG interface. The

Internal Scan Chain and Break Point Scan Chain are used for On-chip Debugging only.

Test Access P ort –

TAP The JTAG interface is accessed thro ugh fou r of t he AVR’s pins . In JTAG termino logy, the se 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

• 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 and the JTD bit in MCUCSR is cleared, the TAP

input signals are internally pulled high and the JTAG is enabled for Boundary-scan and program-

ming. In this case, the TAP output pin (TDO) is left floating in states where the JTAG TAP

controller is not shifting data, and must therefore be connected to a pull-up resistor or other

hardware having pull-ups (for instance the TDI-input of the next device in the scan chain). The

device is shipp ed with this fuse programmed .

For the On-chip Debug system, in addition to the JTAG interface pins, the RESET pin is moni-

tored 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 112. 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 113. TAP Controller State Diagram

TAP Controller The TAP controller is a 1 6- sta te f init e st ate m achin e tha t cont ro ls the o per at ion o f t he Bou nd ary-

scan circuitry, JTAG programming circuitry, or On-chip Debug system. The state transitions

depicted in Figure 11 3 depend on the signa l present on TMS (sho wn adjacent to e ach state tran-

sition) 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 docum e nt, the LSB is shifte d in an d out firs t for all Shift Reg ist er s.

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 o f TCK to enter the Shift

Instruction Register – Shift-IR state. While in this state, shift the four bits of the JTAG

instructions into the JTAG Instruction Register from the TDI input at the rising edge of TCK.

The TMS input must be h eld low dur ing inp ut of the 3 LSBs in or der to rem ain in th e Shif t-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

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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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-I R states are only used for navigating the state machine.

• At the TMS input, apply the sequence 1, 0, 0 at the rising edges of TCK to enter the Shift

Data Register – Shift-DR stat e. 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 in puts to the Data Register cap tured in the Captur e-DR state is shifted out on t he

TDO pin.

• Apply the TMS sequence 1, 1, 0 to re-enter the Run-Test/Idle state. If the selected Data

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 betwe en 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 227.

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 228.

Using the On-chip

Debug System As shown in Figure 112, the hardwar e su pp or t for On- ch ip De bu g gin g co ns ists 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 locat ion which is par t of the communicat ion interface be tween the CPU and t he

JTAG system.

The Break Point Unit implements Break on Change of Program Flow, Single Step Break, 2 Pro-

gram Memory Break Points, and 2 combined Break Points. Together, the 4 Break Points can be

configured as either:

• 4 single Progra m Memory Break Points

• 3 Single Program Memory Break Point + 1 single Data Memory Break Point

• 2 single Program Memory Break Points + 2 single Data Memory Break Points

• 2 single Progr am Memory Break Points + 1 Program Memory Break P oint with ma sk (“range

Break Point”)

• 2 single Program Memory Break Points + 1 Data Memory Break Point with mask (“range

Break Point”)

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A debugger, like the AVR Studio, may however use one or more of these resources for its inter-

nal 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 226.

The JTAGEN Fuse must be programmed to enable the JTAG Test Access Port. In addition, the

OCDEN Fuse must be pro gramm ed and no Lock bit s must be set fo r the On -chip Debu g syst em

to work. As a secur ity feature , the On-ch ip Debug system is disabled when any Lock bit s are set.

Otherwise, the On -chip Debug system would have provided a back-door into a secured device.

The AVR JTAG ICE from Atmel is a powerful developm ent tool for On-chip Debugging of all

AVR 8-bit RISC Microcontrollers with IEEE 1149.1 compliant JTAG interface. The JTAG ICE

and the AVR Studio user interfa ce give the user complete cont rol of the int ernal resour ces of the

microcontroller, helping to reduce development time by making debugging easier. The JTAG

ICE performs real-time emulation of the microcontroller while it is running in a target system.

Please refer to the Support Tools section on the AVR pages on www.atmel.com for a full

description of the AVR JTEG ICE. AVR Studio can be downloaded free from Software section

on the same web site.

All necessary executio n commands are available in AVR Stud io, both on source level and on

disassembly level. T he user can execute the pr ogram, single step throug h the code either by

tracing into or stepping over functions, st ep out of funct ions, place the cur sor on a stateme nt 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 breakpoints (using the BREAK instruc-

tion) and up to two data memory brea kpoints, alternatively combined as a mask (rang e) Break

Point.

On-chip Debug

Specific JTAG

Instructions

The On-chip Debu g support is cons idered being private JTAG instructions, and distributed within

ATMEL and to selected third party vendors only. Instruction opcodes are listed for reference.

PRIVATE0; $8 Private JTAG instruction for accessing On-chip Debug system.

PRIVATE1; $9 Private JTAG instruction for accessing On-chip Debug system.

PRIVATE2; $A Private JTAG instruction for accessing On-chip Debug system.

PRIVATE3; $B Private JTAG instruction for accessing On-chip Debug system.

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On-chip Debug

Related Register in

I/O Memory

On-chip Debug

The OCDR Register provides a communication channel from the running program in the micro-

controller 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 reg ist er has been wr it te n. When th e CPU re ads the OCDR Regist er the

7 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 shar ed with a standard I/O location . In this case, the OCDR

access to the OCDR Register. In all other cases, the standard I/O location is accessed.

Refer to the debugger documentation for further information on how to use this register.

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 program-

ming (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 MCUSR 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 278.

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

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 Value 0 0000000

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IEEE 1149.1

(JTAG)

Boundary-scan

Features •JTAG (IEEE std. 1149.1 Compliant) Interface

•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

System Overview The Boundary-scan chain has the capability of driving and observing the logic levels on the digi-

tal 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 Register. An external controller sets up the devices to

drive values at their output pins, and observe the input values received from other devices. The

controller compares the received data with the expected result. In this way, Boundary-scan pro-

vides 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/PRE-

LOAD, 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 ID CODE is the default JTAG instru ction. It may be desirabl e 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 instruc-

tion with appropriate setting of the Reset Data Register.

The EXTEST instruction is used for sampling external pins and lo ading 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-Register. Therefore, the SAMPLE/PRELOAD should also be used for

setting initial value s to the scan r ing, to avoid da maging the board when issuing the EXT EST

instruction for the first tim e. SAMPLE/PRELOAD can also be used for taking a snapshot of the

external pins duri ng normal operation of th e part.

The JTAGEN Fuse must be programmed and the JTD bit in the I/O Register MCUCSR must be

cleared to enable the JTAG Test Access Port.

When using the JTAG interface for Boundary-scan, using a JTAG T CK clock frequency higher

than the internal chip frequency is possible. The chip clock is not required to run.

Data Registers The Data Registers relevant for Boundary-scan operations are:

• Bypass Register

• Device Identification Registe r

• Reset Register

• Boundary-scan Chain

Bypass Register 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

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controller state. The Bypass Register can be used to shorten the scan chain on a system when

the other devices are to be tested.

Device Identification

Figure 114. The Format of the Device Identification Register

Version Version is a 4-bit number identifying the revision of the com ponent. The JTAG version number

follows the revision of the device. Revision A is 0x0, revision B is 0x1 and so on. However, some

revisions deviate from this rule, and the relevant version number is shown in Table 87.

Par t Number The part number is a 16-bit code identifying the component. The JTAG Part Number for

ATmega16 is listed in Table 88.

Manufacturer ID The Manufacturer ID is a 11 bit code identifying the manufacturer. The JTAG manufacturer ID

for ATMEL is listed in Table 89.

Reset Register The Reset Register is a Test Data Register used to reset the part. Since the AVR tri-states Port

Pins when reset, the Reset Re gister can a lso replace t he functi on of th e unimplement ed opt ional

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 set-

tings for the clock options, the part will remain reset for a Reset Time-Out Period (refer to “Clock

Sources” on page 25) after releasing the Reset Register. The output from this Data Register is

not latched, so the reset will take place immediately, as shown in Figure 115.

MSB LSB

Bit 3128271211 1 0

Device ID Version Part Number Manufacturer ID 1

4 bits 16 bits 11 bits 1 bit

Table 87. JTAG Version Numbers

Versio n JTAG V e rsion Number (He x)

ATmega16 re vision G 0x6

ATmega16 re vision H 0xE

ATmega16 re vision I 0x8

ATmega16 re vision J 0x9

ATmega16 re vision K 0xA

ATmega16 re vision L 0xB

Table 88. AVR JTAG Part Number

P art Number JTAG Part Number (Hex)

ATmega16 0x9403

Table 89. Manufacturer ID

Manufacturer JTAG Manufacturer ID (Hex)

ATMEL 0x01F

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Figure 115. Reset Register

Boundary-scan Chain The Boundary-scan Chain has the capability of driving and observing the logic levels on the dig-

ital 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 232 for a comple te desc rip tio n.

Boundary-scan

Specific JTAG

Instructions

The instruction reg ister is 4-bit wide, suppo rting up t o 16 instruction s. Listed b elow 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-impedant 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 Re gister is selected as path between TDI and TDO for each instruction.

EXTEST; $0 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 Inpu t Data are all accessible in the scan chain. For An alog circuits having Off- chip

connections, the int erface be tween t he analog an d th e digita l logic is in the scan chain . The con-

tents of the latche d outputs of the Bound ary-scan chain is driven out as soon as the JTAG IR-

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 shift ed by the TCK input.

• Update-DR: Data from the scan chain is applied to output pins.

IDCODE; $1 Optional JTAG instruction selecting the 32 -bit I D-re gister as D ata R egi ster . The ID-re giste r c on-

sists 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.

SAMPLE_PRELOAD;

$2 Mandatory JTAG instruction for pre-loading the output latches and talking 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.

From

TDI

ClockDR · AVR_RESET

TDO

From other Internal and

External Reset Sources

Internal Reset

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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.

AVR_RESET; $C 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 instruct ion. 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 Chai n. The output from this chain is not latched.

The active states are:

• Shift-DR: The Reset Register is shifted by the TCK input.

BYPASS; $F Mandatory JTAG instruction selecting the Bypass Register for Data Register.

The active states are:

• Capture-DR: Loa ds a logic “0” into the Bypass Register.

• Shift-DR: The Bypass Register cell between TDI and TDO is shifted.

Boundary-scan

Related Register in I/O

Memory

MCU Control and

Status Register –

MCUCSR

The MCU Control and Status Register contains control bits for general MCU functions, and pro-

vides information on which reset source caused an MCU Reset.

• Bit 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.

If the JTAG interface is left unconnected to other JTAG circuitry, the JTD bit should be set to

one. The reason for this is to avoid static current at the TDO pin in the JTAG interface.

• 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 76543210

JTD ISC2 –JTRFWDRF BORF EXTRF PORF MCUCSR

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

Initial Value 0 0 0 See Bit Description

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Boundary-scan

Chain The Boundary-scan chain has the capability of driving and observing the logic levels on the digi-

tal I/O pins, as well as the boundary between digital and analog logic for analog circuitry having

Off-chip connection.

Scanning the Digital

Port Pins Figure 116 shows the Boundary-scan Cell for a bi-directional port pin with pull-up function. The

cell consists of a standard Boundary-scan cell for the Pull-up Enable – PUExn – function, and a

bi-directional pin cell that combine s 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-sca n logic is not included in the f igures in the d atasheet. Figure 117 shows a sim-

ple digital Port Pin as described in the section “I/O Ports” on page 50. The Boundary-scan

details from Figure 116 replaces the dashed box in Figure 117.

When no alternate port function is present, the Input Data – ID – corresponds to the PINxn Re g-

ister value (but ID has no synchro nizer), Ou tput Data corre sponds to the PORT Regi ster, Ou tput

Control corresponds to the Dat a Direction – DD Register, and the Pull-up Enable – PUExn – cor-

responds to logic expression PUD · DDxn · PORTxn.

Digital alternate port functions are conn ected outside the dotted box in Figure 117 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, and a scan chain is inserted on the interface between the digi-

tal logic and the analog circuitry.

Figure 116. Boundary-scan Cell for Bidirectional Port Pin with Pull-up Function.

DQ DQ

1DQ DQ

Port Pin (PXn)

VccEXTESTTo Next CellShiftDR

Output Control (OC)

Pullup Enable (PUE)

Output Data (OD)

Input Data (ID)

From Last Cell UpdateDRClockDR

FF2 LD2

FF1 LD1

LD0FF0

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Figure 117. General Port Pin Schematic Diagram(1)

Note: 1. See Boundary-scan description for details.

Boundary-scan and

the Two-wire Interface The 2 Two-wire Interface pins SCL and SDA have one additional control signal in the scan-

chain; Two-wire Interface Enable – TWIEN. As shown in Figure 118, the TWIEN signal enables

a tri-state buffer with slew-rate control in parallel with the ordinary digital port pins. A general

scan cell as shown in Figure 122 is attached to the TWIEN signal.

Notes: 1. A separate scan chain for the 50 ns spike filter on the input is not provided. The ordinary scan

support for digital por t pins su ffice for connectivity tests. The only reason for having TWIEN in

the scan path, is to be able to disconnect the slew-rate control buffer when doing boundary-

scan.

2. Make sure the OC and TWIEN signals are not asserted simultaneously, as this will lead to

drive contention.

CLK

RPx

RRx

WPx

RDx

WDx

PUD

SYNCHRONIZER

WDx: WRITE DDRx

WPx: 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

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

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Figure 118. Additional Scan Signal for the Two-wire Interface

Scanning the RESET

Pin The RESET pin accepts 5V active low logic for standard reset operation, and 12V active hig h

logic for High Voltage Parallel Programm ing. An observe-only cell as shown in Figure 119 is

inserted both for the 5V reset signal; RSTT, and the 12V reset signal; RSTHV.

Figure 119. Observe-only Cell

Scanning the Clock

Pins The AVR devices have many clock options selectable by fuses. These are: Internal RC Oscilla-

tor, External RC, Externa l Clock, (High Frequency) Crystal Oscillator, Low Frequency Cry stal

Oscillator, and Ceramic Resonator.

Figure 120 shows how each Oscillator with external connection is supported in the scan chain.

The Enable signal is supported with a general boundary-scan cell, while the Oscillator/Clock out-

put is attached to an observe-only cell. In addition to the main clock, the Timer Oscillator is

scanned in the same way. The output from the internal RC Oscillator is not scanned, as this

Oscillator does not have external connections.

Pxn

PUExn

ODxn

IDxn

TWIEN

OCxn

Slew-rate Limited

SRC

From

Cell

ClockDR

ShiftDR

Cell

From System Pin To System Logic

FF1

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Figure 120. Boundary-scan Cells for Oscillators and Clock Options

Table 90 summaries the scan registers for the external clock pin XTAL1, Oscillators with

XTAL1/XTAL2 connections as well as 32 kHz Timer Oscillator.

Notes: 1. Do not enable more than one clock source as main clock at a time.

2. Scanning an Oscillator output gives unpredictable results as there is a frequency drift between

the Internal Oscillator and the JTAG TCK clock. If possible, scanning an external clock is

preferred.

3. The clock configuration is programmed by fuses. As a fuse is not changed r un-time, the clock

configuration is considered fixed for a given application. The user is advised to scan th e same

clock option as to be used in the final system. The enable signals are suppor ted in the scan

chain because the system logic can disable clock options in sleep modes, thereby disconnect-

ing the Oscillator pins from the scan path if not provided. The INTCAP Fuses are not

supported in the scan-chain, so the boundary scan chain can not make a XTAL Oscillator

requiring internal capacitors to run unless the fuse is correctly programmed.

Scanning the Analog

Comparator The relevant Comparator signals regarding Boundary-scan are shown in Figure 121. The

Boundary-scan cell from Figure 122 is attached to each of these signals. The signals are

described in Tab le 91.

The Comparator need not be used for pure connectivity testing, since all analog inputs are

shared with a digital port pin as well.

Table 90. Scan Signals for the Oscillators(1)(2)(3)

Enable Signal Scanned Clock Line Clock Option Scanned Clock Line

when not Used

EXTCLKEN EXTCLK (XTAL1) External Clock 0

OSCON OSCCK External Crystal

External Ceramic

Resonator

RCOSCEN RCCK External RC 1

OSC32EN OSC32CK Low Freq. External Crystal 0

TOSKON TOSCK 32 kHz Timer Oscillator 0

From

Cell

ClockDR

ShiftDR

Cell

To System Logic

FF1

DQ DQ

From

Cell

ClockDR UpdateDR

ShiftDR

Cell EXTEST

From Digital Logic

XTAL1/TOSC1 XTAL2/TOSC2

Oscillator

ENABLE OUTPUT

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Figure 121. Analog Comparator

Figure 122. General Boundary-scan Cell used for Signals for Comparator and ADC

ACBG

BANDGAP

REFERENCE

ADC MULTIPLEXER

OUTPUT

ACME

AC_IDLE

ACO

ADCEN

DQ DQ

From

Cell

ClockDR UpdateDR

ShiftDR

Cell EXTEST

To Analog Circuitry/

To Digital Logic

From Digital Logic/

From Analog Ciruitry

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Scanning the ADC

Figure 123 shows a block diagram of the ADC with all relevant control and observe signals. The Boundary-scan cell from

Figure 122 is attached to each of these signals. The ADC need not be used for pure connectivity testing, since all analog

inputs are shar ed with a dig ita l port pin as well.

Figure 123. Analog to Digital Converter

The signals are described briefly in Table 92.

Table 91. Boundary-scan Signals for the Analog Comparator

Signal

Name Direction as Seen from

the Comparator Description Recommended Input

when Not in Use Output Values when

Recommended Inputs are Used

AC_IDLE Input Turns off Analog

comparator when true 1 Depends upon µC code being

executed

A CO Output Analog Comparator

Output Will become input to µC

code being executed 0

A C ME Input Uses output signal from

ADC mux when true 0 Depends upon µC code being

executed

ACBG Input Bandgap Reference

enable 0 Depends upon µC code being

executed

10-bit DAC +

AREF

PRECH

DACOUT

COMP

MUXEN_7

ADC_7

MUXEN_6

ADC_6

MUXEN_5

ADC_5

MUXEN_4

ADC_4

MUXEN_3

ADC_3

MUXEN_2

ADC_2

MUXEN_1

ADC_1

MUXEN_0

ADC_0

NEGSEL_2

ADC_2

NEGSEL_1

ADC_1

NEGSEL_0

ADC_0

EXTCH

10x 20x

G10 G20

ACLK

AMPEN

2.56V

ref

IREFEN

AREF

VCCREN

DAC_9..0

ADCEN

HOLD

GNDEN

PASSEN

ACTEN

COMP

SCTEST ADCBGEN

To Comparator

1.22V

ref

AREF

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Table 92. Boundary-scan Signals for the ADC

Signal

Name Direction as Seen

from the ADC Description

Recommended

Input when Not

in Use

Output V alues when Recommended

Inputs are used, and CPU is not

Using the ADC

COMP Output Comparator Output 0 0

A C LK Input Clock signal to gain stages

implemented as Switch-cap filters 00

A C TEN Input Enable path from gain stages to

the compar a to r 00

ADCBGEN Input Enable Band-gap reference as

negative input to comparator 00

ADCEN Input Power-on signal to the ADC 0 0

AMPEN I nput Power-on signal to the gain stages 0 0

DAC_9 Input Bit 9 of digital value to DAC 1 1

DAC_8 Input Bit 8 of digital value to DAC 0 0

DAC_7 Input Bit 7 of digital value to DAC 0 0

DAC_6 Input Bit 6 of digital value to DAC 0 0

DAC_5 Input Bit 5 of digital value to DAC 0 0

DAC_4 Input Bit 4 of digital value to DAC 0 0

DAC_3 Input Bit 3 of digital value to DAC 0 0

DAC_2 Input Bit 2 of digital value to DAC 0 0

DAC_1 Input Bit 1 of digital value to DAC 0 0

DAC_0 Input Bit 0 of digital value to DAC 0 0

EXTCH Input Connect ADC channels 0 - 3 to by-

pass path around gain stages 11

G10 Input Enable 10x gain 0 0

G20 Input Enable 20x gain 0 0

GNDEN Input Ground the negative input to

comparator when true 00

HOLD Input Sample&Hold signal. Sample

analog signal when low. Hold

signal when high. If gain stages

are used, this signal must go

active when ACLK is high.

IREFEN Input Enab l es Ba nd -g a p reference as

AREF signal to DAC 00

MUXEN_7 Input Inp ut Mux bit 7 0 0

MUXEN_6 Input Inp ut Mux bit 6 0 0

MUXEN_5 Input Inp ut Mux bit 5 0 0

MUXEN_4 Input Inp ut Mux bit 4 0 0

MUXEN_3 Input Inp ut Mux bit 3 0 0

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Note: Incorrect setting of the switches in Figure 123 will make signal contention and may damage the par t. There are several input

choices to the S&H circuitr y on the n egative input of the output comparator in Figure 123. Make sure only one path is selected

from either one ADC pin, Bandgap reference source, or Ground.

If the ADC is not to be used during scan, the recommended input values from Table 92 should

be used. The user is recommended not to use the Differential Gain stages during scan. Switch-

cap based gain stages require fast operation and accurate timing which is difficult to obtain

when used in a scan chain. Details concerning operations of the differential gain stage is there-

fore not provided.

The AVR ADC is based on the analog circuitry shown in Figure 123 with a successive approxi-

mation algorithm implemented in the digital logic. When used in Boundary-scan, the problem is

usually to ensure that an applied analog voltage is measured within some limits. This can easily

be done without running a successive approximation algorithm: apply the lower limit on the digi-

tal DAC[9:0] lines, make sure the output from the comp arator is low, then apply the upper limit

on the digital DAC[9:0] lines, and verify the output from the comparator to be high.

The ADC need not be used for pure connectivity testing, since all analog inputs are shared with

a digital port pin as we ll.

When using the ADC, remember the following:

• The Port Pin for the ADC channel in use must be configured to be an input with pull-up

disabled to avoid signal contention.

• In Normal mode, a dummy conversion (consisting of 10 comparisons) is perf ormed when

enab ling th e ADC. The user is advised to wait a t least 20 0 n s af ter ena bling the ADC before

MUXEN_2 Input Inp ut Mux bit 2 0 0

MUXEN_1 Input Inp ut Mux bit 1 0 0

MUXEN_0 Input Inp ut Mux bit 0 1 1

NEGSEL_2 Input Input Mux for negative input for

differential signal, bit 2 00

NEGSEL_1 Input Input Mux for negative input for

differential signal, bit 1 00

NEGSEL_0 Input Input Mux for negative input for

differential signal, bit 0 00

PASSEN Input Enable pass-gate of gain stages. 1 1

PRECH Input Precharge output latch of

comparator. (Active low) 11

SCTEST Input Switch-cap TEST enable. Output

from x10 gain stage send out to

Port Pin having ADC_4

ST Input Output of gain stages will settle

faster if this signal is high first two

ACLK periods after AMPEN goes

high.

VCCREN Input Selects Vcc as the ACC reference

voltage. 00

Table 92. Boundary-scan Signals for th e ADC (Continued)

Signal

Name Direction as Seen

from the ADC Description

Recommended

Input when Not

in Use

Output V alues when Recommended

Inputs are used, and CPU is not

Using the ADC

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controlling/observing any ADC signal, or perform a dummy conversion before using the first

result.

• The DAC values must be stable at the midpoint value 0x200 when having the HOLD signal

low (Sample mode).

As an example, consider the task of verifying a 1.5V ±5% input signal at ADC channel 3 when

the power supply is 5.0V and AREF is externally connected to VCC.

The recommended valu es from Tabl e 92 are used unless othe r values are given in th e algorithm

in Table 93. Only the DAC and Port Pin values of the Scan-chain are shown. The column

“Actions” describes what JTAG instruction to be used before filling the Boundary-scan Register

with the succeeding columns. The verification should be done on the data scanned out when

scanning in the data on the same row in the table.

Using this algorithm, the timi ng constraint on the HOLD signal constrains the TC K clock fre-

quency. As the algorithm keeps HOLD high for five steps, the TCK clock frequency has to be at

least five times the number of scan bits divided by the maximum hold time, thold,max.

Table 93. Algorithm for Using the ADC

Step Actions ADCEN DAC MUXEN HOLD PRECH PA3.

Data PA3.

Control

PA3.

Pullup_

Enable

1 SAMPLE

_PRELO

1 0x200 0x08 1 1 0 0 0

2 EXTEST 1 0x200 0x08 0 1 0 0 0

3 1 0x200 0x08 1 1 0 0 0

4 1 0x123 0x08 1 1 0 0 0

5 1 0x123 0x08 1 0 0 0 0

6Verify the

COMP bit

scanned

out to be

1 0x200 0x08 1 1 0 0 0

7 1 0x200 0x08 0 1 0 0 0

8 1 0x200 0x08 1 1 0 0 0

9 1 0x143 0x08 1 1 0 0 0

10 1 0x143 0x08 1 0 0 0 0

11 Verify the

COMP bit

scanned

out to be

1 0x200 0x08 1 1 0 0 0

The lower limit is: 1024 1,5V0,95 5V⁄⋅⋅ 291 0x123==

The upper limit is: 1024 1,5V1,05 5V⁄⋅⋅ 323 0x143==

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ATmega16

Boundary-scan

Order

Table 94 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. Therefore, the bits of Port A is scanned in

the opposite bit order of the other ports. Exceptions from the rules are the Scan chains for the

analog circuits, which constitu te the most significant bits of the s can chain regardless of which

physical pin they are connected to. In Figure 116, PXn. Data corresponds to FF0, PXn. Control

corresponds to FF1, and PXn. Pullup_enable corresponds to FF2. Bit 2, 3, 4, and 5 of Port C is

not in the scan chain, since these pins constitute th e TAP pins when the JTAG is enabled.

Table 94. ATmega16 Boundary-scan Order

Bit Number Signal Name Module

140 AC_IDLE Comparator

139 ACO

138 ACME

137 ACBG

136 COMP ADC

135 PRIVATE_SIGNAL1(1)

134 ACLK

133 ACTEN

132 PRIVATE_SIGNAL2(2)

131 ADCBGEN

130 ADCEN

129 AMPEN

128 DAC_9

127 DAC_8

126 DAC_7

125 DAC_6

124 DAC_5

123 DAC_4

122 DAC_3

121 DAC_2

120 DAC_1

119 DAC_0

118 EXTCH

117 G10

116 G20

115 GNDEN

114 HOLD

113 IREFEN

112 MUXEN_7

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111 MUXEN_6

110 MUXEN_5

109 MUXEN_4

108 MUXEN_3

107 MUXEN_2

106 MUXEN_1

105 MUXEN_0

104 NEGSEL_2

103 NEGSEL_1

102 NEGSEL_0

101 PASSEN

100 PRECH

99 SCTEST

98 ST

97 VCCREN

96 PB0.Data Port B

95 PB0.Control

94 PB0.Pullup_Enable

93 PB1.Data

92 PB1.Control

91 PB1.Pullup_Enable

90 PB2.Data

89 PB2.Control

88 PB2.Pullup_Enable

87 PB3.Data

86 PB3.Control

85 PB3.Pullup_Enable

84 PB4.Data

83 PB4.Control

82 PB4.Pullup_Enable

81 PB5.Data

80 PB5.Control

79 PB5.Pullup_Enable

78 PB6.Data

77 PB6.Control

76 PB6.Pullup_Enable

Table 94. ATmega16 Boundary-scan Order (Continued)

Bit Number Signal Name Module

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75 PB7.Data

74 PB7.Control

73 PB7.Pullup_Enable

72 RSTT Reset Logic

(Observe-Only)

71 RSTHV

70 EXTCLKEN Enable signals for main clock/Oscillators

69 OSCON

68 RCOSCEN

67 OSC32EN

66 EXTCLK (XTA L1) Clock input and Oscillators for the main clock

(Observe-Only)

65 OSCCK

64 RCCK

63 OSC32CK

62 TWIEN TWI

61 PD0.Data Port D

60 PD0.Control

59 PD0.Pullup_Enable

58 PD1.Data

57 PD1.Control

56 PD1.Pullup_Enable

55 PD2.Data

54 PD2.Control

53 PD2.Pullup_Enable

52 PD3.Data

51 PD3.Control

50 PD3.Pullup_Enable

49 PD4.Data

48 PD4.Control

47 PD4.Pullup_Enable

46 PD5.Data

45 PD5.Control

44 PD5.Pullup_Enable

43 PD6.Data

42 PD6.Control

41 PD6.Pullup_Enable

40 PD7.Data

Table 94. ATmega16 Boundary-scan Order (Continued)

Bit Number Signal Name Module

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ATmega16(L)

39 PD7.Control

38 PD7.Pullup_Enable

37 PC0.Data Port C

36 PC0.Control

35 PC0.Pullup_Enable

34 PC1.Data

33 PC1.Control

32 PC1.Pullup_Enable

31 PC6.Data

30 PC6.Control

29 PC6.Pullup_Enable

28 PC7.Data

27 PC7.Control

26 PC7.Pullup_Enable

25 TOSC 32 kHz Timer Oscillator

24 TOSCON

23 PA7.Data Port A

22 PA7.Control

21 PA7.Pullup_Enable

20 PA6.Data

19 PA6.Control

18 PA6.Pullup_Enable

17 PA5.Data

16 PA5.Control

15 PA5.Pullup_Enable

14 PA4.Data

13 PA4.Control

12 PA4.Pullup_Enable

11 PA3.Data

10 PA3.Control

9 PA3.Pullup_Enable

8PA2.Data

7 PA2.Control

6 PA2.Pullup_Enable

5PA1.Data

Table 94. ATmega16 Boundary-scan Order (Continued)

Bit Number Signal Name Module

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Notes: 1. PRIVATE_SIGNAL1 should always be scanned in as zero.

2. PRIVATE:SIGNAL2 should always be scanned in as zero.

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 Register are included in this description. A BSDL file for ATmega16 is

available.

4 PA1.Control

3 PA1.Pullup_Enable

2PA0.Data

1 PA0.Control

0 PA0.Pullup_Enable

Table 94. ATmega16 Boundary-scan Order (Continued)

Bit Number Signal Name Module

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Boot Loader

Support – Read-

While-Write

Self-

Programming

The Boot Loader Support provides a real Read-While-Write Self-Programming mechanism for

downloading and up loading progr am code by the MCU itsel f. This feature allows flexible applica-

tion 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 r ead code

and write (program) that code into the Flash memory, or read the code from the Program mem-

ory. The program code within the Boot Loader section has the capability to write into the e ntire

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 Loade r has two separate sets of Boot

Lock bits which can be set independently. This gives the user a unique flexibility to select differ-

ent levels of protection.

Features •Read-While-Write Self-Programming

•Flexible Boot Memory size

•High Security (Separate Boot Lock Bits for a Flexible Protection)

•Separate 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 107 on page 262 )

used during programming. The page organization does not affect normal operation.

Application and

Boot Loader Flash

Sections

The Flash memory is organized in two main sections, the Application section and the Boot

Loader sectio n (see Fi gure 125). Th e size of t he dif fere nt sectio ns is con figur ed by t he BOOT SZ

Fuses as sh own in Table 100 on page 257 and Figure 125. These two sections can have differ-

ent level of protection since they have different sets of Lock bits.

Application Section Th e 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), see Table 96 on page 249. The Application section can never store any Boot

Loader code since the SPM instruction is disabled when executed from the Application section.

BLS – Boot Loader

Section While the Application section is used for storing the application code, the The Boot Loader soft-

ware 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), see Table 97 on page 249.

Read-While-Write

and no Read-

While-Write Flash

Sections

Whether the CPU supports Read-While-Write or if the CPU is halted during a Boot Loader soft-

ware update is depende nt on which address that is being progr ammed. 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. The limit between the RWW- and NRWW sections is given in Table 101

on page 257 and Figure 125 on page 248. The main differen ce between the two sections is:

• When erasing o r writing a page locate d inside t he RWW section, the NRWW section can be

read during the operation.

• When erasing or writing a page located inside the NR WW section, the CPU is halted during

the entire operation.

Note that the user software ca n never read any code th at is located in side the RWW section dur-

ing 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.

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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 (that is, by a

call/jmp/lpm 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 sec-

tion. The Boot Loader section is always located in the NRWW section. The RWW Section Busy

bit (RWWSB) in the Store Program Memory Control Register (SPMCR) 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. See

“Store Program Memory Con trol Register – SPMCR” on page 250. for details on how to clear

RWWSB.

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 entir e pa g e er ase or pa g e writ e op er at ion .

Figure 124. Read-While-Write vs. No Read-While-Write

Table 95. Read-While-Write Features

Which Section does the Z-

pointer Address during the

Programming?

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

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

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Figure 125. Memory Sections(1)

Note: 1. The parameters in the figure above are given in Table 100 on page 257.

Boot Loader Lock

Bits If no Boot Loader capability is needed, the entire Flash is available for application code. The

Boot Loader has two separ at e sets of Boot Lock bits which ca n be set indepen de ntly. 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 Table 96 and Ta ble 97 for further details. The Boot Lock bits can be set in software and in

Serial or Parallel Programming mode, but they can be cleared by a Chip Erase command only.

The general Writ e Lock (Lock Bit m ode 2) does not control the prog ramming of th e Flash mem -

ory by SPM instruction. Similarly, the general Read/Write Lock (Lock Bit mode 3) does not

control reading nor writing by LPM/SPM, if it is attempted.

$0000

Flashend

Program Memory

BOOTSZ = '11'

Application Flash Section

Boot Loader Flash Section Flashend

Program Memory

BOOTSZ = '10'

$0000

Program Memory

BOOTSZ = '01'

Program Memory

BOOTSZ = '00'

Application Flash Section

Boot Loader Flash Section

$0000

Flashend

Application Flash Section

Flashend

End RWW

Start NRWW

Application flash Section

Boot Loader Flash Section

End RWW

Start NRWW

End RWW

Start NRWW

$0000

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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Note: 1. “1” means unprogrammed, “0” means programmed

Entering the Boot

Loader Program Entering the Boot Loader takes place by a jump or call from the application program. This may

be initiated by a trigge r such as a command received via USART, or SPI int erface. Alternatively,

the Boot Reset Fuse can be programmed so that the Reset Vector is pointing to the Boot Flash

start address afte r a reset. In th is case, the Boot Loader is star ted after a reset. After the applica-

tion code is loaded, the program can start execut ing the application code. Note that the fuses

cannot be changed by the MCU itself. This means that once the Boot Reset Fuse is pro-

grammed, 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.

Table 96. Boot Lock Bit0 Protection Modes (Application Section)(1)

BLB0 Mode BLB02 BLB01 Protection

111

No restrictions for SPM or LPM accessing the Application

section.

2 1 0 SPM is not allowed to write to the Application section.

300

SPM is not allowed to write to the Application section, and

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.

401

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 97. Boot Lock Bit1 Protection Modes (Boot Loader Section)(1)

BLB1 mode BLB12 BLB11 Pr otection

111

No restrictions for SPM or LPM accessing the Boot Loader

section.

2 1 0 SPM is not allowed to write to the Boot Loader section.

300

SPM is not allowed to write to the Boot Loader section,

and 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 ex ecuting from the Boot Loader section.

401

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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Note: 1. “1” means unprogrammed, “0” means programmed

Store Program

Memory Control

The Store Progr am Memory Cont rol Register co ntains the cont rol bits needed to control the Bo ot

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 SPMCR Register is cleared.

• Bit 6 – RWWSB: Read-While-Write Section Busy

When a self-progra mming (Page Er ase or Page Write) oper ation to the RWW section is initia ted,

the RWWSB will be set (one) by hardware. When the RWWSB bit is set, the RWW section can-

not 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 – Res: Reserved Bit

This bit is a reserved bit in the ATmega16 and always read as zero.

• 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 writ-

ten 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 writt en to one at the same t ime as SPMEN, t he next SPM instru ctio n within f our clock

cycles sets Boot Lock bits, accor ding t o the d ata in R0. The dat a in R1 and th e addr ess in th e 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 LPM instruction within three cycles after BLBSET and SPMEN are set in the SPMCR Regis-

ter, will read either the Lock bits or the Fu se bits (depending on Z0 in the Z-pointer) into the

destination register. See “Reading the Fuse and Lock Bits from Software” on page 254 for

details.

Table 98. Boot Reset Fuse(1)

BOOTRST Reset Address

1 Reset Vector = Application reset (address $0000)

0 Reset Vector = Boot Loader reset (see Table 100 on page 257)

Bit 7 6 5 4 3 210

SPMIE RWWSB – RWWSRE BLBSET PGWRT PGERS SPMEN SPMCR

Read/Write R/W R 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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ATmega16(L)

• Bit 2 – PGWRT: Page Write

If this bit is writt en to one at the same t ime as SPMEN, t he next SPM instru ctio n within f our 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 writt en to one at the same t ime as SPMEN, t he next SPM instru ctio n within f our 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 addr essed.

• Bit 0 – SPMEN: Store Program Memory Enable

This bit enables the SPM instruction for the next four clock cycles. If written to one together with

either RWWSRE, BLBSE T, PGWRT’ or PGERS, the followi ng SPM instruction will have a spe-

cial 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.

Addressing the

Flash during Self-

Programming

The Z-pointer is used to address the SPM commands.

Since the Flash is organized in pages (see T able 107 on page 262), the Program Counter can

be treated as havin g two different sections. One sect ion, consisting of the least signif icant bits, is

addressing the words within a page, while the most significant bits are addressing the pages.

This is shown in Figure 126. Note that the Page Erase and Pa ge Write operat ions are addressed

independen tly. Therefor e it is of major impo rtance that the Boot Loade r software ad dresses the

same page in both the Page Erase and Page Write operation. Once a programming operation is

initiated, the add ress is latched and the Z-pointer can be used for other operatio ns.

The only SPM operation that does no t use the Z-pointer is Setting the Boot Loader Lock bits.

The content of the Z-pointer is ignored and will have no effect on the operation. The LPM

instruction does also use the Z pointer to store the address. Since this instruction addresses the

Flash byte by byte, also the LSB (bit Z0) of the Z-pointer is used.

Bit 151413121110 9 8

ZH (R31) Z15 Z14 Z13 Z12 Z11 Z10 Z9 Z8

ZL (R30) Z7Z6Z5Z4Z3Z2Z1Z0

76543210

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ATmega16(L)

Figure 126. Addressing the Flash during SPM(1)

Notes: 1. The different variables used in Figure 126 are listed in Table 102 on page 258.

2. PCPAGE and PCWORD are listed in Table 107 on page 262.

Self-Programming

the Flash The program memory is updated in a page by page fashion. Before programming a page with

the data stored in th e temporary page buf fer, the page must be erased. The temp orary page bu f-

fer is filled one word at a time using SPM and th e 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 t he page needs to be changed, the rest of the page must be sto red (for example

in the temporary page buffer) before the erase, and then be rewritten. When using alternative 1,

the Boot Loader pr ovides an effe ctive Rea d- Modify- Wr ite fe at ure which allo ws t he use r soft ware

to first read the page, do the necessary changes, and then write back the modified data. If alter-

native 2 is used, it is not p ossible 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 256 for an assembly

code example.

PROGRAM MEMORY

0115

Z - REGISTER

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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ATmega16(L)

Performing Page

Erase by SPM To execute Page Erase, set up the address in the Z-pointer, write “X0000011” to SPMCR and

execute SPM within four clock cycles after writing SPMCR. 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 must

be written zero 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.

Filling the Temporary

Buffer (Page Loadi ng) To write an instruction word, set up the address in the Z-pointer and data in R1:R0, write

“00000001” to SPMCR and execu te SPM within four clo ck cycles after writing SPMCR. The con-

tent of PCWORD in the Z-register is used to ad dress the data in the tempora ry buffe r. The

temporary buffer will auto-erase after a page write operation or by writing the RWWSRE bit in

SPMCR. It is also erased after a syst em reset . No te that it is no t p ossi ble to write mor e than one

time to each address without erasing the temporary buffer.

Note: If the EEPROM is written in the middle of an SPM Page Load operation, all data loaded will be

lost.

Performing a Page

Write To execute Page Write, set up the address in the Z-pointer, write “X0000101” to SPMCR and

execute SPM within four clock cycles after writing SPMCR. 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 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 ope ration.

Using the SPM

Interrupt If the SPM interrupt is enabled, the SPM interrupt will generate a constant interrupt when the

SPMEN bit in SPMCR is cleared. This means that the interrupt can be used instead of polling

the SPMCR Register in software . When using t he SPM int errupt, t he Int errupt Ve ctors sh ould 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 describ ed in “Interrupts” on page 45.

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 r ecommended to progra m the Boot Lock bit11 to

protect the Boot Loader software from any internal software changes.

Prevent Reading the

RWW Section during

Self-Programming

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 SPMCR 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 45, 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 256 for an example.

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ATmega16(L)

Setting the Boo t

Loader Lock Bits by

SPM

To set the Boot Loader Lock bits, write the desired data to R0, write “X0001001” to SPMCR and

execute SPM within four clock cycles after writ ing SPMCR. 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 96 and Table 97 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 SPMCR.

The Z-pointer is don’t care during this operation, but for future compatibility it is recommended to

load the Z-pointer with $0001 (same as used for reading the Lock bits). For future compatibility It

is also recommended to set bit s 7, 6, 1, and 0 in R0 to “1” when writing the Lock bits. When pro-

gramming the Lock bits the entire Flash can be read during t he operation.

EEPROM Write

Preve n ts Writing to

SPMCR

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 (EEWE) in the EECR Register and verifies

that the bit is cleared before writing to the SPMCR Register.

Reading th e Fuse and

Lock Bits from

Software

It is possib le to read bo th the Fuse and Loc k bits from so ftware. To read t he Lock bits, lo ad the

Z-pointer with $0001 and se t the BL BSET and SPMEN bit s in SPMCR. When an LPM instruct ion

is executed within three CPU cycles after the BLBSET and SPMEN bits are set in SPMCR, 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 LPM instruction is executed

within three CPU cycles or no SPM instruction is executed within four CPU cycles. When BLB-

SET and SPMEN are cleared, LPM will work as described in the Instruction set Manual.

The algorithm for readin g the Fuse Low bit s is similar to the on e described above f or reading the

Lock bits. To read the Fuse Low bits, load the Z-pointer with $0000 and set the BLBSET and

SPMEN bits in SPMCR. When an LPM instruction is executed within three cycles after the BLB-

SET and SPMEN bits are set in the SPMCR, the value of the Fuse Low bits (FLB) will be loaded

in the destination register as shown below. Refer to Table 106 on page 261 for a detailed

description and mapping of the Fuse Low bits.

Similarly, when reading the Fuse High bits, load $0003 in the Z-pointer. When an LPM instruc-

tion is executed within three cycles after the BLBSET and SPMEN bits are set in the SPMCR,

the value of the Fuse High bits (FHB ) will be loaded in the destination register as shown below.

Refer to Table 105 on page 260 for detailed description and mapping of the Fuse High bits.

Fuse and Lock bits that are programmed, will be read as zero. Fuse and Lock bits that are

unprogrammed, will be read as one.

Preventing Flash

Corruption During periods of low VCC, the Flash pr ogram can b e corrupted because the supply voltage is t oo

low for the CPU and the Flash to op erate pro perly. These issue s are the sa me as for boa rd leve l

systems using the Flash, and the sa me design solutions should be applied.

Bit 76543210

R0 1 1 BLB12 BLB11 BLB02 BLB01 1 1

Bit 76543210

Rd – – BLB12 BLB11 BLB02 BLB01 LB2 LB1

Bit 76543210

Rd FLB7 FLB6 FLB5 FLB4 FLB3 FLB2 FLB1 FLB0

Bit 76543210

Rd FHB7 FHB6 FHB5 FHB4 FHB3 FHB2 FHB1 FHB0

255

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ATmega16(L)

A Flash program co rr up tion can be cau sed b y two situ a tions when th e voltag e is too low. F irst , 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 recommendatio ns (one is

sufficient):

1. If there is no nee d fo r a Boot Loader update in the system, program the Boot Load er Lock

bits to prevent an y Boot Loader software updates.

2. Keep the AVR RESET active (low) during periods of insufficient power supply voltage .

This can be done b y enabling the internal Brow n-out Det ector ( BOD) if the oper ating volt-

age 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 prog ress, the write operation will be

completed provided that the power supply voltage is sufficie nt.

3. Keep the AVR core in Power-down Sleep mode during periods of low VCC. This will pre-

vent the CPU from attempting to decode and execute instructions, ef fectively protecting

the SPMCR Register and thus the Flash from unintentional writes.

Programming Time for

Flash when using SPM The Calibrated RC Oscillator is used to time Flash accesses. Table 99 shows the typical pro-

gramming time for Flash accesses fr om the CPU.

Table 99. SPM Programming Time.

Symbol Min Programming Time Max Programming Time

Flash write (Page Erase, Page

Write, and write Lock bits by SPM) 3.7 ms 4.5 ms

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ATmega16(L)

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),

; 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:

lpm 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, SPMCR

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ATmega16(L)

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, SPMCR

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, EEWE

rjmp Wait_ee

; SPM timed sequence

out SPMCR, spmcrval

spm

; restore SREG (to enable interrupts if originally enabled)

out SREG, temp2

ret

ATmega16 Boot

Loader Parameters In Table 100 through Table 102, the parameters used in the description of the self programming

are given.

Note: 1. The different BOOTSZ Fuse configurations are shown in Figure 125

Note: 1. For details about these two section, see “NRWW – No Read-While-Write Section” on page

247 and “RWW – Read-W hile-Write Section” on page 247

Table 100. Boot Size Conf iguration(1)

BOOTSZ1 BOOTSZ0 Boot

Size Pages

Application

Flash

Section

Boot

Loader

Flash

Section

End

Application

section

Boot Reset

Address

(start Boot

Loader

Section)

11128

words 2$0000 -

$1F7F $1F80 -

$1FFF $1F7F $1F80

10256

words 4$0000 -

$1EFF $1F00 -

$1FFF $1EFF $1F00

01512

words 8$0000 -

$1DFF $1E00 -

$1FFF $1DFF $1E00

001024

words 16 $0000 -

$1BFF $1C00 -

$1FFF $1BFF $1C00

Table 101. Read-While-Write Limit(1)

Section Pages Address

Read-While-Wr ite section (RWW) 112 $000 0 - $1BFF

No Read-While-Write section (NRWW) 16 $1C00 - $1FFF

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ATmega16(L)

Note: 1. Z15:Z14: always ignored

Z0: should be zero for all SPM commands, byte select for the LPM instruction.

See “Addressing the Flash during Self-Programming” on page 251 for details about the use of

Z-pointer during Self-Programming.

Table 102. Explanation of Different Variables used in Figure 126 and the Mapping to the Z-

pointer

Variable Corresponding

Z-value(1) Description

PCMSB 12 Most significant bit in the Program Counter.

(The Program Counter is 13 bits PC[12:0])

PAGEMSB 5 Most significant bit which is used to address the

words within one page (64 words in a page

requires 6 bits PC [5:0]).

ZPCMSB Z13 Bit in Z-register that is mapped to PCMSB.

Because Z0 is not used, the ZPCMSB equals

PCMSB + 1.

ZPAGEMSB Z6 Bit in Z-register that is mapped to PAGEMSB.

Because Z0 is not used, the ZPAGEMSB

equals PAGEMSB + 1.

PCPAGE PC[12:6] Z13:Z7 Program Counter page address: Page select,

for Page Erase and Page Write

PCWORD PC[5:0] Z6:Z1 Progr am Counter word address: Word select,

for filling temporary buffer (must be zero during

page write op eration)

259

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ATmega16(L)

Memory

Programming

Program And Data

Memory Lock Bits The ATmega16 provides six Lock bits which can be left unprogrammed (“1”) or can be pro-

grammed (“0”) to obtain the add itional features listed in Table 104. The Lock bits can only be

erased to “1” with the Chip Erase command.

Note: 1. “1” means unprogrammed, “0” means programmed

Table 103. Lock Bit Byte(1)

Lock Bit Byte Bit No. Description Default Value

7 – 1 (unprogrammed)

6 – 1 (unprogrammed)

BLB12 5 Boot Lock bit 1 (unprogrammed)

BLB11 4 Boot Lock bit 1 (unprogrammed)

BLB02 3 Boot Lock bit 1 (unprogrammed)

BLB01 2 Boot Lock bit 1 (unprogrammed)

LB2 1 Lock bit 1 (unprogrammed)

LB1 0 Lock bit 1 (unprogrammed)

Table 104. Lock Bit Protection Modes

Memory Lock Bits(2) Protection Type

LB Mode LB2 LB1

1 1 1 No memory lock features enabled.

210

Further programming of the Flash and EEPROM is

disabled in Parallel and SPI/JTAG Serial Programming

mode. The Fuse bits are locked in both Serial and P ar allel

Programming mode.(1)

300

Further programming and verification of the Flash and

EEPROM is disabled in Pa rallel and SPI/JTAG Serial

Programming mode. The Fuse bits are locked in both

Serial and Parallel Programming mode.(1)

BLB0 Mode BLB02 BLB01

111

No restrictions for SPM or LPM accessing the Application

section.

2 1 0 SPM is not allowed to write to the Application section.

300

SPM is not allowed to write to the Application section, and

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.

401

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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ATmega16(L)

Notes: 1. Program the Fuse bits before programming the Lock bits .

2. “1” means unprogrammed, “0” means programmed

Fuse Bits The ATmega16 has two fu se bytes. Tabl e 105 an d Table 10 6 d escribe b riefly t he funct ion ality of

all the fuses and how they are mapped into the fuse bytes. Note that the fuses are read as logi-

cal zero, “0”, if they are pr og rammed.

Notes: 1. The SPIEN Fuse is not accessible in SPI Serial Programming mode.

2. The CKOPT Fuse functionality depends on the setting of the CKSEL bits. See See “Clock

Sources” on page 25. for details.

3. The default value of BOOTSZ1..0 results in maximum Boot Size. See Table 100 on page 257.

4. Ne ver ship a product with the OCDEN Fuse programmed regardless of the setting of Lock bits

and the JTAGEN Fuse. A programmed OCDEN Fuse enables some par ts of the clock system

to be running in all sleep modes. This may increase the power consumption.

5. If the JTAG interface is left unconnected, the JTAGEN fuse should if possible be disabled. This

to avoid static current at the TDO pin in the JTAG interface .

BLB1 Mode BLB12 BLB11

111

No restrictions for SPM or LPM accessing the Boot Loader

section.

2 1 0 SPM is not allowed to write to the Boot Loader section.

300

SPM is not allowed to write to the Boot Loader section,

and 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 ex ecuting from the Boot Loader section.

401

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 104. Lock Bit Protection Modes (Continued)

Memory Lock Bits(2) Protection Type

Table 105. Fuse High Byte

Fuse High

Byte Bit

No. Description Default Value

OCDEN(4) 7 Enable OCD 1 (unprogrammed, OCD disabled)

JTAGEN(5) 6 Enable JTAG 0 (programmed, JTAG enabled)

SPIEN(1) 5Enable SPI Serial Program and

Data Downloading 0 (programmed, SPI prog. enabled)

CKOPT(2) 4 Oscillator options 1 (unprogrammed)

EESAVE 3 EEPROM memory is preserved

through the Chip Erase 1 (unprogrammed, EEPROM not

preserved)

BOOTSZ1 2 Select Boot Size (see Table 100

for details) 0 (programmed)(3)

BOOTSZ0 1 Select Boot Size (see Table 100

for details) 0 (programmed)(3)

BOOTRST 0 Select reset vector 1 (unprogrammed)

261

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ATmega16(L)

Notes: 1. The default value of SUT1..0 results in maximum start-up time. SeeTable 10 on page 29 for

details.

2. The default setting of CKSEL3..0 results in internal RC Oscillator @ 1 MHz. See Table 2 on

page 25 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.

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.

Signature Bytes All Atmel microcontrollers have a three-byte sig nature 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.

For the ATmega16 the signature bytes are:

1. $000: $1E (indicates manufactured by Atmel)

2. $001: $94 (indicates 16 Kbytes Flash memory)

3. $002: $03 (indicates ATmega16 device when $001 is $94)

Calibration Byte The ATmega16 stores four different calibration values for the internal RC Oscillator. These bytes

resides in the signature ro w High Byte of the addresses 0x0000 , 0x0001, 0x0002, and 0x0003

for 1 MHz, 2 MHz, 4 MHz, and 8 Mhz respectively. During Reset, the 1 MHz value is automati-

cally loaded into the OSCCAL Register. If other frequencies are used, the calibration value has

to be loaded manua lly, see “Oscillator Calibration Register – OSCCAL” on page 30 for details.

Table 106. Fuse Low Byte

Fuse Low

Byte Bit

No. Description Default Value

BODLEVEL 7 Brown-out Detector trigger level 1 (unprogrammed)

BODEN 6 Brown-out Detector enable 1 (unprogrammed, BOD disabled)

SUT1 5 Select start-up time 1 (unprogrammed)(1)

SUT0 4 Select start-up time 0 (programmed)(1)

CKSEL3 3 Select Clock source 0 (programmed)(2)

CKSEL2 2 Select Clock source 0 (programmed)(2)

CKSEL1 1 Select Clock source 0 (programmed)(2)

CKSEL0 0 Select Clock source 1 (unprogrammed)(2)

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ATmega16(L)

Page Size

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 ATmega16. Pulses are assumed to be at

least 250 ns unless otherwise noted.

Signal Names In this section, some pins of the ATmega16 are referenced by signal names describing their

functionality during parallel programming, see Figure 127 and Table 109. Pins not described in

the following table are ref erenced 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 111.

When pulsing WR or OE, the com mand loaded determines the action executed. The different

Commands are shown in Table 112.

Figure 127. Parallel Programming

Table 107. No. of Words in a Page and no. of Pages in the Flash

Flash Size Page Size PCWORD No. of Pages PCPAGE PCMSB

8K words (16 Kbytes) 64 words PC[5:0] 128 PC[12:6] 12

Table 108. No. of Words in a Page and no. of Pages in the EEPROM

EEPROM Size Page Size PCWORD No. of Pages PCPAGE EEAMSB

512 bytes 4 bytes EEA[1:0] 128 EEA[8:2] 8

VCC

+5V

GND

XTAL1

PD1

PD2

PD3

PD4

PD5

PD6

PB7 - PB0 DATA

RESET

PD7

+12 V

BS1

XA0

XA1

RDY/BSY

PAGEL

PA0

BS2

AVCC

+5V

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Table 109. Pin Name Mapping

Signal Name in

Programming Mode Pin Name I/O Function

RDY/BSY PD1 O 0: Device is busy programming, 1: Device is ready

for new command

OE PD2 I Output Enable (Active low)

WR PD3 I Write Pulse (Active low)

BS1 PD4 I Byte Select 1 (“0” selects Low byte, “1” selects

High byte)

XA0 PD5 I XTAL Action Bit 0

XA1 PD6 I XTAL Action Bit 1

PAGEL PD7 I Program Memory and EEPROM data Page Load

BS2 PA0 I Byte Select 2 (“0” selects Low byte, “1” selects

2’nd High byte)

DATA PB7-0 I/O Bidirectional Data bus (Output when OE is low)

Table 110. 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 111. XA1 and XA0 Coding

XA1 XA0 Action when XTAL 1 is Pulsed

0 0 Load Flash or EEPROM Address (High or low address b yte determined by BS1)

0 1 Load Data (High or Low data byte for Flash determined by BS1)

1 0 Lo ad Command

1 1 No Action, Idle

Table 112. Command Byte Bit Coding

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

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0000 1000 Read Signature Bytes and Calibration byte

0000 0100 Read Fuse and Lock bits

0000 0010 Read Flash

0000 0011 Read EEPROM

Table 112. Command Byte Bit Coding

Command Byte Command Executed

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Parallel

Programming

Enter Programming

Mode The following algorithm puts the device in Parallel Programming mode:

1. Apply 4.5V - 5.5V between VCC and GND, and wait at least 100 µs.

2. Set RESET to “0” and toggle XTAL1 at least 6 times

3. Set the Prog_enable pins listed in Tab le 110 on p age 263 to “ 0000” and w ait a t least 100

ns.

4. Apply 11.5V - 12.5V to RESET. Any activity on Prog_enable pins within 100 ns after

+12V has been applied to RESET, will cause the device to fail entering Programming

mode.

Note, if External Crystal or External RC configuration is selected, it may not be possible to apply

qualified XTAL1 pulses. In such cases, the following algorithm should be followed:

1. Set Prog_enable pins listed in Table 110 on page 263 to “0000”.

2. Apply 4.5V - 5.5V between VCC and GND simult aneously as 11.5V - 12.5V is applied to

RESET.

3. Wait 100 µs.

4. Re-program the fuses to ensure that External Clock is selected as clock source

(CKSEL3:0 = 0b0000) If Lock bits are pr ogrammed, a Chip Erase command must be

executed before changing the fuses.

5. Exit Programming mode by power the device down or by bringing RESET pin to 0b0.

6. Entering Programming mode with the original algorithm, as described above.

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 $FF, 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 read ing a new 256 word

window in Flash or 256 byte EEPROM. This consideration al so applies to Signature bytes

reading.

Chip Erase The Chip Erase will erase the Flash and EEPROM(1) memories plus Lock bits. The Lock bit s 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 the 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 comma nd.

5. Give WR a negative pulse. This starts the Chip Erase. RDY/BSY goes low.

6. Wait until RDY/BS Y goes high before loading a new command.

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Programming the

Flash The Flash is org anized in pa ges, see Table 107 o n page 2 62. When pr ogramming th e Flash, the

program data is latched into a page buffer. This allows one pag e of program data to be pro-

grammed 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 comma nd.

B. Load Address Low byte

1. Set XA1, XA0 to “00”. This enables address loading.

2. Set BS1 to “0”. This selects low address.

3. Set DATA = Address Low byte ($00 - $FF).

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 ($00 - $FF).

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 ($00 - $FF).

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 129 for signal

waveforms)

F. Repeat B through E unt il the entire buffer is filled or until all data within the page is loaded.

While the lower bits in t he addre ss are mapped t o words within the pa ge, th e higher b its addr ess

the pages within the FLASH. This is illustrated in Figure 128 on page 267. Note that if less than

8 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

1. Set XA1, XA0 to “00”. This enables address loading.

2. Set BS1 to “1”. This selects high address.

3. Set DATA = Address High byte ($00 - $FF).

4. Give XTAL1 a positive pulse. This loads the address High byte.

H. Program Page

1. Set BS1 = “0”

2. Give WR a negative pulse . Th is starts programming of the en tire page of data. RDY/BSY

goes low.

3. Wait until RDY/BS Y goes high. (See Figure 129 for signal wavefor m s)

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I. Repeat B through H until the entire Flash is programmed or until all data has been

programmed.

J. End Page Programming

1. 1. Set XA1, XA0 to “10”. This enables command loadin g.

2. Set DATA to “0000 0000”. This is the command for No Operation.

3. Give XTAL1 a positive pulse. This loads the command, a nd the internal write signals are

reset.

Figure 128. Addressing the Flash which is Organized in Pages

Note: 1. PCPAGE and PCWORD are listed in Table 107 on page 262.

PROGRAM MEMORY

WORD ADDRESS

WITHIN A PAGE

PAGE ADDRESS

WITHIN THE FLASH

INSTRUCTION WORD

PAGE PCWORD[PAGEMSB:0]:

PAGEEND

PAGE

PCWORDPCPAGE

PCMSB PAGEMSB

PROGRAM

COUNTER

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Figure 129. Programming the Flash Waveforms(1)

Note: 1. “XX” is don’t care. The letters refer to the programming description above.

Programming the

EEPROM The EEPROM is organized in pages, see Table 108 on page 262. 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 F lash” on page 266 for details on Com mand, Address and

Data loading):

1. A: Load Command “0001 0001”.

2. G: Load Address High Byte ($00 - $FF)

3. B: Load Address Low Byte ($00 - $FF)

4. C: Load Data ($00 - $FF)

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 BS1 to “0”.

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 progra mming t he next page. (See Figu re 13 0 for

signal waveforms)

RDY/BSY

RESET +12V

PAGEL

BS2

$10 ADDR. LOW ADDR. HIGH

DATA

DATA LOW DATA HIGH ADDR. LOW DATA LOW DATA HIGH

XA1

XA0

BS1

XTAL1

XX XX XX

ABCDEBCDEGH

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Figure 130. Programming the EEPROM Waveforms

Reading the Flash The algorithm for reading the Flash memory is as follows (re fer to “Programming the Flash” on

page 266 for details on Command and Address loading):

1. A: Load Command “0000 0010”.

2. G: Load Address High Byte ($00 - $FF)

3. B: Load Address Low Byte ($00 - $FF)

4. Set OE to “0”, and BS1 to “0”. The Flash word Low byte can now be read at DATA.

5. Set BS1 to “1”. The Flash word High b yte can now be rea d at DATA.

6. Set OE to “1”.

Reading the EEPROM The algorithm for reading the EEPROM memory is as follows (refer to “Programming the Flash”

on page 266 for details on Command and Address loading):

1. A: Load Command “0000 0011”.

2. G: Load Address High Byte ($00 - $FF)

3. B: Load Address Low Byte ($00 - $FF)

4. Set OE to “0”, and BS1 to “0”. The EEPROM Data byte can now be read at DATA.

5. Set OE to “1”.

Programming the

Fuse Low Bits The algorith m for progr amming th e Fuse Lo w bits is as follows (ref er to “ Programmin g the Flash”

on page 266 for details on Command and Data loading):

1. A: Load Command “0100 0000”.

2. C: Load Data Low Byte. Bit n = “0” prog rams and bit n = “1” erases the Fuse bit.

3. Set BS1 to “0” and BS2 to “0”.

4. Give WR a negative pulse and wait for RDY/BSY to go high.

RDY/BSY

RESET +12V

PAGEL

BS2

0x11 ADDR. HIGH

DATA

ADDR. LOW DATA ADDR. LOW DATA XX

XA1

XA0

BS1

XTAL1

AGB CEB CE L

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Programming the

Fuse High Bits The algorithm for programm ing the Fuse high bits is a s follows (refer to “Pr ogramming the Flash”

on page 266 for details on Command and Data loading):

1. A: Load Command “0100 0000”.

2. C: Load Data Low Byte. Bit n = “0” prog rams and bit n = “1” erases the Fuse bit.

3. Set BS1 to “1” and BS2 to “0”. This selects high data byte.

4. Give WR a negative pulse and wait for RDY/BSY to go high.

5. Set BS1 to “0”. This selects low data byte.

Figure 131. Programming the Fuses

Programming the Lock

Bits The algorithm for programming the Lock bits is as follows (refer to “Programming the Flash” on

page 266 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.

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.

Reading th e Fuse and

Lock Bits The algorithm for reading the Fuse and Lock bits is as follows (refer to “Programm ing the Flash”

on page 266 for details on Command loading):

1. A: Load Command “0000 0100”.

2. Set OE to “0”, BS2 to “0” and BS1 to “0”. The status of the Fuse Low bits can now be

read at DATA (“0” means programmed).

3. Set OE to “0”, BS2 to “1” and BS1 to “1”. The status of the Fuse High bits can now be

read at DATA (“0” means programmed).

4. Set OE to “0”, BS2 to “0” and BS1 to “1”. The status of the Lock bits can now be read at

DATA (“0” means programmed).

5. Set OE to “1”.

RDY/BSY

RESET +12V

PAGEL

$40

DATA

DATA XX

XA1

XA0

BS1

XTAL1

$40 DATA XX

Write Fuse Low byte Write Fuse high byte

BS2

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Figure 132. Mapping between BS1, BS2 and the Fuse- and Lock Bits during Read

Reading the Sig nature

Bytes The algorithm for reading the Signature bytes is as follows (refer to “Programming t he F lash” on

page 266 for details on Command and Address loading):

1. A: Load Command “0000 1000”.

2. B: Load Address Low Byte ($00 - $02).

3. Set OE to “0”, and BS1 to “0”. The selected Signature byte can now be read at DATA.

4. Set OE to “1”.

Reading the

Calibration Byte The algorithm for reading the Calibration byte is as follows (refer to “Progr am m ing the Flas h” o n

page 266 for details on Command and Address loading):

1. A: Load Command “0000 1000”.

2. B: Load Address Low Byte, $00.

3. Set OE to “0”, and BS1 to “1”. The Cali bration b yte can now be read at DATA.

4. Set OE to “1”.

Parallel Programming

Characteristics Figure 133. Parallel Programmi ng Timing, Including some General Timing Requirements

Fuse Low Byte

Lock Bits0

BS2

Fuse High Byte

BS1

DATA

Data & Contol

(DATA, XA0/1, BS1, BS2)

XTAL1 tXHXL

tWL WH

tDVXH tXLDX

tPLWL

tWLRH

RDY/BSY

PAGEL tPHPL

tPLBX

tBVPH

tXLWL

tWLBX

tBVWL

WLRL

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Figure 134. Parallel Programming Timing, Loading Sequ ence with Timing Requirements(1)

Note: 1. The timing requirements shown in Figure 133 (that is, tDVXH, tXHXL, and tXLDX) also apply to

loading operation.

Figure 135. Parallel Programming Timing, Reading Sequence (within the Same Page) with

Timing Requirements(1)

Note: 1. The timing requirements shown in Figure 133 (that is, tDVXH, tXHXL, and tXLDX) also apply to

reading operation.

Table 113. Parallel Programming Characteristics, VCC = 5V ±10%

Symbol Parameter Min Typ Max Units

VPP Programming Enable Voltage 11.5 12.5 V

IPP Programming Enable Current 250 μA

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

(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

(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.

Serial

Downloading Both the Flash and EEPROM memory arrays can be programmed using the serial SPI bus while

RESET is pulled to GND. The serial interface consists of pins SCK, MOSI (input), and MISO

(output). After RESET is set low, the Programming Enable instruction needs to be executed first

before program /erase operations can be executed. NOT E, in Table 114 on page 273, the pin

mapping for SPI programming is listed. Not all parts use the SPI pins dedicated for the internal

SPI interface.

SPI Serial

Programming Pin

Mapping

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 PAG E L l ow to XTAL1 high 150

tBVPH BS1 Valid before PAGEL High 67

tPHPL PAGEL Pulse Width High 150

tPLBX BS1 Hold after PAGEL Lo w 67

tWLBX BS2/1 Hold after WR Low 67

tPLWL PAGEL Lo w to WR Low 67

tBVWL BS1 Valid to WR Low 67

tWLWH WR Pulse Width Low 150

tWLRL WR Low to RDY/BSY Low 0 1 μs

tWLRH WR Low to RDY/BSY High(1) 3.7 4.5 ms

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 v alid 0 250

tOLDV OE Low to DATA Valid 250

tOHDZ OE High to DATA Tri-stated 250

Table 113. Parallel Programming Characteristics, VCC = 5V ±10% (Continued)

Symbol Parameter Min Typ Max Units

Table 114. Pin Mapping SPI Serial Programming

Symbol Pins I/O Description

MOSI PB5 I S erial Data in

MISO PB6 O Serial Data out

SCK PB7 I Serial Clock

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Figure 136. SPI Serial Programming and Verify(1)

Notes: 1. If the device is clocked by the Internal Oscil lator, 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 2.7V - 5.5V

When programming the EEPROM, an auto-erase cycle is built into the self-timed programming

operation (in the ser ial mode ONLY) and there is no need to first execut e the Chip Erase instruc-

tion. The Chip Erase operation turns the content of every memory location in both the Program

and EEPROM arrays into $FF.

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: > 2 CPU clock cycles for fck < 12 MHz, 3 CPU clock cycles for fck ≥ 12 MHz

High: > 2 CPU clock cycles for fck < 12 MHz, 3 CPU clock cycles fo r fck ≥ 12 MHz

SPI Serial

Programming

Algorithm

When writing serial data to the ATmega16, data is clocked on the rising edge of SCK.

When reading data from the ATmega16, data is clocked on the falling edge of SCK. See Figure

138 for timing details.

To program and verify the ATmega16 in the SPI Serial Programming mode, the following

sequence is recommended (See four byte instruction formats in Figure 116 on page 276):

1. Power-up sequence:

Apply power between VCC and GND while RESET and SCK are set to “0”. In some sys-

tems, the programmer can not guarantee that SCK is held low during power-up. In this

case, RESET must be given a positive pulse of at least t wo CPU clock cycles duration

after SCK has been set to “0”.

2. Wait for at least 20 ms and enable SPI Serial Programming by sending the Programming

Enable serial instruction to pin MO SI.

3. The SPI Serial Programming instructions will not work if the communication is out of syn-

chronization. When in sync. the second byte ($53), will echo back when issuing the third

byte of the Programming Enable instruction. Whether the echo is correct or not, all four

bytes of the instruction must be transmit ted. If the $53 did not echo back, give RESET a

positive pulse and issue a new Programming Enable command.

VCC

GND

XTAL1

SCK

MISO

MOSI

RESET

PB5

PB6

PB7

+2.7 - 5.5V

AVCC

+2.7 - 5.5V

(2)

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4. The Flash is programmed one page at a time. The page size is f ound in Table 107 on page

262. The memory page is loaded one byte at a time by supplying the 6 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 7 MSB of the address. If polling is not used,

the user must wait at least tWD_FLASH before issuing the next page. (See Table 115).

Accessing the SPI Serial Programming interface before the Flash write operation com-

pletes can result in incorrect programming.

5. The EEPROM arr ay is prog rammed one b yte at a time by su pplying 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 bef ore issuing the ne xt byt e. (See Table 115). In a chip er ased device ,

no $FFs in the data file(s) need to be programmed.

6. An y memory location can be verified by using the Read instruction which returns the con-

tent at the selected address at serial output MISO.

7. At the end of th e pr ogramming se ssio n, RESE T can be set high to commence normal

operation.

8. Power-off sequence ( if needed):

Set RESET to “1”.

Turn VCC power off.

Data Polling Flash When a page is being programmed into the Flash, reading an address location within the page

being programmed will give the value $FF. At the time the device is ready for a new page, the

programmed value will read correctly. This is used to determine when the next page can be writ-

ten. Note that the entire page is written simultaneously and any address within the page can be

used for polling. Data polling of the Flash will not work for the value $FF, so when programming

this value, the user will have to wait for at least tWD_FLASH before programming the next page. As

a chip erased device contains $FF in all locations, programming of addresses that are meant to

contain $FF, can be skipped. See Table 115 for tWD_FLASH value

Data Polling EEPROM When a new byte has been written and is being programmed into EEPROM, reading the

address location being programmed will give the value $FF. At the time the device is ready for a

new byte, the programmed value will read correctly. This is used to determine when the next

byte can be written. This will not work for the value $FF, but the user should have the following in

mind: As a chip erased device contains $FF in all locations, programming of addresses that are

meant to contain $FF, can be skipped. This does not apply if the EEPROM is re-programmed

without chip erasing the device. In this case, data polling cannot be used for the value $FF, and

the user will have to wait at least tWD_EEPROM before programming the next byte. See Table 115

for tWD_EEPROM value.

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Serial Pro gramming

Instruction se t Table 116 on page 276 and Figure 137 on p age 277 describes the Instruction set.

Table 115. Minimum Wait Delay before Writing the Next Flash or EEPROM Location

Symbol Minimum Wait Delay

tWD_FUSE 4.5 ms

tWD_FLASH 4.5 ms

tWD_EEPROM 9.0 ms

tWD_ERASE 9.0 ms

Table 116. Serial Programming Instruction Set (Hexadecimal values)

Instruction(1)/Operation

Instruction Format

Byte 1 Byte 2 Byte 3 Byte 4

Programming Enab le $A C $53 $00 $00

Chip Erase (Program Memory/EEPROM) $AC $80 $00 $00

Poll RDY/BSY $F0 $00 $00 data byte out

Load Instructions

Load Extended Address byte(1) $4D $00 Extended adr $00

Load Program Memory Page, High byte $48 adr MSB adr LSB high data byte in

Load Program Memory Page, Low byte $40 adr MSB adr LSB low data byte in

Load EEPROM Memory Page (page access)(1) $C1 $00 adr LSB data b yte in

Read Instructions

Read Program Memory, High byte $28 adr MSB adr LSB high data byte out

Read Program Memory, Low by t e $20 adr MSB adr LSB low da t a byte out

Read EEPROM Memory $A0 adr MSB adr LSB data byte out

Read Lock bits $58 $00 $00 data byte out

Read Signatu re Byte $30 $00 0000 000aa data byte out

Read Fuse bits $50 $00 $00 data byte out

Read Fuse High bits $58 $08 $00 data byte out

Read Extended Fuse Bits $50 $08 $00 data byte out

Read Calibration Byte $38 $00 $0b00 000bb data byte out

Write Instructions

Write Program Memory Page $4C 000a aaaa aa00 0000 $00

Write EEPROM Memory $C0 adr MSB adr LSB data byte in

Write EEPROM Memory Page (page access)(1) $C2 adr MSB adr LSB $00

Write Lock bits $A C $E0 $00 data by te in

Write Fuse bits $AC $A0 $00 data byte in

Write Fuse High bits $AC $A8 $00 data byte in

Write Extended Fuse Bits $AC $A4 $00 data byte in

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Notes: 1. Not all instructions are applicable for all parts.

2. a = address

3. Bits are programmed ‘0’, unprogrammed ‘1’.

4. To ensure future compatibility, unused Fuses and Lock bits should be unprogrammed (‘1’) .

5. Refer to the correspondig section for Fuse and Lock bits, Calibration and Signature bytes and

Page size.

6. See htt://www.atmel.com/avr for Application Notes regarding programming and programmers.

If the LSB in RDY/BSY data byte out is ‘1’, a programming operation is still pending. Wait until

this bit returns ‘0’ before the next instruction is carried out.

Within the same page, the low data byte must be loaded prior to the high data byte.

After data is loaded to the page buffer, program the EEPROM page, see Figure 137 on page

277.

Figure 137. Serial Programming Instr uction example

Byte 1 Byte 2 Byte 3 Byte 4

Adr MSB

Adr LSB

Bit 15 B 0

Serial Programming Instruction

Program Memory/

EEPROM Memory

Page 0

Page 1

Page 2

Page N-1

Page Buffer

Write Program Memory Page/

Write EEPROM Memory Page

Load Program Memory Page (High/Low Byte)/

Load EEPROM Memory Page (page access)

Byte 1 Byte 2 Byte 3 Byte 4

Bit 15 B 0

Adr MSB

Adr LSB

Page Offset

Page Number

Adr

dr M

r MS

Adr

dr LS

LSB

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SPI Serial

Programming

Characteristics

For characteristics of the SPI module, see “SPI Timi ng Characteristics” on page 295.

Figure 138. SPI Serial Programming Waveforms

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 shippe d with the fu se p rogramme d. In addit ion, the JTD bit in MCUCSR must be cleare d.

Alternatively, if the JTD bit is set, the External Reset can be for ced 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 dedi-

cated for this purpose.

As a definition in this datasheet, the LSB is shifted in and out first of all Shift Registers.

Progra mming 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 Re gister 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 139.

MSB

LSB

SERIAL CLOCK INPUT

(SCK)

SERIAL DATA INPUT

(MOSI)

(MISO)

SAMPLE

SERIAL DATA OUTPUT

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Figure 139. State Machine Sequence for Changing the Instruction Word

AVR_RESET ($C) 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.

PROG_ENABLE ($4) 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 : Th e programmin g en able signature is shifted int o th e D at a Regist er.

• Update-DR: The programming enable signature is compared to the correct v alue, and

Programming mode is entered if the signature is valid.

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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PROG_COMMANDS

($5) 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 fo llowin g :

• Capture-DR: The result of the previous command is loaded into the Data Register.

• Shift-DR : The Dat a Register is shifted by the TCK input, shifting out th e re su lt 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 (not always

required, see Tabl e 11 7 below).

PROG_PAGELOAD

($6) The AVR specific public JTAG instru ctio n to directly load t he Flash data pag e via the JTAG port.

The 1024 bit Vir tual Flash Page Load Register is selecte d as Data Register. This is a virtual

scan chain with length equal to the number of bit s in one Flash page. Int ernally the Shift Register

is 8-bit. Unlike most JTAG instr uctions, the Upda te-DR state is not use d to transf er data fr om the

Shift Register. The dat a are automatically transfer red to the Flash page bu ffer byte by byte in the

Shift-DR state by an internal state machine. This is the only active state:

• Shift-DR: Flash page data are shifted in from TDI by the TCK input, and automatically

loaded into the Flash page one byte at a time.

Note: The JTAG instr uction PROG_PAGELOAD can only be used if the AVR device is the first device in

JTAG scan chain. If the AVR cannot be the first device in the scan chain, the byte-wise program-

ming algorithm must be used.

PROG_PAGEREAD

($7) The AVR specific public JTAG instruction to read on e full Flash data page via the JTAG port.

The 1032 bit Vir tual Flash Page Read Register is selected as Data Register. This is a virtual

scan chain with length equal to the number of bits in one Flash page plus 8. Internally the Shift

data to the Shift Registe r. The dat a are aut omatically tr ansferr ed from t he Flash pa ge buffer byte

by byte in the Shift-DR state by an internal state machine. This is the only active state:

• Shift-DR: Fla sh data are automatically r ead one b yte at a time and shifted out on TDO b y the

TCK input. The TDI input is ignored.

Note: The JTAG instr uction PROG_PAGEREAD can only be used if the AVR device is the first device in

JTAG scan chain. If the AVR cannot be the first device in the scan chain, the byte-wise program-

ming algorithm must be used.

Data Registers The Data Registers are selected by the JTAG Instruction Registers described in section “Pro-

gramming Specific JTAG Instructions” on page 278. The Data Registers relevant for

programming operations are:

• Reset Register

• Programming Enable Register

• Programming Command Register

• Virtual Flash Page Load Regist er

• Virtual Flash Page Read Register

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Reset Register The Reset Register is a Test Data Register used to reset the part during programming. It is

required to reset the part before ent ering 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 set-

tings for the clock options, the part will remain reset for a Reset Time-out Period (refer to “Clock

Sources” on page 25) after releasing the Reset Register. The output from this Data Register is

not latched, so the reset will take place immediately, as shown in Figure 115 on page 230.

Programming Enable

to the programming enable signature, binary code 1010_0011_0 111_0000. When the contents

of the register is equal to the programming enable signature, programming via the JTAG port is

enabled. The regist er is reset to 0 on Power-on Reset , and should al ways be reset when leaving

Programming mode.

Figure 140. Programming Enable Register

TDI

TDO

ClockDR & PROG_ENABLE

Programming Enable

$A370

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Programming

Command Register The Programming Command Register is a 15-bit register. This register is used to serially shift in

programming co mmands, and to serially shift out the result of the pre vious command, if any. The

JTAG Programming Instruction Set is shown in Table 11 7. The state sequ ence when shifting in

the programming commands is illustrated in Figure 142.

Figure 141. Programming Command Register

TDI

TDO

Flash

EEPROM

Fuses

Lock Bits

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Table 117. JTAG Programming Instruction Set

a = address high bits, b = address low bits, H = 0 – Low b yte, 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 High Byte 0000111_aaaaaaaa xxxxxxx_xxxxxxxx (9)

2c. Load Address Low Byte 0000011_bbbbbbbb xxxxxxx_xxxxxxxx

2d. Load Data Low Byte 0010011_iiiiiiii xxxxxxx_xxxxxxxx

2e. Load Data High Byte 0010111_iiiiiiii xxxxxxx_xxxxxxxx

2f. Latch Data 0110111_0000000 0

1110111_00000000

0110111_00000000

xxxxxxx_xxxxxxxx

(1)

2g. Write Flash Page 0110111_00000000

0110101_00000000

0110111_00000000

xxxxxxx_xxxxxxxx

(1)

2h. Poll for Page Write complete 0110111_00000000 xxxxxox_xxxxxxxx (2)

3a. Enter Flash Read 0100011_00000010 xxxxxxx_xxxxxxxx

3b. Load Address High Byte 0000111_aaaaaaaa xxxxxxx_xxxxxxxx (9)

3c. Load Address Low Byte 0000011_bbbbbbbb xxxxxxx_xxxxxxxx

3d. Read Data Low and High Byte 0110010_00000000

0110110_00000000

0110111_00000000

xxxxxxx_xxxxxxxx

xxxxxxx_oooooooo

xxxxxxx_oooooooo Low byte

High byte

4a. Enter EEPROM Write 0100011_00010001 xxxxxxx_xxxxxxxx

4b. Load Address High Byte 0000111_aaaaaaaa xxxxxxx_xxxxxxxx (9)

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)

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 (9)

5c. Load Address Low Byte 0000011_bbbbbbbb xxxxxxx_xxxxxxxx

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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 High byte 0110111_00000000

0110101_00000000

0110111_00000000

xxxxxxx_xxxxxxxx

(1)

6d. Poll for Fuse Write complete 0110111_000 00000 xxxxxox_xxxxxxxx (2)

6e. Load Data Low Byte(7) 0010011_iiiiiiii xxxxxxx_xxxxxxxx (3)

6f. Write Fuse Low byte 0110011_0000000 0

0110001_00000000

0110011_00000000

xxxxxxx_xxxxxxxx

(1)

6g. Poll for Fuse Write complete 0110011_000 00000 xxxxxox_xxxxxxxx (2)

7a. Enter Lock Bit Write 0100011_00100000 xxxxxxx_xxxxxxxx

7b. Load Data Byte(8) 0010011_11iiiiii xxxxxxx_xxxxxxxx (4)

7c. Write Lock Bits 0110011_0000000 0

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 Fuse High Byte(6) 0111110_00000000

0111111_00000000 xxxxxxx_xxxxxxxx

xxxxxxx_oooooooo

8c. Read Fuse Low Byte(7) 0110010_00000000

0110011_00000000 xxxxxxx_xxxxxxxx

xxxxxxx_oooooooo

8d. Read Lock Bits(8) 0110110_00000000

0110111_00000000 xxxxxxx_xxxxxxxx

xxxxxxx_xxoooooo (5)

8e. Read Fuses and Lock Bits 0111110_00000000

0110010_00000000

0110110_00000000

0110111_00000000

xxxxxxx_xxxxxxxx

xxxxxxx_oooooooo

(5)

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

Table 117. JTAG Programming Instr uction Set (Continued)

a = address high bits, b = address low bits, H = 0 – Low b yte, 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 req uired if the seven MSB a re correctly set by the previous command sequence (wh ich is

nor mally 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 High byte is listed in Table 105 on page 260

7. The bit mapping for Fuses Low byte is listed in Table 106 on page 261

8. The bit mapping for Lock bits byte is listed in Table 103 on page 259

9. Address bits exceeding PCMSB and EEAMSB (Table 107 and Table 108 ) are don’t care

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

xxxxxxx_xxxxxxxx

Table 117. JTAG Programming Instr uction Set (Continued)

a = address high bits, b = address low bits, H = 0 – Low b yte, 1 – High Byte, o = data out, i = data in, x = don’t care

Instruction TDI sequence TDO sequence Notes

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Figure 142. State Machine Sequence for Changing/Reading the Data Word

Virtual Flash Page

Load Register The Vir tual Flash Page Load Register is a virtual scan chain with length equal to the number of

bits in one Flash page. Internally the Shift Register is 8-bit, and the data are automatically trans-

ferred to the Flash page buffer byte by byte. Shift in all instructio n words in the page, star ting

with the LSB of the first instruction in the page and ending with the MSB of the last instruction in

the page. This provides an efficient way to load the entire Flash page buffer before executing

Page Write.

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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Figure 143. Virtual Flash Page Load Register

Virtual Flash Page

Read Register The Virtual Flash Page Read Register is a virtual scan chain with length equal to the number of

bits in one Flash pag e plus 8. In te rnally t he Shift Register is 8-bit, and the d ata a re auto matically

transferred from the Flash data page byte by byte. The first 8 cycles are used t o tr ansfer th e fi rst

byte to the internal Shift Register, and the bits that are shifted out during these 8 cycles should

be ignored. Following this initialization, data are shifted out star ting with the LSB of the first

instruction in the page and end ing with the MSB of the la st instruction in the page. This provides

an efficient way to read one full Flash page to verify programming.

Figure 144. Virtual F lash Page Read Register

TDI

TDO

Flash

EEPROM

Fuses

Lock Bits

STROBES

ADDRESS

State

Machine

TDI

TDO

Flash

EEPROM

Fuses

Lock Bits

STROBES

ADDRESS

State

Machine

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Programming

Algorithm All references belo w of type “1a”, “1b”, and so on, refer to Table 117.

Entering Programming

Mode 1. Enter JTAG instruction AVR_RESET and shift 1 in the Reset Register.

2. Enter instruction PROG_ENABLE a nd shift 1010_0011_0111_0000 in the Programming

Enable Register.

Leaving Programming

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 0000_0000_0000_0000 in the programming

Enable Register.

4. Enter JTAG instruction AVR_RESET and shift 0 in the Reset Register.

Performing Chip Erase 1. Enter JTAG instruction PROG_COMMANDS.

2. Start chip erase using programming instruction 1a.

3. P oll f or Chi p Erase complete using progr amming instruction 1b , or w ait f or tWLRH_CE (ref er

to Table 113 on page 272).

Programming the

Flash Before programming the Flash a Chip Erase must be p erformed. See “Perform ing Chip Erase”

on page 288.

1. Enter JTAG instruction PROG_COMMANDS.

2. Enable Flash write using programming instruction 2a.

3. Load address High byte using programming instruction 2b.

4. Load address Low byte using programming instruction 2c.

5. Load data using programming instructions 2d, 2e and 2f.

6. Repeat steps 4 and 5 for all instruction words in the page.

7. Write the page using pr ogramming instruction 2g.

8. Poll for Flash write complete using programming instruction 2h, or w ait for tWLRH (refer to

Table 113 on page 272).

9. Repeat steps 3 to 7 until all data have been programmed.

A more efficient data tra nsfer 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 and 2c. PCWORD (refer to

Table 107 on page 262) 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 a ll instruction word s in the pag e, starting with the LSB

of the first instruction in the page and ending with the MSB of the last instruction in the

page.

6. Enter JTAG instruction PROG_COMMANDS.

7. Write the page using pr ogramming instruction 2g.

8. Poll for Flash write complete using programming instruction 2h, or w ait for tWLRH (refer to

Table 113 on page 272).

9. Repeat steps 3 to 8 until all data have been programmed.

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Reading the Flash 1. Enter JTAG instruction PROG_COMMANDS.

2. Enable Flash read using programming instruction 3a.

3. Load address using programming instructions 3b and 3c.

4. Read data using programming instruction 3d.

5. Repeat steps 3 and 4 until all data have been re ad .

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.

3. Load the page address using programming instructions 3b and 3c. PCWORD (refer to

Table 107 on page 262) is used to address within one page and must be written as 0.

4. Enter JTAG instruction PROG_PAGEREAD.

5. Read the entire page by shifting out all instruction words in the page, starting with the

LSB of the first instruction in the page and ending with the MSB of the last instruction in

the page. Remember that the first 8 bits shifted out should be ignored.

6. Enter JTAG instruction PROG_COMMANDS.

7. Repeat steps 3 to 6 until all data have been read.

Programming the

EEPROM Before programming the EEPROM a Chip Erase must be performed. See “Performing Chip

Erase” on page 288.

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 113 on page 272).

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

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 re ad .

Note that the PROG_PAGEREAD instruction can not be used when reading the EEPROM

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Programming the

Fuses 1. Enter JTAG instruction PROG_COMMANDS.

2. Enable Fuse write using programming instruction 6a.

3. Load data High by te 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 progr amming instruction 6d, or wait for tWLRH (refer to

Table 113 on page 272).

6. Load data Low byte using prog r amming i nstructions 6e. A “0” will progr am the fuse , a “1”

will unprogram the fuse.

7. Write Fuse Low byte using programming instruction 6f.

8. Poll for Fuse write complete using progr amming instruction 6g, or wait for tWLRH (refer to

Table 113 on page 272).

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 corre-

sponding Lock bit, a “1” will leave the Lock bit unchanged.

4. Write Loc k bits using programming instruction 7c.

5. Poll f or Lock bit write complete using programming instruction 7d, or wait for tWLRH (refer

to Table 113 on page 272).

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 Lo ck bits, use programming instruction 8e.

To only read Fuse High byte, use programming instruction 8b.

To only read Fuse Low b yte, use programming instruction 8c.

To only read Lock bits, use programming instruction 8d.

Reading the Sig nature

Bytes 1. Enter JTAG instruction PROG_COMMANDS.

2. Enable Signature byte read using programming instruction 9a.

3. Load address $00 using programming instruction 9b.

4. Read first signature byte using programming instruction 9c.

5. Repeat steps 3 and 4 with address $01 and address $02 to read the second and third

signature bytes, respectively.

Reading the

Calibration Byte 1. Enter JTAG instruction PROG_COMMANDS.

2. Enable Calibration byte read using programming instruction 10a.

3. Load address $00 using programming instruction 10b.

4. Read the calibration byte using prog ramming instruction 10c.

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Electrical Characteristics

Absolute Maximum Ratings*

DC Characteristics

Operating Temperature.................................. -55°C to +125°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 rating

conditions for extended periods may affect

device reliability.

Storage Temperature..................................... -65°C to +150°C

Voltage on any Pin except RESET

with respect to Ground ................................-0.5V to VCC+0.5V

Voltage on RESET with respect to Ground......-0.5V to +13.0V

Maximum Operating Voltage ............................................ 6.0V

DC Current per I/O Pin............................................... 40.0 mA

DC Current VCC and GND Pins................200.0 mA PDIP and

400.0 mA TQFP/MLF

TA = -40°C to 85°C, VCC = 2.7V to 5.5V (Unless Otherwise Noted)

Symbol Parameter Condition Min Typ Max Units

VIL Input Low Voltage except

XTAL1 and RESET pins VCC = 2.7 - 5.5 -0.5 0.2 VCC(1)

VIH Input High V oltage e xcept

XTAL1 and RESET pins VCC = 2.7 - 5.5 0.6 VCC(2) VCC +0.5

VIH1 Input High Voltage

XTAL1 pin VCC = 2.7 - 5.5 0.7 VCC(2) VCC +0.5

VIL1 Input Low Voltage

XTAL1 pin VCC = 2.7 - 5.5 -0.5 0.1 VCC(1)

VIH2 Input High Voltage

RESET pin VCC = 2.7 - 5.5 0.9 VCC(2) VCC +0.5

VIL2 Input Low Voltage

RESET pin VCC = 2.7 - 5.5 -0.5 0.2 VCC

VOL Output Low Voltage(3)

(Ports A,B,C,D) IOL = 20 mA, VCC = 5V

IOL = 10 mA, VCC = 3V 0.7

0.5 V

VOH Output High Voltage(4)

(Ports A,B,C,D) IOH = -20 mA, VCC = 5V

IOH = -10 mA, VCC = 3V 4.2

2.2 V

IIL Input Leakage

Current I/O Pin Vcc = 5.5V, pin low

(absolute value) 1µA

IIH Input Leakage

Current I/O Pin Vcc = 5.5V, pin high

(absolute value) 1

RRST Reset Pull-up Resistor 30 60 kΩ

Rpu I/O Pin Pull-up Resistor 20 50

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Notes: 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 por t can sink more than the test conditions (20 mA at Vcc = 5V, 10 mA at Vcc = 3V) under stead y state

conditions (non-transient), the following must be obser ved:

PDIP Package:

1] The sum of all IOL, for all ports, should not exceed 200 mA.

2] The sum of all IOL, for port A0 - A7 , should not exceed 100 mA.

3] The sum of all IOL, for ports B0 - B7,C0 - C7, D0 - D7 and XTAL2, should not exceed 100 mA.

TQFP and QFN/MLF Package:

1] The sum of all IOL, for all ports, should not exceed 400 mA.

2] The sum of all IOL, for ports A0 - A7, shou ld not exceed 100 mA.

3] The sum of all IOL, for ports B0 - B4, shou ld not exceed 100 mA.

4] The sum of all IOL, for ports B3 - B7, XTAL2, D0 - D2, should not e xceed 100 mA.

5] The sum of all IOL, for ports D3 - D7, should not exceed 100 mA.

6] The sum of all IOL, for ports C0 - C7, should not exceed 100 mA.

If IOL exceeds the test condition, VOL may exceed th e related specificatio n. 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 (20 mA at Vcc = 5V, 10 mA at Vcc = 3V) under steady state

conditions (non-transient), the following must be obser ved:

PDIP Package:

1] The sum of all IOH, for all ports, should not exceed 200 mA.

2] The sum of all IOH, for port A0 - A7, shou ld not exceed 100 mA.

3] The sum of all IOH, for ports B0 - B7,C0 - C7, D0 - D7 and XTAL2, should not exceed 100 mA.

TQFP and QFN/MLF Package:

1] The sum of all IOH, for all ports, should not exceed 400 mA.

2] The sum of all IOH, for ports A0 - A7, should not exceed 100 mA.

3] The sum of all IOH, for ports B0 - B4, should not exceed 100 mA.

4] The sum of all IOH, for ports B3 - B7, XTAL2, D0 - D2, should not exceed 100 mA.

ICC

Power Supply Current

Active 1 MHz, VCC = 3V

(ATmega16L) 1.1

Active 4 MHz, VCC = 3V

(ATmega16L) 3.8 5

Active 8 MHz, VCC = 5V

(ATmega16)12 15

Idle 1 MHz, VCC = 3V

(ATmega16L) 0.35

Idle 4 MHz, VCC = 3V

(ATmega16L) 1.2 2

Idle 8 MHz, VCC = 5V

(ATmega16)5.5 7

Power-down Mode(5) WDT enabled, VCC = 3V <8 15 µA

WDT disabled, VCC = 3V < 1 4

VACIO Analog Comparator

Input Offset Voltage VCC = 5V

Vin = VCC/2 40 mV

IACLK Analog Comparator

Input Leakage Current VCC = 5V

Vin = VCC/2 -50 50 nA

tACPD Analog Comparator

Propagation Delay VCC = 2.7V

VCC = 4.0V 750

500 ns

TA = -40°C to 85°C, VCC = 2.7V to 5.5V (Unless Otherwise Noted) (Continued)

Symbol Parameter Condition Min Typ Max Units

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5] The sum of all IOH, for ports D3 - D7, should not exceed 100 mA.

6] The sum of all IOH, for ports C0 - C7, should not exceed 100 mA.If IOH exceeds the test condition, VOH may exceed the

related specification. Pins are not guaranteed to source current greater than the listed test condition.

5. Minimum VCC for Power-down is 2.5V.

External Cloc k

Drive Waveforms Figure 145. External Clock Drive Waveforms

External Cloc k

Drive

Note: 1. Refer to “External Clock” on page 31 for details.

Notes: 1. R should be in the range 3 kΩ - 100 kΩ, and C should be at least 20 pF.

2. The frequency will vary with package type and board layout.

IL1

IH1

Table 118. External Clock Drive(1)

Symbol Parameter

VCC = 2.7V to 5.5VV

CC = 4.5V to 5.5V

UnitsMin Max Min Max

1/tCLCL Oscillator Frequency 0 8 0 16 MHz

tCLCL Clock Period 125 62.5

nstCHCX High Time 50 25

tCLCX Low Time 50 25

tCLCH Rise Time 1.6 0.5 μs

tCHCL Fall Time 1.6 0.5

ΔtCLCL

Change in period from

one clock cycle to the

next 22%

Table 119. External RC Oscillator, Typical Frequencies (VCC = 5)

R [kΩ](1) C [pF] f(2)

33 22 650 kHz

10 22 2.0 MHz

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ATmega16(L)

Two-wire Serial Interface Characteristics

Table 120 describes the requirements for devices connected to the Two-wire Serial Bus. The ATmega16 Two-wire Serial

Interface meets or exceeds these requirements under the noted conditions.

Timing symbols refer to Figure 146.

Notes: 1. In ATmeg a16, this parameter is characterized and not 100% tested.

2. Required only for fSCL > 100 kHz.

3. Cb = capacitance of one bus line in pF.

4. fCK = CPU clock frequency

Table 120. Two-wire Serial Bus Re quirements

Symbol Parameter Condition Min Max Units

VIL Input Low-voltage -0.5 0.3 VCC

VIH Input High-voltage 0.7 VCC VCC + 0.5

Vhys(1) Hysteresis of Schmitt Trigger Inputs 0.05 VCC(2) –

VOL(1) Output Low-voltage 3 mA sink current 0 0.4

tr(1) Rise Time for both SDA and SCL 20 + 0.1Cb(3)(2) 300

nstof(1) Output Fall Time from VIHmin to VILmax 10 pF < Cb < 400 pF(3) 20 + 0.1Cb(3)(2) 250

tSP(1) Spikes Suppressed by Input Filter 0 50(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) 0 400 kHz

Rp Value of Pull-up resistor

fSCL ≤ 100 kHz

fSCL > 100 kHz

tHD;STA Hold Time (repeated) START Condition fSCL ≤ 100 kHz 4.0 –

µs

fSCL > 100 kHz 0.6 –

tLOW Low Period of th e SCL Clock fSCL ≤ 100 kHz 4.7 –

fSCL > 100 kHz 1.3 –

tHIGH High period of the SCL clock fSCL ≤ 100 kHz 4.0 –

fSCL > 100 kHz 0.6 –

tSU;STA Set-up time for a repeated START condition

fSCL ≤ 100 kHz 4.7 –

fSCL > 100 kHz 0.6 –

tHD;DAT Data hold time fSCL ≤ 100 kHz 0 3.45

fSCL > 100 kHz 0 0.9

tSU;DAT Data setup time fSCL ≤ 100 kHz 250 – ns

fSCL > 100 kHz 100 –

tSU;STO Setup time for STOP condition fSCL ≤ 100 kHz 4.0 –

µs

fSCL > 100 kHz 0.6 –

tBUF Bus free time between a ST OP and START

condition

fSCL ≤ 100 kHz 4.7 –

fSCL > 100 kHz 1.3 –

VCC 0.4V–

3 mA

----------------------------

1000ns

-------------------

VCC 0.4V–

3 mA

----------------------------

300ns

----------------

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5. This requirement applies to all ATmega16 Two-wire Serial Interface operation. Other devices

connected to the Two-wire Serial Bus need only obey the general fSCL requirement.

Figure 146. Two-wire Serial Bus Timing

SPI Timing

Characteristics See Figure 147 and Figure 148 for details.

SU;STA

LOW

HIGH

LOW

HD;STA

HD;DAT

SU;DAT

SU;STO

BUF

SCL

SDA

Table 121. SPI Timing Parameters

Description Mode Min Typ Max

1 SCK peri od Master See Table 58

2 SCK high/low Master 50% duty cycle

3 Rise/Fall time Master 3.6

4 Setup Master 10

5 Hold Master 10

6 Out to SCK Maste r 0.5 • tSCK

7 SCK to out Master 10

8 SCK to out high Master 10

9SS low to out Slave 15

10 SCK per iod Slave 4 • tSCK

11 SCK high/low Slave 2 • tSCK

12 Rise/Fall time Slave 1.6 µs

13 Setup Slave 10

14 Hold Slave 10

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 2 • tSCK

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ATmega16(L)

Figure 147. SPI Interface Timing Requirements (Master Mode)

Figure 148. SPI Interface Timing Requirements (Slave Mode)

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

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ADC Characteristics

Table 122. ADC Characteristics

Symbol Parameter Condition Min(1) Typ(1) Max(1) Units

Resolution

Single Ended Conversion 10

Bits

Differential Conversion

Gain = 1x or 10x 8

Differential Conversion

Gain = 200x 7

Absolute Accuracy (Including INL, DNL,

Quantization Error, Gain, and Offset Error).

Single Ended Conversion

VREF = 4V, VCC = 4V

ADC clock = 200 kHz 1.5 2.5

LSB

Single Ended Conversion

VREF = 4V, VCC = 4V

ADC cloc k = 1 MHz 34

Single Ended Conversion

VREF = 4V, VCC = 4V

ADC clock = 200 kHz

Noise Reduction mode

1.5

Single Ended Conversion

VREF = 4V, VCC = 4V

ADC cloc k = 1 MHz

Noise Reduction mode

Integral Non-linearity (INL) Single Ended Conversion

VREF = 4V, VCC = 4V

ADC clock = 200 kHz 1

Differential Non-line arity (DNL) Single Ended Conversion

VREF = 4V, VCC = 4V

ADC clock = 200 kHz 0.5

Gain Error Single Ended Conversion

VREF = 4V, VCC = 4V

ADC clock = 200 kHz 1

Offset Error Single Ended Conversion

VREF = 4V, VCC = 4V

ADC clock = 200 kHz

Conversion Time Free Running Conversion 13 260 µs

Clock Frequency 50 1000 kHz

AVCC Analog Supply Voltage VCC - 0.3(2) VCC + 0.3(3)

VREF Reference Voltage Single Ended Conversion 2.0 AVCC

Differential Conversion 2.0 AVCC - 0.2

VIN

Input voltage Single ended channels GND VREF

Differential channels 0 VREF

Input bandwidth Single ended channels 38.5 kHz

Differential channels 4

298

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ATmega16(L)

Notes: 1. Values are guidelines only.

2. Minimum for AVCC is 2.7V.

3. Maximum for AVCC is 5.5V.

VINT Internal Voltag e Reference 2.3 2.6 2.9 V

RREF Reference Input Resistance 32 kΩ

RAIN Analog Input Resistance 100 MΩ

Table 122. ADC Characterist ics (Continued)

Symbol Parameter Condition Min(1) Typ(1) Max(1) Units

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ATmega16

Typical

Characteristics

The following charts show typical behavior. These figures are not tested during m anufacturing.

All current consumption 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.

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, loading of I/O pins, switching rate of I/O pins, code executed and ambient tempera-

ture. The dominating factors are operating volt age and frequency.

The current drawn fr om capacitive loaded pin s may be estimated (for on e 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 cur-

rent drawn by the Watchdog Timer.

Active Supply Current Figure 149. Active Supply Current vs. Frequency (0 .1 MHz - 1.0 MHz

0.2

0.4

0.6

0.8

1.2

1.4

1.6

1.8

0 0.1 0.2 0.30.4 0.5 0.6 0.7 0.80.9 1

Frequency (MHz)

(mA)

5.5V

4.5V

4.0V

3.6V

3.3V

3.0V

5.0V

2.7V

300

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ATmega16(L)

Figure 150. Active Supply Current vs. Frequency (1 MHz - 20 MHz)

Figure 151. Active Supply Current vs. VCC (Internal RC Oscillator, 8 MHz)

0246810 12 14 16 1820

Frequency (MHz)

ICC (mA)

5.5V

4.5V

4.0V

3.6V

3.3V

3.0V

5.0V

2.7V

2.5 33.5 4 4.5 5 5.5

(V)

(mA)

°C

-40

°C

301

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ATmega16(L)

Figure 152. Active Supply Current vs. VCC (Internal RC Oscillator, 4 MHz)

Figure 153. Active Supply Current vs. VCC (Internal RC Oscillator, 2 MHz)

2.5 33.5 4 4.5 5 5.5

(V)

(mA)

°C

-40

°C

0.5

1.5

2.5

3.5

4.5

2.5 33.5 4 4.5 5 5.5

(V)

(mA)

85°C

25°C

-40°C

302

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ATmega16(L)

Figure 154. Active Supply Current vs. VCC (Internal RC Oscillator, 1 MHz)

Figure 155. Active Supply Current vs. VCC (32 kHz External Oscillator)

0.5

1.5

2.5

2.5 33.5 4 4.5 5 5.5

(V)

(mA)

°C

-40

°C

100

120

140

160

180

2.5 33.5 4 4.5 5 5.5

(V)

(µA)

25°C

85°C

303

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ATmega16(L)

Idle Supply Current Figure 156. Idle Supply Current vs. Frequency (0.1 MHz - 1.0 MHz)

Figure 157. Idle Supply Current vs. Frequency (1 MHz - 20 MHz)

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 0.1 0.2 0.30.4 0.5 0.6 0.7 0.80.9 1

Frequency (MHz)

(mA)

5.5V

4.5V

4.0

3.6V

3.3V

3.0V

5.0V

2.7V

0246810 12 14 16 1820

Frequency (MHz)

(mA)

5.5V

4.5V

4.0V

3.6V

3.3V

3.0V

5.0V

2.7V

304

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ATmega16(L)

Figure 158. Idle Supply Current vs. VCC (Internal RC Oscillator, 8 MHz)

Figure 159. Idle Supply Current vs. VCC (Internal RC Oscillator, 4 MHz)

2.5 33.5 4 4.5 5 5.5

(V)

(mA)

°C

-40

°C

0.5

1.5

2.5

3.5

2.5 33.5 4 4.5 5 5.5

(V)

(mA)

°C

-40

305

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ATmega16(L)

Figure 160. Idle Supply Current vs. VCC (Internal RC Oscillator, 2 MHz)

Figure 161. Idle Supply Current vs. VCC (Internal RC Oscillator, 1 MHz)

0.2

0.4

0.6

0.8

1.2

1.4

1.6

1.8

2.5 33.5 4 4.5 5 5.5

(V)

(mA)

°C

-40

°C

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

2.5 33.5 4 4.5 5 5.5

(V)

(mA)

°C

306

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ATmega16(L)

Figure 162. Idle Supply Current vs. VCC (32 kHz External Oscillator)

Power-Down Supply

Current Figure 163. Power-Down Supply Current vs. VCC (Watchdog Timer Disabled)

2.5 33.5 4 4.5 5 5.5

(V)

(µA)

°C

0.5

1.5

2.5

2.5 33.5 4 4.5 5 5.5

(V)

(µA)

°C

-40

307

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ATmega16(L)

Figure 164. Power-Down Supply Current vs. VCC (Watchdog Timer Enabled)

Power-Save Supply

Current Figure 165. Power-Save Supply Current vs. VCC (Watchdog Timer Disabled)

2.5 33.5 4 4.5 5 5.5

(V)

(µA)

°C

-40

°C

2.5 33.5 4 4.5 5 5.5

(V)

(µA)

308

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ATmega16(L)

Standby Supply

Current Figure 166. Standby Supply Current vs. VCC (455 kHz Resonator, Watchdog Timer Disabled)

Figure 167. Standby Supply Current vs. VCC (1 MHz Resonator, Watchdog Timer Disabled)

2.5 33.5 4 4.5 5 5.5

(V)

(µA)

2.5 33.5 4 4.5 5 5.5

(V)

(µA)

309

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ATmega16(L)

Figure 168. Standby Supply Current vs. VCC (2 MHz Resonator, Watchdog Timer Disabled)

Figure 169. Standby Supply Current vs. VCC (2 MHz Xtal, Watchdog Timer Disabled)

2.5 33.5 4 4.5 5 5.5

(V)

(µA)

2.5 33.5 4 4.5 5 5.5

(V)

(µA)

310

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ATmega16(L)

Figure 170. Standby Supply Current vs. VCC (4 MHz Resonator, Watchdog Timer Disabled)

Figure 171. Standby Supply Current vs. VCC (4 MHz Xtal, Watchdog Timer Disabled)

100

120

2.5 33.5 4 4.5 5 5.5

(V)

(µA)

100

120

2.5 33.5 4 4.5 5 5.5

(V)

(µA)

311

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ATmega16(L)

Figure 172. Standby Supply Current vs. VCC (6 MHz Resonator, Watchdog Timer Disabled)

Figure 173. Standby Supply Current vs. VCC (6 MHz Xtal, Watchdog Timer Disabled)

100

120

140

2.5 33.5 4 4.5 5 5.5

(V)

(µA)

100

120

140

160

180

2.5 33.5 4 4.5 5 5.5

(V)

(µA)

312

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ATmega16(L)

Pin Pullup Figure 174. I/O Pin Pull-Up Resistor Current vs. Input Voltage (VCC = 5V)

Figure 175. I/O Pin Pull-Up Resistor Current vs. Input Voltage (VCC = 2.7V)

100

120

140

160

0123

VIO (V)

IIO (µA)

85°C 25°C

-40°C

100

0 0.5 1 1.5 2 2.5 3

VIO (V)

IIO (µA)

85°C 25°C

-40°C

313

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ATmega16(L)

Figure 176. Reset Pull-Up Resistor Current vs. Reset Pin Voltage (VCC = 5V)

Figure 177. Reset Pull-Up Resistor Current vs. Reset Pin Voltage (VCC = 2.7V)

100

120

0123

VRESET (V)

IRESET (µA)

85°C

25°C

-40°C

0 0.5 1 1.5 2 2.5 3

VRESET (V)

IRESET (µA)

85°C

25°C

-40°C

314

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ATmega16(L)

Pin Driver Strength Figure 178. I/O Pin Source Current vs. Output Voltage (V CC = 5V)

Figure 179. I/O Pin Source Current vs. Outp ut Voltage (V CC = 2.7V)

01234

VOH (V)

IOH (mA)

85°C

25°C

-40°C

0 0.5 1 1.5 2 2.5 3

VOH (V)

IOH (mA)

85°C

25°C

-40°C

315

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ATmega16(L)

Figure 180. I/O Pin Sink Current vs. Output Voltage (VCC = 5V)

Figure 181. I/O Pin Sink Current vs. Output Voltage (VCC = 2.7V)

0 0.5 1 1.5 2 2.5

VOL (V)

IOL (mA)

85°C

25°C

-40°C

0 0.5 1 1.5 2 2.5

VOL (V)

IOL (mA)

85°C

25°C

-40°C

316

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ATmega16(L)

Pin Thresholds And

Hysteresis Figure 182. I/O Pin Input Thresh o ld Volta g e vs. VCC (VIH, I/O Pin Read As '1')

Figure 183. I/O Pin Input Threshold Voltage vs. VCC (VIL, I/O Pin Read As '0')

0.5

1.5

2.5

2.5 33.5 4 4.5 5 5.5

(V)

Threshold (V)

°C

-40

°C

0.5

1.5

2.5

2.5 33.5 4 4.5 5 5.5

(V)

Threshold (V)

°C

-40

°C

317

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ATmega16(L)

Figure 184. I/O Pin Input Hysteresis vs. VCC

Figure 185. Reset Input Threshold Voltage vs. VCC (VIH, Reset Pin Read As '1')

0.2

0.4

0.6

0.8

2.5 33.5 4 4.5 5 5.5

(V)

Hysteresis (mV)

85°C

°C

-40°C

0.5

1.5

2.5

2.5 33.5 4 4.5 5 5.5

(V)

Threshold (V)

°C

-40

°C

318

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ATmega16(L)

Figure 186. Reset Input Threshold Voltage vs. VCC (VIL, Reset Pin Read As '0')

Figure 187. Reset Input Pin Hysteresis vs. VCC

0.5

1.5

2.5

2.5 33.5 4 4.5 5 5.5

(V)

Threshold (V)

°C

-40

°C

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

2.5 33.5 4 4.5 5 5.5

(V)

Hysteresis (mV)

85°C

25°C

-40°C

319

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ATmega16(L)

Bod Thresholds An d

Analog Comparator

Offset

Figure 188. Bod Thre sholds vs. Temperature (Bo dlevel is 4.0V)

Figure 189. Bod Thre sholds vs. Temperature (Bo dlevel is 2.7V)

3.7

3.8

3.9

4.1

-50 -40 -30 -20 -10 0 10 20 304050607080 90 100

Temperature (°C)

Threshold (V)

Rising V

Falling V

2.4

2.5

2.6

2.7

2.8

2.9

-50 -40 -30 -20 -10 0 10 20 304050607080 90 100

Temperature (°C)

Threshold (V)

Rising V

Falling V

320

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ATmega16(L)

Figure 190. Bandga p Voltage vs. VCC

Figure 191. Analog Comparator Offset Voltage vs. Common Mode Voltage (VCC = 5V)

1.225

1.23

1.235

1.24

1.245

1.25

2.5 33.5 4 4.5 5 5.5

Vcc (V)

Bandgap Voltage (V)

85°

25°

-40°

-0.008

-0.007

-0.006

-0.005

-0.004

-0.003

-0.002

-0.001

0 0.5 1 1.5 2 2.5 33.5 4 4.5 5

Common Mode Voltage (V)

Comparator Offset Voltage (V)

°C

321

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ATmega16(L)

Figure 192. Analog Comparator Offset Voltage vs. Common Mode Voltage (VCC = 3V)

Internal Oscillator

Speed Figure 193. Watchdog Oscillator Frequency vs. VCC

-0.007

-0.006

-0.005

-0.004

-0.003

-0.002

-0.001

0 0.5 1 1.5 2 2.5 3

Common Mode Voltage (V)

Comparator Offset Voltage (V)

85°C

25°C

950

960

970

980

990

1000

1010

1020

1030

1040

1050

1060

2.5 33.5 4 4.5 5 5.5

VCC (V)

FRC (kHz)

-40

322

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ATmega16(L)

Figure 194. Calibrated 8 MHz RC Oscillator Frequency vs. Temperature

Figure 195. Calibrated 8 MHz RC Oscillator Frequency vs. VCC

6.6

7.4

7.8

8.2

8.6

-60 -40 -20 0 20 40 60 80 100

Temperature (°C)

FRC (MHz)

5.5V

2.7V

4.0V

6.5

6.7

6.9

7.1

7.3

7.5

7.7

7.9

8.1

8.3

8.5

2.5 33.5 4 4.5 5 5.5

VCC (V)

FRC (MHz)

-40

323

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ATmega16(L)

Figure 196. Calibrated 8 MHz RC Oscillator Frequency vs. Osccal Value

Figure 197. Calibrated 4 MHz RC Oscillator Frequency vs. Temperature

016324864 80 96 112 128144 160 176 192 208224 240

OSCCAL VALUE

(MHz)

3.6

3.7

3.8

3.9

4.1

-60 -40 -20 0 20 40 60 80 100

Temperature (°C)

FRC (MHz)

5.5V

2.7V

4.0V

324

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ATmega16(L)

Figure 198. Calibrated 4 MHz RC Oscillator Frequency vs. VCC

Figure 199. Calibrated 4 MHz RC Oscillator Frequency vs. Osccal Value

3.5

3.6

3.7

3.8

3.9

4.1

4.2

2.5 33.5 4 4.5 5 5.5

VCC (V)

FRC (MHz)

-40

1.5

2.5

3.5

4.5

5.5

6.5

016324864 80 96 112 128144 160 176 192 208224 240

OSCCAL VALUE

(MHz)

325

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ATmega16(L)

Figure 200. Calibrated 2 MHz RC Oscillator Frequency vs. Temperature

Figure 201. Calibrated 2 MHz RC Oscillator Frequency vs. VCC

1.8

1.85

1.9

1.95

2.05

2.1

-60 -40 -20 0 20 40 60 80 100

Temperature (°C)

FRC (MHz)

5.5V

2.7V

4.0V

1.7

1.8

1.9

2.1

2.2

2.5 33.5 4 4.5 5 5.5

VCC (V)

FRC (MHz)

-40

°C

326

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ATmega16(L)

Figure 202. Calibrated 2 MHz RC Oscillator Frequency vs. Osccal Value

Figure 203. Calibrated 1 MHz RC Oscillator Frequency vs. Temperature

0.8

1.2

1.4

1.6

1.8

2.2

2.4

2.6

2.8

3.2

016324864 80 96 112 128144 160 176 192 208224 240

OSCCAL VALUE

(MHz)

0.93

0.94

0.95

0.96

0.97

0.98

0.99

1.01

1.02

-60 -40 -20 0 20 40 60 80 100

Temperature (°C)

FRC (MHz)

5.5V

2.7V

4.0V

327

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ATmega16(L)

Figure 204. Calibrated 1 MHz RC Oscillator Frequency vs. VCC

Figure 205. Calibrated 1 MHz RC Oscillator Frequency vs. Osccal Value

0.9

0.95

1.05

1.1

2.5 33.5 4 4.5 5 5.5

VCC (V)

FRC (MHz)

°C

-40

0.4

0.6

0.8

1.2

1.4

1.6

016324864 80 96 112 128144 160 176 192 208224 240

OSCCAL VALUE

FRC (MHz)

328

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Current Consumption

Of Peripheral Units Figure 206. Brownout Detector Current vs. VCC

Figure 207. ADC Current vs. VCC(Aref = AVCC)

2.5 33.5 4 4.5 5 5.5

VCC (V)

ICC (µA)

°C

-40

100

150

200

250

300

350

2.5 33.5 4 4.5 5 5.5

(V)

(µA)

°C

-40

°C

329

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Figure 208. Aref External Reference Current vs. V CC

Figure 209. 32khz Tosc Current v s. VCC (Watchdog Timer Disabled)

100

120

140

160

180

2.5 33.5 4 4.5 5 5.5

VCC (V)

IAREF (µA)

°C

-40

°C

2.5 33.5 4 4.5 5 5.5

(V)

(µA)

330

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ATmega16(L)

Figure 210. Watchdog Timer Current vs. VCC

Figure 211. Programming Current vs. VCC

2.5 33.5 4 4.5 5 5.5

(V)

(µA)

°C

-40

2.5 33.5 4 4.5 5 5.5

(V)

(mA)

85°

25°

-40°

331

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Address Name Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0 Page

$3F ($5F) SREG I T H S V N Z C 9

$3E ($5E) SPH – – – – – SP10 SP9 SP8 12

$3D ($5D) SPL SP7 SP6 SP5 SP4 SP3 SP2 SP1 SP0 12

$3C ($5C) OCR0 Timer/Counter0 Output Compare Register 85

$3B ($5B) GICR INT1 INT0 INT2 ––– IVSEL IVCE 48, 69

$3A ($5A) GIFR INTF1 INTF0 INTF2 – – – – –70

$39 ($59) TIMSK OCIE2 TOIE2 TICIE1 OCIE1A OCIE1B TOIE1 OCIE0 TOIE0 85, 115, 133

$38 ($58) TIFR OCF2 TOV2 ICF1 OCF1A OCF1B TOV1 OCF0 TOV0 86, 115, 133

$37 ($57) SPMCR SPMIE RWWSB – RWWSRE BLBSET PGWRT PGERS SPMEN 250

$36 ($56) TWCR TWINT TWEA TWSTA TWSTO TWWC TWEN – TWIE 180

$35 ($55) MCUCR SM2 SE SM1 SM0 ISC11 ISC10 ISC01 ISC00 32, 68

$34 ($54) MCUCSR JTD ISC2 – JTRF WDRF BORF EXTRF PORF 41, 69, 231

$33 ($53) TCCR0 FOC0 WGM00 COM01 COM00 WGM01 CS02 CS01 CS00 83

$32 ($52) TCNT0 Timer/Counter0 (8 Bits) 85

$31(1) ($51)(1) OSCCAL Oscillator Calibration Register 30

OCDR On-Chip Debug Register 227

$30 ($50) SFIOR ADTS2 ADTS1 ADTS0 – ACME PUD PSR2 PSR10 57,88,134,201,221

$2F ($4F) TCCR1A COM1A1 COM1A0 COM1B1 COM1B0 FOC1A FOC1B WGM11 WGM10 110

$2E ($4E) TCCR1B ICNC1 ICES1 – WGM13 WGM12 CS12 CS11 CS10 113

$2D ($4D) TCNT1H Timer/Counter1 – Counter Register High Byte 114

$2C ($4C) TCNT1L Timer/Counter1 – Counter Register Low Byte 114

$2B ($4B) OCR1AH Timer/Counter1 – Output Compare Register A High Byte 114

$2A ($4A) OCR1AL Timer/Counter1 – Output Compare Register A Low Byte 114

$29 ($49) OCR1BH Timer/Counter1 – Output Compare Register B High Byte 114

$28 ($48) OCR1BL Timer/Counter1 – Output Compare Register B Low Byte 114

$27 ($47) ICR1H Timer/Counter1 – Input Capture Register High Byte 114

$26 ($46) ICR1L Timer/Counter1 – Input Capture Register Low Byte 114

$25 ($45) TCCR2 FOC2 WGM20 COM21 COM20 WGM21 CS22 CS21 CS20 128

$24 ($44) TCNT2 Timer/Counter2 (8 Bits) 130

$23 ($43) OCR2 Timer/Counter2 Output Compare Register 130

$22 ($42) ASSR –––– AS2 TCN2UB OCR2UB TCR2UB 131

$21 ($41) WDTCR – – – WDTOE WDE WDP2 WDP1 WDP0 43

$20(2) ($40)(2) UBRRH URSEL ––– UBRR[11:8] 167

UCSRC URSEL UMSEL UPM1 UPM0 USBS UCSZ1 UCSZ0 UCPOL 166

$1F ($3F) EEARH – – – – – – –EEAR8 19

$1E ($3E) EEARL EEPROM Address Register Low Byte 19

$1D ($3D) EEDR EEPROM Data Register 19

$1C ($3C) EECR –––– EERIE EEMWE EEWE EERE 19

$1B ($3B) PORTA PORTA7 PORTA6 PORTA5 PORTA4 PORTA3 PORTA2 PORTA1 PORTA0 66

$1A ($3A) DDRA DDA7 DDA6 DDA5 DDA4 DDA3 DDA2 DDA1 DDA0 66

$19 ($39) PINA PINA7 PINA6 PINA5 PINA4 PINA3 PINA2 PINA1 PINA0 66

$18 ($38) PORTB PORTB7 PORTB6 PORTB5 PORTB4 PORTB3 PORTB2 PORTB1 PORTB0 66

$17 ($37) DDRB DDB7 DDB6 DDB5 DDB4 DDB3 DDB2 DDB1 DDB0 66

$16 ($36) PINB PINB7 PINB6 PINB5 PINB4 PINB3 PINB2 PINB1 PINB0 66

$15 ($35) PORTC PORTC7 PORTC6 PORTC5 PORTC4 PORTC3 PORTC2 PORTC1 PORTC0 67

$14 ($34) DDRC DDC7 DDC6 DDC5 DDC4 DDC3 DDC2 DDC1 DDC0 67

$13 ($33) PINC PINC7 PINC6 PINC5 PINC4 PINC3 PINC2 PINC1 PINC0 67

$12 ($32) PORTD PORTD7 PORTD6 PORTD5 PORTD4 PORTD3 PORTD2 PORTD1 PORTD0 67

$11 ($31) DDRD DDD7 DDD6 DDD5 DDD4 DDD3 DDD2 DDD1 DDD0 67

$10 ($30) PIND PIND7 PIND6 PIND5 PIND4 PIND3 PIND2 PIND1 PIND0 67

$0F ($2F) SPDR SPI Data Register 142

$0E ($2E) SPSR SPIF WCOL – – – – – SPI2X 142

$0D ($2D) SPCR SPIE SPE DORD MSTR CPOL CPHA SPR1 SPR0 140

$0C ($2C) UDR USART I/O Data Register 163

$0B ($2B) UCSRA RXC TXC UDRE FE DOR PE U2X MPCM 164

$0A ($2A) UCSRB RXCIE TXCIE UDRIE RXEN TXEN UCSZ2 RXB8 TXB8 165

$09 ($29) UBRRL USART Baud Rate Register Low Byte 167

$08 ($28) ACSR ACD ACBG ACO ACI ACIE ACIC ACIS1 ACIS0 202

$07 ($27) ADMUX REFS1 REFS0 ADLAR MUX4 MUX3 MUX2 MUX1 MUX0 217

$06 ($26) ADCSRA ADEN ADSC ADATE ADIF ADIE ADPS2 ADPS1 ADPS0 219

$05 ($25) ADCH ADC Data Register High Byte 220

$04 ($24) ADCL ADC Data Register Low Byte 220

$03 ($23) TWDR Two-wire Serial Interface Data Register 182

$02 ($22) TWAR TWA6 TWA5 TWA4 TWA3 TWA2 TWA1 TWA0 TWGCE 182

332

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Notes: 1. When the OCDEN Fu se is unprogrammed, the OSCCAL Registe r is always accessed on this address. Refer to the debug-

ger specific documentation for details on how to use the OCDR Register.

2. Refer to the USART description for details on how to access UBRRH and UCSRC.

3. For co mpatibility with future devices, reser ved bits should be written to zero if accessed. Reserved I/O memory addresses

should never be written.

4. 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 se t, thus clearing the flag. The CBI and SBI instructions

work with registers $00 to $1F only.

$01 ($21) TWSR TWS7 TWS6 TWS5 TWS4 TWS3 –TWPS1 TWPS0 181

$00 ($20) TWBR Two-wire Serial Interface Bit Rate Register 180

Address Name Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0 Page

333

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ATmega16(L)

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 ← $FF − Rd Z,C,N,V 1

NEG Rd Two’s Complement Rd ← $00 − 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 • ($FF - 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 ← $FF 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

JMP k Direct Jump PC ← kNone3

RCALL k Relative Subroutine Call PC ← PC + k + 1 None 3

ICALL Indirect Call to (Z) PC ← ZNone3

CALL k Direct Subroutine Call PC ← kNone4

RET Subroutine Return PC ← STACK None 4

RETI Interrupt Return PC ← STACK I 4

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

BRVC k Branch if Overflow Flag is Cleared if (V = 0) then PC ← PC + k + 1 None 1 / 2

334

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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

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

SPM Store Program Memory (Z) ← R1:R0 None -

IN Rd, P In Port Rd ← PNone1

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

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) ← 1 SREG(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

Mnemonics Operands Description Operation Flags #Clocks

335

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CLH Clear Half Carry Flag in SREG H ← 0 H 1

MCU CONTROL INSTRUCTIONS

NOP No Operation None 1

SLEEP Sleep (see specific descr. for Sleep function) None 1

WDR Watchdog Reset (see specific descr. for WDR/timer) None 1

BREAK Break For On-Chip Debug Only None N/A

Mnemonics Operands Description Operation Flags #Clocks

336

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Ordering Information

Note: 1. Pb-free packaging complies to the European Directive for Restriction of Hazardous Substances (RoHS directive). Also

Halide free and fully Green.

Speed (MHz) Power Supply Ordering Code Package Operation Range

8 2.7V - 5.5V ATmega16L-8AU(1)

ATmega16L-8PU(1)

ATmega16L-8MU(1)

44A

40P6

44M1

Industrial

(-40oC to 85oC)

16 4.5V - 5.5V ATmega16-16AU(1)

ATmega16-16PU(1)

ATmega16-16MU(1)

44A

40P6

44M1

Industrial

(-40oC to 85oC)

Package Type

44A 44-lead, Thin (1.0 mm) Plastic Gull Wing Quad Flat Package (TQFP)

40P6 40-pin, 0.600” Wide, Plastic Dual Inline Package (PDIP)

44M1 44-pad, 7 × 7 × 1.0 mm body, lead pitch 0.50 mm, Quad Flat No-Lead/Micro Lead Frame Package (QFN/MLF)

337

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ATmega16(L)

Packaging Information

44A

2325 Orchard Parkway

San Jose, CA 95131

TITLE DRAWING NO.

REV.

44A, 44-lead, 10 x 10 mm Body Size, 1.0 mm Body Thickness,

0.8 mm Lead Pitch, Thin Profile Plastic Quad Flat Package (TQFP) B

44A

10/5/2001

PIN 1 IDENTIFIER

0˚~7˚

PIN 1

A1 A2 A

eE1 E

COMMON DIMENSIONS

(Unit of Measure = mm)

SYMBOL MIN NOM MAX NOTE

Notes: 1. This package conforms to JEDEC reference MS-026, Variation ACB.

2. Dimensions D1 and E1 do not include mold protrusion. Allowable

protrusion is 0.25 mm per side. Dimensions D1 and E1 are maximum

plastic body size dimensions including mold mismatch.

3. Lead coplanarity is 0.10 mm maximum.

A – – 1.20

A1 0.05 – 0.15

A2 0.95 1.00 1.05

D 11.75 12.00 12.25

D1 9.90 10.00 10.10 Note 2

E 11.75 12.00 12.25

E1 9.90 10.00 10.10 Note 2

B 0.30 – 0.45

C 0.09 – 0.20

L 0.45 – 0.75

e 0.80 TYP

338

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40P6

2325 Orchard Parkway

San Jose, CA 95131

TITLE DRAWING NO.

REV.

40P6, 40-lead (0.600"/15.24 mm Wide) Plastic Dual

Inline Package (PDIP) B

40P6

09/28/01

REF

SEATING PLANE

0º ~ 15º

COMMON DIMENSIONS

(Unit of Measure = mm)

SYMBOL MIN NOM MAX NOTE

A – – 4.826

A1 0.381 – –

D 52.070 – 52.578 Note 2

E 15.240 – 15.875

E1 13.462 – 13.970 Note 2

B 0.356 – 0.559

B1 1.041 – 1.651

L 3.048 – 3.556

C 0.203 – 0.381

eB 15.494 – 17.526

e 2.540 TYP

Notes: 1. This package conforms to JEDEC reference MS-011, Variation AC.

2. Dimensions D and E1 do not include mold Flash or Protrusion.

Mold Flash or Protrusion shall not exceed 0.25 mm (0.010").

339

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44M1

TITLE DRAWING NO.GPC REV.

Package Drawing Contact:

packagedrawings@atmel.com 44M1ZWSH

44M1, 44-pad, 7 x 7 x 1.0 mm Body, Lead

Pitch 0.50 mm, 5.20 mm Exposed Pad, Thermally

Enhanced Plastic Very Thin Quad Flat No

Lead Package (VQFN)

9/26/08

COMMON DIMENSIONS

(Unit of Measure = mm)

SYMBOL MIN NOM MAX NOTE

A 0.80 0.90 1.00

A1 – 0.02 0.05

A3 0.20 REF

b 0.180.230.30

D2 5.00 5.20 5.40

6.90 7.00 7.10

E2 5.00 5.20 5.40

e 0.50 BSC

L 0.59 0.64 0.69

K 0.20 0.26 0.41

Note: JEDEC Standard MO-220, Fig. 1 (SAW Singulation) VKKD-3.

TOP VIEW

SIDE VIEW

BOTTOM VIEW

Marked Pin# 1 ID

Pin #1 Corner

SEATING PLANE

Pin #1

Triangle

Pin #1

Chamfer

(C 0.30)

Option A

Option B

Pin #1

Notch

(0.20 R)

Option C

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Errata The revision letter in this section refer s to the revision of the ATmega16 device.

ATmega16(L) Rev.

M•First Analog Comparator conversion may be delayed

•Interrupts may be lost when writing the timer registers in the asynchronous timer

•IDCODE masks data from TDI input

•Reading EEPROM by using ST or STS to set EERE bit triggers unexpected interrupt request

1. First Analog Comparator conversion may be delayed

If the device is powered by a slow rising VCC, the first Analog Comparator conversion will

take longer than expected on some devices.

Problem Fix/Workaround

When the device has been powered or reset, disable then enable theAnalog Comparator

before the fir st conversion.

2. Interrupts may be lost when writing the timer registers in the asynchronous timer

The interrupt will be lost if a timer register that is synchronized to the asynchronous timer

clock is written when th e asy nc hr on o us Time r/ Co un te r re gis te r(T C N T x) is 0x0 0.

Problem Fix / Workaround

Always check that the asynchronous Timer/Count er register ne ither have the value 0xFF nor

0x00 before writing to the asynchronous Timer Control Register(TCCRx), asynchronous

Timer Counter Register(TCNTx), or asynchronous Output Compare Register(OCRx).

3. IDCODE masks data from TDI input

The JTAG instruction IDCODE is not working correctly. Data to succeeding devices are

replaced by all-ones du ring Update-DR.

Problem Fix / Workaround

– If ATmega16 is the only device in the scan chain, the problem is not visible.

– Select the Device ID Register of th e ATmega16 b y issuing the IDCO DE instruction or

by entering the Test-Logic-Reset state of the TAP controller to read out the contents

of its Device ID Register and possibly data from succeeding devices of the scan

chain. Issue the BYPASS instruction to the ATmega16 while reading the Device ID

Registers of preceding devices of the boundary scan chain.

– If the Device IDs of all devices in the boundary scan chain must be captured

simultaneously, the ATmega16 must be the fist device in the chain.

4. Reading EEPROM by using ST or STS to set EERE bit triggers unexpected interrupt

request.

Reading EEPROM by using the ST or STS command to set the EERE bit in the EECR reg-

ister triggers an unexpected EEPROM interrupt request.

Problem Fix / Workaround

Always use OUT or SBI to set EERE in EECR.

ATmega16(L) Rev.

L•First Analog Comparator conversion may be delayed

•Interrupts may be lost when writing the timer registers in the asynchronous timer

•IDCODE masks data from TDI input

•Reading EEPROM by using ST or STS to set EERE bit triggers unexpected interrupt request

1. First Analog Comparator conversion may be delayed

If the device is powered by a slow rising VCC, the first Analog Comparator conversion will

take longer than expected on some devices.

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ATmega16(L)

Problem Fix/Workaround

When the device has been powered or reset, disable then enable theAnalog Comparator

before the fir st conversion.

2. Interrupts may be lost when writing the timer registers in the asynchronous timer

The interrupt will be lost if a timer register that is synchronized to the asynchronous timer

clock is written when th e asy nc hr on o us Time r/ Co un te r re gis te r(T C N T x) is 0x0 0.

Problem Fix / Workaround

Always check that the asynchronous Timer/Count er register ne ither have the value 0xFF nor

0x00 before writing to the asynchronous Timer Control Register(TCCRx), asynchronous

Timer Counter Register(TCNTx), or asynchronous Output Compare Register(OCRx).

3. IDCODE masks data from TDI input

The JTAG instruction IDCODE is not working correctly. Data to succeeding devices are

replaced by all-ones du ring Update-DR.

Problem Fix / Workaround

– If ATmega16 is the only device in the scan chain, the problem is not visible.

– Select the Device ID Register of th e ATmega16 b y issuing the IDCO DE instruction or

by entering the Test-Logic-Reset state of the TAP controller to read out the contents

of its Device ID Register and possibly data from succeeding devices of the scan

chain. Issue the BYPASS instruction to the ATmega16 while reading the Device ID

Registers of preceding devices of the boundary scan chain.

– If the Device IDs of all devices in the boundary scan chain must be captured

simultaneously, the ATmega16 must be the fist device in the chain.

4. Reading EEPROM by using ST or STS to set EERE bit triggers unexpected interrupt

request.

Reading EEPROM by using the ST or STS command to set the EERE bit in the EECR reg-

ister triggers an unexpected EEPROM interrupt request.

Problem Fix / Workaround

Always use OUT or SBI to set EERE in EECR.

ATmega16(L) Rev.

K•First Analog Comparator conversion may be delayed

•Interrupts may be lost when writing the timer registers in the asynchronous timer

•IDCODE masks data from TDI input

•Reading EEPROM by using ST or STS to set EERE bit triggers unexpected interrupt request

1. First Analog Comparator conversion may be delayed

If the device is powered by a slow rising VCC, the first Analog Comparator conversion will

take longer than expected on some devices.

Problem Fix/Workaround

When the device has been powered or reset, disable then enable theAnalog Comparator

before the fir st conversion.

2. Interrupts may be lost when writing the timer registers in the asynchronous timer

The interrupt will be lost if a timer register that is synchronized to the asynchronous timer

clock is written when th e asy nc hr on o us Time r/ Co un te r re gis te r(T C N T x) is 0x0 0.

342

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ATmega16(L)

Problem Fix / Workaround

Always check that the asynchronous Timer/Count er register ne ither have the value 0xFF nor

0x00 before writing to the asynchronous Timer Control Register(TCCRx), asynchronous

Timer Counter Register(TCNTx), or asynchronous Output Compare Register(OCRx).

3. IDCODE masks data from TDI input

The JTAG instruction IDCODE is not working correctly. Data to succeeding devices are

replaced by all-ones du ring Update-DR.

Problem Fix / Workaround

– If ATmega16 is the only device in the scan chain, the problem is not visible.

– Select the Device ID Register of th e ATmega16 b y issuing the IDCO DE instruction or

by entering the Test-Logic-Reset state of the TAP controller to read out the contents

of its Device ID Register and possibly data from succeeding devices of the scan

chain. Issue the BYPASS instruction to the ATmega16 while reading the Device ID

Registers of preceding devices of the boundary scan chain.

– If the Device IDs of all devices in the boundary scan chain must be captured

simultaneously, the ATmega16 must be the fist device in the chain.

4. Reading EEPROM by using ST or STS to set EERE bit triggers unexpected interrupt

request.

Reading EEPROM by using the ST or STS command to set the EERE bit in the EECR reg-

ister triggers an unexpected EEPROM interrupt request.

Problem Fix / Workaround

Always use OUT or SBI to set EERE in EECR.

ATmega16(L) Rev.

J•First Analog Comparator conversion may be delayed

•Interrupts may be lost when writing the timer registers in the asynchronous timer

•IDCODE masks data from TDI input

•Reading EEPROM by using ST or STS to set EERE bit triggers unexpected interrupt request

1. First Analog Comparator conversion may be delayed

If the device is powered by a slow rising VCC, the first Analog Comparator conversion will

take longer than expected on some devices.

Problem Fix/Workaround

When the device has been powered or reset, disable then enable theAnalog Comparator

before the fir st conversion.

2. Interrupts may be lost when writing the timer registers in the asynchronous timer

The interrupt will be lost if a timer register that is synchronized to the asynchronous timer

clock is written when th e asy nc hr on o us Time r/ Co un te r re gis te r(T C N T x) is 0x0 0.

Problem Fix / Workaround

Always check that the asynchronous Timer/Count er register ne ither have the value 0xFF nor

0x00 before writing to the asynchronous Timer Control Register(TCCRx), asynchronous

Timer Counter Register(TCNTx), or asynchronous Output Compare Register(OCRx).

3. IDCODE masks data from TDI input

The JTAG instruction IDCODE is not working correctly. Data to succeeding devices are

replaced by all-ones du ring Update-DR.

Problem Fix / Workaround

343

2466T–AVR–07/10

ATmega16(L)

– If ATmega16 is the only device in the scan chain, the problem is not visible.

– Select the Device ID Register of th e ATmega16 b y issuing the IDCO DE instruction or

by entering the Test-Logic-Reset state of the TAP controller to read out the contents

of its Device ID Register and possibly data from succeeding devices of the scan

chain. Issue the BYPASS instruction to the ATmega16 while reading the Device ID

Registers of preceding devices of the boundary scan chain.

– If the Device IDs of all devices in the boundary scan chain must be captured

simultaneously, the ATmega16 must be the fist device in the chain.

4. Reading EEPROM by using ST or STS to set EERE bit triggers unexpected interrupt

request.

Reading EEPROM by using the ST or STS command to set the EERE bit in the EECR reg-

ister triggers an unexpected EEPROM interrupt request.

Problem Fix / Workaround

Always use OUT or SBI to set EERE in EECR.

ATmega16(L) Rev.

I•First Analog Comparator conversion may be delayed

•Interrupts may be lost when writing the timer registers in the asynchronous timer

•IDCODE masks data from TDI input

•Reading EEPROM by using ST or STS to set EERE bit triggers unexpected interrupt request

1. First Analog Comparator conversion may be delayed

If the device is powered by a slow rising VCC, the first Analog Comparator conversion will

take longer than expected on some devices.

Problem Fix/Workaround

When the device has been powered or reset, disable then enable theAnalog Comparator

before the fir st conversion.

2. Interrupts may be lost when writing the timer registers in the asynchronous timer

The interrupt will be lost if a timer register that is synchronized to the asynchronous timer

clock is written when th e asy nc hr on o us Time r/ Co un te r re gis te r(T C N T x) is 0x0 0.

Problem Fix / Workaround

Always check that the asynchronous Timer/Count er register ne ither have the value 0xFF nor

0x00 before writing to the asynchronous Timer Control Register(TCCRx), asynchronous

Timer Counter Register(TCNTx), or asynchronous Output Compare Register(OCRx).

3. IDCODE masks data from TDI input

The JTAG instruction IDCODE is not working correctly. Data to succeeding devices are

replaced by all-ones du ring Update-DR.

Problem Fix / Workaround

– If ATmega16 is the only device in the scan chain, the problem is not visible.

– Select the Device ID Register of th e ATmega16 b y issuing the IDCO DE instruction or

by entering the Test-Logic-Reset state of the TAP controller to read out the contents

of its Device ID Register and possibly data from succeeding devices of the scan

chain. Issue the BYPASS instruction to the ATmega16 while reading the Device ID

Registers of preceding devices of the boundary scan chain.

– If the Device IDs of all devices in the boundary scan chain must be captured

simultaneously, the ATmega16 must be the fist device in the chain.

344

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ATmega16(L)

4. Reading EEPROM by using ST or STS to set EERE bit triggers unexpected interrupt

request.

Reading EEPROM by using the ST or STS command to set the EERE bit in the EECR reg-

ister triggers an unexpected EEPROM interrupt request.

Problem Fix / Workaround

Always use OUT or SBI to set EERE in EECR.

ATmega16(L) Rev.

H•First Analog Comparator conversion may be delayed

•Interrupts may be lost when writing the timer registers in the asynchronous timer

•IDCODE masks data from TDI input

•Reading EEPROM by using ST or STS to set EERE bit triggers unexpected interrupt request

1. First Analog Comparator conversion may be delayed

If the device is powered by a slow rising VCC, the first Analog Comparator conversion will

take longer than expected on some devices.

Problem Fix/Workaround

When the device has been powered or reset, disable then enable theAnalog Comparator

before the fir st conversion.

2. Interrupts may be lost when writing the timer registers in the asynchronous timer

The interrupt will be lost if a timer register that is synchronized to the asynchronous timer

clock is written when th e asy nc hr on o us Time r/ Co un te r re gis te r(T C N T x) is 0x0 0.

Problem Fix / Workaround

Always check that the asynchronous Timer/Count er register ne ither have the value 0xFF nor

0x00 before writing to the asynchronous Timer Control Register(TCCRx), asynchronous

Timer Counter Register(TCNTx), or asynchronous Output Compare Register(OCRx).

3. IDCODE masks data from TDI input

The JTAG instruction IDCODE is not working correctly. Data to succeeding devices are

replaced by all-ones du ring Update-DR.

Problem Fix / Workaround

– If ATmega16 is the only device in the scan chain, the problem is not visible.

– Select the Device ID Register of th e ATmega16 b y issuing the IDCO DE instruction or

by entering the Test-Logic-Reset state of the TAP controller to read out the contents

of its Device ID Register and possibly data from succeeding devices of the scan

chain. Issue the BYPASS instruction to the ATmega16 while reading the Device ID

Registers of preceding devices of the boundary scan chain.

– If the Device IDs of all devices in the boundary scan chain must be captured

simultaneously, the ATmega16 must be the fist device in the chain.

4. Reading EEPROM by using ST or STS to set EERE bit triggers unexpected interrupt

request.

Reading EEPROM by using the ST or STS command to set the EERE bit in the EECR reg-

ister triggers an unexpected EEPROM interrupt request.

Problem Fix / Workaround

Always use OUT or SBI to set EERE in EECR.

345

2466T–AVR–07/10

ATmega16(L)

Datasheet

Revision

History

Please note that the referr ing page numbers in this section are referred to this document. The

referring revision in this section are referring to the docu m ent revision.

Rev. 2466T-07/10 1. Corrected use of comma in formula Rp in Table 120, “Two-wire Serial Bus Require-

ments,” on page 294.

2. Updated document according to Atmel’s Technical Terminology

3. Note 6 and Note 7 under Table 120, “Two-wire Serial Bus Requirements,” on

page 294 have been removed.

Rev. 2466S-05/09 1. Updated “Errata” on page 340.

2. Updated the last page with Atmel’s new adresses.

Rev. 2466R-06/08 1. Added “Not recommended for new designs” note in Figure on page 1.

Rev. 2466Q-05/08 1. Updated “Fast PWM Mode” on page 77 in “8-bit Timer/Counter0 with PWM” on page

71:

– Removed the last section describing how to achieve a frequency with 50% duty

cycle waveform output in fast PWM mode.

2. Removed note from Feature list in “Analog to Digital Converter” on page 204.

3. Removed note from Table 84 on page 218.

4. Updated “Ordering Information” on page 336:

- Commercial or de rin g co de s re mo ve d .

- Non Pb-free package option removed.

Rev. 2466P-08/07 1. Updated “Features” on page 1.

2. Added “Data Retention” on page 6.

3. Updated “Errata” on page 340.

4. Updated “Slave Mode” on page 140.

Rev. 2466O-03/07 1. Updated “Calibrated Internal RC Oscillator” on page 29.

2. Updated C code example in “USART Initialization” on page 149.

3. Updated “ATmega16 Boundary-scan Order” on page 241.

4. Removed “premilinary” from “ADC Characteristics” on page 297.

5. Updated from V to mV in “I/O Pin Input Hysteresis vs. VCC” on page 317.

6. Updated from V to mV in “Reset Input Pin Hysteresis vs. VCC” on page 318.

346

2466T–AVR–07/10

ATmega16(L)

Rev. 2466N-10/06 1. Updated “Timer/Counter Osci lla t or” on pa ge 31.

2. Updated “Fast PWM Mode” on pa g e 10 2.

3. Updated Table 38 on page 83, Table 40 on page 84, Table 45 on page 111, Table 47 on

page 112, Table 50 on page 128 and Table 52 on page 129.

4. Updated C code example in “USART Initialization” on page 149.

5. Updated “Errata” on page 340.

Rev. 2466M-04/06 1. Updated typos.

2. Updated “Serial Peripheral Interf ac e – SPI” o n pag e 135.

3. Updated Table 86 on page 221, Table 116 on page 276 ,Table 121 on page 295 and

Table 122 on page 297.

Rev. 2466L-06/05 1. Update d no t e in “Bit Rate Generator Unit” on page 178.

2. Updated values for VINT in “ADC Characteristics” on page 297.

3. Updated “Serial Programming In struction set” on page 276.

4. Updated USART init C-code example in “USART” on page 144.

Rev. 2466K-04/05 1. Updated “Ordering Information” on page 336.

2. MLF-package alternative changed to “Quad Flat No-Lead/Micro Lead Frame Package

QFN/MLF”.

3. Updated “Electrical Characteristics” on page 291.

Rev. 2466J-10/04 1. Updated “Ordering Information” on page 336.

Rev. 2466I-10/04 1. Removed references t o analog ground.

2. Updated Table 7 on page 28, Table 15 on page 38, Table 16 on page 42, Table 81 on

page 209, Table 116 on page 276, and Tabl e 11 9 o n page 293.

3. Updated “Pinout ATmega16” on page 2.

4. Updated features in “Analog to Digital Converter” on page 204.

5. Updated “Version” on page 229.

6. Updated “Calibration Byte” on page 261.

7. Added “Page Size” on page 262.

Rev. 2466H-12/03 1. Updated “Calibrated Internal RC Oscillat or” on page 29.

347

2466T–AVR–07/10

ATmega16(L)

Rev. 2466G-10/03 1. Removed “Preliminary” from the datasheet.

2. Changed ICP to ICP1 in the datasheet.

3. Updated “JTAG Interface and On-chip Debug System” on page 36.

4. Updated assembly and C code examples in “Watchdog Timer Control Register –

WDTCR” on page 43.

5. Updated Figure 46 on page 103.

6. Updated Table 15 on page 38, Table 82 on pag e 217 and Table 115 on page 276.

7. Updated “Test Access Port – TAP” on page 222 regarding JTAGEN.

8. Updated description for the JTD bit on page 231.

9. Added note 2 to Figure 126 on page 252.

10. Added a note regarding JTAGEN fuse to Table 105 on page 260.

11. Updated Absolute Maximum Ra tings* a nd DC Charact eristics in “Elect rical Character-

istics” on page 291.

12. Updated “ATmega16 Typical Char acteristics” on page 299.

13. Fixed typo for 16 MHz QFN/MLF package in “Ordering Information” on page 336.

14. Added a proposal for solving problems regarding the JTAG instruction IDCODE in

“Errata” on page 340.

Rev. 2466F-02/03 1. Added note about masking out unused bits when reading the Program Counter in

“Stack Pointer” on page 12.

2. Added Chip Erase as a first step in “Programming the Flash” on page 288 and “Pro-

gramming the EEPROM” on page 289.

3. Added the section “Unconnected pins” on page 55.

4. Added tips on how to disable the OCD system in “On-chip Debug System” on page

34.

5. Removed reference to the “Multi-purpose Oscillator” application note and “32 kHz

Crystal Oscillator” application note, whic h do not exist.

6. Added information about PWM symmetry for Timer0 and Timer2.

7. Added note in “Filling the Temporary Buffer (Page Loading)” on page 253 about writ-

ing to the EEPROM during an SPM Page Load.

8. Removed ADHSM completely.

348

2466T–AVR–07/10

ATmega16(L)

9. Added Table 73, “TWI Bit Rate Prescaler,” on page 182 to describe the TWPS bits in

the “TWI Status Register – TWSR” on page 181.

10. Added section “Default Clock Source” on page 25.

11. Added note about frequency variation when using an external clock. Note added in

“External Clock” on page 31. An e xtra row and a note added in Table 118 on page 293 .

12. Various minor TWI corrections.

13. Added “Power Consumption” data in “Features” on page 1.

14. Added section “EEPROM Write During Po wer-down Sleep Mode” on page 22.

15. Added note about Differential Mode with Auto Triggering in “Prescaling and Conver-

sion Timing” on page 207.

16. Added updated “Packaging Information” on page 337.

Rev. 2466E-10/02 1. Updated “DC Characteri st ic s” on pag e 291 .

Rev. 2466D-09/02 1. Changed all Flash write/erase cycles from 1,000 to 10,000.

2. Updated the following tables: Table 4 on page 26, Table 15 on page 38, Table 42 on

page 85, Table 45 on page 111, Table 46 on page 111, Table 59 on page 143, Table 67

on page 167, Table 90 on page 235, Table 102 on page 258, “DC Characteristics” on

page 291, Table 119 on page 293, Table 121 on page 295, and Table 122 on pag e 297.

3. Updated “Errata” on page 340.

Rev. 2466C-03/02 1. Updated typical EEPROM programming time, Table 1 on page 20.

2. Updated typical start-up time in the following tables:

Table 3 on page 25, Table 5 on page 27, Table 6 on page 28, Table 8 on page 29, Table 9

on page 29, and Table 10 on page 29.

3. Updated Table 17 on page 43 with typical WDT Time-out.

4. Added Some Preliminary Test Limits and Ch aracterization Data.

Removed some of the TBD's in the following tables and pages:

Table 15 on page 38, T able 16 on page 42, Table 116 on page 272 (table removed in docu-

ment review #D), “Electrical Characteristics” on page 291, Table 119 on page 293, Table

121 on page 295, and Table 122 on page 297.

5. Upd at e d TWI Chapter .

Added the note at the end of the “Bit Rate Generator Unit” on page 178.

6. Corrected description of ADSC b it in “ADC Control and Status Register A – ADCSRA”

on page 219.

7. Improved description on how to do a polarity check of the ADC doff results in “ADC

Conversion Result” on page 216.

349

2466T–AVR–07/10

ATmega16(L)

8. Added JTAG version number for rev. H in Table 87 on page 229.

9. Added not regarding OCDEN Fuse below Table 105 on page 260.

10. Updated Programming Figures:

Figure 12 7 on page 262 and Figure 136 on page 274 are updated to also reflect that AVCC

must be connected during Programming mo de. Figure 131 on page 270 added to illustrate

how to program the fuses.

11. Added a note regarding usage of the “PROG_PAGELOAD ($6)” on page 280 and

“PROG_PAGEREAD ($7)” on page 280.

12. Removed alternative algortihm for leaving JTAG Programming mode.

See “Leaving Programming Mode” on page 288.

13. Added Calibrated RC Oscillator characterization curves in section “ATmega16 Typi-

cal Characteristics” on page 299.

14. Corrected ordering code for Q FN/ML F pa cka ge (16 MHz) i n “Ordering Information” on

page 336.

15. Corrected Table 90, “Scan Signals for the Oscillators(1)(2 )(3),” on page 235.

2466T–AVR–07/10

ATmega16(L)

Table of Contents

Features 1

Pin Configurations 2

Disclaimer 2

Overview 3

Block Diagram 3

Pin Descriptions 4

Resources 6

Data Retention 6

About Code Examples 7

AVR CPU Core 8

Introductio n 8

Architectural Overview 8

ALU – Arithmetic Logic Unit 9

Status Register 9

General Purpose Register File 11

Stack Pointer 12

Instruction Execution Timing 13

Reset and Interrupt Handling 13

AVR ATmega16 Memories 16

In-System Reprog rammable Flash Program Memory 16

SRAM Data Memory 17

EEPROM Data Memory 18

I/O Memory 23

System Clock and Clock Options 24

Clock Systems and their Distribution 24

Clock Sources 25

Default Clock Source 25

Crystal Oscillator 25

Low-frequency Crystal Oscillator 28

External RC Oscillator 28

Calibrated Internal RC Oscillator 29

External Clock 31

Timer/Counter Oscillator 31

Power Management and Sleep Modes 32

ii 2466T–AVR–07/10

ATmega16(L)

Idle Mode 33

ADC Noise Reduction Mode 33

Power-down Mode 33

Power-save Mo d e 33

Standby Mode 34

Extended Standby Mode 34

Minimizing Power Consumption 35

System Control and Reset 37

Internal Volt ag e Refe re n ce 42

Watchdog Timer 42

Interrupts 45

Interrupt Vec to rs in ATm e ga 1 6 45

I/O Ports 50

Introductio n 50

Ports as General Digital I/O 50

Alternate Port Funct ions 55

External Interrupts 68

8-bit Timer/Counter0 with PWM 71

Overview 71

Timer/Counter Clock Sources 72

Counter Unit 72

Output Compare Unit 73

Compare Match Ou tput Unit 74

Modes of Operation 76

Timer/Coun te r Tim in g Dia gr am s 81

8-bit Timer/Counter Register Description 83

Timer/Counter0 and Timer/Counter1 Prescalers 87

16-bit Timer/Counter1 89

Overview 89

Accessing 16-bit Re gisters 92

Timer/Counter Clock Sources 94

Counter Unit 95

Input Captur e Unit 96

Output Compa re Units 98

Compare Match Ou tput Unit 100

Modes of Operation 101

Timer/Coun te r Tim in g Dia gr am s 10 8

16-bit Timer/Counter Register Description 110

iii

2466T–AVR–07/10

ATmega16(L)

8-bit Timer/Counter2 with PWM and Asynchronous Operation 117

Overview 117

Timer/Counter Clock Sources 118

Counter Unit 118

Output Compare Unit 119

Compare Match Ou tput Unit 121

Modes of Operation 122

Timer/Coun te r Tim in g Dia gr am s 12 6

8-bit Timer/Counter Register Description 128

Asynchronous Operation of the Timer/Counter 131

Timer/Counte r Pre sca le r 13 4

Serial Peripheral Interface – SPI 135

SS Pin Functionality 140

Data Modes 143

USART 144

Overview 144

Clock Generation 145

Frame Formats 148

USART Initialization 149

Data Reception – The USART Receiver 154

Asynchronous Data Reception 157

Multi-processor Communication Mode 161

Accessing UBRRH/ UCSRC Registers 162

USART Register Desc rip tio n 16 3

Examples of Baud Rate Setting 168

Two-wire Serial Interface 172

Features 172

Two-wire Serial Interface Bus Definition 172

Data Transfer and Frame Format 173

Multi-master Bus Systems, Arbitration and Synchronization 176

Overview of the TWI Module 178

TWI Register Description 180

Using the TWI 183

Transmission Modes 186

Multi-master Systems and Arbitration 199

Analog Comparator 201

Analog Comparator Multiplexed Input 203

Analog to Digital Converter 204

Features 204

Operation 205

Starting a Conversion 206

iv 2466T–AVR–07/10

ATmega16(L)

Prescaling and Conversion Timing 207

Changing Channel or Reference Selection 210

ADC Noise Canceler 211

ADC Conversion Result 216

JTAG Interface and On-chip Debug System 222

Features 222

Overview 222

Test Access Port – TAP 222

TAP Controller 224

Using the Boundary-scan Chain 225

Using the On-chip Debug System 225

On-chip Debug Sp ecific JTAG Instructions 226

On-chip Debug Related Register in I/O Memory 227

Using the JTAG Programming Capabilities 227

Bibliography 227

IEEE 1149.1 (JTAG) Boundary-scan 228

Features 228

System Overview 228

Data Registers 228

Boundary-scan Specific JTAG Instructions 230

Boundary-scan Chain 232

ATmega16 Boundary-scan Order 241

Boundary-scan Description Language Files 245

Boot Loader Support – Read-While-Write Self-Programming 246

Features 246

Application and Boot Loader Flash Sections 246

Read-While-Write and no Read-While-Write Flash Sections 246

Boot Loader Lock Bits 24 8

Entering the Boot Loader Program 249

Addressing the Flash during Self-Programming 251

Self-Programming the Flash 252

Memory Programming 259

Program And Data Me m ory Lock Bits 25 9

Fuse Bits 260

Signature Bytes 261

Calibration Byte 261

Page Size 262

Parallel Programming Parameters, Pin Mapping, and Commands 262

Parallel Programming 265

Serial Downloading 273

Programming via the JTAG Interface 278

2466T–AVR–07/10

ATmega16(L)

Electrical Characteristics 291

Absolute Maximum Ratings* 291

DC Characteristics 291

External Clock Drive Waveforms 293

External Clock Drive 293

Two-wire Serial Interface Characteristics 294

SPI Timing Characte rist ics 29 5

ADC Characteristics 297

ATmega16 Typical Characteristics 299

Instruction Set Summary 333

Ordering Information 336

Packaging Information 337

44A 337

40P6 338

44M1 339

Errata 340

ATmega16(L) Rev. M 340

ATmega16(L) Rev. L 340

ATmega16(L) Rev. K 341

ATmega16(L) Rev. J 342

ATmega16(L) Rev. I 343

ATmega16(L) Rev. H 344

Datasheet Revision History 345

Rev. 2466T-07/10 345

Rev. 2466S-05/09 345

Rev. 2466R-06/08 345

Rev. 2466Q-05/08 345

Rev. 2466P-08/07 345

Rev. 2466O-03/07 345

Rev. 2466N-10/06 346

Rev. 2466M-04/06 346

Rev. 2466L-06/05 346

Rev. 2466K-04/05 346

Rev. 2466J-10/04 346

Rev. 2466I-10/04 346

Rev. 2466H-12/03 346

Rev. 2466G-10/03 347

Rev. 2466F-02/03 347

vi 2466T–AVR–07/10

ATmega16(L)

Rev. 2466E-10/02 348

Rev. 2466D-09/02 348

Rev. 2466C-03/02 348

Table of Contents i

vii

2466T–AVR–07/10

ATmega16(L)

2466T–AVR–07/10

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