Features
•High-performance, Low-power Atmel®AVR® 8-bit Microcontroller
•Advanced RISC Architecture
– 130 Powerful Instructions – Most Single-clock Cycle Execution
– 32 × 8 General Purpose Working Registers
– Fully Static Operation
– Up to 16MIPS Throughput at 16MHz
– On-chip 2-cycle Multiplier
•High Endurance Non-volatile Memory segments
– 8Kbytes of In-System Self-programmable Flash program memory
– 512Bytes EEPROM
– 1Kbyte Internal SRAM
– 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 Programming by On-chip Boot Program
True Read-While-Write Operation
– Programming Lock for Software Security
•Peripheral Features
– Two 8-bit Timer/Counters with Separate Prescaler, one Compare Mode
– One 16-bit Timer/Counter with Separate Prescaler, Compare Mode, and Capture
Mode
– Real Time Counter with Separate Oscillator
– Three PWM Channels
– 8-channel ADC in TQFP and QFN/MLF package
Eight Channels 10-bit Accuracy
– 6-channel ADC in PDIP package
Six Channels 10-bit Accuracy
– Byte-oriented Two-wire Serial Interface
– Programmable Serial USART
– Master/Slave SPI Serial Interface
– Programmable Watchdog Timer with Separate 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 Interrupt Sources
– Five Sleep Modes: Idle, ADC Noise Reduction, Power-save, Power-down, and
Standby
•I/O and Packages
– 23 Programmable I/O Lines
– 28-lead PDIP, 32-lead TQFP, and 32-pad QFN/MLF
•Operating Voltages
– 2.7V - 5.5V (ATmega8L)
– 4.5V - 5.5V (ATmega8)
•Speed Grades
– 0 - 8MHz (ATmega8L)
– 0 - 16MHz (ATmega8)
•Power Consumption at 4Mhz, 3V, 25°C
– Active: 3.6mA
– Idle Mode: 1.0mA
– Power-down Mode: 0.5µA
8-bit
with 8KBytes
In-System
Programmable
Flash
ATmega8
ATmega8L
Rev.2486Z–AVR–02/11
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2486Z–AVR–02/11
ATmega8(L)
Pin
Configurations
1
2
3
4
5
6
7
8
24
23
22
21
20
19
18
17
(INT1) PD3
(XCK/T0) PD4
GND
VCC
GND
VCC
(XTAL1/TOSC1) PB6
(XTAL2/TOSC2) PB7
PC1 (ADC1)
PC0 (ADC0)
ADC7
GND
AREF
ADC6
AVCC
PB5 (SCK)
32
31
30
29
28
27
26
25
9
10
11
12
13
14
15
16
(T1) PD5
(AIN0) PD6
(AIN1) PD7
(ICP1) PB0
(OC1A) PB1
(SS/OC1B) PB2
(MOSI/OC2) PB3
(MISO) PB4
PD2 (INT0)
PD1 (TXD)
PD0 (RXD)
PC6 (RESET)
PC5 (ADC5/SCL)
PC4 (ADC4/SDA)
PC3 (ADC3)
PC2 (ADC2)
TQFP Top View
1
2
3
4
5
6
7
8
9
10
11
12
13
14
28
27
26
25
24
23
22
21
20
19
18
17
16
15
(RESET) PC6
(RXD) PD0
(TXD) PD1
(INT0) PD2
(INT1) PD3
(XCK/T0) PD4
VCC
GND
(XTAL1/TOSC1) PB6
(XTAL2/TOSC2) PB7
(T1) PD5
(AIN0) PD6
(AIN1) PD7
(ICP1) PB0
PC5 (ADC5/SCL)
PC4 (ADC4/SDA)
PC3 (ADC3)
PC2 (ADC2)
PC1 (ADC1)
PC0 (ADC0)
GND
AREF
AVCC
PB5 (SCK)
PB4 (MISO)
PB3 (MOSI/OC2)
PB2 (SS/OC1B)
PB1 (OC1A)
PDIP
1
2
3
4
5
6
7
8
24
23
22
21
20
19
18
17
32
31
30
29
28
27
26
25
9
10
11
12
13
14
15
16
MLF Top View
(INT1) PD3
(XCK/T0) PD4
GND
VCC
GND
VCC
(XTAL1/TOSC1) PB6
(XTAL2/TOSC2) PB7
PC1 (ADC1)
PC0 (ADC0)
ADC7
GND
AREF
ADC6
AVCC
PB5 (SCK)
(T1) PD5
(AIN0) PD6
(AIN1) PD7
(ICP1) PB0
(OC1A) PB1
(SS/OC1B) PB2
(MOSI/OC2) PB3
(MISO) PB4
PD2 (INT0)
PD1 (TXD)
PD0 (RXD)
PC6 (RESET)
PC5 (ADC5/SCL)
PC4 (ADC4/SDA)
PC3 (ADC3)
PC2 (ADC2)
NOTE:
The large center pad underneath the MLF
packages is made of metal and internally
connected to GND. It should be soldered
or glued to the PCB to ensure good
mechanical stability. If the center pad is
left unconneted, the package might
loosen from the PCB.
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2486Z–AVR–02/11
ATmega8(L)
Overview The Atmel®AVR® ATmega8 is a low-power CMOS 8-bit microcontroller based on the AVR RISC
architecture. By executing powerful instructions in a single clock cycle, the ATmega8 achieves
throughputs approaching 1MIPS per MHz, allowing the system designer to optimize power con-
sumption versus processing speed.
Block Diagram Figure 1. Block Diagram
INTERNAL
OSCILLATOR
OSCILLATOR
WATCHDOG
TIMER
MCU CTRL.
& TIMING
OSCILLATOR
TIMERS/
COUNTERS
INTERRUPT
UNIT
STACK
POINTER
EEPROM
SRAM
STATUS
REGISTER
USART
PROGRAM
COUNTER
PROGRAM
FLASH
INSTRUCTION
REGISTER
INSTRUCTION
DECODER
PROGRAMMING
LOGIC
SPI
ADC
INTERFACE
COMP.
INTERFACE
PORTC DRIVERS/BUFFERS
PORTC DIGITAL INTERFACE
GENERAL
PURPOSE
REGISTERS
X
Y
Z
ALU
+
-
PORTB DRIVERS/BUFFERS
PORTB DIGITAL INTERFACE
PORTD DIGITAL INTERFACE
PORTD DRIVERS/BUFFERS
XTAL1
XTAL2
CONTROL
LINES
VCC
GND
MUX &
ADC
AGND
AREF
PC0 - PC6 PB0 - PB7
PD0 - PD7
AVR CPU
TWI
RESET
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2486Z–AVR–02/11
ATmega8(L)
The Atmel®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 inde-
pendent registers to be accessed in one single instruction executed in one clock cycle. The
resulting architecture is more code efficient while achieving throughputs up to ten times faster
than conventional CISC microcontrollers.
The ATmega8 provides the following features: 8 Kbytes of In-System Programmable Flash with
Read-While-Write capabilities, 512 bytes of EEPROM, 1 Kbyte of SRAM, 23 general purpose
I/O lines, 32 general purpose working registers, three flexible Timer/Counters with compare
modes, internal and external interrupts, a serial programmable USART, a byte oriented Two-
wire Serial Interface, a 6-channel ADC (eight channels in TQFP and QFN/MLF packages) with
10-bit accuracy, a programmable Watchdog Timer with Internal Oscillator, an SPI serial port,
and five software selectable power saving modes. The Idle mode stops the CPU while allowing
the SRAM, Timer/Counters, SPI port, and interrupt system to continue functioning. The Power-
down mode saves the register contents but freezes the Oscillator, disabling all other chip func-
tions until the next Interrupt 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 sleep-
ing. 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/resonator Oscillator is running while the rest of the device is sleeping. This allows very
fast start-up combined with low-power consumption.
The device is manufactured using Atmel’s high density non-volatile memory technology. The
Flash Program memory can be reprogrammed In-System through an SPI serial interface, by a
conventional non-volatile memory programmer, or by an On-chip boot program running on the
AVR core. The boot program can use any interface to download the application program in the
Application Flash memory. Software in the Boot Flash Section will continue to run while the
Application Flash Section is updated, providing true Read-While-Write operation. By combining
an 8-bit RISC CPU with In-System Self-Programmable Flash on a monolithic chip, the Atmel
ATmega8 is a powerful microcontroller that provides a highly-flexible and cost-effective solution
to many embedded control applications.
The ATmega8 is supported with a full suite of program and system development tools, including
C compilers, macro assemblers, program debugger/simulators, In-Circuit Emulators, and evalu-
ation kits.
Disclaimer Typical values contained in this datasheet are based on simulations and characterization of
other AVR microcontrollers manufactured on the same process technology. Minimum and Maxi-
mum values will be available after the device is characterized.
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2486Z–AVR–02/11
ATmega8(L)
Pin Descriptions
VCC Digital supply voltage.
GND Ground.
Port B (PB7..PB0)
XTAL1/XTAL2/TOSC1/
TOSC2
Port B is an 8-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The
Port B output buffers have symmetrical drive characteristics with both high sink and source
capability. As inputs, Port B pins that are externally pulled low will source current if the pull-up
resistors are activated. The Port B pins are tri-stated when a reset condition becomes active,
even if the clock is not running.
Depending on the clock selection fuse settings, PB6 can be used as input to the inverting Oscil-
lator amplifier and input to the internal clock operating circuit.
Depending on the clock selection fuse settings, PB7 can be used as output from the inverting
Oscillator amplifier.
If the Internal Calibrated RC Oscillator is used as chip clock source, PB7..6 is used as TOSC2..1
input for the Asynchronous Timer/Counter2 if the AS2 bit in ASSR is set.
The various special features of Port B are elaborated in “Alternate Functions of Port B” on page
58 and “System Clock and Clock Options” on page 25.
Port C (PC5..PC0) Port C is an 7-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The
Port C output buffers have symmetrical drive characteristics with both high sink and source
capability. As inputs, Port C pins that are externally pulled low will source current if the pull-up
resistors are activated. The Port C pins are tri-stated when a reset condition becomes active,
even if the clock is not running.
PC6/RESET If the RSTDISBL Fuse is programmed, PC6 is used as an I/O pin. Note that the electrical char-
acteristics of PC6 differ from those of the other pins of Port C.
If the RSTDISBL Fuse is unprogrammed, PC6 is used as a 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 are not guaranteed to
generate a Reset.
The various special features of Port C are elaborated 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 high sink and source
capability. As inputs, Port D pins that are externally pulled low will source current if the pull-up
resistors are activated. The Port D pins are tri-stated when a reset condition becomes active,
even if the clock is not running.
Port D also serves the functions of various special features of the ATmega8 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 are not guaranteed to generate a reset.
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2486Z–AVR–02/11
ATmega8(L)
AVCC AVCC is the supply voltage pin for the A/D Converter, Port C (3..0), and ADC (7..6). It should be
externally connected to VCC, even if the ADC is not used. If the ADC is used, it should be con-
nected to VCC through a low-pass filter. Note that Port C (5..4) use digital supply voltage, VCC.
AREF AREF is the analog reference pin for the A/D Converter.
ADC7..6 (TQFP and
QFN/MLF Package
Only)
In the TQFP and QFN/MLF package, ADC7..6 serve as analog inputs to the A/D converter.
These pins are powered from the analog supply and serve as 10-bit ADC channels.
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2486Z–AVR–02/11
ATmega8(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.
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2486Z–AVR–02/11
ATmega8(L)
About Code
Examples
This datasheet contains simple code examples that briefly show how to use various parts of the
device. These code examples assume that the part specific header file is included before compi-
lation. Be aware that not all C compiler vendors include bit definitions in the header files and
interrupt handling in C is compiler dependent. Please confirm with the C compiler documentation
for more details.
9
2486Z–AVR–02/11
ATmega8(L)
Atmel AVR CPU
Core
Introduction This section discusses the Atmel®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 2. 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 program 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 – enabling efficient address calculations. One of the these address pointers
Flash
Program
Memory
Instruction
Register
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
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2486Z–AVR–02/11
ATmega8(L)
can also be used as an address pointer for look up tables in Flash Program memory. These
added function registers are the 16-bit X-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 register operations can also be executed in the ALU. After an arithmetic opera-
tion, the Status Register is updated to reflect information about the result of the operation.
The 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 memory space is divided in two sections, the Boot program section and the
Application program section. Both sections have dedicated Lock Bits for write and read/write
protection. The SPM instruction that writes into the Application Flash memory section must
reside in the Boot program section.
During interrupts and subroutine calls, the return address Program Counter (PC) is stored on the
Stack. The Stack is effectively allocated in the general data SRAM, and consequently the Stack
size is only limited by the total SRAM size and the usage of the SRAM. All user programs must
initialize the SP in the reset routine (before subroutines or interrupts are executed). The Stack
Pointer SP is read/write accessible in the I/O space. The data SRAM can easily be accessed
through the five different addressing modes supported in the AVR architecture.
The memory spaces in the AVR architecture are all linear and regular memory maps.
A flexible interrupt module has its control registers in the I/O space with an additional global
interrupt enable bit in the Status Register. All interrupts have a separate Interrupt Vector in the
Interrupt Vector table. The interrupts have priority in accordance with their Interrupt Vector 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 functions. The I/O Memory can be accessed directly, or as the Data
Space locations following those of the Register File, 0x20 - 0x5F.
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2486Z–AVR–02/11
ATmega8(L)
Arithmetic Logic
Unit – ALU
The high-performance Atmel®AVR® ALU operates in direct connection with all the 32 general
purpose working registers. Within a single clock cycle, arithmetic operations between general
purpose registers or between a register and an immediate are executed. The ALU operations
are divided into three main categories – arithmetic, logical, and bit-functions. Some implementa-
tions of the architecture also provide a powerful multiplier supporting both signed/unsigned
multiplication and fractional format. For a detailed description, see “Instruction Set Summary” on
page 282.
Status Register The Status Register contains information about the result of the most recently executed arithme-
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 Instruction Set Reference. This will in many cases remove the need for using the
dedicated compare instructions, resulting in faster and more compact code.
The Status Register is not automatically stored when entering an interrupt routine and restored
when returning from an interrupt. This must be handled by software.
The AVR Status Register – SREG – is defined as:
• Bit 7 – I: Global Interrupt Enable
The Global Interrupt Enable bit must be set for the interrupts to be enabled. The individual inter-
rupt enable control is then performed in separate control registers. If the Global Interrupt Enable
Register is cleared, none of the interrupts are enabled independent of the individual interrupt
enable settings. The I-bit is cleared by hardware after an interrupt has occurred, and is set by
the RETI instruction to enable subsequent interrupts. The I-bit can also be set and cleared by
the application with the SEI and CLI instructions, as described in the Instruction Set Reference.
• Bit 6 – T: Bit Copy Storage
The Bit Copy instructions BLD (Bit LoaD) and BST (Bit STore) use the T-bit as source or 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 the “Instruction Set Description” for detailed information.
• Bit 4 – S: Sign Bit, S = N ⊕ V
The S-bit is always an exclusive or between the Negative Flag N and the Two’s Complement
Overflow Flag V. See 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 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
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2486Z–AVR–02/11
ATmega8(L)
• 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.
General Purpose
Register File
The Register File is optimized for the AVR Enhanced RISC instruction set. In order to achieve
the required performance and flexibility, the following input/output schemes are supported by the
Register File:
• One 8-bit output operand and one 8-bit result input
• Two 8-bit output operands and one 8-bit result input
• Two 8-bit output operands and one 16-bit result input
• One 16-bit output operand and one 16-bit result input
Figure 3 shows the structure of the 32 general purpose working registers in the CPU.
Figure 3. AVR CPU General Purpose Working Registers
Most of the instructions operating on the Register File have direct access to all registers, and
most of them are single cycle instructions.
As shown in Figure 3, 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 memory organization provides great flexibility in access of the
registers, as the X-pointer, Y-pointer, and Z-pointer Registers can be set to index any register in
the file.
7 0 Addr.
R0 0x00
R1 0x01
R2 0x02
…
R13 0x0D
General R14 0x0E
Purpose R15 0x0F
Working R16 0x10
Registers R17 0x11
…
R26 0x1A X-register Low Byte
R27 0x1B X-register High Byte
R28 0x1C Y-register Low Byte
R29 0x1D Y-register High Byte
R30 0x1E Z-register Low Byte
R31 0x1F Z-register High Byte
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2486Z–AVR–02/11
ATmega8(L)
The X-register, Y-
register and Z-register
The registers R26..R31 have some added functions to their general purpose usage. These reg-
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 4.
Figure 4. The X-register, Y-register and Z-Register
In the different addressing modes these address registers have functions as fixed displacement,
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 the Stack. Note that the Stack is implemented as growing from higher memory loca-
tions to lower memory locations. This implies that a Stack PUSH command decreases the Stack
Pointer.
The Stack Pointer points to the data SRAM Stack area where the Subroutine and Interrupt
Stacks are located. This Stack space in the data SRAM must be defined by the program before
any subroutine calls are executed or interrupts are enabled. The Stack Pointer must be set to
point above 0x60. 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 address is pushed onto
the Stack with subroutine call or interrupt. The Stack Pointer is incremented by one when data is
popped from the Stack with the POP instruction, and it is incremented by two when address 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.
Instruction
Execution Timing
This section describes the general access timing concepts for instruction execution. The
Atmel®AVR® CPU is driven by the CPU clock clkCPU, directly generated from the selected clock
source for the chip. No internal clock division is used.
15 XH XL 0
X-register 7 0 7 0
R27 (0x1B) R26 (0x1A)
15 YH YL 0
Y-register 7 0 7 0
R29 (0x1D) R28 (0x1C)
15 ZH ZL 0
Z-register 7 0 7 0
R31 (0x1F) R30 (0x1E)
Bit 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
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2486Z–AVR–02/11
ATmega8(L)
Figure 5 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 1MIPS per MHz with the corresponding unique results for functions per cost,
functions per clocks, and functions per power-unit.
Figure 5. The Parallel Instruction Fetches and Instruction Executions
Figure 6 shows the internal timing concept for the Register File. In a single clock cycle an ALU
operation using two register operands is executed, and the result is stored back to the destina-
tion register.
Figure 6. Single Cycle ALU Operation
Reset and
Interrupt Handling
The Atmel®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 inter-
rupts are assigned individual enable bits which must be written logic one together with the
Global Interrupt Enable bit in the Status Register in order to enable the interrupt. Depending on
the Program Counter value, interrupts may be automatically disabled when Boot Lock Bits
BLB02 or BLB12 are programmed. This feature improves software security. See the section
“Memory Programming” on page 215 for details.
The lowest addresses in the Program memory space are by default defined as the Reset and
Interrupt Vectors. The complete list of Vectors is shown in “Interrupts” on page 46. The list also
determines the priority levels of the different interrupts. The lower the address the higher is the
priority level. RESET has the highest priority, and next is INT0 – the External Interrupt Request
0. The Interrupt Vectors can be moved to the start of the boot Flash section by setting the Inter-
rupt Vector Select (IVSEL) bit in the General Interrupt Control Register (GICR). Refer to
“Interrupts” on page 46 for more information. The Reset Vector can also be moved to the start of
the boot Flash section by programming the BOOTRST Fuse, see “Boot Loader Support – Read-
While-Write Self-Programming” on page 202.
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
Register Operands Fetch
ALU Operation Execute
Result Write Back
T1 T2 T3 T4
clk
CPU
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2486Z–AVR–02/11
ATmega8(L)
When an interrupt occurs, the Global Interrupt Enable I-bit is cleared and all interrupts 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 automatically set when a
Return from Interrupt instruction – RETI – is executed.
There are basically two types of interrupts. The first type is triggered by an event that sets the
Interrupt Flag. For these interrupts, the Program Counter is vectored to the actual Interrupt 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. These
interrupts do not necessarily have Interrupt Flags. If the interrupt condition disappears before the
interrupt is enabled, the interrupt will not be triggered.
When the AVR exits from an interrupt, it will always return to the main program and execute one
more instruction before any pending interrupt is served.
Note that the Status Register is not automatically stored when entering an interrupt routine, nor
restored when returning from an interrupt routine. This must be handled by software.
When using the CLI instruction to disable interrupts, the interrupts will be immediately disabled.
No interrupt will be executed after the CLI instruction, even if it occurs simultaneously with the
CLI instruction. The following example shows how this can be used to avoid interrupts during the
timed EEPROM write sequence.
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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ATmega8(L)
When using the SEI instruction to enable interrupts, the instruction following SEI will be exe-
cuted before any pending interrupts, as shown in the following example.
Interrupt Response
Time
The interrupt execution response for all the enabled Atmel®AVR® interrupts is four clock cycles
minimum. After four clock cycles, the Program Vector address for the actual interrupt handling
routine is executed. During this 4-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 com-
pleted before the interrupt is served. If an interrupt occurs when the MCU is in sleep mode, the
interrupt execution response time is increased by four clock cycles. This increase comes in addi-
tion 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 (2 bytes) is popped back from the Stack, the Stack Pointer is incre-
mented by 2, and the 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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ATmega8(L)
AVR ATmega8
Memories
This section describes the different memories in the Atmel®AVR® ATmega8. The AVR architec-
ture has two main memory spaces, the Data memory and the Program Memory space. In
addition, the ATmega8 features an EEPROM Memory for data storage. All three memory spaces
are linear and regular.
In-System
Reprogrammable
Flash Program
Memory
The ATmega8 contains 8Kbytes On-chip In-System Reprogrammable Flash memory for pro-
gram storage. Since all AVR instructions are 16-bits or 32-bits wide, the Flash is organized as
4K × 16 bits. For software security, the Flash Program memory space is divided into two sec-
tions, Boot Program section and Application Program section.
The Flash memory has an endurance of at least 10,000 write/erase cycles. The ATmega8 Pro-
gram Counter (PC) is 12 bits wide, thus addressing the 4K Program memory locations. The
operation of Boot Program section and associated Boot Lock Bits for software protection are
described in detail in “Boot Loader Support – Read-While-Write Self-Programming” on page
202. “Memory Programming” on page 215 contains a detailed description on Flash Program-
ming in SPI- or Parallel Programming mode.
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 7. Program Memory Map
$000
$FFF
Application Flash Section
Boot Flash Section
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ATmega8(L)
SRAM Data
Memory
Figure 8 shows how the Atmel®AVR® SRAM Memory is organized.
The lower 1120 Data memory locations address the Register File, the I/O Memory, and the inter-
nal data SRAM. The first 96 locations address the Register File and I/O Memory, and the 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 Displacement mode reaches 63 address 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 ATmega8 are all accessible through all these addressing modes. The Register File
is described in “General Purpose Register File” on page 12.
Figure 8. Data Memory Map
Register File
R0
R1
R2
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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2486Z–AVR–02/11
ATmega8(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 9.
Figure 9. On-chip Data SRAM Access Cycles
EEPROM Data
Memory
The ATmega8 contains 512bytes 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
below, specifying the EEPROM Address Registers, the EEPROM Data Register, and the
EEPROM Control Register.
“Memory Programming” on page 215 contains a detailed description on EEPROM Programming
in SPI- or Parallel Programming mode.
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 on page 21. A self-timing function,
however, lets the user software detect when the next byte can be written. If the user code con-
tains instructions that write the EEPROM, some precautions must be taken. In heavily filtered
power supplies, VCC is likely to rise or fall slowly on Power-up/down. This causes the device for
some period of time to run at a voltage lower than specified as minimum for the clock frequency
used. See “Preventing EEPROM Corruption” on page 23. 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 EEPROM Control Register – EECR” on page 20 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
WR
RD
Data
Data
Address
Address Valid
T1 T2 T3
Compute Address
Read Write
CPU
Memory Vccess Instruction Next Instruction
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2486Z–AVR–02/11
ATmega8(L)
The EEPROM Address
Register – EEARH and
EEARL
• Bits 15..9 – Res: Reserved Bits
These bits are reserved bits in the ATmega8 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
512bytes 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
Register – EEDR
• Bits 7..0 – EEDR7..0: EEPROM Data
For the EEPROM write operation, the EEDR Register contains the data to be written to the
EEPROM in the address given by the EEAR Register. For the EEPROM read operation, the
EEDR contains the data read out from the EEPROM at the address given by EEAR.
The EEPROM Control
Register – EECR
• Bits 7..4 – Res: Reserved Bits
These bits are reserved bits in the Atmel®AVR® ATmega8 and will always read as zero.
• Bit 3 – EERIE: EEPROM Ready Interrupt Enable
Writing EERIE to one enables the EEPROM Ready Interrupt if the I bit in SREG is set. Writing
EERIE to zero disables the interrupt. The EEPROM Ready interrupt generates a constant 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 “Bit 1 – EEWE: EEPROM Write Enable” for an EEPROM write procedure.
• Bit 1 – EEWE: EEPROM Write Enable
The EEPROM Write Enable Signal EEWE is the write strobe to the EEPROM. When address
and data are correctly set up, the EEWE bit must be written to one to write the value into the
EEPROM. The EEMWE bit must be written to one before a logical one is written to EEWE, oth-
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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2486Z–AVR–02/11
ATmega8(L)
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. Write 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 the Flash is never being updated by the CPU, step 2 can be omitted. See “Boot Loader
Support – Read-While-Write Self-Programming” on page 202 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 to fail. It is recommended to have the Global Interrupt Flag cleared
during all the steps to avoid these problems.
When the write access time has elapsed, the EEWE bit is cleared by hardware. The user soft-
ware can poll this bit and wait for a zero before writing the next byte. When EEWE has been set,
the CPU is halted for two cycles before the next instruction is executed.
• Bit 0 – EERE: EEPROM Read Enable
The EEPROM Read Enable Signal EERE is the read strobe to the EEPROM. When the correct
address is set up in the EEAR Register, the EERE bit must be written to a logic one to trigger the
EEPROM read. The EEPROM read access takes one instruction, and the requested data is
available immediately. When the EEPROM is read, the CPU is halted for four cycles before the
next instruction is executed.
The user should poll the 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 1MHz clock, independent of CKSEL Fuse settings
Table 1. EEPROM Programming Time
Symbol
Number of Calibrated RC
Oscillator Cycles(1) Typ Programming Time
EEPROM Write (from CPU) 8448 8.5ms
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ATmega8(L)
The following code examples show one assembly and one C function for writing to the
EEPROM. The examples assume that interrupts are controlled (for example by disabling inter-
rupts globally) so that no interrupts will occur during execution of these functions. The examples
also assume that no Flash boot loader is present in the software. If such code is present, the
EEPROM write function must also wait for any ongoing SPM command to finish.
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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2486Z–AVR–02/11
ATmega8(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 functions.
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 corrupted because the supply voltage is
too low for the CPU and the EEPROM to operate properly. These issues are the same as for
board level systems using EEPROM, and the same design solutions should be applied.
An EEPROM data corruption can be caused by two situations when the voltage is too low. First,
a regular write sequence to the EEPROM requires a minimum voltage to operate correctly. Sec-
ond, the CPU itself can execute instructions incorrectly, if the supply voltage is too low.
EEPROM data corruption can easily be avoided by following this design recommendation:
Keep the AVR RESET active (low) during periods of insufficient power supply voltage. This
can be done by enabling the internal Brown-out Detector (BOD). If the detection level of the
internal BOD does not match the needed detection level, an external low VCC Reset Protec-
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;
}
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2486Z–AVR–02/11
ATmega8(L)
tion circuit can be used. If a reset occurs while a write operation is in progress, the write
operation will be completed provided that the power supply voltage is sufficient.
I/O Memory The I/O space definition of the ATmega8 is shown in “Register Summary” on page 280.
All Atmel®AVR® ATmega8 I/Os and peripherals are placed in the I/O space. The I/O locations
are accessed by the IN and OUT instructions, transferring data between the 32 general purpose
working registers and the I/O space. I/O Registers within the address range 0x00 - 0x1F are
directly bit-accessible using the SBI and CBI instructions. In these registers, the value of single
bits can be checked by using the SBIS and SBIC instructions. Refer to the “Instruction Set Sum-
mary” on page 282 for more details. When using the I/O specific commands IN and OUT, the I/O
addresses 0x00 - 0x3F must be used. When addressing I/O Registers as data space using LD
and ST instructions, 0x20 must be added to these addresses.
For compatibility with future devices, reserved bits should be written to zero if accessed.
Reserved I/O memory addresses should never be written.
Some of the Status Flags are cleared by writing a logical one to them. Note that the CBI and SBI
instructions will operate on all bits in the I/O Register, writing a one back into any flag read as
set, thus clearing the flag. The CBI and SBI instructions work with registers 0x00 to 0x1F only.
The I/O and Peripherals Control Registers are explained in later sections.
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ATmega8(L)
System Clock
and Clock
Options
Clock Systems
and their
Distribution
Figure 10 presents the principal clock systems in the Atmel®AVR® and their distribution. All of
the clocks need not be active at a given time. In order to reduce power consumption, the clocks
to modules not being used can be halted by using different sleep modes, as described in “Power
Management and Sleep Modes” on page 33. The clock systems are detailed Figure 10.
Figure 10. 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 detected 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 The Flash clock controls operation of the Flash interface. The Flash clock is usually active simul-
taneously with the CPU clock.
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ATmega8(L)
Asynchronous Timer
Clock – clkASY
The Asynchronous Timer clock allows the Asynchronous Timer/Counter to be clocked directly
from an external 32kHz clock crystal. The dedicated clock domain allows using this Timer/Coun-
ter as a real-time counter even when the device is in sleep mode. The Asynchronous
Timer/Counter uses the same XTAL pins as the CPU main clock but requires a CPU main clock
frequency of more than four times the Oscillator frequency. Thus, asynchronous operation is
only available while the chip is clocked on the Internal Oscillator.
ADC Clock – clkADC The ADC is provided with a dedicated clock domain. This allows halting the CPU and I/O clocks
in order to reduce noise generated by digital circuitry. This gives more accurate ADC conversion
results.
Clock Sources The device has the following clock source options, selectable by Flash Fuse Bits as shown
below. The clock from the selected source is input to the AVR clock generator, and routed to the
appropriate modules.
Note: 1. For all fuses “1” means unprogrammed while “0” means programmed
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 reach a stable level before com-
mencing 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 “ATmega8 Typical
Characteristics”. The device is shipped with CKSEL = “0001” and SUT = “10” (1MHz Internal RC
Oscillator, slowly rising power).
Table 2. Device Clocking Options Select(1)
Device Clocking Option CKSEL3..0
External Crystal/Ceramic Resonator 1111 - 1010
External Low-frequency Crystal 1001
External RC Oscillator 1000 - 0101
Calibrated Internal RC Oscillator 0100 - 0001
External Clock 0000
Table 3. Number of Watchdog Oscillator Cycles
Typical Time-out (VCC = 5.0V) Typical Time-out (VCC = 3.0V) Number of Cycles
4.1ms 4.3ms 4K (4,096)
65ms 69ms 64K (65,536)
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ATmega8(L)
Crystal Oscillator XTAL1 and XTAL2 are input and output, respectively, of an inverting amplifier which can be con-
figured for use as an On-chip Oscillator, as shown in Figure 11. 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 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 cannot be
used to drive other clock buffers.
For resonators, the maximum frequency is 8MHz with CKOPT unprogrammed and 16MHz 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
choosing capacitors for use with crystals are given in Table 4. For ceramic resonators, the
capacitor values given by the manufacturer should be used.
Figure 11. 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
The CKSEL0 Fuse together with the SUT1..0 Fuses select the start-up times as shown in Table
5 on page 28.
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
C2
C1
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ATmega8(L)
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
Low-frequency
Crystal Oscillator
To use a 32.768kHz watch crystal as the clock source for the device, the Low-frequency Crystal
Oscillator must be selected by setting the CKSEL Fuses to “1001”. The crystal should be con-
nected as shown in Figure 11 on page 27. By programming the CKOPT Fuse, the user can
enable internal capacitors on XTAL1 and XTAL2, thereby removing the need for external capac-
itors. The internal capacitors have a nominal value of 36pF.
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 external RC configuration shown in Figure 12 on page 29
can be used. The frequency is roughly estimated by the equation f = 1/(3RC). C should be at
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) 4.1ms Ceramic resonator, fast
rising power
0 01 258 CK(1) 65ms Ceramic resonator, slowly
rising power
010 1K CK
(2) –Ceramic resonator, BOD
enabled
011 1K CK
(2) 4.1ms Ceramic resonator, fast
rising power
100 1K CK
(2) 65ms Ceramic resonator, slowly
rising power
101 16K CK – Crystal Oscillator, BOD
enabled
1 10 16K CK 4.1ms Crystal Oscillator, fast
rising power
1 11 16K CK 65ms Crystal Oscillator, slowly
rising power
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) 4.1ms Fast rising power or BOD enabled
01 1K CK(1) 65ms Slowly rising power
10 32K CK 65ms Stable frequency at start-up
11 Reserved
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ATmega8(L)
least 22pF. By programming the CKOPT Fuse, the user can enable an internal 36pF capacitor
between XTAL1 and GND, thereby removing the need for an external capacitor.
Figure 12. 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.
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 device
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
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 4.1ms Fast rising power
10 18 CK 65ms Slowly rising power
11 6 CK(1) 4.1ms Fast rising power or BOD enabled
XTAL2
XTAL1
GND
C
R
VCC
NC
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ATmega8(L)
Calibrated Internal
RC Oscillator
The calibrated internal RC Oscillator provides a fixed 1.0MHz, 2.0MHz, 4.0MHz, or 8.0MHz
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 unprogrammed when using
this clock option. During reset, hardware loads the 1MHz calibration byte into the OSCCAL Reg-
ister and thereby automatically calibrates the RC Oscillator. At 5V, 25°C and 1.0MHz Oscillator
frequency selected, this calibration gives a frequency within ±3% of the nominal frequency.
Using run-time 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. For more information on the pre-programmed cali-
bration value, see the section “Calibration Byte” on page 218.
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. PB6 (XTAL1/TOSC1) and PB7(XTAL2/TOSC2) can be used as either general I/O pins
or Timer Oscillator pins..
Note: 1. The device is shipped with this option selected
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 Delay
from Reset
(VCC = 5.0V) Recommended Usage
00 6 CK – BOD enabled
01 6 CK 4.1ms Fast rising power
10(1) 6 CK 65ms Slowly rising power
11 Reserved
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2486Z–AVR–02/11
ATmega8(L)
Oscillator Calibration
Register – OSCCAL
• 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. During Reset, the 1MHz calibration value which is located
in the signature row High byte (address 0x00) is automatically loaded into the OSCCAL Regis-
ter. If the internal RC is used at other frequencies, the calibration values must be loaded
manually. This can be done by first reading the signature row by a programmer, and then store
the calibration values in the Flash or EEPROM. Then the value can be read by software and
loaded into the OSCCAL Register. When OSCCAL is zero, the lowest available frequency is
chosen. Writing non-zero values to this register will increase the frequency of the Internal Oscil-
lator. Writing 0xFF 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 frequency. Otherwise, the EEPROM or Flash write may fail.
Note that the Oscillator is intended for calibration to 1.0MHz, 2.0MHz, 4.0MHz, or 8.0MHz. Tun-
ing 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 Percentage of
Nominal Frequency (%)
0x00 50 100
0x7F 75 150
0xFF 100 200
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2486Z–AVR–02/11
ATmega8(L)
External Clock To drive the device from an external clock source, XTAL1 should be driven as shown in Figure
13. 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 36pF capacitor between
XTAL1 and GND, and XTAL2 and GND.
Figure 13. External Clock Drive Configuration
When this clock source is selected, start-up times are determined by the SUT Fuses 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. By programming the CKOPT Fuse, the user can enable
internal capacitors on XTAL1 and XTAL2, thereby removing the need for external capacitors.
The Oscillator is optimized for use with a 32.768kHz watch 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 36pF
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 4.1ms Fast rising power
10 6 CK 65ms Slowly rising power
11 Reserved
EXTERNAL
CLOCK
SIGNAL
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2486Z–AVR–02/11
ATmega8(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 various sleep modes allowing the user to tailor the power consump-
tion to the application’s requirements.
To enter any of the five 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-down, Power-save, or 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.
Note that the Extended Standby mode present in many other AVR MCUs has been removed in
the ATmega8, as the TOSC and XTAL inputs share the same physical pins.
Figure 10 on page 25 presents the different clock systems in the ATmega8, 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 management.
• Bit 7 – SE: Sleep Enable
The SE bit must be written to logic one to make the MCU enter the sleep mode when the SLEEP
instruction is executed. To avoid the MCU entering the sleep mode unless it is the programmer’s
purpose, it is recommended to set the Sleep Enable (SE) bit just before the execution of the
SLEEP instruction.
• Bits 6..4 – SM2..0: Sleep Mode Select Bits 2, 1, and 0
These bits select between the five available sleep modes as shown in Table 13.
Note: 1. Standby mode is only available with external crystals or resonators
Bit 76543210
SE SM2 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)
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2486Z–AVR–02/11
ATmega8(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/Counters, Watchdog, and the interrupt system to continue operating. This sleep
mode basically halts clkCPU and clkFLASH, while allowing the other clocks to run.
Idle mode enables the MCU to wake up from external triggered interrupts as well as internal
ones like the Timer Overflow and USART Transmit Complete interrupts. If wake-up from the
Analog Comparator interrupt is not required, the Analog Comparator can be powered down by
setting the ACD bit in the Analog Comparator Control and Status Register – ACSR. This will
reduce power consumption in Idle mode. If the ADC is enabled, a conversion starts 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 mode, stopping the CPU but allowing the ADC, the external interrupts, the
Two-wire Serial Interface address watch, Timer/Counter2 and the Watchdog to continue
operating (if enabled). This sleep mode basically halts clkI/O, clkCPU, and clkFLASH, while allowing
the other clocks to run.
This improves the noise environment for the ADC, enabling higher resolution measurements. If
the ADC is enabled, a conversion starts automatically when this mode is entered. Apart form the
ADC Conversion Complete interrupt, only an External Reset, a Watchdog Reset, a Brown-out
Reset, a Two-wire Serial Interface address match interrupt, a Timer/Counter2 interrupt, an
SPM/EEPROM ready interrupt, or an external level interrupt on INT0 or INT1, 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 address watch, and the Watchdog continue operating (if enabled).
Only an External Reset, a Watchdog Reset, a Brown-out Reset, a Two-wire Serial Interface
address match interrupt, or an external level interrupt on INT0 or INT1, can wake up the MCU.
This sleep mode basically halts all generated clocks, allowing operation of asynchronous mod-
ules 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 66
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 26.
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 up from either Timer Overflow or
Output Compare event from Timer/Counter2 if the corresponding Timer/Counter2 interrupt
enable bits are set in TIMSK, and the global interrupt enable bit in SREG is set.
If the asynchronous timer is NOT clocked asynchronously, Power-down mode is recommended
instead of Power-save mode because the contents of the registers in the asynchronous timer
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 operation only of asynchronous
modules, including Timer/Counter 2 if clocked asynchronously.
35
2486Z–AVR–02/11
ATmega8(L)
Standby Mode When the SM2..0 bits are 110 and an external crystal/resonator clock option is selected, the
SLEEP instruction makes the MCU enter Standby mode. This mode is identical to Power-down
with the exception that the Oscillator is kept running. From Standby mode, the device wakes up
in 6 clock cycles.
Notes: 1. External Crystal or resonator selected as clock source
2. If AS2 bit in ASSR is set
3. Only level interrupt INT1 and INT0
Minimizing Power
Consumption
There are several issues to consider when trying to minimize the power consumption in an AVR
controlled system. In general, sleep modes should be used as much as possible, and the sleep
mode should be selected so that as few as possible of the device’s functions are operating. All
functions not needed should be disabled. In particular, the following modules may need special
consideration when trying to achieve the lowest possible power consumption.
Analog-to-Digital
Converter (ADC)
If enabled, the ADC 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 189
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 the Analog Comparator 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 186 for details on how to
configure the Analog Comparator.
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
INT1
INT0
TWI
Address
Match
Timer
2
SPM/
EEPROM
Ready ADC
Other
I/O
Idle X X X X X(2) XX X X XX
ADC Noise
Reduction XX X X
(2) X(3) XX XX
Power
Down X(3) X
Power
Save X(2) X(2) X(3) XX
(2)
Standby(1) XX
(3) X
36
2486Z–AVR–02/11
ATmega8(L)
Brown-out Detector If the Brown-out Detector is not needed in the application, this module should be turned 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 how 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 disabled as described in the sections
above, the internal voltage reference will be disabled and it will not be consuming power. When
turned on again, the user must allow the reference to start up before the output is used. If the
reference is kept on in sleep mode, the output can be used immediately. Refer to “Internal Volt-
age Reference” on page 42 for details on the start-up time.
Watchdog Timer If the Watchdog Timer is not needed in the application, 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 page 43 for details on how to configure the Watchdog Timer.
Port Pins When entering a sleep mode, all port pins should be configured to use minimum power. The
most important thing is then to ensure that no pins drive resistive loads. In sleep modes where
the both the I/O clock (clkI/O) and the ADC clock (clkADC) are stopped, the input buffers of the
device will be disabled. This ensures that no power is consumed by the input logic when not
needed. In some cases, the input logic is needed for detecting wake-up conditions, and it will
then be enabled. Refer to the section “Digital Input Enable and Sleep Modes” on page 55 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.
37
2486Z–AVR–02/11
ATmega8(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. 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 14 on page 38 shows the Reset Logic. Table 15 on
page 38 defines the electrical parameters of the reset circuitry.
The I/O ports of the AVR are immediately reset to their initial state when a reset source goes
active. This does not require any clock source to be running.
After all reset sources have gone inactive, a delay counter is invoked, stretching the internal
reset. This allows the power to reach a stable level before normal operation starts. The time-out
period of the delay counter is defined by the user through the CKSEL Fuses. The different selec-
tions for the delay period are presented in “Clock Sources” on page 26.
Reset Sources The ATmega8 has four sources of Reset:
• Power-on Reset. The MCU is reset when the supply voltage is below the Power-on Reset
threshold (VPOT)
• External Reset. The MCU is reset when a low level is present on the RESET pin for longer
than the minimum pulse length
• Watchdog Reset. The MCU is reset when the Watchdog Timer period expires and the
Watchdog is enabled
• Brown-out Reset. The MCU is reset when the supply voltage VCC is below the Brown-out
Reset threshold (VBOT) and the Brown-out Detector is enabled
38
2486Z–AVR–02/11
ATmega8(L)
Figure 14. 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 case, the device is tested down to VCC = VBOT during the production test. This guar-
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 ATmega8L and BODLEVEL = 0 for ATmega8. BODLEVEL = 1 is not appli-
cable for ATmega8
Table 15. Reset Characteristics
Symbol Parameter Condition Min Typ Max Units
VPOT
Power-on Reset Threshold
Voltage (rising)(1) 1.4 2.3
V
Power-on Reset Threshold
Voltage (falling) 1.3 2.3
VRST RESET Pin Threshold Voltage 0.2 0.9 VCC
tRST
Minimum pulse width on
RESET Pin 1.5 µs
VBOT
Brown-out Reset Threshold
Voltage(2)
BODLEVEL = 1 2.4 2.6 2.9 V
BODLEVEL = 0 3.7 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 130 mV
MCU Control and Status
Register (MCUCSR)
Brown-Out
Reset Circuit
BODEN
BODLEVEL
Delay Counters
CKSEL[3:0]
CK TIMEOUT
WDRF
BORF
EXTRF
PORF
DATA BU S
Clock
Generator
SPIKE
FILTER
Pull-up Resistor
Watchdog
Oscillator
SUT[1:0]
39
2486Z–AVR–02/11
ATmega8(L)
Power-on Reset A Power-on Reset (POR) pulse is generated by an On-chip detection circuit. The detection level
is defined in Table 15 on page 38. The POR is activated whenever VCC is below the detection
level. The POR circuit can be used to trigger the Start-up Reset, as well as to detect a failure in
supply voltage.
A Power-on Reset (POR) circuit ensures that the device is reset from Power-on. Reaching the
Power-on Reset threshold voltage invokes the delay counter, which determines how long the
device is kept in RESET after VCC rise. The RESET signal is activated again, without any delay,
when VCC decreases below the detection level.
Figure 15. MCU Start-up, RESET Tied to VCC
Figure 16. MCU Start-up, RESET Extended Externally
V
RESET
TIME-OUT
INTERNAL
RESET
tTOUT
VPOT
VRST
CC
RESET
TIME-OUT
INTERNAL
RESET
tTOUT
VPOT
VRST
VCC
40
2486Z–AVR–02/11
ATmega8(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 on page 38) will generate a reset, even if the clock is not
running. Shorter pulses are not guaranteed to generate a reset. When the applied signal
reaches the Reset Threshold Voltage – VRST on its positive edge, the delay counter starts the
MCU after the time-out period tTOUT has expired.
Figure 17. External Reset During Operation
Brown-out Detection ATmega8 has an On-chip Brown-out Detection (BOD) circuit for monitoring the VCC level during
operation by comparing it to a fixed trigger level. The trigger level for the BOD can be selected
by the fuse BODLEVEL to be 2.7V (BODLEVEL unprogrammed), or 4.0V (BODLEVEL pro-
grammed). 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 value below the trigger level (VBOT- in Figure
18), the Brown-out Reset is immediately activated. When VCC increases above the trigger level
(VBOT+ in Figure 18), 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 on page 38.
Figure 18. Brown-out Reset During Operation
CC
V
CC
RESET
TIME-OUT
INTERNAL
RESET
V
BOT-
V
BOT+
t
TOUT
41
2486Z–AVR–02/11
ATmega8(L)
Watchdog Reset When the Watchdog times out, it will generate a short reset pulse of 1 CK cycle duration. On the
falling edge of this pulse, the delay timer starts counting the time-out period tTOUT. Refer to page
43 for details on operation of the Watchdog Timer.
Figure 19. Watchdog Reset During Operation
MCU Control and
Status Register –
MCUCSR
The MCU Control and Status Register provides information on which reset source caused an
MCU Reset.
• Bit 7..4 – Res: Reserved Bits
These bits are reserved bits in the ATmega8 and always read as zero.
• Bit 3 – WDRF: Watchdog Reset Flag
This bit is set if a Watchdog Reset occurs. The bit is reset by a Power-on Reset, or by writing a
logic zero to the flag.
• Bit 2 – BORF: Brown-out Reset Flag
This bit is set if a Brown-out Reset occurs. The bit is reset by a Power-on Reset, or by writing a
logic zero to the flag.
• Bit 1 – EXTRF: External Reset Flag
This bit is set if an External Reset occurs. The bit is reset by a Power-on Reset, or by writing a
logic zero to the flag.
• Bit 0 – PORF: Power-on Reset Flag
This bit is set if a Power-on Reset occurs. The bit is reset only by writing a logic zero to the flag.
To make use of the Reset Flags to identify a reset condition, the user should read and then reset
the MCUCSR as early as possible in the program. If the register is cleared before another reset
occurs, the source of the reset can be found by examining the Reset Flags.
CK
CC
Bit 76543210
–––– WDRF BORF EXTRF PORF MCUCSR
Read/Write R R R R R/W R/W R/W R/W
Initial Value 0 0 0 0 See Bit Description
42
2486Z–AVR–02/11
ATmega8(L)
Internal Voltage
Reference
ATmega8 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 that may influence the way it should be used. The
start-up time is given in Table 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.
Table 16. Internal Voltage Reference Characteristics
Symbol Parameter Min Typ Max Units
VBG Bandgap reference voltage 1.15 1.30 1.40 V
tBG Bandgap reference start-up time 40 70 µs
IBG Bandgap reference current consumption 10 µA
43
2486Z–AVR–02/11
ATmega8(L)
Watchdog Timer The Watchdog Timer is clocked from a separate On-chip Oscillator which runs at 1MHz. This is
the typical value at VCC = 5V. See characterization data for typical values at other VCC levels. By
controlling the Watchdog Timer prescaler, the Watchdog Reset interval can be adjusted as
shown in Table 17 on page 44. 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 ATmega8 resets and executes from the
Reset Vector. For timing details on the Watchdog Reset, refer to page 41.
To prevent unintentional disabling of the Watchdog, a special turn-off sequence must be fol-
lowed when the Watchdog is disabled. Refer to “Watchdog Timer Control Register – WDTCR”
for details.
Figure 20. Watchdog Timer
Watchdog Timer
Control Register –
WDTCR
• Bits 7..5 – Res: Reserved Bits
These bits are reserved bits in the ATmega8 and will always read as zero.
• Bit 4 – WDCE: Watchdog Change Enable
This bit must be set when the WDE bit is written to logic zero. Otherwise, the Watchdog will not
be disabled. Once written to one, hardware will clear this bit after four clock cycles. Refer to “Bit
3 – WDE: Watchdog Enable” on page 44for a Watchdog disable procedure. In Safety Level 1
and 2, this bit must also be set when changing the prescaler bits. See the Code Examples on
page 45.
WATCHDOG
OSCILLATOR
Bit 76543210
– – – WDCE WDE WDP2 WDP1 WDP0 WDTCR
Read/Write R R R R/W R/W R/W R/W R/W
Initial Value00000000
44
2486Z–AVR–02/11
ATmega8(L)
• 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 WDCE 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 one to WDCE and WDE. A logic one must be written
to WDE even though it is set to one 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.
The following code example shows one assembly and one C function for turning off the WDT.
The example assumes that interrupts are controlled (for example, by disabling interrupts glob-
ally) so that no interrupts will occur during execution of these functions.
Table 17. Watchdog Timer Prescale Select
WDP2 WDP1 WDP0
Number of WDT
Oscillator Cycles
Typical Time-out
at VCC = 3.0V
Typical Time-out
at VCC = 5.0V
0 0 0 16K (16,384) 17.1ms 16.3ms
0 0 1 32K (32,768) 34.3ms 32.5ms
0 1 0 64K (65,536) 68.5ms 65ms
0 1 1 128K (131,072) 0.14s 0.13s
1 0 0 256K (262,144) 0.27s 0.26s
1 0 1 512K (524,288) 0.55s 0.52s
1 1 0 1,024K (1,048,576) 1.1s 1.0s
1 1 1 2,048K (2,097,152) 2.2s 2.1s
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2486Z–AVR–02/11
ATmega8(L)
Timed Sequences
for Changing the
Configuration of
the Watchdog
Timer
The sequence for changing the Watchdog Timer configuration differs slightly between the safety
levels. Separate procedures are described for each level.
Safety Level 1
(WDTON Fuse
Unprogrammed)
In this mode, the Watchdog Timer is initially disabled, but can be enabled by writing the WDE bit
to 1 without any restriction. A timed sequence is needed when changing the Watchdog Time-out
period or disabling an enabled Watchdog Timer. To disable an enabled Watchdog Timer and/or
changing the Watchdog Time-out, the following procedure must be followed:
1. In the same operation, write a logic one to WDCE and WDE. A logic one must be written
to WDE regardless of the previous value of the WDE bit
2. Within the next four clock cycles, in the same operation, write the WDE and WDP bits as
desired, but with the WDCE bit cleared
Safety Level 2
(WDTON Fuse
Programmed)
In this mode, the Watchdog Timer is always enabled, and the WDE bit will always read as one. A
timed sequence is needed when changing the Watchdog Time-out period. To change the
Watchdog Time-out, the following procedure must be followed:
1. In the same operation, write a logical one to WDCE and WDE. Even though the WDE
always is set, the WDE must be written to one to start the timed sequence
Within the next four clock cycles, in the same operation, write the WDP bits as desired, but with
the WDCE bit cleared. The value written to the WDE bit is irrelevant.
Assembly Code Example
WDT_off:
; reset WDT
WDR
; Write logical one to WDCE and WDE
in r16, WDTCR
ori r16, (1<<WDCE)|(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 WDCE and WDE */
WDTCR |= (1<<WDCE) | (1<<WDE);
/* Turn off WDT */
WDTCR = 0x00;
}
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Interrupts This section describes the specifics of the interrupt handling performed by the ATmega8. For a
general explanation of the AVR interrupt handling, refer to “Reset and Interrupt Handling” on
page 14.
Interrupt Vectors
in ATmega8
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 202
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 on page 47 shows reset and Interrupt Vectors placement for the various combinations
of BOOTRST and IVSEL settings. If the program never enables an interrupt source, the Interrupt
Vectors are not used, and regular program code can be placed at these locations. This is also
the case if the Reset Vector is in the Application section while the Interrupt Vectors are in the
boot section or vice versa.
Table 18. Reset and Interrupt Vectors
Vector No.
Program
Address(2) Source Interrupt Definition
1 0x000(1) RESET External Pin, Power-on Reset, Brown-out
Reset, and Watchdog Reset
2 0x001 INT0 External Interrupt Request 0
3 0x002 INT1 External Interrupt Request 1
4 0x003 TIMER2 COMP Timer/Counter2 Compare Match
5 0x004 TIMER2 OVF Timer/Counter2 Overflow
6 0x005 TIMER1 CAPT Timer/Counter1 Capture Event
7 0x006 TIMER1 COMPA Timer/Counter1 Compare Match A
8 0x007 TIMER1 COMPB Timer/Counter1 Compare Match B
9 0x008 TIMER1 OVF Timer/Counter1 Overflow
10 0x009 TIMER0 OVF Timer/Counter0 Overflow
11 0x00A SPI, STC Serial Transfer Complete
12 0x00B USART, RXC USART, Rx Complete
13 0x00C USART, UDRE USART Data Register Empty
14 0x00D USART, TXC USART, Tx Complete
15 0x00E ADC ADC Conversion Complete
16 0x00F EE_RDY EEPROM Ready
17 0x010 ANA_COMP Analog Comparator
18 0x011 TWI Two-wire Serial Interface
19 0x012 SPM_RDY Store Program Memory Ready
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ATmega8(L)
Note: 1. The Boot Reset Address is shown in Table 82 on page 213. 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
ATmega8 is:
addressLabels Code Comments
$000 rjmp RESET ; Reset Handler
$001 rjmp EXT_INT0 ; IRQ0 Handler
$002 rjmp EXT_INT1 ; IRQ1 Handler
$003 rjmp TIM2_COMP ; Timer2 Compare Handler
$004 rjmp TIM2_OVF ; Timer2 Overflow Handler
$005 rjmp TIM1_CAPT ; Timer1 Capture Handler
$006 rjmp TIM1_COMPA ; Timer1 CompareA Handler
$007 rjmp TIM1_COMPB ; Timer1 CompareB Handler
$008 rjmp TIM1_OVF ; Timer1 Overflow Handler
$009 rjmp TIM0_OVF ; Timer0 Overflow Handler
$00a rjmp SPI_STC ; SPI Transfer Complete Handler
$00b rjmp USART_RXC ; USART RX Complete Handler
$00c rjmp USART_UDRE ; UDR Empty Handler
$00d rjmp USART_TXC ; USART TX Complete Handler
$00e rjmp ADC ; ADC Conversion Complete Handler
$00f rjmp EE_RDY ; EEPROM Ready Handler
$010 rjmp ANA_COMP ; Analog Comparator Handler
$011 rjmp TWSI ; Two-wire Serial Interface Handler
$012 rjmp SPM_RDY ; Store Program Memory Ready Handler
;
$013 RESET: ldi r16,high(RAMEND); Main program start
$014 out SPH,r16 ; Set Stack Pointer to top of RAM
$015 ldi r16,low(RAMEND)
$016 out SPL,r16
$017 sei ; Enable interrupts
$018 <instr> xxx
... ... ...
Table 19. Reset and Interrupt Vectors Placement
BOOTRST(1) IVSEL Reset Address Interrupt Vectors Start Address
1 0 0x000 0x001
1 1 0x000 Boot Reset Address + 0x001
0 0 Boot Reset Address 0x001
0 1 Boot Reset Address Boot Reset Address + 0x001
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ATmega8(L)
When the BOOTRST Fuse is unprogrammed, the boot section size set to 2Kbytes 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:
AddressLabels Code Comments
$000 rjmp RESET ; Reset handler
;
$001 RESET:ldi r16,high(RAMEND); Main program start
$002 out SPH,r16 ; Set Stack Pointer to top of RAM
$003 ldi r16,low(RAMEND)
$004 out SPL,r16
$005 sei ; Enable interrupts
$006 <instr> xxx
;
.org $c01
$c01 rjmp EXT_INT0 ; IRQ0 Handler
$c02 rjmp EXT_INT1 ; IRQ1 Handler
... ... ... ;
$c12 rjmp SPM_RDY ; Store Program Memory Ready Handler
When the BOOTRST Fuse is programmed and the boot section size set to 2Kbytes, the most
typical and general program setup for the Reset and Interrupt Vector Addresses is:
AddressLabels Code Comments
.org $001
$001 rjmp EXT_INT0 ; IRQ0 Handler
$002 rjmp EXT_INT1 ; IRQ1 Handler
... ... ... ;
$012 rjmp SPM_RDY ; Store Program Memory Ready Handler
;
.org $c00
$c00 rjmp RESET ; Reset handler
;
$c01 RESET:ldi r16,high(RAMEND); Main program start
$c02 out SPH,r16 ; Set Stack Pointer to top of RAM
$c03 ldi r16,low(RAMEND)
$c04 out SPL,r16
$c05 sei ; Enable interrupts
$c06 <instr> xxx
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ATmega8(L)
When the BOOTRST Fuse is programmed, the boot section size set to 2Kbytes, 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:
AddressLabels Code Comments
;
.org $c00
$c00 rjmp RESET ; Reset handler
$c01 rjmp EXT_INT0 ; IRQ0 Handler
$c02 rjmp EXT_INT1 ; IRQ1 Handler
... ... ... ;
$c12 rjmp SPM_RDY ; Store Program Memory Ready Handler
$c13 RESET: ldi r16,high(RAMEND); Main program start
$c14 out SPH,r16 ; Set Stack Pointer to top of RAM
$c15 ldi r16,low(RAMEND)
$c16 out SPL,r16
$c17 sei ; Enable interrupts
$c18 <instr> xxx
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 section of the Flash. The actual address of the start of the boot Flash section is deter-
mined by the BOOTSZ Fuses. Refer to the section “Boot Loader Support – Read-While-Write
Self-Programming” on page 202 for details. To avoid unintentional changes of Interrupt Vector
tables, a special write procedure must be followed to change the IVSEL bit:
1. Write the Interrupt Vector Change Enable (IVCE) bit to one
2. Within four cycles, write the desired value to IVSEL while writing a zero to IVCE
Interrupts will automatically be disabled while this sequence is executed. Interrupts are disabled
in the cycle IVCE is set, and they remain disabled until after the instruction following the write to
IVSEL. If IVSEL is not written, interrupts remain disabled for four cycles. The I-bit in the Status
Register is unaffected by the automatic disabling.
Note: If Interrupt Vectors are placed in the Boot Loader section and Boot Lock bit BLB02 is pro-
grammed, interrupts are disabled while executing from the Application section. If Interrupt
Vectors are placed in the Application section and Boot Lock bit BLB12 is programed, interrupts
are disabled while executing from the Boot Loader section. Refer to the section “Boot Loader
Support – Read-While-Write Self-Programming” on page 202 for details on Boot Lock Bits.
Bit 76543210
INT1 INT0 –––– IVSEL IVCE GICR
Read/Write R/W R/W R R R R R/W R/W
Initial Value 0 0 0 0 0 0 0 0
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ATmega8(L)
• Bit 0 – IVCE: Interrupt Vector Change Enable
The IVCE bit must be written to logic one to enable change of the IVSEL bit. IVCE is cleared by
hardware four cycles after it is written or when IVSEL is written. Setting the IVCE bit will disable
interrupts, as explained in the IVSEL description above. See Code Example below.
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);
}
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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 configured as output) or enabling/disabling of pull-up resistors (if configured as
input). Each output buffer has symmetrical drive characteristics with both high sink and source
capability. The pin driver is strong enough to drive LED displays directly. All port pins have 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 21. Refer to “Electrical Charac-
teristics” on page 235 for a complete list of parameters.
Figure 21. 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 port, and a lower case “n” represents the bit number. However,
when using the register or bit defines in a program, the precise form must be used (that is,
PORTB3 for bit 3 in Port B, here documented generally as PORTxn). The physical I/O Registers
and bit locations are listed in “Register Description for I/O Ports” on page 65.
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 General Digital I/O is described in “Ports as General Digital I/O” . Most port
pins are multiplexed with alternate functions for the peripheral features on the device. How each
alternate function interferes with the port pin is described in “Alternate Port Functions” on page
56. Refer to the individual module sections for a full description of the alternate functions.
Note that enabling the alternate function of some of the port pins does not affect the use of the
other pins in the port as general digital I/O.
Cpin
Logic
Rpu
See Figure
"General Digital I/O" for
Details
Pxn
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2486Z–AVR–02/11
ATmega8(L)
Ports as General
Digital I/O
The ports are bi-directional I/O ports with optional internal pull-ups. Figure 22 on page 52 shows
a functional description of one I/O port pin, here generically called Pxn.
Figure 22. General Digital I/O(1)
Note: 1. WPx, WDx, RRx, RPx, and RDx are common to all pins within the same port. clkI/O, SLEEP,
and PUD are common to all ports
Configuring the Pin Each port pin consists of 3 Register bits: DDxn, PORTxn, and PINxn. As shown in “Register
Description for I/O Ports” on page 65, the DDxn bits are accessed at the DDRx I/O address, the
PORTxn bits at the PORTx I/O address, and the PINxn bits at the PINx I/O address.
The DDxn bit in the DDRx Register selects the direction of this pin. If DDxn is written logic one,
Pxn is configured as an output pin. If DDxn is written logic zero, Pxn is configured as an input
pin.
If PORTxn is written logic one when the pin is configured as an input pin, the pull-up resistor is
activated. To switch the pull-up resistor off, PORTxn has to be written logic zero or the pin has to
be configured as an output pin. The port pins are tri-stated when 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).
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
clkI/O: I/O CLOCK
RDx: READ DDRx
D
L
Q
Q
RESET
RESET
Q
Q
D
Q
QD
CLR
PORTxn
Q
QD
CLR
DDxn
PINxn
DATA BU S
SLEEP
SLEEP: SLEEP CONTROL
Pxn
I/O
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2486Z–AVR–02/11
ATmega8(L)
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 difference 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.
Switching between input with pull-up and output low generates the same problem. The user
must use either the tri-state ({DDxn, PORTxn} = 0b00) or the output high state ({DDxn, PORTxn}
= 0b11) as an intermediate step.
Table 20 summarizes the control signals for the pin value.
Reading the Pin Value Independent of the setting of Data Direction bit DDxn, the port pin can be read through the
PINxn Register Bit. As shown in Figure 22 on page 52, the PINxn Register bit and the preceding
latch constitute a synchronizer. This is needed to avoid metastability if the physical pin changes
value near the edge of the internal clock, but it also introduces a delay. Figure 23 shows a timing
diagram of the synchronization when reading an externally applied pin value. The maximum and
minimum propagation delays are denoted tpd,max and tpd,min, respectively.
Figure 23. Synchronization when Reading an Externally Applied Pin Value
Consider the clock period starting shortly after the first falling edge of the system clock. The latch
is closed when the clock is low, and goes transparent when the clock is high, as indicated by the
shaded region of the “SYNC LATCH” signal. The signal value is latched when the system clock
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 external
pulled low.
0 1 1 Input No Tri-state (Hi-Z)
1 0 X Output No Output Low (Sink)
1 1 X Output No Output High (Source)
XXX in r17, PINx
0x00 0xFF
INSTRUCTIONS
SYNC LATCH
PINxn
r17
XXX
SYSTEM CLK
t
pd, max
t
pd, min
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2486Z–AVR–02/11
ATmega8(L)
goes low. It is clocked into the PINxn Register at the succeeding positive clock edge. As indi-
cated by the two arrows tpd,max and tpd,min, a single signal transition on the pin will be delayed
between ½ and 1-½ system clock period depending upon the time of assertion.
When reading back a software assigned pin value, a nop instruction must be inserted as indi-
cated in Figure 24. The out instruction sets the “SYNC LATCH” signal at the positive edge of the
clock. In this case, the delay tpd through the synchronizer is 1 system clock period.
Figure 24. Synchronization when Reading a Software Assigned Pin Value
out PORTx, r16 nop in r17, PINx
0xFF
0x00 0xFF
SYSTEM CLK
r16
INSTRUCTIONS
SYNC LATCH
PINxn
r17
tpd
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ATmega8(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 22 on page 52, 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 Control-
ler in Power-down mode, Power-save mode, and Standby mode to avoid high power
consumption if some input signals are left floating, or have an analog signal level close to VCC/2.
SLEEP is overridden for port pins enabled as External Interrupt pins. If the External Interrupt
Request is not enabled, SLEEP is active also for these pins. SLEEP is also overridden by vari-
ous other alternate functions as described in “Alternate Port Functions” on page 56.
If a logic high level (“one”) is present on an Asynchronous External Interrupt pin configured as
“Interrupt on Rising Edge, Falling Edge, or Any Logic Change on Pin” while the external interrupt
is not enabled, the corresponding External Interrupt Flag will be set when resuming from the
above mentioned sleep 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;
...
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2486Z–AVR–02/11
ATmega8(L)
Unconnected pins If some pins are unused, it is recommended to ensure that these pins have a defined level. Even
though most of the digital inputs are disabled in the deep sleep modes as described above, float-
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, since this may cause excessive currents 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 25 shows
how the port pin control signals from the simplified Figure 22 on page 52 can be overridden by
alternate functions. The overriding signals may not be present in all port pins, but the figure
serves as a generic description applicable to all port pins in the AVR microcontroller family.
Figure 25. Alternate Port Functions(1)
Note: 1. WPx, WDx, RRx, RPx, and RDx are common to all pins within the same port. clkI/O, SLEEP,
and PUD are common to all ports. All other signals are unique for each pin
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
D
L
Q
Q
SET
CLR
0
1
0
1
0
1
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
RESET
Q
QD
CLR
Q
QD
CLR
Q
Q
D
CLR
PINxn
PORTxn
DDxn
DATA BUS
0
1
DIEOVxn
SLEEP
DIEOExn: Pxn DIGITAL INPUT-ENABLE OVERRIDE ENABLE
DIEOVxn: Pxn DIGITAL INPUT-ENABLE OVERRIDE VALUE
SLEEP: SLEEP CONTROL
Pxn
I/O
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Table 21 summarizes the function of the overriding signals. The pin and port indexes from Fig-
ure 25 on page 56 are not shown in the succeeding tables. The overriding signals are generated
internally 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 DDOV signal. If this signal is cleared,
the Output driver is enabled by the DDxn Register bit.
DDOV Data Direction
Override Value
If DDOE is set, the Output Driver is enabled/disabled
when DDOV is set/cleared, regardless of the setting of
the DDxn Register bit.
PVOE Port Value
Override Enable
If this signal is set and the Output Driver is enabled,
the port value is controlled by the PVOV signal. If
PVOE is cleared, and the Output Driver is enabled, the
port Value is controlled by the PORTxn Register bit.
PVOV Port Value
Override Value
If PVOE is set, the port value is set to PVOV,
regardless of the setting of the PORTxn Register bit.
DIEOE Digital Input Enable
Override Enable
If this bit is set, the Digital Input Enable is controlled by
the DIEOV signal. If this signal is cleared, the Digital
Input Enable is determined by MCU-state (Normal
mode, sleep 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 (Normal 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 before the synchronizer. Unless the
Digital Input is used as a clock source, the module with
the alternate function will use its own synchronizer.
AIO Analog Input/output This is the Analog Input/output to/from alternate
functions. The signal is connected directly to the pad,
and can be used bi-directionally.
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ATmega8(L)
Special Function IO
Register – SFIOR
• Bit 2 – PUD: Pull-up Disable
When this bit is written to one, the pull-ups in the I/O ports are disabled even if the DDxn and
PORTxn Registers are configured to enable the pull-ups ({DDxn, PORTxn} = 0b01). See “Con-
figuring the Pin” on page 52 for more details about this feature.
Alternate Functions of
Port B
The Port B pins with alternate functions are shown in Table 22.
The alternate pin configuration is as follows:
• XTAL2/TOSC2 – Port B, Bit 7
XTAL2: Chip clock Oscillator pin 2. Used as clock pin for crystal Oscillator or Low-frequency
crystal Oscillator. When used as a clock pin, the pin can not be used as an I/O pin.
TOSC2: Timer Oscillator pin 2. Used only if internal calibrated RC Oscillator is selected as chip
clock source, and the asynchronous timer is enabled by the correct setting in ASSR. When the
AS2 bit in ASSR is set (one) to enable asynchronous clocking of Timer/Counter2, pin PB7 is dis-
connected from the port, and becomes the inverting output of the Oscillator amplifier. In this
mode, a crystal Oscillator is connected to this pin, and the pin cannot be used as an I/O pin.
If PB7 is used as a clock pin, DDB7, PORTB7 and PINB7 will all read 0.
• XTAL1/TOSC1 – Port B, Bit 6
XTAL1: Chip clock Oscillator pin 1. Used for all chip clock sources except internal calibrated RC
Oscillator. When used as a clock pin, the pin can not be used as an I/O pin.
TOSC1: Timer Oscillator pin 1. Used only if internal calibrated RC Oscillator is selected as chip
clock source, and the asynchronous timer is enabled by the correct setting in ASSR. When the
AS2 bit in ASSR is set (one) to enable asynchronous clocking of Timer/Counter2, pin PB6 is dis-
connected from the port, and becomes the 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.
If PB6 is used as a clock pin, DDB6, PORTB6 and PINB6 will all read 0.
Bit 7 6 5 4 3 2 1 0
ACME PUD PSR2 PSR10 SFIOR
Read/Write R R R R R/W R/W R/W R/W
Initial Value 0 0 0 0 0 0 0 0
Table 22. Port B Pins Alternate Functions
Port Pin Alternate Functions
PB7 XTAL2 (Chip Clock Oscillator pin 2)
TOSC2 (Timer Oscillator pin 2)
PB6 XTAL1 (Chip Clock Oscillator pin 1 or External clock input)
TOSC1 (Timer Oscillator pin 1)
PB5 SCK (SPI Bus Master clock Input)
PB4 MISO (SPI Bus Master Input/Slave Output)
PB3 MOSI (SPI Bus Master Output/Slave Input)
OC2 (Timer/Counter2 Output Compare Match Output)
PB2 SS (SPI Bus Master Slave select)
OC1B (Timer/Counter1 Output Compare Match B Output)
PB1 OC1A (Timer/Counter1 Output Compare Match A Output)
PB0 ICP1 (Timer/Counter1 Input Capture Pin)
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• SCK – Port B, Bit 5
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 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.
• MISO – Port B, Bit 4
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 DDB4. When the SPI is
enabled as a Slave, 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.
• MOSI/OC2 – Port B, Bit 3
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 DDB3. When the SPI is
enabled as a Master, the data direction of this pin is controlled by DDB3. When the pin is forced
by the SPI to be an input, the pull-up can still be controlled by the PORTB3 bit.
OC2, Output Compare Match Output: The PB3 pin can serve as an external output for the
Timer/Counter2 Compare Match. The PB3 pin has to be configured as an output (DDB3 set
(one)) to serve this function. The OC2 pin is also the output pin for the PWM mode timer
function.
•SS
/OC1B – Port B, Bit 2
SS: Slave Select input. When the SPI is enabled as a Slave, this pin is configured as an input
regardless of the setting of DDB2. As a Slave, the SPI is activated when this pin is driven low.
When the SPI is enabled as a Master, the data direction of this pin is controlled by DDB2. When
the pin is forced by the SPI to be an input, the pull-up can still be controlled by the PORTB2 bit.
OC1B, Output Compare Match output: The PB2 pin can serve as an external output for the
Timer/Counter1 Compare Match B. The PB2 pin has to be configured as an output (DDB2 set
(one)) to serve this function. The OC1B pin is also the output pin for the PWM mode timer
function.
• OC1A – Port B, Bit 1
OC1A, Output Compare Match output: The PB1 pin can serve as an external output for the
Timer/Counter1 Compare Match A. The PB1 pin has to be configured as an output (DDB1 set
(one)) to serve this function. The OC1A pin is also the output pin for the PWM mode timer
function.
• ICP1 – Port B, Bit 0
ICP1 – Input Capture Pin: The PB0 pin can act as an Input Capture Pin for Timer/Counter1.
Table 23 on page 60 and Table 24 on page 60 relate the alternate functions of Port B to the
overriding signals shown in Figure 25 on page 56. 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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Notes: 1. INTRC means that the internal RC Oscillator is selected (by the CKSEL Fuse)
2. EXT means that the external RC Oscillator or an external clock is selected (by the CKSEL
Fuse)
Table 23. Overriding Signals for Alternate Functions in PB7..PB4
Signal
Name
PB7/XTAL2/
TOSC2(1)(2)
PB6/XTAL1/
TOSC1(1) PB5/SCK PB4/MISO
PUOE EXT • (INTRC +
AS2)
INTRC + AS2 SPE • MSTR SPE • MSTR
PUO 0 0 PORTB5 • PUD PORTB4 • PUD
DDOE EXT • (INTRC +
AS2)
INTRC + AS2 SPE • MSTR SPE • MSTR
DDOV 0 0 0 0
PVOE 0 0 SPE • MSTR SPE • MSTR
PVOV 0 0 SCK OUTPUT SPI SLAVE OUTPUT
DIEOE EXT • (INTRC +
AS2)
INTRC + AS2 0 0
DIEOV 0 0 0 0
DI – – SCK INPUT SPI MSTR INPUT
AIO Oscillator Output Oscillator/Clock
Input
––
Table 24. Overriding Signals for Alternate Functions in PB3..PB0
Signal
Name PB3/MOSI/OC2 PB2/SS/OC1B PB1/OC1A PB0/ICP1
PUOE SPE • MSTR SPE • MSTR 00
PUO PORTB3 • PUD PORTB2 • PUD 00
DDOE SPE • MSTR SPE • MSTR 00
DDOV 0 0 0 0
PVOE SPE • MSTR +
OC2 ENABLE
OC1B ENABLE OC1A ENABLE 0
PVOV SPI MSTR OUTPUT + OC2 OC1B OC1A 0
DIEOE0 000
DIEOV0 000
DI SPI SLAVE INPUT SPI SS –ICP1 INPUT
AIO– –––
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Alternate Functions of
Port C
The Port C pins with alternate functions are shown in Table 25.
The alternate pin configuration is as follows:
• RESET – Port C, Bit 6
RESET, Reset pin: When the RSTDISBL Fuse is programmed, this pin functions as a normal I/O
pin, and the part will have to rely on Power-on Reset and Brown-out Reset as its reset sources.
When the RSTDISBL Fuse is unprogrammed, the reset circuitry is connected to the pin, and the
pin can not be used as an I/O pin.
If PC6 is used as a reset pin, DDC6, PORTC6 and PINC6 will all read 0.
• SCL/ADC5 – Port C, Bit 5
SCL, Two-wire Serial Interface Clock: When the TWEN bit in TWCR is set (one) to enable the
Two-wire Serial Interface, pin PC5 is disconnected from the port and becomes the Serial Clock
I/O pin for the Two-wire Serial Interface. In this mode, there 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.
PC5 can also be used as ADC input Channel 5. Note that ADC input channel 5 uses digital
power.
• SDA/ADC4 – Port C, Bit 4
SDA, Two-wire Serial Interface Data: When the TWEN bit in TWCR is set (one) to enable the
Two-wire Serial Interface, pin PC4 is disconnected from the port and becomes the Serial Data
I/O pin for the Two-wire Serial Interface. In this mode, there 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.
PC4 can also be used as ADC input Channel 4. Note that ADC input channel 4 uses digital
power.
• ADC3 – Port C, Bit 3
PC3 can also be used as ADC input Channel 3. Note that ADC input channel 3 uses analog
power.
• ADC2 – Port C, Bit 2
PC2 can also be used as ADC input Channel 2. Note that ADC input channel 2 uses analog
power.
Table 25. Port C Pins Alternate Functions
Port Pin Alternate Function
PC6 RESET (Reset pin)
PC5 ADC5 (ADC Input Channel 5)
SCL (Two-wire Serial Bus Clock Line)
PC4 ADC4 (ADC Input Channel 4)
SDA (Two-wire Serial Bus Data Input/Output Line)
PC3 ADC3 (ADC Input Channel 3)
PC2 ADC2 (ADC Input Channel 2)
PC1 ADC1 (ADC Input Channel 1)
PC0 ADC0 (ADC Input Channel 0)
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• ADC1 – Port C, Bit 1
PC1 can also be used as ADC input Channel 1. Note that ADC input channel 1 uses analog
power.
• ADC0 – Port C, Bit 0
PC0 can also be used as ADC input Channel 0. Note that ADC input channel 0 uses analog
power.
Table 26 and Table 27 relate the alternate functions of Port C to the overriding signals shown in
Figure 25 on page 56.
Note: 1. When enabled, the Two-wire Serial Interface enables slew-rate controls on the output pins
PC4 and PC5. This is not shown in the figure. In addition, spike filters are connected between
the AIO outputs shown in the port figure and the digital logic of the TWI module
Table 26. Overriding Signals for Alternate Functions in PC6..PC4
Signal Name PC6/RESET PC5/SCL/ADC5 PC4/SDA/ADC4
PUOE RSTDISBL TWEN TWEN
PUOV 1 PORTC5 • PUD PORTC4 • PUD
DDOE RSTDISBL TWEN TWEN
DDOV 0 SCL_OUT SDA_OUT
PVOE 0 TWEN TWEN
PVOV 0 0 0
DIEOE RSTDISBL 00
DIEOV 0 0 0
DI – – –
AIO RESET INPUT ADC5 INPUT / SCL INPUT ADC4 INPUT / SDA INPUT
Table 27. Overriding Signals for Alternate Functions in PC3..PC0(1)
Signal Name PC3/ADC3 PC2/ADC2 PC1/ADC1 PC0/ADC0
PUOE0000
PUOV0000
DDOE0000
DDOV0000
PVOE0000
PVOV0000
DIEOE0000
DIEOV0000
DI––––
AIO ADC3 INPUT ADC2 INPUT ADC1 INPUT ADC0 INPUT
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Alternate Functions of
Port D
The Port D pins with alternate functions are shown in Table 28.
The alternate pin configuration is as follows:
• AIN1 – Port D, Bit 7
AIN1, Analog Comparator Negative 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.
• AIN0 – Port D, Bit 6
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.
• T1 – Port D, Bit 5
T1, Timer/Counter1 counter source.
• XCK/T0 – Port D, Bit 4
XCK, USART external clock.
T0, Timer/Counter0 counter source.
• 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 29 on page 64 and Table 30 on page 64 relate the alternate functions of Port D to the
overriding signals shown in Figure 25 on page 56.
Table 28. Port D Pins Alternate Functions
Port Pin Alternate Function
PD7 AIN1 (Analog Comparator Negative Input)
PD6 AIN0 (Analog Comparator Positive Input)
PD5 T1 (Timer/Counter 1 External Counter Input)
PD4 XCK (USART External Clock Input/Output)
T0 (Timer/Counter 0 External Counter Input)
PD3 INT1 (External Interrupt 1 Input)
PD2 INT0 (External Interrupt 0 Input)
PD1 TXD (USART Output Pin)
PD0 RXD (USART Input Pin)
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Table 29. Overriding Signals for Alternate Functions PD7..PD4
Signal Name PD7/AIN1 PD6/AIN0 PD5/T1 PD4/XCK/T0
PUOE0000
PUO0000
OOE0000
OO0000
PVOE 0 0 0 UMSEL
PVO 0 0 0 XCK OUTPUT
DIEOE0000
DIEO0000
DI – – T1 INPUT XCK INPUT / T0 INPUT
AIO AIN1 INPUT AIN0 INPUT – –
Table 30. Overriding Signals for Alternate Functions in PD3..PD0
Signal Name PD3/INT1 PD2/INT0 PD1/TXD PD0/RXD
PUOE 0 0 TXEN RXEN
PUO 0 0 0 PORTD0 • PUD
OOE 0 0 TXEN RXEN
OO0010
PVOE 0 0 TXEN 0
PVO 0 0 TXD 0
DIEOE INT1 ENABLE INT0 ENABLE 0 0
DIEO1100
DI INT1 INPUT INT0 INPUT – RXD
AIO––––
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Register Description for I/O Ports
The Port B Data
Register – PORTB
The Port B Data
Direction Register –
DDRB
The Port B Input Pins
Address – PINB
The Port C Data
Register – PORTC
The Port C Data
Direction Register –
DDRC
The Port C Input Pins
Address – PINC
The Port D Data
Register – PORTD
The Port D Data
Direction Register –
DDRD
The Port D Input Pins
Address – PIND
Bit 76543210
PORTB7 PORTB6 PORTB5 PORTB4 PORTB3 PORTB2 PORTB1 PORTB0 PORTB
Read/WriteR/WR/WR/WR/WR/WR/WR/WR/W
Initial Value00000000
Bit 76543210
DDB7 DDB6 DDB5 DDB4 DDB3 DDB2 DDB1 DDB0 DDRB
Read/WriteR/WR/WR/WR/WR/WR/WR/WR/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
Bit 76543210
– PORTC6 PORTC5 PORTC4 PORTC3 PORTC2 PORTC1 PORTC0 PORTC
Read/Write R R/WR/WR/WR/WR/WR/WR/W
Initial Value00000000
Bit 76543210
– DDC6 DDC5 DDC4 DDC3 DDC2 DDC1 DDC0 DDRC
Read/Write R R/WR/WR/WR/WR/WR/WR/W
Initial Value00000000
Bit 76543210
– PINC6 PINC5 PINC4 PINC3 PINC2 PINC1 PINC0 PINC
Read/WriteRRRRRRRR
Initial Value 0 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/WriteR/WR/WR/WR/WR/WR/WR/WR/W
Initial Value00000000
Bit 76543210
DDD7 DDD6 DDD5 DDD4 DDD3 DDD2 DDD1 DDD0 DDRD
Read/WriteR/WR/WR/WR/WR/WR/WR/WR/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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External
Interrupts
The external interrupts are triggered by the INT0, and INT1 pins. Observe that, if enabled, the
interrupts will trigger even if the INT0..1 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 ris-
ing edge or a low level. This is set up as indicated in the specification for the MCU Control
Register – MCUCR. When the external interrupt is enabled and is configured as level triggered,
the interrupt will trigger as long as the pin is held low. Note that recognition of falling or rising
edge interrupts on INT0 and INT1 requires the presence of an I/O clock, described in “Clock
Systems and their Distribution” on page 25. Low level interrupts on INT0/INT1 are detected
asynchronously. This implies that these interrupts can be used for waking the part also from
sleep modes other than Idle mode. The I/O clock is halted in all sleep modes except Idle mode.
Note that if a level triggered interrupt is used for wake-up from Power-down 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 Watchdog 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 235. 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 time. The start-up time is defined by the SUT Fuses as described in “System Clock and
Clock Options” on page 25. 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 required level must be held long enough for the MCU to complete the wake up to
trigger the level interrupt.
MCU Control Register
– MCUCR
The MCU Control Register contains control 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 defined in Table 31. 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 interrupt. Shorter pulses are not guaranteed to generate an interrupt. If
low level interrupt is selected, the low level must be held until the completion of the currently
executing instruction to generate an interrupt.
Bit 76543210
SE SM2 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 31. 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 interrupt request
1 1 The rising edge of INT1 generates an interrupt request
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• 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 32. 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 generate an interrupt. If low level interrupt is
selected, the low level must be held until the completion of the currently executing instruction to
generate an interrupt.
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 interrupt is enabled. The Interrupt Sense Control1 bits 1/0 (ISC11 and ISC10) in the MCU
general Control Register (MCUCR) define whether the external interrupt is activated on rising
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 corresponding interrupt of External Interrupt
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 interrupt is enabled. The Interrupt Sense Control0 bits 1/0 (ISC01 and ISC00) in the 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 corresponding interrupt of External Interrupt
Request 0 is executed from the INT0 Interrupt Vector.
General Interrupt Flag
Register – GIFR
• Bit 7 – INTF1: External Interrupt Flag 1
When an event on the INT1 pin triggers an interrupt request, INTF1 becomes set (one). If the I-
bit in SREG and the INT1 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. 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.
Table 32. 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 interrupt request
1 1 The rising edge of INT0 generates an interrupt request
Bit 76543210
INT1 INT0 – – – – IVSEL IVCE GICR
Read/Write R/W R/W R R R R R/W R/W
Initial Value 0 0 0 0 0 0 0 0
Bit 76543210
INTF1 INTF0 ––––––GIFR
Read/WriteR/WR/WRRRRRR
Initial Value 0 0 0 0 0 0 0 0
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• Bit 6 – INTF0: External Interrupt Flag 0
When an event on the INT0 pin triggers an interrupt request, INTF0 becomes set (one). If the I-
bit in SREG and the INT0 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. 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.
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8-bit
Timer/Counter0
Timer/Counter0 is a general purpose, single channel, 8-bit Timer/Counter module. The main
features are:
•Single Channel Counter
•Frequency Generator
•External Event Counter
•10-bit Clock Prescaler
Overview A simplified block diagram of the 8-bit Timer/Counter is shown in Figure 26. For the actual place-
ment of I/O pins, refer to “Pin Configurations” 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 loca-
tions are listed in the “8-bit Timer/Counter Register Description” on page 71.
Figure 26. 8-bit Timer/Counter Block Diagram
Registers The Timer/Counter (TCNT0) is an 8-bit register. Interrupt request (abbreviated to Int. Req. in the
figure) signals are all visible in the Timer Interrupt Flag Register (TIFR). All interrupts are individ-
ually masked with the Timer Interrupt Mask Register (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 internally or via the prescaler, or by an external clock source
on the T0 pin. The Clock Select logic block controls which clock source and edge the
Timer/Counter uses to increment 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).
Definitions Many register and bit references in this document are written in general form. A lower 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/Counter0 counter value and so on.
The definitions in Table 33 are also used extensively throughout this datasheet.
Timer/Counter
DATA BUS
TCNTn
Control Logic
= 0xFF
count
TOVn
(Int.Req.)
TCCRn
Clock Select
Tn
Edge
Detector
( From Prescaler )
clkTn
Table 33. Definitions
BOTTOM The counter reaches the BOTTOM when it becomes 0x00
MAX The counter reaches its MAXimum when it becomes 0xFF (decimal 255)
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Timer/Counter
Clock Sources
The Timer/Counter can be clocked by an internal or an external clock source. The clock source
is selected by the clock select logic which is controlled by the clock select (CS02:0) bits located
in the Timer/Counter Control Register (TCCR0). For details on clock sources and prescaler, see
“Timer/Counter0 and Timer/Counter1 Prescalers” on page 73.
Counter Unit The main part of the 8-bit Timer/Counter is the programmable counter unit. Figure 27 shows a
block diagram of the counter and its surroundings.
Figure 27. Counter Unit Block Diagram
Signal description (internal signals):
count Increment TCNT0 by 1
clkTnTimer/Counter clock, referred to as clkT0 in the following
max Signalize that TCNT0 has reached maximum value
The counter is incremented at each timer clock (clkT0). clkT0 can be generated from an external
or internal clock source, selected by the clock select bits (CS02:0). When no clock source is
selected (CS02:0 = 0) the timer is stopped. However, the TCNT0 value can be accessed by the
CPU, regardless of whether clkT0 is present or not. A CPU write overrides (has priority over) all
counter clear or count operations.
Operation 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 (MAX = 0xFF) and then restarts
from the bottom (0x00). In normal operation the Timer/Counter Overflow Flag (TOV0) will be set
in the same timer clock cycle as the TCNT0 becomes zero. The TOV0 Flag in this case behaves
like a ninth bit, except that it is only set, not cleared. However, combined with the timer overflow
interrupt that automatically clears the TOV0 Flag, the timer resolution can be increased by soft-
ware. A new counter value can be written anytime.
Timer/Counter
Timing Diagrams
The Timer/Counter is a synchronous design and the timer clock (clkT0) is therefore shown as a
clock enable signal in the following figures. The figures include information on when Interrupt
Flags are set. Figure 28 on page 71 contains timing data for basic Timer/Counter operation. The
figure shows the count sequence close to the MAX value.
DATA BUS
TCNTn Control Logic
count
TOVn
(Int. Req.)
Clock Select
max
Tn
Edge
Detector
( From Prescaler )
clk
Tn
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Figure 28. Timer/Counter Timing Diagram, No Prescaling
Figure 29 shows the same timing data, but with the prescaler enabled.
Figure 29. Timer/Counter Timing Diagram, with Prescaler (fclk_I/O/8)
8-bit
Timer/Counter
Register
Description
Timer/Counter Control
Register – TCCR0
• Bit 2:0 – CS02:0: Clock Select
The three clock select bits select the clock source to be used by the Timer/Counter.
clk
Tn
(clk
I/O
/1)
TOVn
clk
I/O
TCNTn
MAX - 1 MAX BOTTOM BOTTOM + 1
TOVn
TCNTn MAX - 1 MAX BOTTOM BOTTOM + 1
clk
I/O
clk
Tn
(clk
I/O
/8)
Bit 76543210
––––– CS02 CS01 CS00 TCCR0
Read/Write R R R R R R/W R/W R/W
Initial Value00000000
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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
Register – TCNT0
The Timer/Counter Register gives direct access, both for read and write operations, to the
Timer/Counter unit 8-bit counter.
Timer/Counter
Interrupt Mask
Register – TIMSK
• 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 0 – TOV0: Timer/Counter0 Overflow Flag
The bit TOV0 is set (one) when an overflow occurs in Timer/Counter0. TOV0 is cleared by hard-
ware when executing the corresponding interrupt Handling Vector. Alternatively, TOV0 is
cleared by writing a logic one to the flag. When the SREG I-bit, TOIE0 (Timer/Counter0 Overflow
Interrupt Enable), and TOV0 are set (one), the Timer/Counter0 Overflow interrupt is executed.
Table 34. Clock Select Bit Description
CS02 CS01 CS00 Description
0 0 0 No clock source (Timer/Counter stopped)
001clk
I/O/(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 External 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
OCIE2 TOIE2 TICIE1 OCIE1A OCIE1B TOIE1 – TOIE0 TIMSK
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial Value00000000
Bit 76543210
OCF2 TOV2 ICF1 OCF1A OCF1B TOV1 –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/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 system clock (by setting the CSn2:0 = 1). This
provides the fastest operation, with a maximum Timer/Counter clock frequency equal to system
clock frequency (fCLK_I/O). Alternatively, one of four taps from the prescaler can be used as a
clock source. The prescaled clock has a 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 by Timer/Counter1 and Timer/Counter0. Since the prescaler is
not affected by the Timer/Counter’s clock select, the state of the prescaler will have implications
for situations where a prescaled clock is used. One example of prescaling artifacts occurs when
the timer is enabled and clocked by the prescaler (6 > CSn2:0 > 1). The number of system clock
cycles from when the timer is enabled to the first count occurs can be from 1 to N+1 system
clock cycles, where N equals the prescaler divisor (8, 64, 256, or 1024).
It is possible to use the prescaler reset for synchronizing the Timer/Counter to program 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 pin synchronization
logic. The synchronized (sampled) signal is then passed through the edge detector. Figure 30
shows a functional equivalent block diagram of the T1/T0 synchronization and edge detector
logic. The registers are clocked at the positive edge of the internal system clock (clkI/O). The latch
is transparent in the high period of the internal system clock.
The edge detector generates one clkT1/clkT0 pulse for each positive (CSn2:0 = 7) or negative
(CSn2:0 = 6) edge it detects.
Figure 30. 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 applied 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 risk that a false Timer/Counter clock pulse is generated.
Each half period of the external clock applied must be longer than one system clock cycle to
ensure correct sampling. The external clock must be guaranteed to have less than half the sys-
tem clock frequency (fExtClk < fclk_I/O/2) given a 50/50% duty cycle. Since the edge detector uses
Tn_sync
(To Clock
Select Logic)
Edge DetectorSynchronization
DQDQ
LE
DQ
Tn
clkI/O
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sampling, the maximum frequency of an external clock it can detect is half the sampling fre-
quency (Nyquist sampling theorem). However, due 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 prescaled.
Figure 31. Prescaler for Timer/Counter0 and Timer/Counter1(1)
Note: 1. The synchronization logic on the input pins (T1/T0) is shown in Figure 30 on page 73
Special Function IO
Register – SFIOR
• 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
clk
T1
clk
T0
T1
T0
clk
I/O
Synchronization
Synchronization
Bit 7 6 5 4 3 2 1 0
– – – – ACME PUD PSR2 PSR10 SFIOR
Read/Write R R R R R/W R/W R/W R/W
Initial Value 0 0 0 0 0 0 0 0
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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 case “x” replaces the Output Compare unit
channel. However, when using the register or bit defines in a program, the precise form must be
used, that is, TCNT1 for accessing Timer/Counter1 counter value and so on.
A simplified block diagram of the 16-bit Timer/Counter is shown in Figure 32 on page 76. For the
actual placement of I/O pins, refer to “Pin Configurations” on page 2. CPU accessible I/O Regis-
ters, 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 96.
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Figure 32. 16-bit Timer/Counter Block Diagram(1)
Note: 1. Refer to “Pin Configurations” on page 2, Table 22 on page 58, and Table 28 on page 63 for
Timer/Counter1 pin placement and description
Registers The Timer/Counter (TCNT1), Output Compare Registers (OCR1A/B), and Input Capture Regis-
ter (ICR1) are all 16-bit registers. Special procedures must be followed when accessing the 16-
bit registers. These procedures are described in the section “Accessing 16-bit Registers” on
page 77. The Timer/Counter Control Registers (TCCR1A/B) are 8-bit registers and have no CPU
access restrictions. Interrupt requests (abbreviated to Int.Req. in the figure) 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 regis-
ters are shared by other timer units.
The Timer/Counter can be clocked internally, via the prescaler, or by an external clock source on
the T1 pin. The Clock Select logic block controls which clock source and edge the Timer/Counter
uses to increment (or decrement) its value. The Timer/Counter is inactive when no clock source
is selected. The output from the clock select logic is referred to as the timer clock (clkT1).
The double buffered Output Compare Registers (OCR1A/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 83. 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 B U S
OCRnA
OCRnB
ICRn
=
=
TCNTn
Waveform
Generation
Waveform
Generation
OCnA
OCnB
Noise
Canceler
ICPn
=
Fixed
TOP
Values
Edge
Detector
Control Logic
= 0
TOP BOTTOM
Count
Clear
Direction
TOVn
(Int. Req.)
OCFnA
(Int. Req.)
OCFnB
(Int.Req.)
ICFn (Int.Req.)
TCCRnA TCCRnB
( From Analog
Comparator Ouput )
Tn
Edge
Detector
( From Prescaler )
clk
Tn
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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 186). The Input Capture unit includes a digital filtering unit (Noise
Canceler) for reducing the chance of capturing noise spikes.
The TOP value, or maximum Timer/Counter value, can in some modes of operation be defined
by either the 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 run time. If a fixed TOP value is required, the ICR1 Register can be used
as an alternative, freeing the OCR1A to be used as PWM output.
Definitions The following definitions are used extensively throughout the document:
Compatibility The 16-bit Timer/Counter has been 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 address locations, including Timer Interrupt
Registers
• Bit locations inside all 16-bit Timer/Counter Registers, including Timer Interrupt Registers
• Interrupt Vectors
The following control bits have changed name, but have same functionality and register location:
• 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 TCCR1B
The 16-bit Timer/Counter has improvements that will affect the compatibility in some special
cases.
Accessing 16-bit
Registers
The TCNT1, OCR1A/B, and ICR1 are 16-bit registers that can be accessed by the AVR CPU via
the 8-bit data bus. The 16-bit register must be byte accessed using two read or write operations.
The 16-bit timer has a single 8-bit register for temporary storing of the High byte of the 16-bit
access. The same temporary register is shared between all 16-bit registers within the 16-bit
timer. Accessing the Low byte triggers the 16-bit read or write operation. When the Low byte of a
16-bit register is written by the CPU, the High byte stored in the temporary register, and the Low
byte written are both copied into the 16-bit register in the same clock cycle. When the Low byte
Table 35. 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 Register. The assignment is dependent of the mode
of operation.
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of a 16-bit register is read by the CPU, the High byte of the 16-bit register is copied into the tem-
porary register in the same clock cycle as the Low byte is read.
Not all 16-bit accesses uses the temporary register for the High byte. Reading the OCR1A/B 16-
bit registers does not involve using the temporary register.
To do a 16-bit write, the High byte must be written before the Low byte. For a 16-bit read, the
Low byte must be read before the High byte.
The following code examples show how to access the 16-bit Timer Registers assuming that no
interrupts updates the temporary register. The same principle can be used directly for accessing
the OCR1A/B and ICR1 Registers. Note that when using “C”, the compiler handles the 16-bit
access.
Note: 1. See “About Code Examples” on page 8
The assembly code example 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 interrupt code update the temporary register, the main code must disable
the interrupts during the 16-bit access.
The following code examples show how to do an atomic read of the TCNT1 Register contents.
Reading any of the OCR1A/B or ICR1 Registers can be done by using the same principle.
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;
...
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Note: 1. See “About Code Examples” on page 8
The assembly code example 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;
}
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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 Examples” on page 8
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
Register
If writing to more than one 16-bit register where the High byte is the same for all registers 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 clock 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 73.
Counter Unit The main part of the 16-bit Timer/Counter is the programmable 16-bit bi-directional counter unit.
Figure 33 on page 81 shows a block diagram of the counter and its surroundings.
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;
}
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Figure 33. Counter Unit Block Diagram
Signal description (internal signals):
count Increment or decrement TCNT1 by 1
direction Select between increment 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 (TCNT1H) con-
taining the upper eight bits of the counter, and Counter Low (TCNT1L) containing the lower eight
bits. The TCNT1H Register can only be indirectly accessed by the CPU. When the CPU does an
access to the TCNT1H I/O location, the CPU accesses the High byte temporary register
(TEMP). The temporary register is updated 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 mode of operation used, the counter is cleared, incremented, or decremented
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 (has 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 connections between how the counter behaves (counts) and how waveforms
are generated on the Output Compare Outputs OC1x. For more details about advanced count-
ing sequences and waveform generation, see “Modes of Operation” on page 87.
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.
Input Capture Unit The Timer/Counter incorporates an Input Capture unit that can capture external events and give
them a time-stamp indicating time of occurrence. The external signal indicating an event, or mul-
tiple events, can be applied via the ICP1 pin or alternatively, via the Analog Comparator unit.
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
Tn
Edge
Detector
( From Prescaler )
clk
Tn
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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 34. The elements of
the block diagram that are not directly a part of the Input Capture unit are gray shaded. The
small “n” in register and bit names indicates the Timer/Counter number.
Figure 34. Input Capture Unit Block Diagram
When a change of the logic level (an event) occurs on the Input Capture Pin (ICP1), alternatively
on the Analog Comparator Output (ACO), and this change confirms to the setting of the edge
detector, a capture will be triggered. When a capture is triggered, the 16-bit value of the counter
(TCNT1) is written to the Input Capture Register (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
Register. When writing the ICR1 Register the High byte must be written to the ICR1H I/O loca-
tion before the Low byte is written to ICR1L.
For more information on how to access the 16-bit registers refer to “Accessing 16-bit Registers”
on page 77.
Input Capture Pin
Source
The main trigger source for the Input Capture unit is the Input Capture Pin (ICP1). Timer/Counter
1 can alternatively use the Analog Comparator Output as trigger source for the Input Capture
ICFn (Int. Req.)
Analog
Comparator
WRITE ICRn (16-bit Register)
ICRnH (8-bit)
Noise
Canceler
ICPn
Edge
Detector
TEMP (8-bit)
DATA BUS
(8-bit)
ICRnL (8-bit)
TCNTn (16-bit Counter)
TCNTnH (8-bit) TCNTnL (8-bit)
ACIC* ICNC ICES
ACO*
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unit. The Analog Comparator is selected as trigger source by setting the Analog Comparator
Input Capture (ACIC) bit in the Analog Comparator Control and Status Register (ACSR). Be
aware that changing trigger source can trigger a capture. The Input Capture Flag must therefore
be cleared after the change.
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 30 on page 73). The edge detector is also
identical. However, when the noise canceler is enabled, additional logic is inserted before the
edge detector, which increases the delay by four system clock cycles. Note that the input of the
noise canceler and edge detector is always enabled unless the Timer/Counter is set in a 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 monitored over four samples, and all four must be equal for changing the
output that in turn is used by the edge detector.
The noise canceler is enabled by setting the Input Capture Noise Canceler (ICNC1) bit in
Timer/Counter Control Register B (TCCR1B). When enabled the noise canceler introduces 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 system clock and is therefore not affected by the
prescaler.
Using the Input
Capture Unit
The main challenge when using the Input Capture unit is to assign enough processor capacity
for handling the incoming events. The time between two events is critical. If the processor has
not read the captured value in the 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 interrupt has relatively high
priority, the maximum interrupt response time is dependent on the maximum number of clock
cycles it takes to handle any of the other interrupt requests.
Using the Input Capture unit in any mode of operation when the TOP value (resolution) is
actively changed during operation, is not recommended.
Measurement of an external signal’s duty cycle requires that the trigger edge is changed after
each capture. Changing the edge sensing must be done as early as possible after the ICR1
Register has been read. After a change of the edge, the Input Capture Flag (ICF1) must be
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).
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 the Output
Compare Flag (OCF1x) at the next timer clock cycle. If enabled (OCIE1x = 1), the Output Com-
pare Flag generates an Output Compare interrupt. The OCF1x Flag is automatically cleared
when the interrupt 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 87).
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.
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Figure 35 shows a block diagram of the Output Compare unit. The small “n” in the register and
bit names indicates the device number (n = 1 for Timer/Counter 1), and the “x” indicates Output
Compare unit (A/B). The elements of the block diagram that are not directly a part of the Output
Compare unit are gray shaded.
Figure 35. Output Compare Unit, Block Diagram
The OCR1x Register is double buffered when using any of the twelve Pulse Width Modulation
(PWM) modes. For the normal and Clear Timer on Compare (CTC) modes of operation, the dou-
ble buffering is disabled. The double buffering synchronizes the update of the OCR1x Compare
Register to either TOP or BOTTOM of the counting sequence. The synchronization prevents the
occurrence of odd-length, non-symmetrical PWM pulses, thereby making the output glitch-free.
The OCR1x Register access may seem complex, but this is not case. When the double buffering
is enabled, the CPU has access to the OCR1x Buffer Register, and if double buffering is dis-
abled the CPU will access the OCR1x directly. The content of the OCR1x (Buffer or Compare)
Register is only changed by a write operation (the Timer/Counter does not update this register
automatically as the TCNT1 and ICR1 Register). Therefore OCR1x is not read via the High byte
temporary register (TEMP). However, it is a good practice to read the Low byte first as when
accessing other 16-bit registers. Writing the OCR1x Registers must be done via the TEMP Reg-
ister since the compare of all 16-bit is done continuously. The High byte (OCR1xH) has to be
written first. When the High byte I/O location is written by the CPU, the TEMP Register will be
updated by the value written. Then when the Low byte (OCR1xL) is written to the lower eight
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 77.
OCFnx (Int.Req.)
=
(16-bit Comparator )
OCRnx Buffer (16-bit Register)
OCRnxH Buf. (8-bit)
OCnx
TEMP (8-bit)
DATA BUS
(8-bit)
OCRnxL Buf. (8-bit)
TCNTn (16-bit Counter)
TCNTnH (8-bit) TCNTnL (8-bit)
COMnx1:0WGMn3:0
OCRnx (16-bit Register)
OCRnxH (8-bit) OCRnxL (8-bit)
Waveform Generator
TOP
BOTTOM
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Force Output
Compare
In non-PWM Waveform Generation modes, the match output of the comparator can be forced by
writing a one to the Force Output Compare (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
channels, 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 wave-
form 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 Com-
pare (FOC1x) strobe bits in Normal mode. The OC1x Register keeps its value even when
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.
Compare Match
Output Unit
The Compare Output mode (COM1x1:0) bits have two functions. The waveform generator 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 36 on page 86 shows a
simplified schematic of the logic affected 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 System
Reset occur, the OC1x Register is reset to “0”.
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Figure 36. 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
Register bit for the OC1x pin (DDR_OC1x) must be set as output before the OC1x value is visi-
ble on the pin. The port override function is generally independent of the Waveform Generation
mode, but there are some exceptions. Refer to Table 36 on page 96, Table 37 on page 96 and
Table 38 on page 97 for details.
The design of the Output Compare Pin logic allows initialization of the OC1x state before the
output is enabled. Note that some COM1x1:0 bit settings are reserved for certain modes of oper-
ation. See “16-bit Timer/Counter Register Description” on page 96.
The COM1x1:0 bits have no effect on the Input Capture unit.
PORT
DDR
DQ
DQ
OCnx
Pin
OCnx
DQ
Waveform
Generator
COMnx1
COMnx0
0
1
DATA BU S
FOCnx
clk
I/O
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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 to be performed on the next Compare Match. For compare output actions in
the non-PWM modes refer to Table 36 on page 96. For fast PWM mode refer to Table 37 on
page 96, and for phase correct and phase and frequency correct PWM refer to Table 38 on page
97.
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.
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 Generation mode (WGM13:0) and Compare Out-
put mode (COM1x1:0) bits. The Compare Output mode bits do not affect the counting sequence,
while the Waveform Generation mode bits do. The COM1x1:0 bits control whether the PWM out-
put generated should be inverted or not (inverted or non-inverted PWM). For non-PWM modes
the COM1x1:0 bits control whether the output should be set, cleared or toggle at a Compare
Match. See “Compare Match Output Unit” on page 85.
For detailed timing information refer to “Timer/Counter Timing Diagrams” on page 94.
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, observe that the maximum
interval between the external events must not exceed the resolution of the counter. If the interval
between events are too long, the timer overflow interrupt or the prescaler must be used to
extend the resolution for the capture unit.
The Output Compare units can be used to generate interrupts at some given time. Using the
Output Compare to generate waveforms in Normal mode is not recommended, since this will
occupy too much of the CPU time.
Clear Timer on
Compare Match (CTC)
Mode
In Clear Timer on Compare or CTC mode (WGM13:0 = 4 or 12), the OCR1A or ICR1 Register
are used to manipulate the counter resolution. In CTC mode the counter is cleared to zero when
the counter value (TCNT1) matches either the 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 greater control of the Compare Match output frequency. It also simplifies the oper-
ation of counting external events.
The timing diagram for the CTC mode is shown in Figure 37 on page 88. The counter value
(TCNT1) increases until a Compare Match occurs with either OCR1A or ICR1, and then counter
(TCNT1) is cleared.
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Figure 37. CTC Mode, Timing Diagram
An interrupt can be generated at each time the counter value reaches 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 the TOP to a value close to BOTTOM when the counter is running with none or a
low prescaler value must be done with care since the CTC mode does not have the double 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 wrap around starting at 0x0000 before the Compare Match can occur.
In many cases this feature is not desirable. An alternative will then be to use the fast PWM mode
using OCR1A for defining TOP (WGM13:0 = 15) since the OCR1A then will be double buffered.
For generating a waveform output in CTC mode, the OC1A output can be set to toggle its logical
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 waveform frequency is
defined by the following equation:
The N variable represents the prescaler factor (1, 8, 64, 256, or 1024).
As for the Normal mode of operation, the TOV1 Flag is set in the same timer clock cycle that the
counter counts from MAX to 0x0000.
Fast PWM Mode The fast Pulse Width Modulation or fast PWM mode (WGM13:0 = 5, 6, 7, 14, or 15) provides a
high frequency PWM waveform generation option. The fast PWM differs from the other PWM
options by its single-slope operation. The counter counts from BOTTOM to TOP then restarts
from BOTTOM. In non-inverting Compare Output mode, the Output Compare (OC1x) is cleared
on the Compare Match between TCNT1 and OCR1x, and set at BOTTOM. In inverting Compare
Output mode output is set on Compare Match and cleared at BOTTOM. Due to the single-slope
operation, the operating frequency of the fast PWM mode can be twice as high as the phase 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 resolution for fast PWM 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), and the
TCNTn
OCnA
(Toggle)
OCnA Interrupt Flag Set
or ICFn Interrupt Flag Set
(Interrupt on TOP)
1 4
Period
2 3
(COMnA1:0 = 1)
fOCnA
fclk_I/O
2N1OCRnA+()⋅⋅
---------------------------------------------------=
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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 38. The figure shows
fast 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 single-slope operation. The diagram includes
non-inverted and inverted PWM outputs. The small horizontal 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.
Figure 38. Fast PWM Mode, Timing Diagram
The Timer/Counter Overflow Flag (TOV1) is set each time the counter reaches TOP. In addition
the OCF1A or ICF1 Flag is set at the same timer clock cycle as TOV1 is set when either OCR1A
or ICR1 is used for defining the TOP value. If one of the interrupts are enabled, the interrupt han-
dler routine can be used for updating the TOP and compare values.
When changing the TOP value the program must ensure that the new TOP value is higher or
equal to the value of all of the Compare Registers. If the TOP value is lower than any of the
Compare Registers, a Compare Match will never occur between the 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 running with none or a low prescaler value, there is a risk that the 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.
RFPWM
TOP 1+()log
2()log
-----------------------------------=
TCNTn
OCRnx / TOP Update
and TOVn Interrupt Flag
Set and OCnA Interrupt
Flag Set or ICFn
Interrupt Flag Set
(Interrupt on TOP)
1 7
Period
2 3 4 5 6 8
OCnx
OCnx
(COMnx1:0 = 2)
(COMnx1:0 = 3)
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Using the ICR1 Register for defining TOP works 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 generation 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 37 on page 96. 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, and clearing (or setting) the OC1x Register at
the timer clock cycle the counter is cleared (changes from TOP to BOTTOM).
The PWM frequency for the output can be calculated by the following equation:
The N variable represents the prescaler divider (1, 8, 64, 256, or 1024).
The extreme values for the OCR1x Register represents special cases when generating a PWM
waveform output in the fast PWM mode. If the OCR1x is set equal to BOTTOM (0x0000) the out-
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 value (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 the double buffer feature of the Output Com-
pare unit is enabled in the fast PWM mode.
Phase Correct PWM
Mode
The phase correct Pulse Width 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 repeatedly from BOTTOM (0x0000) to TOP and then 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 mode, the operation is
inverted. The dual-slope operation has lower maximum operation frequency than single slope
operation. However, due to the symmetric feature of the dual-slope PWM modes, these modes
are preferred for motor control applications.
The PWM resolution for the phase correct PWM mode can be fixed to 8-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), and the maximum resolution is 16-bit (ICR1 or OCR1A set to MAX). The PWM reso-
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 39 on page 91.
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 opera-
fOCnxPWM
fclk_I/O
N1TOP+()⋅
-----------------------------------=
RPCPWM
TOP 1+()log
2()log
-----------------------------------=
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tion. The diagram includes non-inverted and inverted PWM outputs. The small horizontal 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.
Figure 39. Phase Correct PWM Mode, Timing Diagram
The Timer/Counter Overflow Flag (TOV1) is set each time the counter reaches BOTTOM. 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 program must ensure that the new TOP value is higher or
equal to the value of all of the Compare Registers. If the TOP value is lower than any of the
Compare Registers, a Compare Match will never occur between the 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 third period shown in Figure 39 illustrates, changing the
TOP actively while the Timer/Counter is running in the Phase Correct mode can result in an
unsymmetrical output. The reason for this can be found in the time of update of the OCR1x Reg-
ister. Since the OCR1x update occurs at TOP, the PWM period starts and ends at TOP. This
implies that the length of the falling slope is determined by the previous TOP value, while the
length of the rising slope is determined by the new TOP value. When these two values differ the
two slopes of the period will differ in length. The difference in length gives the unsymmetrical
result on the output.
It is recommended to use the Phase and Frequency Correct mode instead of the Phase Correct
mode when changing the TOP value while the Timer/Counter is running. When using a static
TOP value there are practically no differences between the two modes of operation.
In phase correct PWM mode, the compare units allow generation of PWM waveforms on the
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 38 on page 97. 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 Regis-
ter at the Compare Match between OCR1x and TCNT1 when the counter increments, and
clearing (or setting) the OC1x 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
OCnx
(COMnx1:0 = 2)
(COMnx1:0 = 3)
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the counter decrements. The PWM frequency for the output when using phase correct PWM can
be calculated by the following equation:
The N variable represents the prescaler divider (1, 8, 64, 256, or 1024).
The extreme values for the OCR1x Register represent special cases when generating a PWM
waveform output in the phase 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 (WMG13:0 = 11) and COM1A1:0 = 1, the OC1A out-
put will toggle with a 50% duty cycle.
Phase and Frequency
Correct PWM Mode
The phase and frequency correct Pulse Width Modulation, or phase and frequency correct 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 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 Compare 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 modes, these modes are preferred for motor control applications.
The main difference between the phase correct, and the phase and frequency correct PWM
mode is the time the OCR1x Register is updated by the OCR1x Buffer Register, (see Figure 39
on page 91 and Figure 40 on page 93).
The PWM resolution for the phase and frequency correct PWM mode can be defined 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 to MAX). The PWM resolution in bits can
be calculated using the following equation:
In phase and frequency correct PWM mode the counter is incremented until the counter value
matches either the value in ICR1 (WGM13:0 = 8), or the value in OCR1A (WGM13:0 = 9). 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 and frequency
correct PWM mode is shown on Figure 40 on page 93. The figure shows phase and frequency
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 represent 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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Figure 40. Phase and Frequency Correct PWM Mode, Timing Diagram
The Timer/Counter Overflow Flag (TOV1) is set at the same timer clock cycle as the OCR1x
Registers are updated with the double buffer value (at BOTTOM). When either OCR1A or ICR1
is used for defining the TOP value, the OC1A or ICF1 Flag set when TCNT1 has reached TOP.
The Interrupt Flags can then be used to generate an interrupt each time the counter reaches the
TOP or BOTTOM value.
When changing the TOP value the program must ensure that the new TOP value is higher or
equal to the value of all of the Compare Registers. If the TOP value is lower than any of the
Compare Registers, a Compare Match will never occur between the TCNT1 and the OCR1x.
As Figure 40 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 Register for defining TOP works 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 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 output can be generated by setting the COM1x1:0 to 3. See Table 38 on page
97. 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 represents the prescaler divider (1, 8, 64, 256, or 1024).
The extreme values for the OCR1x Register represents special cases when generating a PWM
waveform output in the phase 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
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 signal in the following figures. The figures include information on when Interrupt
Flags are set, and when the OCR1x Register is updated with the OCR1x buffer value (only for
modes utilizing double buffering). Figure 41 shows a timing diagram for the setting of OCF1x.
Figure 41. Timer/Counter Timing Diagram, Setting of OCF1x, no Prescaling
Figure 42 shows the same timing data, but with the prescaler enabled.
Figure 42. Timer/Counter Timing Diagram, Setting of OCF1x, with Prescaler (fclk_I/O/8)
Figure 43 on page 95 shows the count sequence close to TOP in various modes. When using
phase and frequency correct PWM mode the OCR1x Register is updated at BOTTOM. The tim-
clk
Tn
(clk
I/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
Tn
(clk
I/O
/8)
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ing diagrams will be the same, but TOP should be replaced by BOTTOM, TOP-1 by BOTTOM+1
and so on. The same renaming applies for modes that set the TOV1 Flag at BOTTOM.
Figure 43. Timer/Counter Timing Diagram, no Prescaling
Figure 44 shows the same timing data, but with the prescaler enabled.
Figure 44. 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
Tn
(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
Tn
(clk
I/O
/8)
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ATmega8(L)
16-bit
Timer/Counter
Register
Description
Timer/Counter 1
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 OC1B pin must be set in order to enable the output driver.
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 36 shows the COM1x1:0 bit functionality when the
WGM13:0 bits are set to a normal or a CTC mode (non-PWM).
Table 37 shows the COM1x1:0 bit functionality when the WGM13:0 bits are set to the fast PWM
mode.
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 88 for more details
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 Value 0 0 0 0 0 0 0 0
Table 36. 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)
Table 37. 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 WGM1
settings, normal port operation, OC1A/OC1B disconnected.
1 0 Clear OC1A/OC1B on Compare Match, set OC1A/OC1B at
BOTTOM, (non-inverting mode)
1 1 Set OC1A/OC1B on Compare Match, clear OC1A/OC1B at
BOTTOM, (inverting mode)
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ATmega8(L)
Table 38 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/OCR1B equals TOP and COM1A1/COM1B1 is set. See
“Phase Correct PWM Mode” on page 90 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.
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 bits found in the TCCR1B Register, these bits control 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 39. 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 Operation” on page 87).
Table 38. 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 OC1A on Compare Match, OC1B
disconnected (normal port operation). For all other WGM1
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.
Table 39. Waveform Generation Mode Bit Description
Mode WGM13
WGM12
(CTC1)
WGM11
(PWM11)
WGM10
(PWM10)
Timer/Counter Mode of
Operation(1) TOP
Update of
OCR1x
TOV1 Flag
Set on
0 0 0 0 0 Normal 0xFFFF Immediate MAX
1 0 0 0 1 PWM, Phase Correct, 8-bit 0x00FF TOP BOTTOM
2 0 0 1 0 PWM, Phase Correct, 9-bit 0x01FF TOP BOTTOM
3 0 0 1 1 PWM, Phase Correct, 10-bit 0x03FF TOP BOTTOM
4 0 1 0 0 CTC OCR1A Immediate MAX
5 0 1 0 1 Fast PWM, 8-bit 0x00FF BOTTOM TOP
6 0 1 1 0 Fast PWM, 9-bit 0x01FF BOTTOM TOP
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Note: 1. The CTC1 and PWM11:0 bit definition names are obsolete. Use the WGM12:0 definitions. However, the functionality and
location of these bits are compatible with previous versions of the timer
Timer/Counter 1
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 valued 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 Input 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 (positive) 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 Register), the ICP1 is disconnected and consequently the Input Cap-
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
41 on page 94 and Figure 42 on page 94.
7 0 1 1 1 Fast PWM, 10-bit 0x03FF BOTTOM TOP
8 1 0 0 0 PWM, Phase and Frequency Correct ICR1 BOTTOM BOTTOM
9 1 0 0 1 PWM, Phase and Frequency Correct OCR1A BOTTOM BOTTOM
10 1 0 1 0 PWM, Phase Correct 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
Table 39. Waveform Generation Mode Bit Description (Continued)
Mode WGM13
WGM12
(CTC1)
WGM11
(PWM11)
WGM10
(PWM10)
Timer/Counter Mode of
Operation(1) TOP
Update of
OCR1x
TOV1 Flag
Set on
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 Value00000000
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ATmega8(L)
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/Counter 1 –
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 77.
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 (removes) the Compare Match on the following timer clock
for all compare units.
Output Compare
Register 1 A –
OCR1AH and OCR1AL
Output Compare
Register 1 B –
OCR1BH and OCR1BL
Table 40. 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 External clock source on T1 pin. Clock on falling edge
1 1 1 External clock source on T1 pin. Clock on rising edge
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
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ATmega8(L)
The Output Compare Registers contain a 16-bit value that is continuously compared with the
counter value (TCNT1). A match can be used to generate an Output Compare 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 temporary High byte Register (TEMP). This temporary register is shared by all the
other 16-bit registers. See “Accessing 16-bit Registers” on page 77.
Input Capture Register
1 – ICR1H and ICR1L
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 Cap-
ture can be used for defining the counter TOP value.
The Input Capture Register is 16-bit in size. To ensure that both the high and Low bytes are read
simultaneously when the CPU accesses these registers, the access is performed using an 8-bit
temporary High byte Register (TEMP). This temporary register is shared by all the other 16-bit
registers. See “Accessing 16-bit Registers” on page 77.
Timer/Counter
Interrupt Mask
Register – TIMSK(1)
Note: 1. This register contains interrupt control 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 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/Counter1 Input Capture Interrupt is enabled. The corresponding Interrupt
Vector (see “Interrupts” on page 46) 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), the Timer/Counter1 Output Compare A match interrupt is enabled. The corresponding
Interrupt Vector (see “Interrupts” on page 46) 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), the Timer/Counter1 Output Compare B match interrupt is enabled. The corresponding
Interrupt Vector (see “Interrupts” on page 46) 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/Counter1 Overflow Interrupt is enabled. The corresponding Interrupt Vector
(see “Interrupts” on page 46) is executed when the TOV1 Flag, located in TIFR, is set.
Bit 76543210
ICR1[15:8] ICR1H
ICR1[7:0] ICR1L
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial Value00000000
Bit 76543210
OCIE2 TOIE2 TICIE1 OCIE1A OCIE1B TOIE1 –TOIE0 TIMSK
Read/Write R/W R/W R/W R/W R/W R/W R R/W
Initial Value00000000
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ATmega8(L)
Timer/Counter
Interrupt Flag Register
– TIFR(1)
Note: 1. 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 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 WGM13:0 to be used as the TOP value, the ICF1 Flag is set when the coun-
ter reaches the TOP value.
ICF1 is automatically cleared when the Input Capture Interrupt Vector is executed. Alternatively,
ICF1 can be cleared by writing 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 this flag is dependent of the WGM13:0 bits setting. In normal and CTC modes, the
TOV1 Flag is set when the timer overflows. Refer to Table 39 on page 97 for the TOV1 Flag
behavior when using another WGM13:0 bit setting.
TOV1 is automatically cleared when the Timer/Counter1 Overflow Interrupt Vector is executed.
Alternatively, TOV1 can be cleared by writing a logic one to its bit location.
Bit 76543210
OCF2 TOV2 ICF1 OCF1A OCF1B TOV1 –TOV0 TIFR
Read/Write R/W R/W R/W R/W R/W R/W R R/W
Initial Value00000000
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ATmega8(L)
8-bit
Timer/Counter2
with PWM and
Asynchronous
Operation
Timer/Counter2 is a general purpose, single channel, 8-bit Timer/Counter module. The main
features are:
•Single Channel 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 32kHz Watch Crystal Independent of the I/O Clock
Overview A simplified block diagram of the 8-bit Timer/Counter is shown in Figure 45. For the actual place-
ment of I/O pins, refer to “Pin Configurations” 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 loca-
tions are listed in the “8-bit Timer/Counter Register Description” on page 114.
Figure 45. 8-bit Timer/Counter Block Diagram
Timer/Counter
DATA BU S
=
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
Tn
clk
I/O
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ATmega8(L)
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 shared 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/Counter uses to increment (or decrement) its value. The Timer/Counter is inac-
tive when no clock source is selected. The output from the clock select logic is referred to as the
timer clock (clkT2).
The double buffered Output Compare Register (OCR2) is compared with the Timer/Counter
value at all times. The result of the compare can be used by the waveform generator to generate
a PWM or variable frequency output on the Output Compare Pin (OC2). For details, see “Output
Compare Unit” on page 105. 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 are written in general form. A lower 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 41 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. When the AS2
bit in the ASSR Register is written to logic one, the clock source is taken from the Timer/Counter
Oscillator connected to TOSC1 and TOSC2. For details on asynchronous operation, see “Asyn-
chronous Status Register – ASSR” on page 117. For details on clock sources and prescaler, see
“Timer/Counter Prescaler” on page 120.
Table 41. 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.
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ATmega8(L)
Counter Unit The main part of the 8-bit Timer/Counter is the programmable bi-directional counter unit. Figure
46 shows a block diagram of the counter and its surrounding environment.
Figure 46. Counter Unit Block Diagram
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
BOTTOM Signalizes that TCNT2 has reached minimum value (zero)
Depending on the mode of operation used, the counter is cleared, incremented, or decremented
at each timer clock (clkT2). clkT2 can be generated from an external or internal 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 are generated on the Output Compare Output
OC2. For more details about advanced counting sequences and waveform generation, see
“Modes of Operation” on page 108.
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.
DATA BU S
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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ATmega8(L)
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 max and bottom signals are used by the waveform generator for han-
dling the special cases of the extreme values in some modes of operation (see “Modes of
Operation” on page 108).
Figure 47 shows a block diagram of the Output Compare unit.
Figure 47. 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 Timer on Compare (CTC) modes of operation, the double buff-
ering is disabled. The double buffering synchronizes the update of the OCR2 Compare Register
to either top or bottom of the counting sequence. The synchronization prevents the occurrence
of odd-length, non-symmetrical PWM pulses, thereby making the output glitch-free.
The OCR2 Register access may seem complex, but this is not case. When the double buffering
is enabled, the CPU has access to the OCR2 Buffer Register, and if double buffering is disabled
the CPU will access the OCR2 directly.
OCFn (Int. Req.)
= (8-bit Comparator )
OCRn
OCxy
DATA B U S
TCNTn
WGMn1:0
Waveform Generator
TOP
FOCn
COMn1:0
BOTTOM
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ATmega8(L)
Force Output
Compare
In non-PWM Waveform Generation modes, the match output of the comparator can be forced by
writing a one to the Force Output Compare (FOC2) bit. Forcing Compare 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 define 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 timer is stopped. This feature 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 channel,
independently of whether the Timer/Counter is running or not. If the value written to TCNT2
equals the OCR2 value, the Compare Match will be missed, resulting in incorrect waveform gen-
eration. Similarly, do not write the TCNT2 value equal to BOTTOM when the counter is
downcounting.
The setup of the OC2 should be performed before setting the Data Direction Register for the port
pin to output. The easiest way of setting the OC2 value is to use the Force Output Compare
(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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ATmega8(L)
Compare Match
Output Unit
The Compare Output mode (COM21:0) bits have two functions. The waveform generator uses
the COM21:0 bits for defining the Output Compare (OC2) state at the next Compare Match.
Also, the COM21:0 bits control the OC2 pin output source. Figure 48 shows a simplified sche-
matic of the logic affected by the COM21:0 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 COM21:0 bits are shown. When referring to the OC2 state, the
reference is for the internal OC2 Register, not the OC2 pin.
Figure 48. Compare Match Output Unit, Schematic
The general I/O port function is overridden by the Output 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 114.
PORT
DDR
DQ
DQ
OCn
Pin
OCn
DQ
Waveform
Generator
COMn1
COMn0
0
1
DATAB U S
FOCn
clkI/O
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Compare Output Mode
and Waveform
Generation
The Waveform Generator uses the COM21:0 bits 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
Register is to be performed on the next Compare Match. For compare output actions in the non-
PWM modes refer to Table 43 on page 115. For fast PWM mode, refer to Table 44 on page 115,
and for phase correct PWM refer to Table 45 on page 116.
A change of the COM21:0 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
FOC2 strobe bits.
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 Generation mode (WGM21:0) and 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 107).
For detailed timing information refer to “Timer/Counter Timing Diagrams” on page 112.
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 overflow interrupt
that automatically clears the TOV2 Flag, the timer resolution can be increased by software.
There are no special cases to consider in the Normal mode, a new counter value can be written
anytime.
The Output Compare unit can be used to generate interrupts at some given time. Using the Out-
put Compare to generate waveforms in Normal mode is not recommended, since this will
occupy too much of the CPU time.
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Clear Timer on
Compare Match (CTC)
Mode
In Clear Timer on Compare or CTC mode (WGM21:0 = 2), the OCR2 Register is used to manip-
ulate the counter resolution. In CTC mode the counter is cleared to zero when the counter value
(TCNT2) matches the OCR2. The OCR2 defines the top value for the counter, hence also its
resolution. This mode allows greater control of the Compare Match output frequency. It also sim-
plifies the operation of counting external events.
The timing diagram for the CTC mode is shown in Figure 49. The counter value (TCNT2)
increases until a Compare Match occurs between TCNT2 and OCR2, and then counter (TCNT2)
is cleared.
Figure 49. 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 prescaler 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
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 waveform generated will have a maximum 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.
TCNTn
OCn
(Toggle)
OCn Interrupt Flag Set
1 4
Period
2 3
(COMn1:0 = 1)
fOCn
fclk_I/O
2N1OCRn+()⋅⋅
-----------------------------------------------=
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ATmega8(L)
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 Output Compare (OC2) is cleared on the Compare
Match between TCNT2 and OCR2, and set at BOTTOM. In inverting Compare Output mode, the
output is set on Compare Match and cleared at BOTTOM. Due to the single-slope operation, the
operating frequency of the fast PWM mode can be twice as high as the phase correct PWM
mode that uses 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 50. 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 between OCR2 and TCNT2.
Figure 50. Fast PWM Mode, Timing Diagram
The Timer/Counter Overflow Flag (TOV2) is set each time the counter reaches MAX. If the inter-
rupt is enabled, the interrupt handler routine can be used for updating the compare value.
In fast PWM mode, the compare unit allows generation of PWM waveforms on the 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 44 on page 115). 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 the following equation:
The N variable represents the prescale factor (1, 8, 32, 64, 128, 256, or 1024).
TCNTn
OCRn Update
and
TOVn Interrupt Flag Set
1
Period 23
OCn
OCn
(COMn1:0 = 2)
(COMn1:0 = 3)
OCRn Interrupt Flag Set
4 5 6 7
fOCnPWM
fclk_I/O
N256⋅
------------------=
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ATmega8(L)
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 fea-
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 correct PWM mode (WGM21:0 = 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 Compare 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 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 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 51.
The TCNT2 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 TCNT2 slopes represent compare matches between OCR2 and TCNT2.
Figure 51. Phase Correct PWM Mode, Timing Diagram
TOVn Interrupt Flag Set
OCn Interrupt Flag Set
1 2 3
TCNTn
Period
OCn
OCn
(COMn1:0 = 2)
(COMn1:0 = 3)
OCRn Update
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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 45 on page 116). 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 clearing (or setting) the OC2 Register at the Compare Match
between OCR2 and TCNT2 when the counter increments, and setting (or clearing) the OC2
Register at Compare Match between OCR2 and TCNT2 when the counter decrements. The
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 mode. If the OCR2 is set equal to BOTTOM, 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 inverted PWM the output will have the opposite logic values.
At the very start of period 2 in Figure 51 on page 111 OCn has a transition from high to low even
though there is no Compare Match. The point of this transition is to guarantee symmetry around
BOTTOM. There are two cases that give a transition without Compare Match:
• OCR2A changes its value from MAX, like in Figure 51 on page 111. When the OCR2A value
is MAX the OCn pin value is the same as the result of a down-counting Compare Match. To
ensure symmetry around BOTTOM the OCn value at MAX must correspond to the result of
an up-counting Compare Match
• The timer starts counting from a value higher than the one in OCR2A, and for that reason
misses the Compare Match and hence the OCn change that would have happened on the
way up
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 signal. In Asynchronous mode, clkI/O should be replaced by
the Timer/Counter Oscillator clock. The figures include information on when Interrupt Flags are
set. Figure 52 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 52. Timer/Counter Timing Diagram, no Prescaling
fOCnPCPWM
fclk_I/O
N510⋅
------------------=
clk
Tn
(clkI/O/1)
TOVn
clk
I/O
TCNTn MAX - 1 MAX BOTTOM BOTTOM + 1
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ATmega8(L)
Figure 53 shows the same timing data, but with the prescaler enabled.
Figure 53. Timer/Counter Timing Diagram, with Prescaler (fclk_I/O/8)
Figure 54 shows the setting of OCF2 in all modes except CTC mode.
Figure 54. Timer/Counter Timing Diagram, Setting of OCF2, with Prescaler (fclk_I/O/8)
TOVn
TCNTn MAX - 1 MAX BOTTOM BOTTOM + 1
clk
I/O
clk
Tn
(clkI/O/8)
OCFn
OCRn
TCNTn
OCRn Value
OCRn - 1 OCRn OCRn + 1 OCRn + 2
clk
I/O
clk
Tn
(clkI/O/8)
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ATmega8(L)
Figure 55 shows the setting of OCF2 and the clearing of TCNT2 in CTC mode.
Figure 55. Timer/Counter Timing Diagram, Clear Timer on Compare Match Mode, with Pres-
caler (fclk_I/O/8)
8-bit
Timer/Counter
Register
Description
Timer/Counter Control
Register – TCCR2
• 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 the 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 6:3 – WGM21:0: Waveform Generation Mode
These bits control the counting 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 42 on page 115 and “Modes of
Operation” on page 108.
OCFn
OCRn
TCNTn
(CTC)
TOP
TOP - 1 TOP BOTTOM BOTTOM + 1
clk
I/O
clk
Tn
(clk
I/O
/8)
Bit 76543210
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 Value00000000
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ATmega8(L)
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.
When OC2 is connected to the pin, the function of the COM21:0 bits depends on the WGM21:0
bit setting.
Table 43 shows the COM21:0 bit functionality when the WGM21:0 bits are set to a normal or
CTC mode (non-PWM).
Table 44 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 equals 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 110
for more details
Table 42. Waveform Generation Mode Bit Description
Mode
WGM21
(CTC2)
WGM20
(PWM2)
Timer/Counter Mode
of Operation(1) TOP
Update of
OCR2
TOV2 Flag
Set
0 0 0 Normal 0xFF Immediate MAX
1 0 1 PWM, Phase Correct 0xFF TOP BOTTOM
2 1 0 CTC OCR2 Immediate MAX
3 1 1 Fast PWM 0xFF BOTTOM MAX
Table 43. 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 44. 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)
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Table 45 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 equals TOP and COM21 is set. In this case, the Compare
Match is ignored, but the set or clear is done at TOP. See “Phase Correct PWM Mode” on page
111 for more details
• Bit 2:0 – CS22:0: Clock Select
The three clock select bits select the clock source to be used by the Timer/Counter, see Table
46.
Timer/Counter
Register – TCNT2
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
Register – OCR2
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 Compare interrupt, or to
generate a waveform output on the OC2 pin.
Table 45. Compare Output Mode, Phase Correct PWM Mode(1)
COM21 COM20 Description
0 0 Normal port operation, OC2 disconnected
01Reserved
10
Clear OC2 on Compare Match when up-counting. Set OC2 on Compare
Match when downcounting
11
Set OC2 on Compare Match when up-counting. Clear OC2 on Compare
Match when downcounting
Table 46. Clock Select Bit Description
CS22 CS21 CS20 Description
0 0 0 No clock source (Timer/Counter stopped)
001clk
T2S/(No prescaling)
010clk
T2S/8 (From prescaler)
011clk
T2S/32 (From prescaler)
100clk
T2S/64 (From prescaler)
101clk
T2S/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
Register – ASSR
• 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/Counter 2 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 corrupted.
• Bit 2 – TCN2UB: Timer/Counter2 Update Busy
When Timer/Counter2 operates asynchronously 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 updated from the temporary storage register, this 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/Counter2 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, OCR2, and TCCR2
4. To switch to asynchronous operation: Wait for TCN2UB, OCR2UB, and TCR2UB
5. Clear the Timer/Counter2 Interrupt Flags
6. Enable interrupts, if needed
• The Oscillator is optimized for use with a 32.768kHz watch crystal. Applying an external
clock to the TOSC1 pin may result in incorrect Timer/Counter2 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
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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ATmega8(L)
write a new value before the contents of the temporary register have been transferred to its
destination. Each of the three mentioned registers have their individual temporary register,
which means that, for example, writing to TCNT2 does not disturb an OCR2 write in
progress. To detect that a transfer to the destination register has taken place, the
Asynchronous Status Register – ASSR has been implemented
• When entering Power-save mode after having written to TCNT2, OCR2, or TCCR2, the user
must wait until the written register 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 Compare2 interrupt is used to wake up the device,
since the Output Compare function is disabled during writing to OCR2 or TCNT2. If the write
cycle is not finished, and the MCU enters sleep mode before the OCR2UB bit returns to
zero, the device will never receive a Compare Match interrupt, and the MCU will not wake up
• If Timer/Counter2 is used to wake the device up from Power-save 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 before 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 returns to zero
3. Enter Power-save or Extended Standby mode
• When the asynchronous operation is selected, the 32.768kHZ Oscillator for Timer/Counter2
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 aware 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 second before using Timer/Counter2 after Power-up or Wake-up from Power-down
or Standby mode. The contents of all Timer/Counter2 Registers must be considered lost
after a wake-up from Power-down or Standby mode 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
clocked asynchronously: When the interrupt condition is met, the wake up process is started
on the following cycle of the timer clock, that is, the timer is always advanced by at least one
before the processor can read the counter value. After wake-up, the MCU is halted for four
cycles, it executes the interrupt routine, and resumes execution 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 through a register synchronized to the internal I/O clock domain.
Synchronization takes place for every rising TOSC1 edge. When waking up from Power-
save mode, and the I/O clock (clkI/O) again becomes active, TCNT2 will read as the previous
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 wake-up time. The recommended procedure for reading TCNT2 is thus as follows:
1. Write any value to either of the registers OCR2 or TCCR2
2. Wait for the corresponding Update Busy Flag to be cleared
3. Read TCNT2
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• During asynchronous operation, the synchronization of the Interrupt Flags for the
asynchronous timer takes three processor cycles plus one timer cycle. The timer is therefore
advanced by at least one before the processor can read the timer value causing the 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
Register – TIMSK
• Bit 7 – OCIE2: Timer/Counter2 Output 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/Counter2 Compare Match interrupt is enabled. The corresponding interrupt is executed if
a Compare Match in Timer/Counter2 occurs (that is, when the OCF2 bit is set in the
Timer/Counter 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. When the I-bit in SREG, OCIE2 (Timer/Counter2 Compare Match Interrupt Enable), and
OCF2 are set (one), the Timer/Counter2 Compare Match Interrupt is executed.
• Bit 6 – TOV2: Timer/Counter2 Overflow Flag
The TOV2 bit is set (one) when an overflow occurs in Timer/Counter2. TOV2 is cleared by hard-
ware when executing the corresponding interrupt Handling Vector. Alternatively, TOV2 is
cleared by writing a logic one to the flag. When the SREG I-bit, TOIE2 (Timer/Counter2 Overflow
Interrupt 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 direction at 0x00.
Bit 76543210
OCIE2 TOIE2 TICIE1 OCIE1A OCIE1B TOIE1 –TOIE0 TIMSK
Read/Write R/W R/W R/W R/W R/W R/W R R/W
Initial Value00000000
Bit 76543210
OCF2 TOV2 ICF1 OCF1A OCF1B TOV1 –TOV0 TIFR
Read/Write R/W R/W R/W R/W R/W R/W R R/W
Initial Value00000000
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ATmega8(L)
Timer/Counter
Prescaler
Figure 56. 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 clkI/O. 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 B. A crystal can
then be connected between the TOSC1 and TOSC2 pins to serve as an independent clock
source for Timer/Counter2. The Oscillator is optimized for use with a 32.768kHz 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
Register – SFIOR
• 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
/8
clk
T2S
/64
clk
T2S
/128
clk
T2S
/1024
clk
T2S
/256
clk
T2S
/32
0
PSR2
Clear
clk
T2
Bit 7 6 5 4 3 2 1 0
– – – – ACME PUD PSR2 PSR10 SFIOR
Read/Write R R R 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
ATmega8 and peripheral devices or between several AVR devices. The ATmega8 SPI includes
the following features:
•Full-duplex, Three-wire Synchronous Data Transfer
•Master or Slave Operation
•LSB First or MSB First Data Transfer
•Seven Programmable Bit Rates
•End of Transmission Interrupt Flag
•Write Collision Flag Protection
•Wake-up from Idle Mode
•Double Speed (CK/2) Master SPI Mode
Figure 57. SPI Block Diagram(1)
Note: 1. Refer to “Pin Configurations” on page 2, and Table 22 on page 58 for SPI pin placement
The interconnection between Master and Slave CPUs with SPI is shown in Figure 58 on page
122. The system consists of two Shift Registers, and a Master clock generator. The SPI Master
initiates the communication cycle when pulling low the Slave Select SS pin of the desired Slave.
Master and Slave prepare the data to be sent in their respective Shift Registers, and the Master
generates the required clock pulses on the SCK line to interchange data. Data is always shifted
from Master to Slave on the Master Out – Slave In, MOSI, line, and from Slave to Master on the
Master In – Slave Out, MISO, line. After each data packet, the Master will synchronize the Slave
by pulling high the Slave Select, SS, line.
When configured as a Master, the SPI interface has no automatic control of the SS line. This
must be handled by user software before communication can start. When this is done, writing a
SPI2X
SPI2X
DIVIDER
/2/4/8/16/32/64/128
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byte to the SPI Data Register starts the SPI clock generator, and the hardware shifts the eight
bits into the Slave. After shifting one byte, the SPI clock generator stops, setting the end of
Transmission Flag (SPIF). If the SPI interrupt enable bit (SPIE) in the SPCR Register is set, an
interrupt is requested. The Master may continue to shift the next byte by writing it into SPDR, or
signal the end of packet by pulling high the Slave Select, SS line. The last incoming byte will be
kept in the Buffer Register for later use.
When configured as a Slave, the SPI interface will remain sleeping with MISO tri-stated as long
as the SS pin is driven high. In this state, software may update the contents of the SPI Data
Register, SPDR, but the data will not be shifted out by incoming clock pulses on the SCK pin
until the SS pin is driven low. As one byte has been completely shifted, the end of Transmission
Flag, SPIF is set. If the SPI interrupt enable bit, SPIE, in the SPCR 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 58. 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 high periods should be:
Low period: longer than 2 CPU clock cycles
High period: longer than 2 CPU clock cycles
When the SPI is enabled, the data direction of the MOSI, MISO, SCK, and SS pins is overridden
according to Table 47. For more details on automatic port overrides, refer to “Alternate Port
Functions” on page 56.
Note: 1. See “Port B Pins Alternate Functions” on page 58 for a detailed description of how to define
the direction of the user defined SPI pins
Table 47. SPI Pin Overrides(1)
Pin Direction, Master SPI Direction, Slave SPI
MOSI User Defined Input
MISO Input User Defined
SCK User Defined Input
SS User Defined Input
MSB MASTER LSB
8 BIT SHIFT REGISTER
MSB SLAVE LSB
8 BIT SHIFT REGISTER
MISO
MOSI
SPI
CLOCK GENERATOR
SCK
SS
MISO
MOSI
SCK
SS
V
CC
SHIFT
ENABLE
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The following code examples show how to initialize the SPI as a Master and how to perform a
simple transmission. DDR_SPI in the examples must be replaced by the actual Data Direction
Register controlling the SPI pins. DD_MOSI, DD_MISO and DD_SCK must be replaced by the
actual data direction bits for these pins. For example if MOSI is placed on pin PB5, replace
DD_MOSI with DDB5 and DDR_SPI with DDRB.
Note: 1. See “About Code Examples” on page 8
Assembly Code Example(1)
SPI_MasterInit:
; Set MOSI and SCK output, all others input
ldi r17,(1<<DD_MOSI)|(1<<DD_SCK)
out DDR_SPI,r17
; Enable SPI, Master, set clock rate fck/16
ldi r17,(1<<SPE)|(1<<MSTR)|(1<<SPR0)
out SPCR,r17
ret
SPI_MasterTransmit:
; Start transmission of data (r16)
out SPDR,r16
Wait_Transmit:
; Wait for transmission complete
sbis SPSR,SPIF
rjmp Wait_Transmit
ret
C Code Example(1)
void SPI_MasterInit(void)
{
/* Set MOSI and SCK output, all others input */
DDR_SPI = (1<<DD_MOSI)|(1<<DD_SCK);
/* Enable SPI, Master, set clock rate fck/16 */
SPCR = (1<<SPE)|(1<<MSTR)|(1<<SPR0);
}
void SPI_MasterTransmit(char cData)
{
/* Start transmission */
SPDR = cData;
/* Wait for transmission complete */
while(!(SPSR & (1<<SPIF)))
;
}
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ATmega8(L)
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 8
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, the Slave Select (SS) pin is always input. When SS is
held low, the SPI is activated, and MISO becomes an output if configured so by the user. All
other pins are inputs. When SS is driven high, all pins are inputs 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 will 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 as an input, it must be held high to ensure Master SPI operation. If the SS pin
is driven low by peripheral circuitry when the SPI is configured as a Master with the SS pin
defined as an input, the SPI system interprets this as another Master selecting the SPI as a
Slave and starting to send data to it. To avoid bus contention, the SPI system takes the following
actions:
1. The MSTR bit in SPCR is cleared and the SPI system becomes a Slave. As a result of
the SPI 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 cleared by a Slave Select, it must be set by the user to 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 one, and Slave SPI mode when written logic
zero. If SS is configured as an input and is driven low while MSTR is set, MSTR will be cleared,
and SPIF in SPSR will become set. The user will then have to set MSTR to re-enable SPI 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 59 on page 128 and Figure 60 on page 128 for an example. The
CPOL functionality is summarized below:
• Bit 2 – CPHA: Clock Phase
The settings of the clock phase bit (CPHA) determine if data is sampled on the leading (first) or
trailing (last) edge of SCK. Refer to Figure 59 on page 128 and Figure 60 on page 128 for an
example. The CPHA functionality is summarized below:
• Bits 1, 0 – SPR1, SPR0: SPI Clock Rate Select 1 and 0
These two bits control the SCK rate of the device configured as a Master. SPR1 and SPR0 have
no effect on the Slave. The relationship between SCK and the Oscillator Clock frequency fosc is
shown in the following table:
SPI Status Register –
SPSR
Table 48. CPOL Functionality
CPOL Leading Edge Trailing Edge
0 Rising Falling
1 Falling Rising
Table 49. CPHA Functionality
CPHA Leading Edge Trailing Edge
0 Sample Setup
1Setup Sample
Table 50. 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
Bit 76543210
SPIFWCOL–––––SPI2XSPSR
Read/WriteRRRRRRRR/W
Initial Value00000000
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• Bit 7 – SPIF: SPI Interrupt Flag
When a serial transfer is complete, the SPIF Flag is set. An interrupt is generated if SPIE in
SPCR is set and global interrupts are enabled. If SS is an input and is driven low when the SPI is
in Master mode, this will also set the SPIF Flag. SPIF is cleared by hardware when executing the
corresponding interrupt Handling Vector. Alternatively, the SPIF bit is cleared by first reading the
SPI Status Register with SPIF set, then accessing the SPI Data Register (SPDR).
• Bit 6 – WCOL: Write COLlision Flag
The WCOL bit is set if the SPI Data Register (SPDR) is written during a data transfer. The
WCOL bit (and the SPIF bit) are cleared by first reading the SPI Status Register with WCOL set,
and then accessing the SPI Data Register.
• Bit 5..1 – Res: Reserved Bits
These bits are reserved bits in the ATmega8 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 50 on page 126). This means that the minimum SCK period will be
2 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 ATmega8 is also used for Program memory and EEPROM download-
ing or uploading. See page 230 for 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 Shift Register Receive buffer to be read.
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
59 on page 128 and Figure 60 on page 128. 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 48 on page 126 and Table 49 on page 126, as done below:
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 Undefined
Table 51. 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
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Figure 59. SPI Transfer Format with CPHA = 0
Figure 60. SPI Transfer Format with CPHA = 1
Bit 1
Bit 6
LSB
MSB
SCK (CPOL = 0)
mode 0
SAMPLE I
MOSI/MISO
CHANGE 0
MOSI PIN
CHANGE 0
MISO PIN
SCK (CPOL = 1)
mode 2
SS
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
SS
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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ATmega8(L)
USART The Universal Synchronous and Asynchronous serial Receiver and Transmitter (USART) is a
highly-flexible serial communication 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 Databits and 1 or 2 Stop Bits
•Odd or Even Parity Generation and Parity Check Supported 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 Transmitter is shown in Figure 61. CPU accessible I/O
Registers and I/O pins are shown in bold.
Figure 61. USART Block Diagram(1)
Note: 1. Refer to “Pin Configurations” on page 2, Table 30 on page 64, and Table 29 on page 64 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
PIN
CONTROL
UDR (Receive)
PIN
CONTROL
XCK
DATA
RECOVERY
CLOCK
RECOVERY
PIN
CONTROL
TX
CONTROL
RX
CONTROL
PARITY
CHECKER
DATABUS
OSC
SYNC LOGIC
Clock Generator
Transmitter
Receiver
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ATmega8(L)
The dashed boxes in the block diagram separate the three main parts of the USART (listed from
the top): Clock generator, Transmitter and Receiver. Control Registers are shared by all units.
The clock generation logic consists of synchronization logic for external clock input used by 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). The 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
important is the fact that the Error Flags (FE and DOR) and the ninth data bit (RXB8) are
buffered with the data in the receive buffer. Therefore the status bits must always be read
before the UDR Register is read. Otherwise the error status will be lost since the buffer state
is lost
• The Receiver Shift Register can now act as a third buffer level. This is done by allowing the
received data to remain in the serial Shift Register (see Figure 61 on page 129) if the Buffer
Registers are full, until a new start bit is detected. The USART is therefore more resistant to
Data OverRun (DOR) error conditions
The following control bits have changed name, but have same functionality and register location:
• CHR9 is changed to UCSZ2
• OR is changed to DOR
Clock Generation The clock generation logic generates the base clock for the Transmitter and Receiver. The
USART supports four modes of clock operation: normal asynchronous, double speed asynchro-
nous, Master synchronous and Slave Synchronous mode. The UMSEL bit in USART Control
and Status Register C (UCSRC) selects between asynchronous and synchronous operation.
Double speed (Asynchronous mode only) is controlled by the U2X found in the UCSRA Regis-
ter. When using Synchronous mode (UMSEL = 1), the Data Direction Register for the XCK 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 62 on page 131 shows a block diagram of the clock generation logic.
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Figure 62. 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 description in this section refers to Figure 62.
The USART Baud Rate Register (UBRR) and the down-counter connected to it function as a
programmable prescaler or baud rate generator. The down-counter, running at system clock
(fosc), is loaded with the 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 Transmitter divides the
baud rate generator clock output by 2, 8, or 16 depending on mode. The baud rate generator
output is used directly by the Receiver’s clock and data recovery units. However, the recovery
units use a state machine that uses 2, 8, or 16 states depending on mode set by the state of the
UMSEL, U2X and DDR_XCK bits.
Table 52 on page 132 contains equations for calculating the baud rate (in bits per second) and
for calculating 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
Register
OSC
XCK
Pin
txclk
U2X
UMSEL
DDR_XCK
0
1
0
1
xcki
xcko
DDR_XCK rxclk
0
1
1
0
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 60 on
page 153.
Double Speed
Operation (U2X)
The transfer rate can be doubled by setting the U2X bit in UCSRA. Setting this bit only has effect
for the asynchronous operation. Set this bit to zero when using synchronous operation.
Setting this bit will reduce the divisor of the baud rate divider from 16 to 8, effectively doubling
the transfer rate for asynchronous communication. Note however that the Receiver will in this
case only use half the number of samples (reduced from 16 to 8) for data sampling and clock
recovery, and therefore a more accurate baud rate setting and system clock are required when
this mode is used. For the Transmitter, there are no downsides.
External Clock External clocking is used by the Synchronous Slave modes of operation. The description in this
section refers to Figure 62 on page 131 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 intro-
duces a two CPU clock period delay and therefore the maximum external XCK clock frequency
is limited by the following equation:
Note that fosc depends on the stability of the system clock source. It is therefore recommended to
add some margin to avoid possible loss of data due to frequency variations.
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.
Table 52. 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
4
-----------
<
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Figure 63. 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 63 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 defined to be one character of data bits with synchronization bits (start and stop
bits), and optionally a parity bit for error checking. The USART accepts all 30 combinations of
the following as valid frame formats:
• 1 start bit
• 5, 6, 7, 8, or 9 data bits
• no, even or odd parity bit
• 1 or 2 stop bits
A frame starts with the start bit followed by the least significant data bit. Then the next data bits,
up to a total of nine, are succeeding, ending with the most significant bit. If enabled, the parity bit
is inserted after the data bits, before the stop bits. When a complete frame is transmitted, it can
be directly followed by a new frame, or the communication line can be set to an idle (high) state.
Figure 64 illustrates the possible combinations of the frame formats. Bits inside brackets are
optional.
Figure 64. 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
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.
RxD / TxD
XCK
RxD / TxD
XCK
UCPOL = 0
UCPOL = 1
Sample
Sample
10 2 3 4 [5] [6] [7] [8] [P]St Sp1 [Sp2] (St / IDLE)(IDLE)
FRAME
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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 character
If used, the parity bit is located between the last data bit and first stop bit of a serial frame.
USART
Initialization
The USART has to be initialized before any communication can take place. The initialization 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-initialization with changed baud rate or frame format, be sure that there 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 initialization code examples show one assembly and one C func-
tion that are equal in functionality. The examples assume asynchronous operation using polling
(no interrupts enabled) and a fixed frame format. The baud rate is given as a function parameter.
For the assembly code, the baud rate parameter is assumed to be stored in the r17:r16 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 Examples” on page 8
More advanced initialization routines can be made that include frame format as parameters, 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.
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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Data Transmission
– The USART
Transmitter
The USART Transmitter is enabled by setting the Transmit Enable (TXEN) bit in the UCSRB
Register. When the Transmitter is enabled, the normal port operation of the TxD pin is overrid-
den by the USART and given the function as the Transmitter’s serial output. The baud rate,
mode of operation and frame format must be set up once before doing any transmissions. If syn-
chronous operation is used, the clock on the XCK pin will be overridden and used as
transmission clock.
Sending Frames with
5 to 8 Data Bits
A data transmission is initiated by loading the transmit buffer with the data to be transmitted. The
CPU can load the transmit buffer by writing to the UDR I/O location. The buffered data in the
transmit buffer will be moved to the Shift Register when the Shift Register is ready to send a new
frame. The Shift Register is loaded with new data if it is in idle state (no ongoing transmission) or
immediately after the last stop bit of the previous frame is transmitted. When the Shift Register is
loaded with new data, it will transfer one complete frame at the rate given by the Baud Register,
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 frames 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 sent is assumed to be stored in Register
R16
Note: 1. See “About Code Examples” on page 8
The function simply waits for the transmit buffer 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 Bits
If 9-bit characters are used (UCSZ = 7), the ninth bit must be written to the 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 other protocol handling as for example synchronization.
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 been 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
Assembly Code Example(1)
USART_Transmit:
; Wait for empty transmit buffer
sbis UCSRA,UDRE
rjmp USART_Transmit
; Copy ninth 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 ninth 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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UDR in order to clear UDRE or disable the Data Register empty Interrupt, otherwise a new inter-
rupt will occur once the interrupt routine terminates.
The Transmit Complete (TXC) Flag bit is set one when the entire frame in the transmit Shift Reg-
ister has been shifted out and there are no new data currently present in the transmit buffer. 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
interfaces (like the RS485 standard), where a transmitting application must enter Receive mode
and free the communication bus immediately after completing the transmission.
When the Transmit Compete Interrupt Enable (TXCIE) bit in 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 bit for the serial frame data. When parity bit is enabled
(UPM1 = 1), the Transmitter control logic inserts the parity bit between the last data bit and the
first stop bit of the frame that is sent.
Disabling the
Transmitter
The disabling of the Transmitter (setting the TXEN to zero) will not become effective until ongo-
ing 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 pin.
Data Reception –
The USART
Receiver
The USART Receiver 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 given the function as the Receiver’s serial input. The baud rate, mode of
operation and frame format must be set up once before any serial reception can be done. If syn-
chronous operation is used, the clock on the XCK pin will be used as transfer clock.
Receiving Frames with
5 to 8 Data Bits
The Receiver starts data reception when it detects a valid start bit. Each bit that follows the start
bit will be sampled at the baud rate or XCK clock, and shifted into the Receive Shift Register until
the first stop bit of a frame is received. A second stop bit will be ignored by the Receiver. When
the first stop bit is received (that is, a complete serial frame is present in the Receive Shift Reg-
ister), 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
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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 Examples” on page 8
The function simply waits for data to be present in the receive buffer by checking the RXC Flag,
before reading the buffer and returning the value.
Receiving Frames with
9 Data Bits
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.
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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The following code example shows a simple USART receive function that handles both 9-bit
characters and the status bits.
Note: 1. See “About Code Examples” on page 8
The receive function example reads all the 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.
Assembly Code Example(1)
USART_Receive:
; Wait for data to be received
sbis UCSRA, RXC
rjmp USART_Receive
; Get status and ninth 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 ninth 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 ninth 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 ninth bit, then return */
resh = (resh >> 1) & 0x01;
return ((resh << 8) | resl);
}
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Receive Compete Flag
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 (that is, does not contain any unread data). If the Receiver is disabled (RXEN =
0), the receive buffer will be flushed and consequently the RXC bit will become zero.
When the Receive Complete 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 the 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 status. 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 by software doing a write to the flag location. However,
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 frame
stored in the receive buffer. The FE Flag is zero when 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 Shift Register, and a new start bit is detected. If the DOR Flag is 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 writing to UCSRA.
The DOR Flag is cleared when the frame received 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 134 and “Parity Checker” .
Parity Checker The Parity Checker is active when the high USART Parity mode (UPM1) bit is set. Type of parity
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 from the serial frame. The result of the check is stored in the receive buffer together
with the received data and stop bits. The Parity Error (PE) Flag can then be read by software to
check if the frame had a parity 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 ongoing
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). Unread data will be lost. If the buffer has to be flushed during normal
operation, due to for instance an error condition, read the UDR I/O location until the RXC Flag is
cleared. The following code example shows how to flush the receive buffer.
Note: 1. See “About Code Examples” on page 8
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 bit, thereby improving the noise immunity of the
Receiver. The asynchronous reception operational range depends on the accuracy of the inter-
nal baud rate clock, the rate of the incoming frames, and the frame size in number of bits.
Asynchronous Clock
Recovery
The clock recovery logic synchronizes internal clock to the incoming serial frames. Figure 65
illustrates the sampling process of the start bit of an incoming frame. The sample rate is 16 times
the baud rate for Normal mode, and eight times the baud rate for Double Speed mode. The hor-
izontal arrows illustrate the synchronization variation due to the sampling process. Note the
larger time variation when using the Double Speed mode (U2X = 1) of operation. Samples
denoted zero are samples done when the RxD line is idle (that is, no communication activity).
Figure 65. Start Bit Sampling
When the clock recovery logic detects a high (idle) to low (start) transition on the RxD line, the
start bit detection sequence is initiated. Let sample 1 denote the first zero-sample as shown in
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
00
BIT 0
3
1234 5 678120
RxD
Sample
(U2X = 0)
Sample
(U2X = 1)
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the figure. The clock recovery logic then uses samples 8, 9 and 10 for Normal mode, and
samples 4, 5 and 6 for Double Speed mode (indicated with sample numbers inside boxes on the
figure), to decide if a valid start bit is received. If two or more of these three samples have logical
high levels (the majority wins), the start bit is rejected as a noise spike and the Receiver starts
looking for the next high to low-transition. If however, a valid start bit is detected, the clock
recovery logic is synchronized and the data recovery can begin. The synchronization process is
repeated for each start bit.
Asynchronous Data
Recovery
When the Receiver clock is synchronized to the start bit, the data recovery can begin. The data
recovery unit uses a state machine that has 16 states for each bit in Normal mode and eight
states for each bit in Double Speed mode. Figure 66 shows the sampling of the data bits and the
parity bit. Each of the samples is given a number that is equal to the state of the recovery unit.
Figure 66. Sampling of Data and Parity Bit
The decision of the logic level of the received bit is taken by doing a majority voting of the logic
value to the three samples in the center of the received bit. The center samples are emphasized
on the figure by having the sample number inside boxes. The majority voting process is done as
follows: If two or all three samples have high levels, the received bit is registered to be a logic 1.
If two or all three samples have low levels, the received bit is registered to be a logic 0. This
majority voting process acts as a low pass filter for the incoming signal on the 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 67 shows the sampling of the stop bit and the earliest possible beginning of the start bit of
the next frame.
Figure 67. Stop Bit Sampling and Next Start Bit Sampling
The same majority voting is done to the stop bit as done for the other bits in the frame. If the stop
bit is registered to have a logic 0 value, the Frame Error (FE) Flag will be set.
A new high to low transition indicating the start bit of a new frame can come right after the last of
the bits used for majority voting. For Normal Speed mode, the first low level sample can be at
point marked (A) in Figure 67. For Double Speed mode the first low level must be delayed to (B).
(C) marks a stop bit of full length. The early start bit detection influences the operational range of
the Receiver.
12345678 9 10 11 12 13 14 15 16 1
BIT n
1234 5 6781
RxD
Sample
(U2X = 0)
Sample
(U2X = 1)
12345678 9 10 0/1 0/1 0/1
STOP 1
1234 5 6 0/1
RxD
Sample
(U2X = 0)
Sample
(U2X = 1)
(A) (B) (C)
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Asynchronous
Operational Range
The operational range of the Receiver is dependent on the mismatch between the received bit
rate and the internally generated baud rate. If the Transmitter is sending frames at too fast or too
slow bit rates, or the internally generated baud rate of the Receiver does not have a similar (see
Table 53) base frequency, the Receiver will not be able to synchronize the frames to the start bit.
The following equations can be used to calculate the ratio of the incoming data rate and internal
Receiver baud rate.
DSum of character size and parity size (D = 5-bit to 10-bit)
SSamples per bit. S = 16 for Normal Speed mode and S = 8 for Double Speed mode
SFFirst sample number used for majority voting. SF = 8 for Normal Speed and SF = 4 for
Double Speed mode
SMMiddle sample number used for majority voting. SM = 9 for Normal Speed and SM = 5 for
Double Speed mode
Rslow is the ratio of the slowest incoming data rate that can be accepted in relation to the
Receiver baud rate. Rfast is the ratio of the fastest incoming data rate that can be
accepted in relation to the Receiver baud rate
Table 53 and Table 54 list the maximum Receiver baud rate error that can be tolerated. Note
that Normal Speed mode has higher toleration of baud rate variations.
Table 53. Recommended Maximum Receiver Baud Rate Error for Normal Speed Mode
(U2X = 0)
D#
(Data + Parity Bit)
Rslow
(%)
Rfast
(%)
Max Total
Error (%)
Recommended Max
Receiver Error (%)
5 93.20 106.67 +6.67/-6.8 ±3.0
6 94.12 105.79 +5.79/-5.88 ±2.0
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 54. Recommended Maximum Receiver Baud Rate Error for Double Speed Mode
(U2X = 1)
D#
(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
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
Rslow
D1+()S
S1–DS⋅SF
++
-------------------------------------------=
Rfast
D2+()S
D1+()SS
M
+
-----------------------------------=
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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 for the Receivers Baud Rate error. The Receiver’s system clock
(XTAL) will always have some minor instability over the supply voltage range and the tempera-
ture range. When using a crystal to generate the system clock, this is rarely a problem, but for a
resonator the system clock may differ more than 2% depending of the resonators tolerance. The
second source for the error is more controllable. The baud rate generator can not always do an
exact division of the system frequency to get the baud rate wanted. In this case an UBRR value
that gives an acceptable low error can be used if possible.
Multi-processor
Communication
Mode
Setting the Multi-processor Communication mode (MPCM) bit in UCSRA enables a filtering
function of incoming frames received by the USART Receiver. Frames that do not contain
address information will be ignored and not put into the receive buffer. This effectively reduces
the number of incoming frames that has to be handled by the CPU, in a system with multiple
MCUs that communicate via the same serial bus. The Transmitter is unaffected by the 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 set up to receive frames 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 stop or the ninth bit) is one, the frame contains an address. When the
frame type bit is zero the frame is a data frame.
The Multi-processor Communication mode enables several Slave MCUs to receive data from a
Master MCU. This is done by first decoding an address frame to find out which MCU has been
addressed. If a particular Slave MCU has been addressed, it will receive the following data
frames as normal, while the other Slave MCUs will ignore the received frames until another
address frame is received.
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 address frame (TXB8 = 1) or cleared when a data frame
(TXB = 0) is being transmitted. The Slave MCUs must in this case be set to use a 9-bit character
frame format.
The following procedure should be used to exchange data in Multi-processor Communication
mode:
1. All Slave MCUs are in Multi-processor Communication mode (MPCM in UCSRA is set)
2. The Master MCU sends an address frame, and all slaves receive and read this frame. In
the Slave MCUs, the RXC Flag in UCSRA will be set as normal
3. Each Slave 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 byte 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 waits for a new address frame from 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 between using n and n+1 character frame formats. This makes full-
duplex operation difficult since the Transmitter and Receiver uses the same character size set-
ting. 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.
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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.
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 Examples” on page 8
As the code examples illustrate, write accesses of the two registers are relatively unaffected of
the sharing of I/O location.
Assembly Code Examples(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 Examples(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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Read Access Doing a read access to the UBRRH or the UCSRC Register is a more complex operation. How-
ever, in most applications, it is rarely necessary to read any of these registers.
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 read 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 reading the UCSRC is an atomic operation. Interrupts must therefore be controlled
(for example, by disabling interrupts globally) during the read operation.
The following code example shows how to read the UCSRC Register contents.
Note: 1. See “About Code Examples” on page 8
The assembly code example returns the UCSRC value in r16.
Reading the UBRRH contents is not an atomic operation and therefore it can be read as an ordi-
nary register, as long as the previous instruction did not access the register location.
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;
}
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USART Register
Description
USART I/O Data
Register – UDR
The USART Transmit Data Buffer Register and USART Receive Data Buffer Registers share the
same I/O address referred to as USART Data Register or 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 will change its state whenever the
receive buffer is accessed. Due to this behavior of the receive buffer, do not use Read-Modify-
Write instructions (SBI and CBI) on this location. Be careful when using bit test instructions
(SBIC and SBIS), since these also will change the state of the FIFO.
USART Control and
Status Register A –
UCSRA
• Bit 7 – RXC: USART Receive Complete
This flag bit is set when there are unread data in the receive buffer and cleared when the receive
buffer is empty (that is, does not contain any unread data). If the Receiver is disabled, the
receive 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 description of the “Bit 7 – RXCIE: RX
Complete Interrupt Enable” on page 149).
• 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 complete interrupt is executed, or it can be cleared by writing
a one to its bit location. The TXC Flag can generate a Transmit Complete interrupt (see descrip-
tion of the “Bit 6 – TXCIE: TX Complete Interrupt Enable” on page 149).
• 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
Register Empty interrupt (see description of the “Bit 5 – UDRIE: USART Data Register Empty
Interrupt Enable” on page 149).
UDRE is set after a reset to indicate that the Transmitter is ready.
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 Value00000000
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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• Bit 4 – FE: Frame Error
This bit is set if the next character in the receive buffer had a Frame Error when received (that 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 the
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 operation. Write this bit to zero when using syn-
chronous operation.
Writing this bit to one will reduce the divisor of the baud rate divider from 16 to 8 effectively dou-
bling the transfer rate for asynchronous communication.
• Bit 0 – MPCM: Multi-processor Communication Mode
This bit enables the Multi-processor Communication 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 Communication Mode” on page 145.
USART Control and
Status Register B –
UCSRB
• Bit 7 – RXCIE: RX Complete Interrupt Enable
Writing this bit to one enables interrupt on 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 one enables interrupt on the TXC Flag. A USART Transmit Complete interrupt
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 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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• 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 ninth data bit of the received character when operating with serial frames with nine
data bits. Must be read before reading the low bits from UDR.
• 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 the same I/O location as the UBRRH Register. See the “Accessing
UBRRH/UCSRC Registers” on page 146 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 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 55. UMSEL Bit Settings
UMSEL Mode
0 Asynchronous Operation
1 Synchronous Operation
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• Bit 5:4 – UPM1:0: Parity Mode
These bits enable and set type of Parity Generation and Check. If enabled, the Transmitter will
automatically generate and send the parity of the transmitted data bits within each frame. The
Receiver will generate a parity value for the incoming data and compare it to the 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 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.
Table 56. UPM Bits Settings
UPM1 UPM0 Parity Mode
0 0 Disabled
01Reserved
1 0 Enabled, Even Parity
1 1 Enabled, Odd Parity
Table 57. USBS Bit Settings
USBS Stop Bit(s)
01-bit
12-bit
Table 58. UCSZ Bits Settings
UCSZ2 UCSZ1 UCSZ0 Character Size
0005-bit
0016-bit
0107-bit
0118-bit
100Reserved
101Reserved
110Reserved
1119-bit
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• Bit 0 – UCPOL: Clock Polarity
This bit is used for Synchronous mode only. Write this bit to zero when Asynchronous mode 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 UBRRHs
The UBRRH Register shares the same I/O location as the UCSRC Register. See the “Accessing
UBRRH/UCSRC Registers” on page 146 section which describes how to access this register.
• Bit 15 – URSEL: Register Select
This bit selects between accessing the UBRRH or the UCSRC Register. It is read as zero when
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.
• Bit 11:0 – UBRR11:0: USART Baud Rate Register
This is a 12-bit register which contains the USART baud rate. The UBRRH contains the four
most significant bits, and the UBRRL contains the eight least significant bits of the USART baud
rate. Ongoing transmissions by the Transmitter and Receiver will be corrupted if the baud rate is
changed. Writing UBRRL will trigger an immediate update of the baud rate prescaler.
Table 59. 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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Examples of Baud
Rate Setting
For standard crystal and resonator frequencies, the most commonly used baud rates for asyn-
chronous operation can be generated by using the UBRR settings in Table 60. UBRR values
which yield an actual baud rate differing less than 0.5% from the target baud rate, are bold in the
table. Higher error ratings are acceptable, but the Receiver will have less noise resistance when
the error ratings are high, especially for large serial frames (see “Asynchronous Operational
Range” on page 144). The error values are calculated using the following equation:
Error[%] BaudRateClosest Match
BaudRate
-------------------------------------------------------- 1–
⎝⎠
⎛⎞
100%•=
Table 60. Examples of UBRR Settings for Commonly Used Oscillator Frequencies
Baud
Rate
(bps)
fosc = 1.0000MHz fosc = 1.8432MHz fosc = 2.0000MHz
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 25 0.2% 51 0.2% 47 0.0% 95 0.0% 51 0.2% 103 0.2%
4800 12 0.2% 25 0.2% 23 0.0% 47 0.0% 25 0.2% 51 0.2%
9600 6 -7.0% 12 0.2% 11 0.0% 23 0.0% 12 0.2% 25 0.2%
14.4k 3 8.5% 8 -3.5% 70.0%150.0%8 -3.5% 16 2.1%
19.2k 2 8.5% 6 -7.0% 50.0%110.0%6 -7.0% 12 0.2%
28.8k 1 8.5% 3 8.5% 3 0.0% 7 0.0% 38.5%8-3.5%
38.4k 1 -18.6% 2 8.5% 2 0.0% 5 0.0% 28.5%6-7.0%
57.6k 0 8.5% 1 8.5% 1 0.0% 3 0.0% 18.5%38.5%
76.8k – – 1 -18.6% 1 -25.0% 20.0%1 -18.6% 2 8.5%
115.2k – – 0 8.5% 0 0.0% 1 0.0% 08.5%18.5%
230.4k – – – – – – 00.0%–-––
250k––––––––––00.0%
Max (1) 62.5kbps 125kbps 115.2kbps 230.4kbps 125kbps 250kbps
1. UBRR = 0, Error = 0.0%
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Table 61. Examples of UBRR Settings for Commonly Used Oscillator Frequencies (Continued)
Baud
Rate
(bps)
fosc = 3.6864MHz fosc = 4.0000MHz fosc = 7.3728MHz
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 23 0.0% 47 0.0% 25 0.2% 51 0.2% 47 0.0% 95 0.0%
14.4k 15 0.0% 31 0.0% 16 2.1% 34 -0.8% 31 0.0% 63 0.0%
19.2k 11 0.0% 23 0.0% 12 0.2% 25 0.2% 23 0.0% 47 0.0%
28.8k 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% 38.5%8-3.5%70.0%150.0%
76.8k 2 0.0% 5 0.0% 28.5%6-7.0%50.0%110.0%
115.2k 1 0.0% 3 0.0% 1 8.5% 3 8.5% 30.0%70.0%
230.4k 0 0.0% 1 0.0% 0 8.5% 1 8.5% 10.0%30.0%
250k 0 -7.8% 1 -7.8% 0 0.0% 1 0.0% 1 -7.8% 3 -7.8%
0.5M – – 0 -7.8% – – 00.0%0 -7.8% 1 -7.8%
1M ––––––––––0-7.8%
Max (1) 230.4kbps 460.8kbps 250kbps 0.5Mbps 460.8kbps 921.6kbps
1. UBRR = 0, Error = 0.0%
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Table 62. Examples of UBRR Settings for Commonly Used Oscillator Frequencies (Continued)
Baud
Rate
(bps)
fosc = 8.0000MHz fosc = 11.0592MHz fosc = 14.7456MHz
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 – – 00.0%– – – – 0 -7.8% 1 -7.8%
Max (1) 0.5Mbps 1Mbps 691.2kbps 1.3824Mbps 921.6kbps 1.8432Mbps
1. UBRR = 0, Error = 0.0%
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Table 63. Examples of UBRR Settings for Commonly Used Oscillator Frequencies (Continued)
Baud
Rate
(bps)
fosc = 16.0000MHz fosc = 18.4320MHz fosc = 20.0000MHz
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% 90.0%190.0%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% 40.0%90.0%
0.5M 1 0.0% 3 0.0% – – 4 -7.8% – – 40.0%
1M 0 0.0% 1 0.0% – – – – – – – –
Max (1) 1Mbps 2Mbps 1.152Mbps 2.304Mbps 1.25Mbps 2.5Mbps
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 400kHz Data Transfer Speed
•Slew-rate Limited Output Drivers
•Noise Suppression Circuitry Rejects Spikes on Bus Lines
•Fully Programmable Slave Address with General Call Support
•Address Recognition Causes Wake-up When AVR is in Sleep Mode
Two-wire Serial
Interface Bus
Definition
The Two-wire Serial Interface (TWI) is ideally suited for typical microcontroller applications. The
TWI protocol allows the systems designer to interconnect up to 128 different devices using only
two bi-directional bus lines, one for clock (SCL) and one for data (SDA). The only external hard-
ware needed to implement the bus is a single pull-up resistor for each of the TWI bus lines. All
devices connected to the bus have individual addresses, and mechanisms for resolving bus
contention are inherent in the TWI protocol.
Figure 68. 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 64. TWI Terminology
Term Description
Master The device that initiates and terminates a transmission. The Master also
generates the SCL clock
Slave The device addressed by a Master
Transmitter The device placing data on the bus
Receiver The device reading data from the bus
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Electrical
Interconnection
As depicted in Figure 68 on page 157, both bus lines are connected to the positive supply volt-
age 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 resis-
tors to pull the line high. Note that all AVR devices connected to the TWI bus must be powered
in order to allow any bus operation.
The number of devices that can be connected to the bus is only limited by the bus capacitance
limit of 400pF and the 7-bit slave address space. A detailed specification of the electrical charac-
teristics of the TWI is given in “Two-wire Serial Interface Characteristics” on page 238. Two
different sets of specifications are presented there, one relevant for bus speeds below 100kHz,
and one valid for bus speeds up to 400kHz.
Data Transfer and
Frame Format
Transferring Bits Each data bit transferred on the TWI bus is accompanied by a pulse on the clock line. The level
of the data line must be stable when the clock line is high. The only exception to this rule is for
generating start and stop conditions.
Figure 69. 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 relin-
quishing control of the bus. After a REPEATED START, the bus is considered busy until the next
STOP. This is identical to the START behavior, and therefore START is used to describe both
START and REPEATED START for the remainder of this datasheet, unless otherwise noted. As
depicted below, START and STOP conditions are signalled by changing the level of the SDA
line when the SCL line is high.
SDA
SCL
Data Stable Data Stable
Data Change
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Figure 70. START, REPEATED START and STOP conditions
Address Packet
Format
All address packets transmitted on the TWI bus are 9 bits long, consisting of 7 address bits, one
READ/WRITE control bit and an acknowledge bit. If the READ/WRITE bit is set, a read opera-
tion 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 address byte is transmitted first. Slave addresses can freely be allocated by the
designer, but the address 0000 000 is reserved for a general call.
When a general call is issued, all slaves should respond by pulling the SDA line low in the ACK
cycle. A general call is used when a Master wishes to transmit the same message to several
slaves in the system. When the general call address followed by a Write bit is transmitted on the
bus, all slaves set up to acknowledge the general call will pull the SDA line low in the ack cycle.
The following data packets will then be received by all the slaves that acknowledged the general
call. Note that transmitting the general call address followed by a Read bit is meaningless, as
this would cause contention if several slaves started transmitting different data.
All addresses of the format 1111 xxx should be reserved for future purposes.
Figure 71. 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 Receiver 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 72. 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 empty message, consisting of a START followed by a STOP condi-
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 consequence, the Slave can reduce the TWI data transfer speed by prolonging the
SCL duty cycle.
Figure 73 shows a typical data transmission. Note that several data bytes can be transmitted
between the SLA+R/W and the STOP condition, depending on the software protocol imple-
mented by the application software.
Figure 73. Typical Data Transmission
12 789
Data MSB Data LSB ACK
Aggregate
SDA
SDA from
Transmitter
SDA from
Receiver
SCL from
Master
SLA+R/W Data Byte
STOP, REPEATED
START or Next
Data Byte
12 789
Data Byte
Data MSB Data LSB ACK
SDA
SCL
START
12 789
Addr MSB Addr LSB R/W ACK
SLA+R/W STOP
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Multi-master Bus
Systems,
Arbitration and
Synchronization
The TWI protocol allows bus systems with several masters. Special concerns have been taken
in order to ensure that transmissions will proceed as normal, even if two or more masters initiate
a transmission at the same time. Two problems arise in multi-master systems:
• An algorithm must be implemented allowing only one of the masters to complete the
transmission. All other masters should cease transmission when they discover that they
have lost the selection process. This selection process is called arbitration. When a
contending master discovers that it has lost the arbitration process, it should immediately
switch to Slave mode to check whether it is being addressed by the winning master. The fact
that multiple masters have started transmission at the same time should not be detectable to
the slaves, that is, the data being transferred on the bus must not be corrupted
• Different masters may use different SCL frequencies. A scheme must be devised to
synchronize the serial clocks from all masters, in order to let the transmission proceed in a
lockstep fashion. This will facilitate the arbitration process
The wired-ANDing of the bus lines is used to solve both these problems. The serial clocks from
all masters will be wired-ANDed, yielding a combined clock with a high period equal to the one
from the Master with the shortest high period. The low period of the combined clock is equal to
the low period of the Master with the longest low period. Note that all masters listen to the SCL
line, effectively starting to count their SCL high and low time-out periods when the combined
SCL line goes high or low, respectively.
Figure 74. SCL Synchronization Between Multiple Masters
Arbitration is carried out by all masters continuously monitoring the SDA line after outputting
data. If the value read from the SDA line does not match the value the Master had output, it has
lost the arbitration. Note that a Master can only lose arbitration when it outputs a high SDA value
while another Master outputs a low value. The losing Master should immediately go to Slave
mode, checking if it is being addressed by the winning Master. The SDA line should be left high,
but losing masters are allowed to generate a clock signal until the end of the current data or
address packet. Arbitration will continue until only one Master remains, and this may take many
bits. If several masters are trying to address the same Slave, arbitration will continue into the
data packet.
TA
low
TA
high
SCL from
Master A
SCL from
Master B
SCL Bus
Line
TB
low
TB
high
Masters Start
Counting Low Period
Masters Start
Counting High Period
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Figure 75. Arbitration Between Two Masters
Note that arbitration is not allowed between:
• A REPEATED START condition and a data bit
• A STOP condition and a data bit
• A REPEATED START and a STOP condition
It is the user software’s responsibility to ensure that these illegal arbitration conditions never
occur. This implies that in multi-master systems, all data transfers must use the same 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
A
SDA
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Overview of the
TWI Module
The TWI module is comprised of several submodules, as shown in Figure 76. All registers drawn
in a thick line are accessible through the AVR data bus.
Figure 76. Overview of the TWI Module
SCL and SDA Pins These pins interface the AVR TWI with the rest of the MCU system. The output drivers contain a
slew-rate limiter in order to conform to the TWI specification. The input stages contain a spike
suppression unit removing spikes shorter than 50ns. Note that the internal pull-ups in the AVR
pads can be enabled by setting the PORT bits corresponding to the SCL and SDA pins, as
explained in the I/O Port section. The internal pull-ups can in some systems eliminate the need
for external ones.
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
Register (TWDR)
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Bit Rate Generator
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
Register (TWSR). Slave operation does not depend on Bit Rate or Prescaler settings, but the
CPU clock frequency in the Slave must be at least 16 times higher than the SCL frequency. Note
that slaves may prolong the SCL low period, thereby reducing the average TWI bus clock
period. The SCL frequency is generated according to the following equation:
• TWBR = Value of the TWI Bit Rate Register
• TWPS = Value of the prescaler bits in the TWI Status Register
Note: Pull-up resistor values should be selected according to the SCL frequency and the capacitive bus
line load. See Table 101 on page 238 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 detection hardware. The TWDR contains the address or data bytes to be transmitted,
or the address or data bytes received. In addition to the 8-bit TWDR, the Bus Interface Unit also
contains a register containing the (N)ACK bit to be transmitted or received. This (N)ACK Regis-
ter is not directly accessible by the application software. However, when receiving, it can be set
or cleared by manipulating the TWI Control Register (TWCR). When in Transmitter mode, the
value of the received (N)ACK bit can be determined by the value in the TWSR.
The START/STOP Controller is responsible for generation and detection of START, REPEATED
START, and STOP conditions. The START/STOP controller is able to detect START and STOP
conditions even when the AVR MCU is in one of the sleep modes, enabling the MCU to wake up
if addressed by a Master.
If the TWI has initiated a transmission as Master, the Arbitration Detection hardware continu-
ously monitors the transmission trying to determine if arbitration is in process. If the TWI has lost
an arbitration, the Control Unit is informed. Correct action can then be taken and appropriate
status codes generated.
Address Match Unit The Address Match unit checks if received address bytes match the seven-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. If another interrupt (for example,
INT0) occurs during TWI Power-down address match and wakes up the CPU, the TWI aborts
operation and return to it’s idle state. If this cause any problems, ensure that TWI Address Match
is the only enabled interrupt when entering Power-down.
Control Unit The Control unit monitors the TWI bus and generates responses corresponding to settings in the
TWI Control Register (TWCR). When an event requiring the attention of the application occurs
on the TWI bus, the TWI Interrupt Flag (TWINT) is asserted. In the next 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 TWINT Flag is set, the SCL line is held low. This allows the application
software to complete its tasks before allowing the TWI transmission to continue.
SCL frequency CPU Clock frequency
16 2(TWBR) 4TWPS
⋅+
-----------------------------------------------------------=
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The TWINT Flag is set in the following situations:
• After the TWI has transmitted a START/REPEATED START condition
• After the TWI has transmitted SLA+R/W
• After the TWI has transmitted an address byte
• After the TWI has lost arbitration
• After the TWI has been addressed by own slave address or general call
• After the TWI has received a data byte
• After a STOP or REPEATED START has been received while still addressed as a Slave
• When a bus error has occurred due to an illegal START or STOP condition
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 164 for calculating bit rates.
TWI Control Register –
TWCR
The TWCR is used to control the operation of the TWI. It is used to enable the TWI, to initiate a
Master access by applying a START condition to the bus, to generate a Receiver acknowledge,
to generate a stop condition, and to control halting of the bus while the data to be written to the
bus are written to the TWDR. It also indicates a write collision if data is attempted written to
TWDR while the register is inaccessible.
• Bit 7 – TWINT: TWI Interrupt Flag
This bit is set by hardware when the TWI has finished its current job and expects application
software response. If the I-bit in SREG and TWIE in TWCR are set, the MCU will jump to the
TWI Interrupt Vector. While the TWINT Flag is set, the SCL low period is stretched. The TWINT
Flag must be cleared by software by writing a logic one to it. Note that this flag is not automati-
cally cleared by hardware when executing the interrupt routine. Also note that clearing this flag
starts the operation of the TWI, so all accesses to the TWI Address Register (TWAR), TWI Sta-
tus Register (TWSR), and TWI Data Register (TWDR) must be complete before clearing this
flag.
Bit 76543210
TWBR7 TWBR6 TWBR5 TWBR4 TWBR3 TWBR2 TWBR1 TWBR0 TWBR
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial Value00000000
Bit 76543210
TWINT TWEA TWSTA TWSTO TWWC TWEN – TWIE TWCR
Read/Write R/W R/W R/W R/W R R/W R R/W
Initial Value00000000
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• Bit 6 – TWEA: TWI Enable Acknowledge Bit
The TWEA bit controls the generation of the acknowledge pulse. If the TWEA bit is written to
one, the ACK pulse is generated on the TWI bus if the following conditions are met:
1. The device’s own slave address has been received
2. A general call has been received, while the TWGCE bit in the TWAR is set
3. A data byte has been received in Master Receiver or Slave Receiver mode
By writing the TWEA bit to zero, the device can be virtually disconnected from the Two-wire
Serial Bus temporarily. Address recognition can then be resumed by writing the TWEA bit to one
again.
• Bit 5 – TWSTA: TWI START Condition Bit
The application writes the TWSTA bit to one when it desires to become a Master on the Two-
wire Serial Bus. The TWI hardware checks if the bus is available, and generates a START con-
dition on the bus if it is free. However, if the bus is not free, the TWI waits until a STOP condition
is detected, and then generates a new START condition to claim the bus Master status. TWSTA
must be cleared by software when the START condition has been transmitted.
• Bit 4 – TWSTO: TWI STOP Condition Bit
Writing the TWSTO bit to one in Master 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 TWSTO bit can be used to recover from an error condition.
This will not generate a STOP condition, but the TWI returns to a well-defined unaddressed
Slave mode and releases the SCL and SDA lines to a high impedance state.
• Bit 3 – TWWC: TWI Write Collision Flag
The TWWC bit is set when attempting to write to the TWI Data Register – TWDR when TWINT is
low. This flag is cleared by writing the TWDR Register when TWINT is high.
• Bit 2 – TWEN: TWI Enable Bit
The TWEN bit enables TWI operation and activates the TWI interface. When TWEN is written to
one, the TWI takes control over the I/O pins connected to the SCL and SDA pins, enabling the
slew-rate limiters and spike filters. If this bit is written to zero, the TWI is switched off and all TWI
transmissions are terminated, regardless of any ongoing operation.
• Bit 1 – Res: Reserved Bit
This bit is a reserved bit and will always read as zero.
• Bit 0 – TWIE: TWI Interrupt Enable
When this bit is written to one, and the I-bit in SREG is set, the TWI interrupt request will be acti-
vated for as long as the TWINT Flag is high.
TWI Status Register –
TWSR
• Bits 7..3 – TWS: TWI Status
These 5 bits reflect the status of the TWI logic and the Two-wire Serial Bus. The different status
codes are described later in this section. Note that the value read from TWSR contains both the
5-bit status value and the 2-bit prescaler value. The application designer should mask the pres-
caler bits to zero when checking the Status bits. This makes status checking independent of
prescaler setting. This approach is used in this datasheet, unless otherwise noted.
Bit 76543210
TWS7 TWS6 TWS5 TWS4 TWS3 – TWPS1 TWPS0 TWSR
Read/WriteRRRRRRR/WR/W
Initial Value11111000
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• Bit 2 – Res: Reserved Bit
This bit is reserved and will always read as zero.
• Bits 1..0 – TWPS: TWI Prescaler Bits
These bits can be read and written, and control the bit rate prescaler.
To calculate bit rates, see “Bit Rate Generator Unit” on page 164. 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 the 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 by the user before the first interrupt occurs. The data in TWDR remains
stable as long as TWINT is set. While data is shifted out, data on the bus is simultaneously
shifted in. TWDR always contains the last byte present on the bus, except after a wake up from
a sleep mode by the TWI interrupt. In this case, the contents of TWDR is undefined. In the case
of a lost bus arbitration, no data is lost in the transition from Master to Slave. Handling of the
ACK bit is controlled automatically by the TWI logic, the CPU cannot access the ACK bit directly.
• Bits 7..0 – TWD: TWI Data Register
These eight bits constitute the next data byte to be transmitted, or the latest data byte received
on the Two-wire Serial Bus.
TWI (Slave) Address
Register – TWAR
The TWAR should be loaded with the 7-bit Slave address (in the seven most significant bits of
TWAR) to which the TWI will respond when programmed as a Slave Transmitter or Receiver,
and not needed in the Master modes. In multimaster systems, TWAR must be set in masters
which can be addressed as Slaves by other Masters.
The LSB of TWAR is used to enable recognition of the general call address (0x00). There is an
associated address comparator that looks for the slave address (or general call address if
enabled) in the received serial address. If a match is found, an interrupt request is generated.
Table 65. TWI Bit Rate Prescaler
TWPS1 TWPS0 Prescaler Value
001
014
1016
1164
Bit 76543210
TWD7 TWD6 TWD5 TWD4 TWD3 TWD2 TWD1 TWD0 TWDR
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial Value11111111
Bit 76543210
TWA6 TWA5 TWA4 TWA3 TWA2 TWA1 TWA0 TWGCE TWAR
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial Value11111110
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• Bits 7..1 – TWA: TWI (Slave) Address Register
These seven bits constitute the slave address of the TWI unit.
• 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 byte or transmission of a START condition. Because the TWI is interrupt-based,
the application software is free to carry on other operations during a TWI byte transfer. Note that
the TWI Interrupt Enable (TWIE) bit in TWCR together with the Global Interrupt Enable bit in
SREG allow the application to decide whether or not assertion of the TWINT Flag should gener-
ate an interrupt request. If the TWIE bit is cleared, the application must poll the TWINT Flag in
order to detect actions on the TWI bus.
When the TWINT Flag is asserted, the TWI has finished an operation and awaits application
response. In this case, the TWI Status Register (TWSR) contains a value indicating the current
state of the TWI bus. The application software can then decide how the TWI should behave in
the next TWI bus cycle by manipulating the TWCR and TWDR Registers.
Figure 77 is a simple example of how the application can interface to the TWI hardware. In this
example, a Master wishes to transmit a single data byte to a Slave. This description is quite
abstract, a more detailed explanation follows later in this section. A simple code example imple-
menting the desired behavior is also presented.
Figure 77. 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
START SLA+W A Data A STOP
1. Application
writes to TWCR to
initiate
transmission of
START
2. TWINT set.
Status code indicates
START condition sent
4. TWINT set.
Status code indicates
SLA+W sent, ACK
received
6. TWINT set.
Status code indicates
data sent, ACK received
3. Check TWSR to see if START was
sent. Application loads SLA+W into
TWDR, and loads appropriate control
signals into TWCR, makin sure that
TWINT is written to one,
and TWSTA is written to zero.
5. Check TWSR to see if SLA+W was
sent and ACK received.
Application loads data into TWDR, and
loads appropriate control signals into
TWCR, making sure that TWINT is
written to one
7. Check TWSR to see if data was sent
and ACK received.
Application loads appropriate control
signals to send STOP into TWCR,
making sure that TWINT is written to one
TWI bus
Indicates
TWINT set
Application
Action
TWI
Hardware
Action
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3. The application software should now examine the value of TWSR, to make sure that the
START condition was successfully transmitted. If TWSR indicates otherwise, the applica-
tion software might take some special action, like calling an error routine. Assuming that
the status code is as expected, the application must load SLA+W into TWDR. Remember
that TWDR is used both for address and data. After TWDR has been loaded with the
desired SLA+W, a specific value must be written to TWCR, instructing the TWI hardware
to transmit the SLA+W present in TWDR. Which value to write is described later on.
However, it is important that the TWINT bit is set in the value written. Writing a one to
TWINT clears the flag. The TWI will not start any operation as long as the TWINT bit in
TWCR is set. Immediately after the application has cleared TWINT, the TWI will initiate
transmission of the address packet
4. When the address packet has been transmitted, the TWINT Flag in TWCR is set, and
TWSR is updated with a status code indicating that the address packet has successfully
been sent. The status code will also reflect whether a Slave acknowledged the packet or
not
5. The application software should now examine the value of TWSR, to make sure that the
address packet was successfully transmitted, and that the value of the ACK bit was as
expected. If TWSR indicates otherwise, the application software might take some special
action, like calling an error routine. Assuming that the status code is as expected, the
application must load a data packet into TWDR. Subsequently, a specific value must be
written to TWCR, instructing the TWI hardware to transmit the data packet present in
TWDR. Which value to write is described later on. However, it is important that the
TWINT bit is set in the value written. Writing a one to TWINT clears the flag. The TWI will
not start any operation as long as the TWINT bit in TWCR is set. Immediately after the
application has cleared TWINT, the TWI will initiate transmission of the data packet
6. When the data packet has been transmitted, the TWINT Flag in TWCR is set, and TWSR
is updated with a status code indicating that the data packet has successfully been sent.
The status code will also reflect whether a Slave acknowledged the packet or not
7. The application software should now examine the value of TWSR, to make sure that the
data packet was successfully transmitted, and that the value of the ACK bit was as
expected. If TWSR indicates otherwise, the application software might take some special
action, like calling an error routine. Assuming that the status code is as expected, the
application must write a specific value to TWCR, instructing the TWI hardware to transmit
a STOP condition. Which value to write is described later on. However, it is important that
the TWINT bit is set in the value written. Writing a one to TWINT clears the flag. The TWI
will not start any operation as long as the TWINT bit in TWCR is set. Immediately after
the application has cleared TWINT, the TWI will initiate transmission of the STOP condi-
tion. Note that TWINT is NOT set after a STOP condition has been sent
Even though this example is simple, it shows the principles involved in all TWI transmissions.
These can be summarized as follows:
• When the TWI has finished an operation and expects application response, the TWINT Flag
is set. The SCL line is pulled low until TWINT is cleared
• When the TWINT Flag is set, the user must update all TWI Registers with the value relevant
for the next TWI bus cycle. As an example, TWDR must be loaded with the value to be
transmitted in the next bus cycle
• After all TWI Register updates and other pending application software tasks have been
completed, TWCR is written. When writing TWCR, the TWINT bit should be set. Writing a
one to TWINT clears the flag. The TWI will then commence executing whatever operation
was specified by the TWCR setting
In the following an assembly and C implementation of the example is given. Note that the code
below assumes that several definitions have been made, for example by using include-files.
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Assembly Code Example C Example Comments
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 TWINT 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 value 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 TWINT Flag set. This
indicates that the SLA+W has been
transmitted, and ACK/NACK has
been received.
5in r16,TWSR
andi r16, 0xF8
cpi r16, MT_SLA_ACK
brne ERROR
if ((TWSR & 0xF8) !=
MT_SLA_ACK)
ERROR();
Check value 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 TWINT 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 value 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
171
2486Z–AVR–02/11
ATmega8(L)
Transmission
Modes
The TWI can operate in one of four major modes. These are named Master Transmitter (MT),
Master Receiver (MR), Slave Transmitter (ST) and Slave Receiver (SR). Several of these
modes can be used in the same application. As an example, the TWI can use MT mode to write
data into a TWI EEPROM, MR mode to read the data back from the EEPROM. If other masters
are present in the system, some of these might transmit data to the TWI, and then SR mode
would be used. It is the application software that decides which modes are legal.
The following sections describe each of these modes. Possible status codes are described
along with figures detailing data transmission in each of the modes. These figures contain the
following abbreviations:
S: START condition
Rs: REPEATED START condition
R: Read bit (high level at SDA)
W: Write bit (low level at SDA)
A: Acknowledge bit (low level at SDA)
A: Not acknowledge bit (high level at SDA)
Data: 8-bit data byte
P: STOP condition
SLA: Slave Address
In Figure 79 on page 174 to Figure 85 on page 183, circles are used to indicate that the TWINT
Flag is set. The numbers in the circles show the status code held in TWSR, with the prescaler
bits masked to zero. At these points, actions must be taken by the application to continue or
complete the TWI transfer. The TWI transfer is suspended until the TWINT Flag is cleared by
software.
When the TWINT Flag is set, the status code in TWSR is used to determine the appropriate soft-
ware action. For each status code, the required software action and details of the following serial
transfer are given in Table 66 on page 173 to Table 69 on page 182. 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 78 on page 172). In order to enter a Master mode, a START condition must be
transmitted. The format of the following address packet determines whether Master Transmitter
or Master Receiver mode is to be entered. If SLA+W is transmitted, MT mode is entered, if
SLA+R is transmitted, MR mode is entered. All the status codes mentioned in this section
assume that the prescaler bits are zero or are masked to zero.
172
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ATmega8(L)
Figure 78. Data Transfer in Master Transmitter Mode
A START condition is sent by writing the following value to TWCR:
TWEN must be set to enable the Two-wire Serial Interface, TWSTA must be written to one to
transmit a START condition and TWINT must be written to one to clear the TWINT Flag. The
TWI will then test the 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 0x08 (see Table 66 on page 173). In order to enter
MT mode, SLA+W must be transmitted. This is done by writing SLA+W to TWDR. Thereafter the
TWINT bit should be cleared (by writing it to one) to continue the transfer. This is accomplished
by writing the following value to TWCR:
When SLA+W have been transmitted and an acknowledgement bit has been received, TWINT is
set again and a number of status codes in TWSR are possible. Possible status codes in Master
mode are 0x18, 0x20, or 0x38. The appropriate action to be taken for each of these status codes
is detailed in Table 66 on page 173.
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 0x10) the Two-wire Serial Interface can access the
same Slave again, or a new Slave without transmitting a STOP condition. Repeated START
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
Device 1
MASTER
TRANSMITTER
Device 2
SLAVE
RECEIVER
Device 3 Device n
SDA
SCL
........ R1 R2
V
CC
173
2486Z–AVR–02/11
ATmega8(L)
enables the Master to switch between Slaves, Master Transmitter mode and Master Receiver
mode without losing control of the bus.
Table 66. Status codes for Master Transmitter 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
0x08 A START condition has been
transmitted
Load SLA+W 0 0 1 X SLA+W will be transmitted;
ACK or NOT ACK will be received
0x10 A repeated START condition
has been transmitted
Load SLA+W or
Load SLA+R
0
0
0
0
1
1
X
X
SLA+W will be transmitted;
ACK or NOT ACK will be received
SLA+R will be transmitted;
Logic will switch to Master Receiver mode
0x18 SLA+W has been transmitted;
ACK has been received
Load data byte or
No TWDR action or
No TWDR action or
No TWDR action
0
1
0
1
0
0
1
1
1
1
1
1
X
X
X
X
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
0x20 SLA+W has been transmitted;
NOT ACK has been received
Load data byte or
No TWDR action or
No TWDR action or
No TWDR action
0
1
0
1
0
0
1
1
1
1
1
1
X
X
X
X
Data byte will be transmitted and ACK or NOT ACK will
be received
Repeated START will be transmitted
STOP condition will be transmitted and
TWSTO Flag will be reset
STOP condition followed by a START condition will be
transmitted and TWSTO Flag will be reset
0x28 Data byte has been transmitted;
ACK has been received
Load data byte or
No TWDR action or
No TWDR action or
No TWDR action
0
1
0
1
0
0
1
1
1
1
1
1
X
X
X
X
Data byte will be transmitted and ACK or NOT ACK will
be received
Repeated START will be transmitted
STOP condition will be transmitted and
TWSTO Flag will be reset
STOP condition followed by a START condition will be
transmitted and TWSTO Flag will be reset
0x30 Data byte has been transmitted;
NOT ACK has been received
Load data byte or
No TWDR action or
No TWDR action or
No TWDR action
0
1
0
1
0
0
1
1
1
1
1
1
X
X
X
X
Data byte will be transmitted and ACK or NOT ACK will
be received
Repeated START will be transmitted
STOP condition will be transmitted and
TWSTO Flag will be reset
STOP condition followed by a START condition will be
transmitted and TWSTO Flag will be reset
0x38 Arbitration lost in SLA+W or data
bytes
No TWDR action or
No TWDR action
0
1
0
0
1
1
X
X
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
174
2486Z–AVR–02/11
ATmega8(L)
Figure 79. Formats and States in the Master Transmitter Mode
S SLA W A DATA A P
$08 $18 $28
R SLA W
$10
AP
$20
P
$30
A or A
$38
A
Other master
continues
A or A
$38
Other master
continues
R
A
$68
Other master
continues
$78 $B0
To corresponding
states in slave mode
MT
MR
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
n
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
S
175
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ATmega8(L)
Master Receiver Mode In the Master Receiver mode, a number of data bytes are received from a Slave Transmitter
(see Figure 80). In order to enter a Master mode, a START condition must be transmitted. The
format of the following address packet determines whether Master Transmitter or Master
Receiver mode is to be entered. If SLA+W is transmitted, MT mode is entered, if SLA+R is 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 80. Data Transfer in Master Receiver Mode
A START condition is sent by writing the following value to TWCR:
TWEN must be written to one to enable the Two-wire Serial Interface, TWSTA must be written to
one to transmit a START condition and TWINT must be set to clear the TWINT Flag. The TWI
will then test the 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 0x08 (see Table 66 on page 173). In order to enter
MR mode, SLA+R must be transmitted. This is done by writing SLA+R to TWDR. Thereafter the
TWINT bit should be cleared (by writing it to one) to continue the transfer. This is accomplished
by writing the following value to TWCR:
When SLA+R have been transmitted and an acknowledgement bit has been received, TWINT is
set again and a number of status codes in TWSR are possible. Possible status codes in Master
mode are 0x38, 0x40, or 0x48. The appropriate action to be taken for each of these status codes
is detailed in Table 67 on page 176. Received data can be read from the TWDR Register when
the TWINT Flag is set high by hardware. This scheme is repeated until the last byte has been
received. After the last byte has been received, the MR should inform the ST by sending a
NACK after the last received data byte. The transfer is ended by generating a STOP condition or
a repeated START condition. A STOP condition is generated by writing the following value to
TWCR:
A REPEATED START condition is generated by writing the following value to TWCR:
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
176
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ATmega8(L)
After a repeated START condition (state 0x10) the Two-wire Serial Interface can access the
same Slave again, or a new Slave without transmitting a STOP condition. Repeated START
enables the Master to switch between Slaves, Master Transmitter mode and Master Receiver
mode without losing control over the bus.
Table 67. 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
0x08 A START condition has been
transmitted
Load SLA+R 0 0 1 X SLA+R will be transmitted
ACK or NOT ACK will be received
0x10 A repeated START condition
has been transmitted
Load SLA+R or
Load SLA+W
0
0
0
0
1
1
X
X
SLA+R will be transmitted
ACK or NOT ACK will be received
SLA+W will be transmitted
Logic will switch to Master Transmitter mode
0x38 Arbitration lost in SLA+R or NOT
ACK bit
No TWDR action or
No TWDR action
0
1
0
0
1
1
X
X
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
0x40 SLA+R has been transmitted;
ACK has been received
No TWDR action or
No TWDR action
0
0
0
0
1
1
0
1
Data byte will be received and NOT ACK will be
returned
Data byte will be received and ACK will be returned
0x48 SLA+R has been transmitted;
NOT ACK has been received
No TWDR action or
No TWDR action or
No TWDR action
1
0
1
0
1
1
1
1
1
X
X
X
Repeated START will be transmitted
STOP condition will be transmitted and TWSTO Flag will
be reset
STOP condition followed by a START condition will be
transmitted and TWSTO Flag will be reset
0x50 Data byte has been received;
ACK has been returned
Read data byte or
Read data byte
0
0
0
0
1
1
0
1
Data byte will be received and NOT ACK will be
returned
Data byte will be received and ACK will be returned
0x58 Data byte has been received;
NOT ACK has been returned
Read data byte or
Read data byte or
Read data byte
1
0
1
0
1
1
1
1
1
X
X
X
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
177
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ATmega8(L)
Figure 81. Formats and States in the Master Receiver Mode
Slave Receiver Mode In the Slave Receiver mode, a number of data bytes are received from a Master Transmitter
(see Figure 82). All the status codes mentioned in this section assume that the prescaler bits are
zero or are masked to zero.
Figure 82. Data transfer in Slave Receiver mode
To initiate the Slave Receiver mode, TWAR and TWCR must be initialized as follows:
S SLA R A DATA A
$08 $40 $50
SLA R
$10
AP
$48
A or A
$38
Other master
continues
$38
Other master
continues
W
A
$68
Other master
continues
$78 $B0
To corresponding
states in slave mode
MR
MT
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
n
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
A
R
S
TWAR TWA6 TWA5 TWA4 TWA3 TWA2 TWA1 TWA0 TWGCE
value Device’s Own Slave Address
Device 3 Device n
SDA
SCL
........
R1 R2
V
CC
Device 2
MASTER
TRANSMITTER
Device 1
SLAVE
RECEIVER
178
2486Z–AVR–02/11
ATmega8(L)
The upper 7 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 (0x00),
otherwise it will ignore the general call address.
TWEN must be written to one to enable the TWI. The TWEA bit must be written to one to enable
the acknowledgement of the device’s own slave address or the general call address. TWSTA
and TWSTO must be written to zero.
When TWAR and TWCR have been initialized, the TWI waits until it is addressed by its own
slave address (or the general call address if enabled) followed by the data direction bit. If the
direction bit is “0” (write), the TWI will operate in SR mode, otherwise ST mode is entered. After
its own slave address and the write bit have been received, the TWINT Flag is set and a valid
status code can be read from TWSR. The status code is used to determine the appropriate soft-
ware action. The appropriate action to be taken for each status code is detailed in Table 68 on
page 179. The Slave Receiver mode may also be entered if arbitration is lost while the TWI is in
the Master mode (see states 0x68 and 0x78).
If the TWEA bit is reset during a transfer, the TWI will return a “Not Acknowledge” (“1”) to SDA
after the next received data byte. This can be used to indicate that the Slave is not able to
receive any more bytes. While TWEA is zero, the TWI does not acknowledge its own slave
address. However, the 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 to temporarily
isolate the TWI from the Two-wire Serial Bus.
In all sleep modes other than Idle mode, the clock system to the TWI is turned off. If the TWEA
bit is set, the interface can still acknowledge its own slave address or the general call address by
using the 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 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
179
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ATmega8(L)
Table 68. 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
0x60 Own SLA+W has been received;
ACK has been returned
No TWDR action or
No TWDR action
X
X
0
0
1
1
0
1
Data byte will be received and NOT ACK will be re-
turned
Data byte will be received and ACK will be returned
0x68 Arbitration lost in SLA+R/W as
Master; own SLA+W has been
received; ACK has been returned
No TWDR action or
No TWDR action
X
X
0
0
1
1
0
1
Data byte will be received and NOT ACK will be re-
turned
Data byte will be received and ACK will be returned
0x70 General call address has been
received; ACK has been returned
No TWDR action or
No TWDR action
X
X
0
0
1
1
0
1
Data byte will be received and NOT ACK will be re-
turned
Data byte will be received and ACK will be returned
0x78 Arbitration lost in SLA+R/W as
Master; General call address has
been received; ACK has been
returned
No TWDR action or
No TWDR action
X
X
0
0
1
1
0
1
Data byte will be received and NOT ACK will be re-
turned
Data byte will be received and ACK will be returned
0x80 Previously addressed with own
SLA+W; data has been received;
ACK has been returned
Read data byte or
Read data byte
X
X
0
0
1
1
0
1
Data byte will be received and NOT ACK will be re-
turned
Data byte will be received and ACK will be returned
0x88 Previously addressed with own
SLA+W; data has been received;
NOT ACK has been returned
Read data byte or
Read data byte or
Read data byte or
Read data byte
0
0
1
1
0
0
0
0
1
1
1
1
0
1
0
1
Switched to the not addressed Slave mode;
no recognition of own SLA or GCA
Switched to the not addressed Slave mode;
own SLA will be recognized;
GCA will be recognized if TWGCE = “1”
Switched to the not addressed Slave mode;
no recognition of own SLA or GCA;
a START condition will be transmitted when the bus
becomes free
Switched to the not addressed Slave mode;
own SLA will be recognized;
GCA will be recognized if TWGCE = “1”;
a START condition will be transmitted when the bus
becomes free
0x90 Previously addressed with
general call; data has been re-
ceived; ACK has been returned
Read data byte or
Read data byte
X
X
0
0
1
1
0
1
Data byte will be received and NOT ACK will be re-
turned
Data byte will be received and ACK will be returned
0x98 Previously addressed with
general call; data has been
received; NOT ACK has been
returned
Read data byte or
Read data byte or
Read data byte or
Read data byte
0
0
1
1
0
0
0
0
1
1
1
1
0
1
0
1
Switched to the not addressed Slave mode;
no recognition of own SLA or GCA
Switched to the not addressed Slave mode;
own SLA will be recognized;
GCA will be recognized if TWGCE = “1”
Switched to the not addressed Slave mode;
no recognition of own SLA or GCA;
a START condition will be transmitted when the bus
becomes free
Switched to the not addressed Slave mode;
own SLA will be recognized;
GCA will be recognized if TWGCE = “1”;
a START condition will be transmitted when the bus
becomes free
0xA0 A STOP condition or repeated
START condition has been
received while still addressed as
Slave
No action 0
0
1
1
0
0
0
0
1
1
1
1
0
1
0
1
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
180
2486Z–AVR–02/11
ATmega8(L)
Figure 83. Formats and States in the Slave Receiver Mode
S SLA W A DATA A
$60 $80
$88
A
$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
n
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
A
$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
181
2486Z–AVR–02/11
ATmega8(L)
Slave Transmitter
Mode
In the Slave Transmitter mode, a number of data bytes are transmitted to a Master Receiver
(see Figure 84). All the status codes mentioned in this section assume that the prescaler bits are
zero or are masked to zero.
Figure 84. Data Transfer in Slave Transmitter Mode
To initiate the Slave Transmitter 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 (0x00),
otherwise it will ignore the general call address.
TWEN must be written to one to enable the TWI. The TWEA bit must be written to one to enable
the acknowledgement of the device’s own slave address or the general call address. TWSTA
and TWSTO must be written to zero.
When TWAR and TWCR have been initialized, the TWI waits until it is addressed by its own
slave address (or the general call address if enabled) followed by the data direction bit. If the
direction bit is “1” (read), the TWI will operate in ST mode, otherwise SR mode is entered. After
its own slave address and the write bit have been received, the TWINT Flag is set and a valid
status code can be read from TWSR. The status code is used to determine the appropriate soft-
ware action. The appropriate action to be taken for each status code is detailed in Table 69 on
page 182. The Slave Transmitter mode may also be entered if arbitration is lost while the TWI is
in the Master mode (see state 0xB0).
If the TWEA bit is written to zero during a transfer, the TWI will transmit the last byte of the trans-
fer. State 0xC0 or state 0xC8 will be entered, depending on whether the Master Receiver
transmits a NACK or ACK after the final byte. The TWI is switched to the not addressed Slave
mode, and will ignore the Master if it continues the transfer. Thus the Master Receiver receives
all “1” as serial data. State 0xC8 is entered if the Master demands additional data bytes (by
transmitting ACK), even though the Slave has transmitted the last byte (TWEA zero and expect-
ing 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 to temporarily isolate the TWI from the Two-wire
Serial Bus.
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
V
CC
Device 2
MASTER
RECEIVER
Device 1
SLAVE
TRANSMITTER
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In all sleep modes other than Idle mode, the clock system to the TWI is turned off. If the TWEA
bit is set, the interface can still acknowledge its own slave address or the general call address by
using the 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.
Table 69. 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
0xA8 Own SLA+R has been received;
ACK has been returned
Load data byte or
Load data byte
X
X
0
0
1
1
0
1
Last data byte will be transmitted and NOT ACK should
be received
Data byte will be transmitted and ACK should be re-
ceived
0xB0 Arbitration lost in SLA+R/W as
Master; own SLA+R has been
received; ACK has been returned
Load data byte or
Load data byte
X
X
0
0
1
1
0
1
Last data byte will be transmitted and NOT ACK should
be received
Data byte will be transmitted and ACK should be re-
ceived
0xB8 Data byte in TWDR has been
transmitted; ACK has been
received
Load data byte or
Load data byte
X
X
0
0
1
1
0
1
Last data byte will be transmitted and NOT ACK should
be received
Data byte will be transmitted and ACK should be re-
ceived
0xC0 Data byte in TWDR has been
transmitted; NOT ACK has been
received
No TWDR action or
No TWDR action or
No TWDR action or
No TWDR action
0
0
1
1
0
0
0
0
1
1
1
1
0
1
0
1
Switched to the not addressed Slave mode;
no recognition of own SLA or GCA
Switched to the not addressed Slave mode;
own SLA will be recognized;
GCA will be recognized if TWGCE = “1”
Switched to the not addressed Slave mode;
no recognition of own SLA or GCA;
a START condition will be transmitted when the bus
becomes free
Switched to the not addressed Slave mode;
own SLA will be recognized;
GCA will be recognized if TWGCE = “1”;
a START condition will be transmitted when the bus
becomes free
0xC8 Last data byte in TWDR has been
transmitted (TWEA = “0”); ACK
has been received
No TWDR action or
No TWDR action or
No TWDR action or
No TWDR action
0
0
1
1
0
0
0
0
1
1
1
1
0
1
0
1
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 85. 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 70.
Status 0xF8 indicates that no relevant information is available because the TWINT Flag is not
set. This occurs between other states, and when the TWI is not involved in a serial transfer.
Status 0x00 indicates that a bus error has occurred during a 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
A
$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
n
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
A
$C8
P or SAll 1's
A
Table 70. 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
0xF8 No relevant state information
available; TWINT = “0”
No TWDR action No TWCR action Wait or proceed current transfer
0x00 Bus error due to an illegal
START or STOP condition
No TWDR action 0 1 1 X Only the internal hardware is affected, no STOP condi-
tion is sent on the bus. In all cases, the bus is released
and TWSTO is cleared.
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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 transmitted both from Master to Slave and vice versa. The Master must instruct
the Slave what location it wants to read, requiring the use of the MT mode. Subsequently, data
must be read from the Slave, implying the use of the MR mode. Thus, the transfer direction must
be changed. The Master must keep control of the bus during all these steps, and the steps
should be carried out as an atomical operation. If this principle is violated in a multimaster 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 ownership of the bus. The following
figure shows the flow in this transfer.
Figure 86. Combining Several TWI Modes to Access a Serial EEPROM
Multi-master
Systems and
Arbitration
If multiple masters are connected to the same bus, transmissions may be initiated simultane-
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 87. An Arbitration Example
Several different scenarios may arise during arbitration, as described below:
• Two or more masters are performing identical communication with the same Slave. In this
case, neither the Slave nor any of the masters will know about the bus contention
Master Transmitter Master Receiver
S = START Rs = REPEATED START P = STOP
Transmitted from master to slave Transmitted from slave to master
S SLA+W A ADDRESS A Rs SLA+R A DATA A P
Device 1
MASTER
TRANSMITTER
Device 2
MASTER
TRANSMITTER
Device 3
SLAVE
RECEIVER
Device n
SDA
SCL
........
R1 R2
V
CC
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• Two or more masters are accessing the same Slave with different data or direction bit. In this
case, arbitration will occur, either in the READ/WRITE bit or in the data bits. The masters
trying to output a one on SDA while another Master outputs a zero will lose the arbitration.
Losing masters will switch to not addressed Slave mode or wait until the bus is free and
transmit a new START condition, depending on application software action
• Two or more masters are accessing different slaves. In this case, arbitration will occur in the
SLA bits. Masters trying to output a one on SDA while another Master outputs a zero will
lose the arbitration. Masters losing arbitration in SLA will switch to Slave mode to check if
they are being addressed by the winning Master. If addressed, they will switch to SR or ST
mode, depending on the value of the READ/WRITE bit. If they are not being addressed, they
will switch to not addressed Slave mode or wait until the bus is free and transmit a new
START condition, depending on application software action
This is summarized in Figure 88. Possible status values are given in circles.
Figure 88. 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
No
Arbitration lost in Data
Direction
Yes
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
B0
68/78
38
SLASTART Data STOP
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Analog
Comparator
The Analog Comparator 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. The 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 89.
Figure 89. Analog Comparator Block Diagram(2)
Notes: 1. See Table 72 on page 188
2. Refer to “Pin Configurations” on page 2 and Table 28 on page 63 for Analog Comparator pin
placement
Special Function IO
Register – SFIOR
• 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 188.
Analog Comparator
Control and Status
Register – ACSR
ACBG
BANDGAP
REFERENCE
ADC MULTIPLEXER
OUTPUT
ACME
ADEN
(1)
Bit 7 6 5 4 3 2 1 0
– – – – ACME PUD PSR2 PSR10 SFIOR
Read/Write R R R R R/W R/W R/W R/W
Initial Value 0 0 0 0 0 0 0 0
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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• Bit 7 – ACD: Analog Comparator Disable
When this bit is written logic one, the power to the Analog Comparator is switched off. This bit
can be set at any time to turn off the Analog Comparator. This will reduce power consumption in
Active and Idle mode. When changing the ACD bit, the Analog Comparator Interrupt must be
disabled by clearing the ACIE bit in ACSR. Otherwise an interrupt can occur when the bit is
changed.
• Bit 6 – ACBG: Analog Comparator Bandgap Select
When this bit is set, a fixed bandgap reference voltage replaces the positive input to the Analog
Comparator. When this bit is cleared, AIN0 is applied to the positive input of the Analog 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 and ACIS0. The Analog Comparator Interrupt routine is executed if the ACIE bit is set
and the I-bit in SREG is set. ACI is cleared by hardware when executing the corresponding inter-
rupt Handling Vector. Alternatively, ACI is cleared by writing a logic one to the flag.
• Bit 3 – ACIE: Analog Comparator Interrupt Enable
When the ACIE bit is written logic one and the I-bit in the Status Register is set, the Analog 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 Analog Comparator. The comparator output is in this case directly connected to the
Input Capture front-end logic, making the comparator utilize the noise canceler and edge select
features of the Timer/Counter1 Input Capture interrupt. When written logic zero, no connection
between the Analog Comparator and the Input Capture function exists. To make the comparator
trigger the Timer/Counter1 Input Capture interrupt, the TICIE1 bit in the Timer Interrupt Mask
Register (TIMSK) must be set.
• Bits 1,0 – ACIS1, ACIS0: Analog Comparator Interrupt Mode Select
These bits determine which comparator events that trigger the Analog Comparator interrupt. The
different settings are shown in Table 71.
When changing the ACIS1/ACIS0 bits, the Analog Comparator Interrupt must be disabled by
clearing its Interrupt Enable bit in the ACSR Register. Otherwise an interrupt can occur when the
bits are changed.
Table 71. 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 Rising Output Edge
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ATmega8(L)
Analog
Comparator
Multiplexed Input
It is possible to select any of the ADC7..0(1) pins to replace the negative input to the Analog
Comparator. The ADC multiplexer is used to select this input, and consequently the ADC must
be switched off to utilize this feature. If the Analog Comparator Multiplexer Enable bit (ACME in
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
72. If ACME is cleared or ADEN is set, AIN1 is applied to the negative input to the Analog
Comparator.
Note: 1. ADC7..6 are only available in TQFP and QFN/MLF Package
Table 72. Analog Comparator Multiplexed Input(1)
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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ATmega8(L)
Analog-to-
Digital
Converter
Features •10-bit Resolution
•0.5 LSB Integral Non-linearity
•±2 LSB Absolute Accuracy
•13µs - 260µs Conversion Time
•Up to 15 kSPS at Maximum Resolution
•6 Multiplexed Single Ended Input Channels
•2 Additional Multiplexed Single Ended Input Channels (TQFP and QFN/MLF Package only)
•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
•Interrupt on ADC Conversion Complete
•Sleep Mode Noise Canceler
The ATmega8 features a 10-bit successive approximation ADC. The ADC is connected to an 8-
channel Analog Multiplexer which allows eight single-ended voltage inputs constructed from the
pins of Port C. The single-ended voltage inputs refer to 0V (GND).
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 90 on
page 190.
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 195 on how to connect this pin.
Internal reference voltages of nominally 2.56V or AVCC are provided On-chip. The voltage refer-
ence may be externally decoupled at the AREF pin by a capacitor for better noise performance.
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Figure 90. Analog to Digital Converter Block Schematic 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 con-
nected to the AREF pin by writing to the REFSn bits in the ADMUX Register. The internal
voltage reference may thus be decoupled by an external capacitor at the AREF pin to improve
noise immunity.
The analog input channel is selected by writing to the MUX bits in ADMUX. Any of the ADC input
pins, as well as GND and a fixed bandgap voltage reference, can be selected as single ended
inputs to the ADC. The ADC is enabled by setting the ADC Enable bit, ADEN in ADCSRA. Volt-
age reference and input channel selections will not go into effect until ADEN is set. The ADC
does not consume power when ADEN is cleared, so it is recommended to switch off the ADC
before entering power saving sleep modes.
The ADC generates a 10-bit result which is presented in the ADC Data Registers, ADCH and
ADCL. By default, the result is presented right adjusted, but can optionally be presented left
adjusted by setting the ADLAR bit in ADMUX.
ADC CONVERSION
COMPLETE IRQ
8-BIT DATA BUS
15 0
ADC MULTIPLEXER
SELECT (ADMUX) ADC CTRL. & STATUS
REGISTER (ADCSRA) ADC DATA REGISTER
(ADCH/ADCL)
MUX2
ADIE
ADFR
ADSC
ADEN
ADIF ADIF
MUX1
MUX0
ADPS0
ADPS1
ADPS2
MUX3
CONVERSION LOGIC
10-BIT DAC
+
-
SAMPLE & HOLD
COMPARATOR
INTERNAL 2.56V
REFERENCE
MUX DECODER
AVCC
ADC7
ADC6
ADC5
ADC4
ADC3
ADC2
ADC1
ADC0
REFS0
REFS1
ADLAR
CHANNEL SELECTION
ADC[9:0]
ADC MULTIPLEXER
OUTPUT
AREF
BANDGAP
REFERENCE
PRESCALER
GND
INPUT
MUX
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If the result is left adjusted and no more than 8-bit precision is required, it is sufficient to read
ADCH. Otherwise, ADCL must be read first, then ADCH, to ensure that the content of the Data
Registers belongs to the same conversion. Once ADCL is read, ADC access to Data Registers
is blocked. This means that if ADCL has been read, and a conversion completes before ADCH is
read, neither register is updated and the result from the conversion is lost. When ADCH is read,
ADC access to the ADCH and ADCL Registers is re-enabled.
The ADC has its own interrupt which can be triggered when a conversion completes. When ADC
access to the Data Registers is prohibited between reading of ADCH and ADCL, the interrupt
will trigger even if the result is lost.
Starting a
Conversion
A single conversion is started by writing a logical one to the ADC Start Conversion bit, ADSC.
This bit stays high as long as the conversion is in progress and will be cleared by hardware
when the conversion is completed. If a different data channel is selected while a conversion is in
progress, the ADC will finish the current conversion before performing the channel change.
In Free Running mode, the ADC is constantly sampling and updating the ADC Data Register.
Free Running mode is selected by writing the ADFR bit in ADCSRA to one. 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.
Prescaling and
Conversion Timing
Figure 91. ADC Prescaler
By default, the successive approximation circuitry requires an input clock frequency between
50kHz and 200kHz to get maximum resolution. If a lower resolution than 10 bits is needed, the
input clock frequency to the ADC can be higher than 200kHz to get a higher sample rate.
The ADC module contains a prescaler, which generates an acceptable ADC clock frequency
from any CPU frequency above 100kHz. The prescaling is set by the ADPS bits in ADCSRA.
The prescaler starts counting from the moment the ADC is switched on by setting the ADEN bit
in ADCSRA. The prescaler keeps running for as long as the ADEN bit is set, and is continuously
reset when ADEN is low.
When initiating a single ended conversion by setting the ADSC bit in ADCSRA, the conversion
starts at the following rising edge of the ADC clock cycle. 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.
7-BIT ADC PRESCALER
ADC CLOCK SOURCE
CK
ADPS0
ADPS1
ADPS2
CK/128
CK/2
CK/4
CK/8
CK/16
CK/32
CK/64
Reset
ADEN
START
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The actual sample-and-hold takes place 1.5 ADC clock cycles after the start of a normal conver-
sion and 13.5 ADC clock cycles after the start of an first conversion. When a conversion is
complete, the result is written to the ADC Data Registers, and ADIF is set. In single conversion
mode, ADSC is cleared simultaneously. The software may then set ADSC again, and a new
conversion will be initiated on the first rising ADC clock edge.
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 73 on page
193.
Figure 92. ADC Timing Diagram, First Conversion (Single Conversion Mode)
Figure 93. 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
3
MUX and REFS
Update
MUX and REFS
Update
Conversion
Complete
12 3 4 5 6 7 8 9 10 11 12 13
MSB of Result
LSB of Result
ADC Clock
ADSC
ADIF
ADCH
ADCL
Cycle Number 12
One Conversion Next Conversion
3
Sample & Hold
MUX and REFS
Update
Conversion
Complete MUX and REFS
Update
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Figure 94. ADC Timing Diagram, Free Running Conversion
Table 73. ADC Conversion Time
Condition
Sample & Hold (Cycles
from Start of Conversion)
Conversion Time
(Cycles)
Extended conversion 13.5 25
Normal conversions, single ended 1.5 13
11 12 13
MSB of Result
LSB of Result
ADC Clock
ADSC
ADIF
ADCH
ADCL
Cycle Number 12
One Conversion Next Conversion
34
Conversion
Complete
Sample &Hold
MUX and REFS
Update
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Changing Channel
or Reference
Selection
The MUXn and REFS1:0 bits in the ADMUX Register are single buffered through a temporary
register to which the CPU has random access. This ensures that the channels and reference
selection only takes place at a safe point during the conversion. The channel and reference
selection is continuously updated until a conversion is started. Once the conversion starts, the
channel and reference selection is locked to ensure a sufficient sampling time for the ADC. Con-
tinuous updating resumes in the last ADC clock cycle before the conversion completes (ADIF in
ADCSRA is set). Note that the conversion starts on the following rising ADC clock edge after
ADSC is written. The user is thus advised not to write new channel or reference selection values
to ADMUX until one ADC clock cycle after ADSC is written.
If both ADFR and ADEN is written to one, an interrupt event can occur at any time. If the
ADMUX Register is changed in this period, the user cannot tell if the next conversion is based
on the old or the new settings. ADMUX can be safely updated in the following ways:
1. When ADFR or ADEN is cleared
2. During conversion, minimum 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.
ADC Input Channels When changing channel selections, the user should observe the following guidelines to ensure
that the correct channel is selected:
In Single Conversion mode, always select the channel before starting the conversion. The chan-
nel selection may be changed one ADC clock cycle after writing one to ADSC. However, the
simplest method is to wait for the conversion to complete before changing the channel selection.
In Free Running mode, always select the channel before starting the first conversion. The 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 channel
selection. Since the next conversion has already started automatically, the next result will reflect
the previous channel selection. Subsequent conversions will reflect the new channel selection.
ADC Voltage
Reference
The reference voltage for the ADC (VREF) indicates the conversion range for the ADC. Single
ended channels that exceed VREF will result in codes close to 0x3FF. VREF can be selected as
either AVCC, internal 2.56V reference, or external AREF pin.
AVCC is connected to the ADC through a passive switch. The internal 2.56V reference is gener-
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 ground. VREF can
also be measured at the AREF pin with a high impedant voltmeter. Note that VREF is a high
impedant source, and only a capacitive load should be connected in a system.
If the user has a fixed voltage source connected to the AREF pin, the user may not use the other
reference voltage options in the application, as they will be shorted to the external voltage. If no
external voltage is applied to the AREF pin, the user may switch between AVCC and 2.56V as
reference selection. The first ADC conversion result after switching reference voltage source
may be inaccurate, and the user is advised to discard this result.
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ATmega8(L)
ADC Noise
Canceler
The ADC features a noise canceler that enables conversion during sleep mode to reduce noise
induced from the CPU core and other I/O peripherals. The noise canceler can be used with ADC
Noise Reduction and Idle mode. To make use of this feature, the following procedure should be
used:
1. Make sure that the ADC is enabled and is not busy converting. Single Conversion
mode must be selected and the ADC conversion complete interrupt must be enabled
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 wake up the CPU and execute the ADC Conversion Complete interrupt routine. If
another interrupt wakes up the CPU before the ADC conversion is complete, that
interrupt will be executed, and an ADC Conversion Complete interrupt request will be
generated when the ADC conversion completes. The CPU will remain in Active mode
until a new sleep command is executed
Note that the ADC will not be automatically turned off when entering other sleep modes than Idle
mode and ADC Noise Reduction mode. The user is advised to write zero to ADEN before enter-
ing such sleep modes to avoid excessive power consumption.
Analog Input Circuitry The analog input circuitry for single ended channels is illustrated in Figure 95. An analog source
applied to ADCn is subjected to the pin capacitance and input leakage of that pin, regardless of
whether that channel is selected as input for the ADC. When the channel is selected, the source
must drive the S/H capacitor through the series resistance (combined resistance in the input
path).
The ADC is optimized for analog signals with an output impedance of approximately 10 kΩ or
less. If such a source is used, the sampling time will be negligible. If a source with higher imped-
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.
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 95. Analog Input Circuitry
ADCn
I
IH
1..100kΩ
C
S/H
= 14pF
V
CC
/2
I
IL
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ATmega8(L)
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. Make sure analog tracks run over the
ground plane, and 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 voltage via
an LC network as shown in Figure 96.
3. Use the ADC noise canceler function to reduce induced noise from the CPU.
4. If any ADC [3..0] port pins are used as digital outputs, it is essential that these do not
switch while a conversion is in progress. However, using the Two-wire Interface
(ADC4 and ADC5) will only affect the conversion on ADC4 and ADC5 and not the
other ADC channels.
Figure 96. ADC Power Connections
ADC Accuracy
Definitions
An n-bit single-ended ADC converts a voltage linearly between GND and VREF in 2n steps
(LSBs). The lowest code is read as 0, and the highest code is read as 2n-1.
Several parameters describe the deviation from the ideal behavior:
• Offset: The deviation of the first transition (0x000 to 0x001) compared to the ideal transition
(at 0.5 LSB). Ideal value: 0 LSB
GND
VCC
PC5 (ADC5/SCL)
PC4 (ADC4/SDA)
PC3 (ADC3)
PC2 (ADC2)
PC1 (ADC1)
PC0 (ADC0)
ADC7
GND
AREF
AVCC
ADC6
PB5
10μH
100nF Analog Ground Plane
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Figure 97. 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 98. Gain Error
Output Code
V
REF
Input Voltage
Ideal ADC
Actual ADC
Offset
Error
Output Code
VREF Input Voltage
Ideal ADC
Actual ADC
Gain
Error
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• 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
Figure 99. Integral Non-linearity (INL)
• Differential Non-linearity (DNL): The maximum deviation of the actual code width (the
interval between two adjacent transitions) from the ideal code width (1 LSB). Ideal value: 0
LSB.
Figure 100. Differential Non-linearity (DNL)
• Quantization Error: Due to the quantization of the input voltage into a finite number of codes,
a range of input voltages (1 LSB wide) will code to the same value. Always ±0.5 LSB.
• Absolute accuracy: The maximum deviation of an actual (unadjusted) transition compared to
an ideal transition for any code. This is the compound effect of offset, gain error, differential
error, non-linearity, and quantization error. Ideal value: ±0.5 LSB.
Output Code
VREF Input Voltage
Ideal ADC
Actual ADC
INL
Output Code
0x3FF
0x000
0VREF Input Voltage
DNL
1 LSB
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ATmega8(L)
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 74 and Table 75). 0x000 represents ground, and 0x3FF represents the selected reference
voltage minus one LSB.
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 74. If these bits are
changed during a conversion, the change will not go in effect until this conversion is complete
(ADIF in ADCSRA is set). The internal voltage reference options may not be used if an external
reference voltage is being applied to the AREF pin.
• Bit 5 – ADLAR: ADC Left Adjust Result
The ADLAR bit affects the presentation of the ADC conversion result in the ADC Data Register.
Write one to ADLAR to left adjust the result. Otherwise, the result is right adjusted. Changing the
ADLAR bit will affect the ADC Data Register immediately, regardless of any ongoing conver-
sions. For a complete description of this bit, see “The ADC Data Register – ADCL and ADCH” on
page 201.
• Bits 3:0 – MUX3:0: Analog Channel Selection Bits
The value of these bits selects which analog inputs are connected to the ADC. See Table 75 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).
ADC VIN 1024⋅
VREF
--------------------------=
Bit 76543210
REFS1 REFS0 ADLAR – MUX3 MUX2 MUX1 MUX0 ADMUX
Read/Write R/W R/W R/W R R/W R/W R/W R/W
Initial Value00000000
Table 74. Voltage Reference Selections for ADC
REFS1 REFS0 Voltage Reference Selection
0 0 AREF, Internal Vref turned off
01
AVCC with external capacitor at AREF pin
10Reserved
1 1 Internal 2.56V Voltage Reference with external capacitor at AREF pin
Table 75. Input Channel Selections
MUX3..0 Single Ended Input
0000 ADC0
0001 ADC1
0010 ADC2
0011 ADC3
0100 ADC4
0101 ADC5
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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 mode, write this bit to one to start each conversion. In Free Running mode,
write this bit to one to start the first conversion. The first conversion after ADSC has been written
after the ADC has been enabled, or if ADSC is written at the same time as the ADC is enabled,
will take 25 ADC clock cycles instead of the normal 13. This first conversion performs 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 – ADFR: ADC Free Running Select
When this bit is set (one) the ADC operates in Free Running mode. In this mode, the ADC sam-
ples and updates the Data Registers continuously. Clearing this bit (zero) will terminate Free
Running mode.
• Bit 4 – ADIF: ADC Interrupt Flag
This bit is set when an ADC conversion completes and the Data Registers are updated. The
ADC Conversion Complete Interrupt is executed if the ADIE bit and the I-bit in SREG are set.
ADIF is cleared by hardware when executing the corresponding interrupt Handling Vector. 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 are used.
• Bit 3 – ADIE: ADC Interrupt Enable
When this bit is written to one and the I-bit in SREG is set, the ADC Conversion Complete Inter-
rupt is activated.
0110 ADC6
0111 ADC7
1000
1001
1010
1011
1100
1101
1110 1.30V (VBG)
1111 0V (GND)
Table 75. Input Channel Selections (Continued)
MUX3..0 Single Ended Input
Bit 76543210
ADEN ADSC ADFR 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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• 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.
The ADC Data
Register – ADCL and
ADCH
ADLAR = 0
ADLAR = 1
When an ADC conversion is complete, the result is found in these two registers.
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 ADLAR is set, the result is left adjusted. If ADLAR is cleared (default), the result
is right adjusted.
• ADC9:0: ADC Conversion result
These bits represent the result from the conversion, as detailed in “ADC Conversion Result” on
page 199.
Table 76. 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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ATmega8(L)
Boot Loader
Support – Read-
While-Write
Self-
Programming
The Boot Loader Support provides a real Read-While-Write Self-Programming mechanism for
downloading and uploading program code by the MCU itself. This feature allows flexible 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 read 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 entire
Flash, including the Boot Loader Memory. The Boot Loader can thus even modify itself, and it
can also erase itself from the code if the feature is not needed anymore. The size of the Boot
Loader Memory is configurable with fuses and the Boot Loader has two separate sets of Boot
Lock Bits which can be set independently. This gives the user a unique flexibility to select differ-
ent levels of protection.
Boot Loader
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 89 on page 218) 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 section (see Figure 102 on page 204). The size of the different sections is configured by
the BOOTSZ Fuses as shown in Table 82 on page 213 and Figure 102 on page 204. These two
sections can have different level of protection since they have different sets of Lock Bits.
Application Section The application section is the section of the Flash that is used for storing the application code.
The protection level for the application section can be selected by the application boot Lock Bits
(Boot Lock Bits 0), see Table 78 on page 205. 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 79 on page 205.
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 dependent on which address that is being programmed. In addition to the two
sections that are configurable by the BOOTSZ Fuses as described above, the Flash is also
divided into two fixed sections, the Read-While-Write (RWW) section and the No Read-While-
Write (NRWW) section. The limit between the RWW- and NRWW sections is given in Table 83
on page 214 and Figure 102 on page 204. The main difference between the two sections is:
• When erasing or writing a page located inside the RWW section, the NRWW section can be
read during the operation
• When erasing or writing a page located inside the NRWW section, the CPU is halted during
the entire operation
Note that the user software can never read any code that is located inside the RWW section 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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ATmega8(L)
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/rjmp/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 Control Register – SPMCR” on page 206. 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 entire page erase or page write operation.
Figure 101. Read-While-Write vs. No Read-While-Write
Table 77. 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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ATmega8(L)
Figure 102. Memory Sections(1)
Note: 1. The parameters in the figure are given in Table 82 on page 213
Boot Loader Lock
Bits
If no Boot Loader capability is needed, the entire Flash is available for application code. The
Boot Loader has two separate sets of Boot Lock Bits which can be set independently. This gives
the user a unique flexibility to select different levels of protection.
The user can select:
• To protect the entire Flash from a software update by the MCU
• To protect only the Boot Loader Flash section from a software update by the MCU
• To protect only the Application Flash section from a software update by the MCU
• Allow software update in the entire Flash
See Table 78 on page 205 and Table 79 on page 205 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 Write Lock (Lock bit mode 2) does not control the program-
ming of the Flash memory 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
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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ATmega8(L)
Note: 1. “1” means unprogrammed, “0” means programmed
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 trigger such as a command received via USART, or SPI interface. Alternatively,
the Boot Reset Fuse can be programmed so that the Reset Vector is pointing to the Boot Flash
start address after a reset. In this case, the Boot Loader is started after a reset. After the applica-
tion code is loaded, the program can start executing the application code. Note that the fuses
cannot be changed by the MCU itself. This means that once the Boot Reset Fuse is 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 78. Boot Lock Bit0 Protection Modes (Application Section)(1)
BLB0
Mode
BLB02
Mode
BLB01
Mode Protection
11 1
No restrictions for SPM or LPM accessing the Application
section
2 1 0 SPM is not allowed to write to the Application section
30 0
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
40 1
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 79. Boot Lock Bit1 Protection Modes (Boot Loader Section)(1)
BLB1
Mode
BLB12
Mode
BLB11
Mode Protection
11 1
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
30 0
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 executing
from the Boot Loader section
40 1
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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ATmega8(L)
Note: 1. “1” means unprogrammed, “0” means programmed
Store Program
Memory Control
Register – SPMCR
The Store Program memory Control Register contains the control bits needed to control the Boot
Loader operations.
• Bit 7 – SPMIE: SPM Interrupt Enable
When the SPMIE bit is written to one, and the I-bit in the Status Register is set (one), the SPM
ready interrupt will be enabled. The SPM ready Interrupt will be executed as long as the SPMEN
bit in the SPMCR Register is cleared.
• Bit 6 – RWWSB: Read-While-Write Section Busy
When a Self-Programming (page erase or page write) operation to the RWW section is initiated,
the RWWSB will be set (one) by hardware. When the RWWSB bit is set, the RWW section 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 ATmega8 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 (The page buffer will be cleared when the Read-While-Write section is re-enabled).
• Bit 3 – BLBSET: Boot Lock Bit Set
If this bit is written to one at the same time as SPMEN, the next SPM instruction within four clock
cycles sets Boot Lock Bits, according to the data in R0. The data in R1 and the address in the Z-
pointer are ignored. The BLBSET bit will automatically be cleared upon completion of the lock bit
set, or if no SPM instruction is executed within four clock cycles.
An LPM instruction within three cycles after BLBSET and SPMEN are set in the SPMCR Regis-
ter, will read either the Lock Bits or the Fuse Bits (depending on Z0 in the Z-pointer) into the
destination register. See “Reading the Fuse and Lock Bits from Software” on page 210 for
details.
Table 80. Boot Reset Fuse(1)
BOOTRST Reset Address
1 Reset Vector = Application Reset (address 0x0000)
0 Reset Vector = Boot Loader Reset (see Table 82 on page 213)
Bit 7 65 4 3210
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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• Bit 2 – PGWRT: Page Write
If this bit is written to one at the same time as SPMEN, the next SPM instruction within four clock
cycles executes page write, with the data stored in the temporary buffer. The page address is
taken from the high part of the Z-pointer. The data in R1 and R0 are ignored. The PGWRT bit
will auto-clear upon completion of a page write, or if no SPM instruction is executed within four
clock cycles. The CPU is halted during the entire page write operation if the NRWW section is
addressed.
• Bit 1 – PGERS: Page Erase
If this bit is written to one at the same time as SPMEN, the next SPM instruction within four clock
cycles executes page erase. The page address is taken from the high part of the Z-pointer. The
data in R1 and R0 are ignored. The PGERS bit will auto-clear upon completion of a page erase,
or if no SPM instruction is executed within four clock cycles. The CPU is halted during the entire
page write operation if the NRWW section is addressed.
• Bit 0 – SPMEN: Store Program Memory Enable
This bit enables the SPM instruction for the next four clock cycles. If written to one together with
either RWWSRE, BLBSET, PGWRT’ or PGERS, the following SPM instruction will have a 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 Table 89 on page 218), the Program Counter can be
treated as having two different sections. One section, consisting of the least significant bits, is
addressing the words within a page, while the most significant bits are addressing the pages.
This is shown in Figure 103 on page 208. Note that the page erase and page write operations
are addressed independently. Therefore it is of major importance that the Boot Loader software
addresses the same page in both the page erase and page write operation. Once a program-
ming operation is initiated, the address is latched and the Z-pointer can be used for other
operations.
The only SPM operation that does not 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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Figure 103. Addressing the Flash during SPM(1)
Notes: 1. The different variables used in the figure are listed in Table 84 on page 214
2. PCPAGE and PCWORD are listed in Table 89 on page 218
Self-Programming
the Flash
The Program memory is updated in a page by page fashion. Before programming a page with
the data stored in the temporary page buffer, the page must be erased. The temporary page buf-
fer is filled one word at a time using SPM and the buffer can be filled either before the page
erase command or between a page erase and a page write operation:
Alternative 1, fill the buffer before a page erase.
• Fill temporary page buffer
• Perform a page erase
• Perform a page write
Alternative 2, fill the buffer after page erase.
• Perform a page erase
• Fill temporary page buffer
• Perform a page write
If only a part of the page needs to be changed, the rest of the page must be stored (for example
in the temporary page buffer) before the erase, and then be rewritten. When using alternative 1,
the boot loader provides an effective Read-Modify-Write feature which allows the user software
to first read the page, do the necessary changes, and then write back the modified data. If alter-
native 2 is used, it is not possible to read the old data while loading since the page is already
erased. The temporary page buffer can be accessed in a random sequence. It is essential that
the page address used in both the page erase and page write operation is addressing the same
page. See “Simple Assembly Code Example for a Boot Loader” on page 212 for an assembly
code example.
PROGRAM MEMORY
0115
Z - REGISTER
BIT
0
ZPAGEMSB
WORD ADDRESS
WITHIN A PAGE
PAGE ADDRESS
WITHIN THE FLASH
ZPCMSB
INSTRUCTION WORD
PAGE PCWORD[PAGEMSB:0]:
00
01
02
PAGEEND
PAGE
PCWORDPCPAGE
PCMSB PAGEMSB
PROGRAM
COUNTER
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ATmega8(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 will
be ignored during this operation.
• Page Erase to the RWW section: The NRWW section can be read during the page erase
• Page Erase to the NRWW section: The CPU is halted during the operation
Note: If an interrupt occurs in the timed sequence, the four cycle access cannot be guaranteed.
In order to ensure atomic operation, disable interrupts before writing to SPMCSR.
Filling the Temporary
Buffer (Page Loading)
To write an instruction word, set up the address in the Z-pointer and data in R1:R0, write
“00000001” to SPMCR and execute SPM within four clock cycles after writing SPMCR. The con-
tent of PCWORD in the Z-register is used to address the data in the temporary page buffer. 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 System Reset. Note that it is not possible to write more 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 to
zero during this operation.
• Page Write to the RWW section: The NRWW section can be read during the page write
• Page Write to the NRWW section: The CPU is halted during the operation
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 the SPM interrupt, the Interrupt Vectors should be
moved to the BLS section to avoid that an interrupt is accessing the RWW section when it is
blocked for reading. How to move the interrupts is described in “Interrupts” on page 46.
Consideration While
Updating BLS
Special care must be taken if the user allows the Boot Loader section to be updated by leaving
Boot Lock bit11 unprogrammed. An accidental write to the Boot Loader itself can corrupt the
entire Boot Loader, and further software updates might be impossible. If it is not necessary to
change the Boot Loader software itself, it is recommended to program the Boot Lock bit11 to
protect the Boot Loader software from any internal software changes.
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 46, 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 212 for
an example.
Setting the Boot
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 writing 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.
Bit 76543210
R0 1 1 BLB12 BLB11 BLB02 BLB01 1 1
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ATmega8(L)
See Table 78 on page 205 and Table 79 on page 205 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 0x0001 (same as used for reading the Lock Bits). For future compatibility
It is also recommended to set bits 7, 6, 1, and 0 in R0 to “1” when writing the Lock Bits. When
programming the Lock Bits the entire Flash can be read during the operation.
EEPROM Write
Prevents 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 the Fuse and
Lock Bits from
Software
It is possible to read both the Fuse and Lock Bits from software. To read the Lock Bits, load the
Z-pointer with 0x0001 and set the BLBSET and SPMEN bits in SPMCR. When an LPM instruc-
tion 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 reading the Fuse Low bits is similar to the one described above for reading the
Lock Bits. To read the Fuse Low bits, load the Z-pointer with 0x0000 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 88 on page 217 for a detailed descrip-
tion and mapping of the fuse low bits.
Similarly, when reading the Fuse High bits, load 0x0003 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 87 on page 216 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 program can be corrupted because the supply voltage is too
low for the CPU and the Flash to operate properly. These issues are the same as for board level
systems using the Flash, and the same design solutions should be applied.
A Flash program corruption can be caused by two situations when the voltage is too low. First, a
regular write sequence to the Flash requires a minimum voltage to operate correctly. Secondly,
the CPU itself can execute instructions incorrectly, if the supply voltage for executing instructions
is too low.
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
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Flash corruption can easily be avoided by following these design recommendations (one is
sufficient):
1. If there is no need for a Boot Loader update in the system, program the Boot Loader Lock
Bits to prevent any Boot Loader software updates
2. Keep the AVR RESET active (low) during periods of insufficient power supply voltage.
This can be done by enabling the internal Brown-out Detector (BOD) if the operating 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 progress, the write operation will be
completed provided that the power supply voltage is sufficient
3. Keep the AVR core in Power-down sleep mode during periods of low VCC. This will pre-
vent the CPU from attempting to decode and execute instructions, effectively 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 81 shows the typical pro-
gramming time for Flash accesses from the CPU.
Note: 1. Minimum and maximum programming time is per individual operation
Table 81. SPM Programming Time(1)
Symbol Min Programming Time Max Programming Time
Flash write (page erase, page write,
and write Lock Bits by SPM) 3.7ms 4.5ms
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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)
rcallDo_spm
; re-enable the RWW section
ldi spmcrval, (1<<RWWSRE) | (1<<SPMEN)
rcallDo_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)
rcallDo_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)
rcallDo_spm
; re-enable the RWW section
ldi spmcrval, (1<<RWWSRE) | (1<<SPMEN)
rcallDo_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
rjmp Error
sbiw loophi:looplo, 1 ;use subi for PAGESIZEB<=256
brne Rdloop
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ATmega8(L)
; return to RWW section
; verify that RWW section is safe to read
Return:
in temp1, SPMCR
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)
rcallDo_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
ATmega8 Boot Loader
Parameters
In Table 82 through Table 84 on page 214, the parameters used in the description of the self
programming are given.
Note: The different BOOTSZ Fuse configurations are shown in Figure 102 on page 204
Table 82. Boot Size Configuration
BOOTSZ1 BOOTSZ0
Boot
Size Pages
Application
Flash
Section
Boot
Loader
Flash
Section
End
Application
Section
Boot Reset
Address
(Start Boot
Loader
Section)
11
128
words 40x000 -
0xF7F
0xF80 -
0xFFF 0xF7F 0xF80
10
256
words 80x000 -
0xEFF
0xF00 -
0xFFF 0xEFF 0xF00
01
512
words 16 0x000 -
0xDFF
0xE00 -
0xFFF 0xDFF 0xE00
00
1024
words 32 0x000 -
0xBFF
0xC00 -
0xFFF 0xBFF 0xC00
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ATmega8(L)
For details about these two section, see “NRWW – No Read-While-Write Section” on page 203
and “RWW – Read-While-Write Section” on page 203.
Note: 1. Z15:Z13: 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 207 for details about the use of
Z-pointer during Self-Programming
Table 83. Read-While-Write Limit
Section Pages Address
Read-While-Write section (RWW) 96 0x000 - 0xBFF
No Read-While-Write section (NRWW) 32 0xC00 - 0xFFF
Table 84. Explanation of Different Variables used in Figure 103 on page 208 and the Mapping
to the Z-pointer
Variable
Corresponding
Z-value(1) Description
PCMSB 11 Most significant bit in the Program Counter.
(The Program Counter is 12 bits PC[11:0])
PAGEMSB 4 Most significant bit which is used to address the
words within one page (32 words in a page
requires 5 bits PC [4:0])
ZPCMSB Z12 Bit in Z-register that is mapped to PCMSB.
Because Z0 is not used, the ZPCMSB equals
PCMSB + 1
ZPAGEMSB Z5 Bit in Z-register that is mapped to PAGEMSB.
Because Z0 is not used, the ZPAGEMSB
equals PAGEMSB + 1
PCPAGE PC[11:5] Z12:Z6 Program counter page address: Page select,
for page erase and page write
PCWORD PC[4:0] Z5:Z1 Program counter word address: Word select, for
filling temporary buffer (must be zero during
page write operation)
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Memory
Programming
Program And Data
Memory Lock Bits
The ATmega8 provides six Lock Bits which can be left unprogrammed (“1”) or can be pro-
grammed (“0”) to obtain the additional features listed in Table 86. The Lock Bits can only be
erased to “1” with the Chip Erase command.
Note: 1. “1” means unprogrammed, “0” means programmed
Table 85. Lock Bit Byte
Lock Bit Byte Bit No. Description Default Value(1)
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 86. Lock Bit Protection Modes(2)
Memory Lock Bits 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 Serial Programming mode. The
Fuse Bits are locked in both Serial and Parallel
Programming mode (1)
300
Further programming and verification of the Flash and
EEPROM is disabled in parallel and Serial Programming
mode. The Fuse Bits are locked in both Serial and Parallel
Programming modes (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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Notes: 1. Program the Fuse Bits before programming the Lock Bits
2. “1” means unprogrammed, “0” means programmed
Fuse Bits The ATmega8 has two fuse bytes. Table 87 and Table 88 on page 217 describe briefly the func-
tionality of all the fuses and how they are mapped into the fuse bytes. Note that the fuses are
read as logical zero, “0”, if they are programmed.
Notes: 1. The SPIEN Fuse is not accessible in Serial Programming mode
2. The CKOPT Fuse functionality depends on the setting of the CKSEL bits, see “Clock Sources”
on page 26 for details
3. The default value of BOOTSZ1..0 results in maximum Boot Size. See Table 82 on page 213
4. When programming the RSTDISBL Fuse Parallel Programming has to be used to change
fuses or perform further programming
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 executing 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 86. Lock Bit Protection Modes(2) (Continued)
Memory Lock Bits Protection Type
Table 87. Fuse High Byte
Fuse High
Byte
Bit
No. Description Default Value
RSTDISBL(4) 7Select if PC6 is I/O pin or RESET pin
1 (unprogrammed, PC6 is
RESET-pin)
WDTON 6 WDT always on
1 (unprogrammed, WDT
enabled by WDTCR)
SPIEN(1) 5Enable 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 82 on
page 213 for details) 0 (programmed)(3)
BOOTSZ0 1 Select Boot Size (see Table 82 on
page 213 for details) 0 (programmed)(3)
BOOTRST 0 Select Reset Vector 1 (unprogrammed)
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Notes: 1. The default value of SUT1..0 results in maximum start-up time. See Table 10 on page 30 for
details
2. The default setting of CKSEL3..0 results in internal RC Oscillator @ 1MHz. See Table 2 on
page 26 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.
Table 88. 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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Signature Bytes All Atmel microcontrollers have a 3-byte signature code which identifies the device. This code
can be read in both Serial and Parallel mode, also when the device is locked. The three bytes
reside in a separate address space.
For the ATmega8 the signature bytes are:
1. 0x000: 0x1E (indicates manufactured by Atmel)
2. 0x001: 0x93 (indicates 8KB Flash memory)
3. 0x002: 0x07 (indicates ATmega8 device)
Calibration Byte The ATmega8 stores four different calibration values for the internal RC Oscillator. These bytes
resides in the signature row High byte of the addresses 0x0000, 0x0001, 0x0002, and 0x0003
for 1MHz, 2MHz, 4MHz, and 8Mhz respectively. During Reset, the 1MHz value is automatically
loaded into the OSCCAL Register. If other frequencies are used, the calibration value has to be
loaded manually, see “Oscillator Calibration Register – OSCCAL” on page 31 for details.
Page Size
Table 89. No. of Words in a Page and no. of Pages in the Flash
Flash Size Page Size PCWORD No. of Pages PCPAGE PCMSB
4K words (8 Kbytes) 32 words PC[4:0] 128 PC[11:5] 11
Table 90. 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
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ATmega8(L)
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 ATmega8. Pulses are assumed to be at
least 250ns unless otherwise noted.
Signal Names In this section, some pins of the ATmega8 are referenced by signal names describing their func-
tionality during parallel programming, see Figure 104 and Table 91. Pins not described in the
following table are referenced by pin names.
The XA1/XA0 pins determine the action executed when the XTAL1 pin is given a positive pulse.
The bit coding is shown in Table 93 on page 220.
When pulsing WR or OE, the command loaded determines the action executed. The different
Commands are shown in Table 94 on page 220.
Figure 104. Parallel Programming
Table 91. 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
VCC
+5V
GND
XTAL1
PD1
PD2
PD3
PD4
PD5
PD6
PC[1:0]:PB[5:0]
DATA
RESET
PD7
+12 V
BS1
XA0
XA1
OE
RDY/BSY
PAGEL
PC2
WR
BS2
AVCC
+5V
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PAG E L P D 7 I Program memory and EEPROM Data
Page Load
BS2 PC2 I Byte Select 2 (“0” selects Low byte, “1”
selects 2’nd High byte)
DATA {PC[1:0]: PB[5:0]} I/O Bi-directional Data bus (Output when OE is
low)
Table 92. 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 93. XA1 and XA0 Coding
XA1 XA0 Action when XTAL1 is Pulsed
0 0 Load Flash or EEPROM Address (High or low address byte determined by BS1)
0 1 Load Data (High or Low data byte for Flash determined by BS1)
1 0 Load Command
1 1 No Action, Idle
Table 94. 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
0000 1000 Read Signature Bytes and Calibration byte
0000 0100 Read Fuse and Lock Bits
0000 0010 Read Flash
0000 0011 Read EEPROM
Table 91. Pin Name Mapping (Continued)
Signal Name in
Programming Mode Pin Name I/O Function
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ATmega8(L)
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 100µs
2. Set RESET to “0” and toggle XTAL1 at least 6 times
3. Set the Prog_enable pins listed in Table 92 on page 220 to “0000” and wait at least
100ns
4. Apply 11.5V - 12.5V to RESET
. Any activity on Prog_enable pins within 100ns after +12V
has been applied to RESET
, will cause the device to fail entering Programming mode
Note, if the RESET pin is disabled by programming the RSTDISBL Fuse, it may not be possible
to follow the proposed algorithm above. The same may apply when External Crystal or External
RC configuration is selected because it is not possible to apply qualified XTAL1 pulses. In such
cases, the following algorithm should be followed:
1. Set Prog_enable pins listed in Table 92 on page 220 to “0000”
2. Apply 4.5V - 5.5V between VCC and GND simultaneously as 11.5V - 12.5V is applied to
RESET
3. Wait 100ns
4. Re-program the fuses to ensure that External Clock is selected as clock source
(CKSEL3:0 = 0’b0000) and RESET pin is activated (RSTDISBL unprogrammed). If Lock
Bits are programmed, 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 0’b0
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 0xFF, that is the contents of the entire EEPROM (unless the
EESAVE Fuse is programmed) and Flash after a Chip Erase
• Address High byte needs only be loaded before programming or reading a new 256 word
window in Flash or 256 byte EEPROM. This consideration also applies to Signature bytes
reading
Chip Erase The Chip Erase will erase the Flash and EEPROM(1) memories plus Lock Bits. The Lock Bits are
not reset until the Program memory has been completely erased. The Fuse Bits are not
changed. A Chip Erase must be performed before the Flash and/or 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 command
5. Give WR a negative pulse. This starts the Chip Erase. RDY/BSY goes low
6. Wait until RDY/BSY goes high before loading a new command
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Programming the
Flash
The Flash is organized in pages, see Table 89 on page 218. When programming the Flash, the
program data is latched into a page buffer. This allows one page 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 command
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 (0x00 - 0xFF)
4. Give XTAL1 a positive pulse. This loads the address Low byte
C. Load Data Low byte
1. Set XA1, XA0 to “01”. This enables data loading
2. Set DATA = Data Low byte (0x00 - 0xFF)
3. Give XTAL1 a positive pulse. This loads the data byte
D. Load Data High byte
1. Set BS1 to “1”. This selects high data byte
2. Set XA1, XA0 to “01”. This enables data loading
3. Set DATA = Data High byte (0x00 - 0xFF)
4. Give XTAL1 a positive pulse. This loads the data byte
E. Latch Data
1. Set BS1 to “1”. This selects high data byte
2. Give PAGEL a positive pulse. This latches the data bytes (see Figure 106 on page 224
for signal waveforms)
F. Repeat B through E until the entire buffer is filled or until all data within the page is loaded
While the lower bits in the address are mapped to words within the page, the higher bits address
the pages within the FLASH. This is illustrated in Figure 105 on page 223. Note that if less than
eight bits are required to address words in the page (pagesize <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 (0x00 - 0xFF)
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. This starts programming of the entire page of data. RDY/BSY
goes low
3. Wait until RDY/BSY goes high. (See Figure 106 on page 224 for signal waveforms)
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ATmega8(L)
I. Repeat B through H until the entire Flash is programmed or until all data has been
programmed.
J. End Page Programming
1. Set XA1, XA0 to “10”. This enables command loading
2. Set DATA to “0000 0000”. This is the command for No Operation
3. Give XTAL1 a positive pulse. This loads the command, and the internal write signals are
reset
Figure 105. Addressing the Flash which is Organized in Pages(1)
Note: 1. PCPAGE and PCWORD are listed in Table 89 on page 218
PROGRAM MEMORY
WORD ADDRESS
WITHIN A PAGE
PAGE ADDRESS
WITHIN THE FLASH
INSTRUCTION WORD
PAGE PCWORD[PAGEMSB:0]:
00
01
02
PAGEEND
PAGE
PCWORDPCPAGE
PCMSB PAGEMSB
PROGRAM
COUNTER
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Figure 106. 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 90 on page 218. When programming the
EEPROM, the program data is latched into a page buffer. This allows one page of data to be
programmed simultaneously. The programming algorithm for the EEPROM Data memory is as
follows (refer to “Programming the Flash” on page 222 for details on Command, Address and
Data loading):
1. A: Load Command “0001 0001”
2. G: Load Address High byte (0x00 - 0xFF)
3. B: Load Address Low byte (0x00 - 0xFF)
4. C: Load Data (0x00 - 0xFF)
5. E: Latch data (give PAGEL a positive pulse)
K: Repeat 3 through 5 until the entire buffer is filled
L: Program EEPROM page
1. Set 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 programming the next page (see Figure 107 on
page 225 for signal waveforms)
RDY/BSY
WR
OE
RESET +12V
PAGEL
BS2
0x10
ADDR. LOW ADDR. HIGH
DATA
DATA L OW DATA H IG H ADDR. LOW DATA LOW DATA HIGH
XA1
XA0
BS1
XTAL1
XX XX XX
AB CDEB C D E G H
F
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Figure 107. Programming the EEPROM Waveforms
Reading the Flash The algorithm for reading the Flash memory is as follows (refer to “Programming the Flash” on
page 222 for details on Command and Address loading):
1. A: Load Command “0000 0010”
2. G: Load Address High byte (0x00 - 0xFF)
3. B: Load Address Low byte (0x00 - 0xFF)
4. Set OE to “0”, and BS1 to “0”. The Flash word Low byte can now be read at DATA
5. Set BS1 to “1”. The Flash word High byte can now be read 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 222 for details on Command and Address loading):
1. A: Load Command “0000 0011”
2. G: Load Address High byte (0x00 - 0xFF)
3. B: Load Address Low byte (0x00 - 0xFF)
4. Set OE to “0”, and BS1 to “0”. The EEPROM Data byte can now be read at DATA
5. Set OE to “1”
Programming the
Fuse Low Bits
The algorithm for programming the Fuse Low bits is as follows (refer to “Programming the Flash”
on page 222 for details on Command and Data loading):
1. A: Load Command “0100 0000”
2. C: Load Data Low byte. Bit n = “0” programs and bit n = “1” erases the Fuse bit
3. Set BS1 and BS2 to “0”
4. Give WR a negative pulse and wait for RDY/BSY to go high
RDY/BSY
WR
OE
RESET +12V
PAGEL
BS2
0x11 ADDR. HIGH
DATA
ADDR. LOW DATA ADDR. LOW DATA XX
XA1
XA0
BS1
XTAL1
XX
AGB CEB C E L
K
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Programming the
Fuse High Bits
The algorithm for programming the Fuse high bits is as follows (refer to “Programming the Flash”
on page 222 for details on Command and Data loading):
1. A: Load Command “0100 0000”
2. C: Load Data Low byte. Bit n = “0” programs and bit n = “1” erases the Fuse bit
3. Set 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
Programming the Lock
Bits
The algorithm for programming the Lock Bits is as follows (refer to “Programming the Flash” on
page 222 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 the Fuse and
Lock Bits
The algorithm for reading the Fuse and Lock Bits is as follows (refer to “Programming the Flash”
on page 222 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”
Figure 108. Mapping Between BS1, BS2 and the Fuse- and Lock Bits During Read
Fuse low byte
Lock bits 0
1
BS2
Fuse high byte
0
1
BS1
DATA
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ATmega8(L)
Reading the Signature
Bytes
The algorithm for reading the Signature bytes is as follows (refer to “Programming the Flash” on
page 222 for details on Command and Address loading):
1. A: Load Command “0000 1000”
2. B: Load Address Low byte (0x00 - 0x02)
3. Set OE to “0”, and 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 bytes is as follows (refer to “Programming the Flash” on
page 222 for details on Command and Address loading):
1. A: Load Command “0000 1000”
2. B: Load Address Low byte, (0x00 - 0x03)
3. Set OE to “0”, and BS1 to “1”. The Calibration byte can now be read at DATA
4. Set OE to “1”
Parallel Programming
Characteristics
Figure 109. Parallel Programming Timing, Including some General Timing Requirements
Figure 110. Parallel Programming Timing, Loading Sequence with Timing Requirements(1)
Note: 1. The timing requirements shown in Figure 109 (that is, tDVXH, tXHXL, and tXLDX) also apply to
loading operation
Data & Contol
(DATA, XA0/1, BS1, BS2)
XTAL1 t
XHXL
t
WL WH
t
DVXH
t
XLDX
t
PLWL
t
WLRH
WR
RDY/BSY
PAGEL t
PHPL
t
PLBX
t
BVPH
t
XLWL
t
WLBX
t
BVWL
WLRL
XTAL1
PAGEL
t
PLXH
XLXH
tt
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)
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Figure 111. Parallel Programming Timing, Reading Sequence (within the same Page) with Tim-
ing Requirements(1)
Note: 1. The timing requirements shown in Figure 109 on page 227 (that is, tDVXH, tXHXL, and tXLDX)
also apply to reading operation
Table 95. 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
tDVXH Data and Control Valid before XTAL1 High 67
ns
tXLXH XTAL1 Low to XTAL1 High 200
tXHXL XTAL1 Pulse Width High 150
tXLDX Data and Control Hold after XTAL1 Low 67
tXLWL XTAL1 Low to WR Low 0
tXLPH XTAL1 Low to PAGEL high 0
tPLXH PAGEL low to XTAL1 high 150
tBVPH BS1 Valid before PAGEL High 67
tPHPL PAGEL Pulse Width High 150
tPLBX BS1 Hold after PAGEL Low 67
tWLBX BS2/1 Hold after WR Low 67
tPLWL PAGEL Low to WR Low 67
tBVWL 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
XTAL1
OE
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)
t
BVDV
t
OLDV
t
XLOL
t
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
tXLOL XTAL1 Low to OE Low 0
ns
tBVDV BS1 Valid to DATA valid 0 250
tOLDV OE Low to DATA Valid 250
tOHDZ OE High to DATA Tri-stated 250
Table 95. Parallel Programming Characteristics, VCC = 5V ±10% (Continued)
Symbol Parameter Min Typ Max Units
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ATmega8(L)
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 (out-
put). After RESET is set low, the Programming Enable instruction needs to be executed first
before program/erase operations can be executed. NOTE, in Table 96, the pin mapping for SPI
programming is listed. Not all parts use the SPI pins dedicated for the internal SPI interface.
Serial
Programming Pin
Mapping
Figure 112. Serial Programming and Verify(1)
Notes: 1. If the device is clocked by the Internal Oscillator, it is no need to connect a clock source to the
XTAL1 pin
2. VCC - 0.3 < AVCC < VCC + 0.3, 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 Serial mode ONLY) and there is no need to first execute the Chip Erase
instruction. The Chip Erase operation turns the content of every memory location in both the
Program and EEPROM arrays into 0xFF.
Depending on CKSEL Fuses, a valid clock must be present. The minimum low and high periods
for the Serial Clock (SCK) input are defined as follows:
Low:> 2 CPU clock cycles for fck <12MHz, 3 CPU clock cycles for fck >=12MHz
High:> 2 CPU clock cycles for fck <12MHz, 3 CPU clock cycles for fck >=12MHz
Table 96. Pin Mapping Serial Programming
Symbol Pins I/O Description
MOSI PB3 I Serial data in
MISO PB4 O Serial data out
SCK PB5 I Serial clock
VCC
GND
XTAL1
SCK
MISO
MOSI
RESET
PB3
PB4
PB5
+2.7V - 5.5V
AVCC
+2.7V - 5.5V
(2)
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Serial Programming
Algorithm
When writing serial data to the ATmega8, data is clocked on the rising edge of SCK.
When reading data from the ATmega8, data is clocked on the falling edge of SCK. See Figure
113 on page 232 for timing details.
To program and verify the ATmega8 in the Serial Programming mode, the following sequence is
recommended (see four byte instruction formats in Table 98 on page 233):
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 two CPU clock cycles duration
after SCK has been set to “0”
2. Wait for at least 20ms and enable Serial Programming by sending the Programming
Enable serial instruction to pin MOSI
3. The Serial Programming instructions will not work if the communication is out of synchro-
nization. When in sync. the second byte (0x53), 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 transmitted. If the 0x53 did not echo back, give RESET a
positive pulse and issue a new Programming Enable command
4. The Flash is programmed one page at a time. The page size is found in Table 89 on
page 218. The memory page is loaded one byte at a time by supplying the 5 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 7MSB of the address. If polling is not used,
the user must wait at least tWD_FLASH before issuing the next page (see Table 97 on page
232).
Note: If other commands than polling (read) are applied before any write operation (FLASH,
EEPROM, Lock Bits, Fuses) is completed, it may result in incorrect programming
5. The EEPROM array is programmed one byte at a time by supplying the address and data
together with the appropriate Write instruction. An EEPROM memory location is first
automatically erased before new data is written. If polling is not used, the user must wait
at least tWD_EEPROM before issuing the next byte (see Table 97 on page 232). In a chip
erased device, no 0xFFs in the data file(s) need to be programmed
6. Any memory location can be verified by using the Read instruction which returns the con-
tent at the selected address at serial output MISO
7. At the end of the programming session, RESET can be set high to commence normal
operation
8. Power-off sequence (if needed):
Set RESET to “1”
Tu r n V CC 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 0xFF. 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 0xFF, 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 0xFF in all locations, programming of addresses that are meant to
contain 0xFF, can be skipped. See Table 97 on page 232 for tWD_FLASH value.
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ATmega8(L)
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 0xFF. 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 0xFF, but the user should have the following
in mind: As a chip-erased device contains 0xFF in all locations, programming of addresses that
are meant to contain 0xFF, can be skipped. This does not apply if the EEPROM is Re-pro-
grammed without chip-erasing the device. In this case, data polling cannot be used for the value
0xFF, and the user will have to wait at least tWD_EEPROM before programming the next byte. See
Table 97 for tWD_EEPROM value.
Figure 113. Serial Programming Waveforms
Table 97. Minimum Wait Delay Before Writing the Next Flash or EEPROM Location
Symbol Minimum Wait Delay
tWD_FUSE 4.5ms
tWD_FLASH 4.5ms
tWD_EEPROM 9.0ms
tWD_ERASE 9.0ms
MSB
MSB
LSB
LSB
SERIAL CLOCK INPUT
(SCK)
SERIAL DATA INPUT
(MOSI)
(MISO)
SAMPLE
SERIAL DATA OUTPUT
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ATmega8(L)
Note: a = address high bits
b = address low bits
H = 0 – Low byte, 1 – High byte
o = data out
i = data in
x = don’t care
Table 98. Serial Programming Instruction Set
Instruction
Instruction Format
OperationByte 1 Byte 2 Byte 3 Byte 4
Programming Enable 1010 1100 0101 0011 xxxx xxxx xxxx xxxx Enable Serial Programming after
RESET goes low
Chip Erase 1010 1100 100x xxxx xxxx xxxx xxxx xxxx Chip Erase EEPROM and Flash
Read Program Memory 0010 H000 0000 aaaa bbbb bbbb oooo oooo Read H (high or low) data o from
Program memory at word address
a:b
Load Program Memory
Page
0100 H000 0000 xxxx xxxb bbbb iiii iiii Write H (high or low) data i to
Program memory page at word
address b. Data Low byte must be
loaded before Data High byte is
applied within the same address
Write Program Memory
Page
0100 1100 0000 aaaa bbbx xxxx xxxx xxxx Write Program memory Page at
address a:b
Read EEPROM Memory 1010 0000 00xx xxxa bbbb bbbb oooo oooo Read data o from EEPROM
memory at address a:b
Write EEPROM Memory 1100 0000 00xx xxxa bbbb bbbb iiii iiii Write data i to EEPROM memory
at address a:b
Read Lock Bits 0101 1000 0000 0000 xxxx xxxx xxoo oooo Read Lock Bits. “0” = programmed,
“1” = unprogrammed. See Table
85 on page 215 for details
Write Lock Bits 1010 1100 111x xxxx xxxx xxxx 11ii iiii Write Lock Bits. Set bits = “0” to
program Lock Bits. See Table 85
on page 215 for details
Read Signature Byte 0011 0000 00xx xxxx xxxx xxbb oooo oooo Read Signature Byte o at address
b
Write Fuse Bits 1010 1100 1010 0000 xxxx xxxx iiii iiii Set bits = “0” to program, “1” to
unprogram. See Table 88 on
page 217 for details
Write Fuse High Bits 1010 1100 1010 1000 xxxx xxxx iiii iiii Set bits = “0” to program, “1” to
unprogram. See Table 87 on
page 216 for details
Read Fuse Bits 0101 0000 0000 0000 xxxx xxxx oooo oooo Read Fuse Bits. “0” = programmed,
“1” = unprogrammed. See Table
88 on page 217 for details
Read Fuse High Bits 0101 1000 0000 1000 xxxx xxxx oooo oooo Read Fuse high bits. “0” = pro-
grammed, “1” = unprogrammed.
See Table 87 on page 216 for
details
Read Calibration Byte 0011 1000 00xx xxxx 0000 00bb oooo oooo Read Calibration Byte
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SPI Serial
Programming
Characteristics
For characteristics of the SPI module, see “SPI Timing Characteristics” on page 239.
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ATmega8(L)
Electrical Characteristics
Note: Typical values contained in this datasheet are based on simulations and characterization of other AVR microcontrollers manu-
factured on the same process technology. Min and Max values will be available after the device is characterized.
DC Characteristics
Absolute Maximum Ratings*
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 ................................................ 40.0mA
DC Current VCC and GND Pins................................. 300.0mA
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 = 2.7V - 5.5V -0.5 0.2 VCC(1)
V
VIH
Input High Voltage except
XTAL1 and RESET pins VCC = 2.7V - 5.5V 0.6VCC(2) VCC + 0.5
VIL1
Input Low Voltage
XTAL1 pin VCC = 2.7V - 5.5V -0.5 0.1VCC(1)
VIH1
Input High Voltage
XTAL 1 pin VCC = 2.7V - 5.5V 0.8VCC(2) VCC + 0.5
VIL2
Input Low Voltage
RESET pin VCC = 2.7V - 5.5V -0.5 0.2 VCC
VIH2
Input High Voltage
RESET pin VCC = 2.7V - 5.5V 0.9VCC(2) VCC + 0.5
VIL3
Input Low Voltage
RESET pin as I/O VCC = 2.7V - 5.5V -0.5 0.2VCC
VIH3
Input High Voltage
RESET pin as I/O VCC = 2.7V - 5.5V 0.6VCC(2)
0.7VCC(2) VCC + 0.5
VOL
Output Low Voltage(3)
(Ports B,C,D)
IOL = 20mA, VCC = 5V
IOL = 10mA, VCC = 3V
0.9
0.6
VOH
Output High Voltage(4)
(Ports B,C,D)
IOH = -20mA, VCC = 5V
IOH = -10mA, VCC = 3V
4.2
2.2
IIL
Input Leakage
Current I/O Pin
Vcc = 5.5V, p i n l ow
(absolute value) 1
µA
IIH
Input Leakage
Current I/O Pin
Vcc = 5.5V, pin high
(absolute value) 1
RRST Reset Pull-up Resistor 30 80 kΩ
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ATmega8(L)
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 port can sink more than the test conditions (20mA at Vcc = 5V, 10mA at Vcc = 3V) under steady state
conditions (non-transient), the following must be observed:
PDIP, TQFP, and QFN/MLF Package:
1] The sum of all IOL, for all ports, should not exceed 300mA.
2] The sum of all IOL, for ports C0 - C5 should not exceed 100mA.
3] The sum of all IOL, for ports B0 - B7, C6, D0 - D7 and XTAL2, should not exceed 200mA.
If IOL exceeds the test condition, VOL may exceed the related specification. Pins are not guaranteed to sink current greater
than the listed test condition
4. Although each I/O port can source more than the test conditions (20mA at Vcc = 5V, 10mA at Vcc = 3V) under steady state
conditions (non-transient), the following must be observed:
PDIP, TQFP, and QFN/MLF Package:
1] The sum of all IOH, for all ports, should not exceed 300mA.
2] The sum of all IOH, for port C0 - C5, should not exceed 100mA.
3] The sum of all IOH, for ports B0 - B7, C6, D0 - D7 and XTAL2, should not exceed 200mA.
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
Rpu I/O Pin Pull-up Resistor 20 50 kΩ
ICC
Power Supply Current
Active 4MHz, VCC = 3V
(ATmega8L) 35
mA
Active 8MHz, VCC = 5V
(ATmega8) 11 15
Idle 4MHz, VCC = 3V
(ATmega8L) 12
Idle 8MHz, VCC = 5V
(ATmega8) 4.5 7
Power-down mode(5) WDT enabled, VCC = 3V < 22 28 µA
WDT disabled, VCC = 3V < 1 3
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 = 5.0V
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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ATmega8(L)
External Clock
Drive Waveforms
Figure 114. External Clock Drive Waveforms
External Clock
Drive
Notes: 1. R should be in the range 3kΩ - 100kΩ, and C should be at least 20pF. The C values given in
the table includes pin capacitance. This will vary with package type
2. The frequency will vary with package type and board layout
VIL1
VIH1
Table 99. External Clock Drive
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 100. External RC Oscillator, Typical Frequencies
R [kΩ](1) C [pF] f(2)
33 22 650kHz
10 22 2.0MHz
238
2486Z–AVR–02/11
ATmega8(L)
Two-wire Serial Interface Characteristics
Table 101 describes the requirements for devices connected to the Two-wire Serial Bus. The ATmega8 Two-wire Serial
Interface meets or exceeds these requirements under the noted conditions.
Timing symbols refer to Figure 115 on page 239.
Notes: 1. In ATmega8, this parameter is characterized and not 100% tested
2. Required only for fSCL > 100kHz
3. Cb = capacitance of one bus line in pF
4. fCK = CPU clock frequency
Table 101. Two-wire Serial Bus Requirements
Symbol Parameter Condition Min Max Units
VIL Input Low-voltage -0.5 0.3VCC
V
VIH Input High-voltage 0.7VCC VCC + 0.5
Vhys(1) Hysteresis of Schmitt Trigger Inputs 0.05VCC(2) –
VOL(1) Output Low-voltage 3mA sink Current 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 10pF < Cb < 400pF(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 ≤ 100kHz
fSCL > 100kHz
tHD;STA Hold Time (repeated) START Condition fSCL ≤ 100kHz 4.0 –
µs
fSCL > 100kHz 0.6 –
tLOW Low Period of the SCL Clock fSCL ≤ 100kHz(6) 4.7 –
fSCL > 100kHz(7) 1.3 –
tHIGH High period of the SCL clock fSCL ≤ 100kHz 4.0 –
fSCL > 100kHz 0.6 –
tSU;STA Set-up time for a repeated START condition fSCL ≤ 100kHz 4.7 –
fSCL > 100kHz 0.6 –
tHD;DAT Data hold time fSCL ≤ 100kHz 0 3.45
fSCL > 100kHz 0 0.9
tSU;DAT Data setup time fSCL ≤ 100kHz 250 – ns
fSCL > 100kHz 100 –
tSU;STO Setup time for STOP condition fSCL ≤ 100kHz 4.0 –
µs
fSCL > 100kHz 0.6 –
tBUF
Bus free time between a STOP and START
condition
fSCL ≤ 100kHz 4.7 –
fSCL > 100kHz 1.3 –
VCC 0.4V–
3mA
----------------------------
1000ns
Cb
-------------------
Ω
VCC 0.4V–
3mA
----------------------------
300ns
Cb
----------------
239
2486Z–AVR–02/11
ATmega8(L)
5. This requirement applies to all ATmega8 Two-wire Serial Interface operation. Other devices connected to the Two-wire Serial
Bus need only obey the general fSCL requirement
6. The actual low period generated by the ATmega8 Two-wire Serial Interface is (1/fSCL - 2/fCK), thus fCK must be greater than
6MHz for the low time requirement to be strictly met at fSCL = 100kHz
7. The actual low period generated by the ATmega8 Two-wire Serial Interface is (1/fSCL - 2/fCK), thus the low time requirement
will not be strictly met for fSCL > 308kHz when fCK = 8MHz. Still, ATmega8 devices connected to the bus may communicate at
full speed (400kHz) with other ATmega8 devices, as well as any other device with a proper tLOW acceptance margin
Figure 115. Two-wire Serial Bus Timing
SPI Timing
Characteristics
See Figure 116 on page 240 and Figure 117 on page 240 for details.
Note: 1. In SPI Programming mode the minimum SCK high/low period is:
- 2tCLCL for fCK < 12MHz
- 3tCLCL for fCK > 12MHz
t
SU;STA
t
LOW
t
HIGH
t
LOW
t
of
t
HD;STA
t
HD;DAT
t
SU;DAT
t
SU;STO
t
BUF
SCL
SDA
t
r
Table 102. SPI Timing Parameters
Description Mode Min Typ Max
1 SCK period Master See Table 50 on
page 126
ns
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 Master 0.5 • tSCK
7 SCK to out Master 10
8 SCK to out high Master 10
9SS
low to out Slave 15
10 SCK period Slave 4 • tck
11 SCK high/low(1) Slave 2 • tck
12 Rise/Fall time Slave 1600
13 Setup Slave 10
14 Hold Slave 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 Salve 2 • tck
240
2486Z–AVR–02/11
ATmega8(L)
Figure 116. SPI interface timing requirements (Master Mode)
Figure 117. SPI interface timing requirements (Slave Mode)
MOSI
(Data Output)
SCK
(CPOL = 1)
MISO
(Data Input)
SCK
(CPOL = 0)
SS
MSBLSB
LSBMSB
...
...
61
22
345
8
7
MISO
(Data Output)
SCK
(CPOL = 1)
MOSI
(Data Input)
SCK
(CPOL = 0)
SS
MSBLSB
LSBMSB
...
...
10
11 11
121314
17
15
9
X
16
18
241
2486Z–AVR–02/11
ATmega8(L)
ADC Characteristics
Notes: 1. Values are guidelines only
2. Minimum for AVCC is 2.7V
3. Maximum for AVCC is 5.5V
4. Maximum conversion time is 1/50kHz × 25 = 0.5ms
Table 103. ADC Characteristics
Symbol Parameter Condition Min(1) Typ(1) Max(1) Units
Resolution Single Ended Conversion 10 Bits
Absolute accuracy
(including INL, DNL,
Quantization Error, Gain,
and Offset Error)
Single Ended Conversion
VREF = 4V, VCC = 4V
ADC clock = 200kHz
1.75
LSB
Single Ended Conversion
VREF = 4V, VCC = 4V
ADC clock = 1MHz
3
Integral Non-linearity (INL)
Single Ended Conversion
VREF = 4V, VCC = 4V
ADC clock = 200kHz 0.75
Differential Non-linearity
(DNL)
Single Ended Conversion
VREF = 4V, VCC = 4V
ADC clock = 200kHz 0.5
Gain Error Single Ended Conversion
VREF = 4V, VCC = 4V
ADC clock = 200kHz
1
Offset Error Single Ended Conversion
VREF = 4V, VCC = 4V
ADC clock = 200kHz
1
Conversion Time(4) Free Running Conversion 13 260 µs
Clock Frequency 50 1000 kHz
AVCC Analog Supply Voltage VCC - 0.3(2) VCC + 0.3(3)
V
VREF Reference Voltage 2.0 AVCC
VIN Input voltage GND VREF
Input bandwidth 38.5 kHz
VINT Internal Voltage Reference 2.3 2.56 2.9 V
RREF Reference Input Resistance 32 kΩ
RAIN Analog Input Resistance 55 100 MΩ
242
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ATmega8(L)
ATmega8
Typical
Characteristics
The following charts show typical behavior. These figures are not tested during manufacturing.
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 voltage and frequency.
The current drawn from capacitive loaded pins may be estimated (for one pin) as:
CL × VCC × f
where CL = load capacitance, VCC = operating voltage and f = average switching frequency of I/O
pin.
The parts are characterized at frequencies higher than test limits. Parts are not guaranteed to
function properly at frequencies higher than the ordering code indicates.
The difference between current consumption in Power-down mode with Watchdog Timer
enabled and Power-down mode with Watchdog Timer disabled represents the differential cur-
rent drawn by the Watchdog Timer.
Active Supply Current Figure 118. Active Supply Current vs. Frequency (0.1MHz - 1.0MHz)
0
0.5
1
1.5
2
2.5
3
0 0.1 0.2 0.30.4 0.5 0.6 0.7 0.80.9 1
Frequency (MHz)
I
CC
(mA)
5.5V
5.0V
4.5V
3.3V
3.0V
2.7V
4.0V
243
2486Z–AVR–02/11
ATmega8(L)
Figure 119. Active Supply Current vs. Frequency (1MHz - 20MHz)
Figure 120. Active Supply Current vs. VCC (Internal RC Oscillator, 8MHz)
0
5
10
15
20
25
30
0246810 12 14 16 1820
Frequency (MHz)
I
CC
(mA)
5.5V
5.0V
4.5V
3.3V
2.7V
3.0V
0
2
4
6
8
10
12
14
16
18
2.5 33.5 4 4.5 5 5.5
V
CC
(V)
I
CC
(mA)
85
°C
25
°C
-40
°C
244
2486Z–AVR–02/11
ATmega8(L)
Figure 121. Active Supply Current vs. VCC (Internal RC Oscillator, 4MHz)
Figure 122. Active Supply Current vs. VCC (Internal RC Oscillator, 2MHz)
0
2
4
6
8
10
12
2.5 33.5 4 4.5 5 5.5
V
CC
(V)
I
CC
(mA)
85
°C
25
°C
-40
°C
0
1
2
3
4
5
6
2.5 33.5 4 4.5 5 5.5
V
CC
(V)
I
CC
(mA)
85
°C
25
°C
-40
°C
245
2486Z–AVR–02/11
ATmega8(L)
Figure 123. Active Supply Current vs. VCC (Internal RC Oscillator, 1MHz)
Figure 124. Active Supply Current vs. VCC (32kHz External Oscillator)
0
0.5
1
1.5
2
2.5
3
3.5
2.5 33.5 4 4.5 5 5.5
V
CC
(V)
I
CC
(mA)
85
°C
25
°C
-40
°C
0
20
40
60
80
100
120
2.5 33.5 4 4.5 5 5.5
V
CC
(V)
I
CC
(µA)
25
°C
246
2486Z–AVR–02/11
ATmega8(L)
Idle Supply Current Figure 125. Idle Supply Current vs. Frequency (0.1MHz - 1.0MHz)
Figure 126. Idle Supply Current vs. Frequency (1MHz - 20MHz)
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0 0.1 0.2 0.30.4 0.5 0.6 0.7 0.80.9 1
Frequency (MHz)
I
CC
(mA)
5.5V
4.5V
4.0V
3.3V
3.0V
2.7V
5.0V
0
2
4
6
8
10
12
14
0246810 12 14 16 1820
Frequency (MHz)
ICC (mA)
5.5V
4.5V
4.0V
3.3V
3.0V
2.7V
5.0V
247
2486Z–AVR–02/11
ATmega8(L)
Figure 127. Idle Supply Current vs. VCC (Internal RC Oscillator, 8MHz)
Figure 128. Idle Supply Current vs. VCC (Internal RC Oscillator, 4MHz)
0
1
2
3
4
5
6
7
8
2.5 33.5 4 4.5 5 5.5
VCC (V)
ICC (mA)
85
°C
25
°C
-40
°C
0
0.5
1
1.5
2
2.5
3
3.5
4
2.5 33.5 4 4.5 5 5.5
V
CC
(V)
I
CC
(mA)
85
°C
25
°C
-40
°C
248
2486Z–AVR–02/11
ATmega8(L)
Figure 129. Idle Supply Current vs. VCC (Internal RC Oscillator, 2MHz)
Figure 130. Idle Supply Current vs. VCC (Internal RC Oscillator, 1MHz)
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2.5 33.5 4 4.5 5 5.5
V
CC
(V)
I
CC
(mA)
85
°C
25
°C
-40
°C
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
2.5 33.5 4 4.5 5 5.5
V
CC
(V)
I
CC
(mA)
85
°C
25
°C
-40
°C
249
2486Z–AVR–02/11
ATmega8(L)
Figure 131. Idle Supply Current vs. VCC (32kHz External Oscillator)
Power-down Supply
Current
Figure 132. Power-down Supply Current vs. VCC (Watchdog Timer Disabled)
0
5
10
15
20
25
30
35
40
2.5 33.5 4 4.5 5 5.5
V
CC
(V)
I
CC
(µA)
25
°C
0
0.5
1
1.5
2
2.5
2.5 33.5 4 4.5 5 5.5
V
CC
(V)
I
CC
(µA)
85
°C
25
°C
-40
°C
250
2486Z–AVR–02/11
ATmega8(L)
Figure 133. Power-down Supply Current vs. VCC (Watchdog Timer Enabled)
Power-save Supply
Current
Figure 134. Power-save Supply Current vs. VCC (Watchdog Timer Disabled)
0
10
20
30
40
50
60
70
80
2.5 3 3.5 4 4.5 5 5.5
V
CC
(V)
I
CC
(µA)
85°C
25°C
-40°C
0
5
10
15
20
25
2.5 33.5 4 4.5 5 5.5
V
CC
(V)
I
CC
(µA)
25
°C
251
2486Z–AVR–02/11
ATmega8(L)
Standby Supply
Current
Figure 135. Standby Supply Current vs. VCC (455kHz Resonator, Watchdog Timer Disabled)
Figure 136. Standby Supply Current vs. VCC (1MHz Resonator, Watchdog Timer Disabled)
0
10
20
30
40
50
60
70
80
2.5 33.5 4 4.5 5 5.5
VCC (V)
ICC (µA)
0
10
20
30
40
50
60
70
2.5 33.5 4 4.5 5 5.5
V
CC
(V)
I
CC
(µA)
252
2486Z–AVR–02/11
ATmega8(L)
Figure 137. Standby Supply Current vs. VCC (2MHz Resonator, Watchdog Timer Disabled)
Figure 138. Standby Supply Current vs. VCC (2MHz Xtal, Watchdog Timer Disabled)
0
10
20
30
40
50
60
70
80
90
2.5 33.5 4 4.5 5 5.5
V
CC
(V)
I
CC
(µA)
0
10
20
30
40
50
60
70
80
90
2.5 33.5 4 4.5 5 5.5
V
CC
(V)
I
CC
(µA)
253
2486Z–AVR–02/11
ATmega8(L)
Figure 139. Standby Supply Current vs. VCC (4MHz Resonator, Watchdog Timer Disabled)
Figure 140. Standby Supply Current vs. VCC (4MHz Xtal, Watchdog Timer Disabled)
0
20
40
60
80
100
120
140
2.5 33.5 4 4.5 5 5.5
V
CC
(V)
I
CC
(µA)
0
20
40
60
80
100
120
140
2.5 33.5 4 4.5 5 5.5
V
CC
(V)
I
CC
(µA)
254
2486Z–AVR–02/11
ATmega8(L)
Figure 141. Standby Supply Current vs. VCC (6MHz Resonator, Watchdog Timer Disabled)
Figure 142. Standby Supply Current vs. VCC (6MHz Xtal, Watchdog Timer Disabled)
0
20
40
60
80
100
120
140
160
2.5 33.5 4 4.5 5 5.5
V
CC
(V)
I
CC
(µA)
0
20
40
60
80
100
120
140
160
180
200
2.5 33.5 4 4.5 5 5.5
V
CC
(V)
I
CC
(µA)
255
2486Z–AVR–02/11
ATmega8(L)
Pin Pull-up Figure 143. I/O Pin Pull-up Resistor Current vs. Input Voltage (VCC = 5V)
Figure 144. I/O Pin Pull-up Resistor Current vs. Input Voltage (VCC = 2.7V)
0
20
40
60
80
100
120
140
160
0123456
V
OP
(V)
I
IO
(µA)
85°C
25°C
-40°C
0
10
20
30
40
50
60
70
80
90
0 0.5 1 1.5 2 2.5 3
V
OP
(V)
I
IO
(µA)
85
°C
25
°C
-40
°C
256
2486Z–AVR–02/11
ATmega8(L)
Figure 145. Reset Pull-up Resistor Current vs. Reset Pin Voltage (VCC = 5V)
Figure 146. Reset Pull-up Resistor Current vs. Reset Pin Voltage (VCC = 2.7V)
0
20
40
60
80
100
012
VRESET (V)
IRESET (µA)
85°C
25°C
- 40°C
0
5
10
15
20
25
30
35
40
45
0 0.5 1 1.5 2 2.5
V
RESET
(V)
I
RESET
(µA)
85
°C
25
°C
-40
°C
257
2486Z–AVR–02/11
ATmega8(L)
Pin Driver Strength Figure 147. I/O Pin Source Current vs. Output Voltage (VCC = 5V)
Figure 148. I/O Pin Source Current vs. Output Voltage (VCC = 2.7V)
0
10
20
30
40
50
60
70
80
V
OH
(V)
I
OH
(mA)
85
°C
25
°C
-40
°C
0
5
10
15
20
25
30
0 0.5 1 1.5 2 2.5 3
V
OH
(V)
I
OH
(mA)
85
°C
25
°C
-40
°C
258
2486Z–AVR–02/11
ATmega8(L)
Figure 149. I/O Pin Sink Current vs. Output Voltage (VCC = 5V)
Figure 150. I/O Pin Sink Current vs. Output Voltage (VCC = 2.7V)
0
10
20
30
40
50
60
70
80
90
0 0.5 1 1.5 2 2.5
V
OL
(V)
I
OL
(mA)
85
°C
25
°C
-40
°C
0
5
10
15
20
25
30
35
0 0.5 1 1.5 2 2.5
V
OL
(V)
I
OL
(mA)
85
°C
25
°C
-40
°C
259
2486Z–AVR–02/11
ATmega8(L)
Figure 151. Reset Pin as I/O – Pin Source Current vs. Output Voltage (VCC = 5V)
Figure 152. Reset Pin as I/O – Pin Source Current vs. Output Voltage (VCC = 2.7V)
0
0.5
1
1.5
2
2.5
3
3.5
4
2 2.5 33.5 4 4.5
V
OH
(V)
Current (mA)
85
°C
25
°C
-40
°C
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
0 0.5 1 1.5 2 2.5
V
OH
(V)
Current (mA)
85
°C
25
°C
-40
°C
260
2486Z–AVR–02/11
ATmega8(L)
Figure 153. Reset Pin as I/O – Pin Sink Current vs. Output Voltage (VCC = 5V)
Figure 154. Reset Pin as I/O – Pin Sink Current vs. Output Voltage (VCC = 2.7V)
0
2
4
6
8
10
12
14
0 0.5 1 1.5 2 2.5
V
OL
(V)
Current (mA)
85
°C
25
°C
-40
°C
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
0 0.5 1 1.5 2 2.5
V
OL
(V)
Current (mA)
85
°C
25
°C
-40
°C
261
2486Z–AVR–02/11
ATmega8(L)
Pin Thresholds and
Hysteresis
Figure 155. I/O Pin Input Threshold Voltage vs. VCC (VIH, I/O Pin Read as “1”)
Figure 156. I/O Pin Input Threshold Voltage vs. VCC (VIL, I/O Pin Read as “0”)
0
0.5
1
1.5
2
2.5
2.5 33.5 4 4.5 5 5.5
V
CC
(V)
Threshold (V)
85
°C
25
°C
-40
°C
0
0.5
1
1.5
2
2.5 33.5 4 4.5 5 5.5
V
CC
(V)
Threshold (V)
85
°C
25
°C
-40
°C
262
2486Z–AVR–02/11
ATmega8(L)
Figure 157. I/O Pin Input Hysteresis vs. VCC
Figure 158. Reset Pin as I/O – Input Threshold Voltage vs. VCC (VIH, I/O Pin Read as “1”)
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
2.5 33.5 4 4.5 5 5.5
V
CC
(V)
Input Hysteresis (V)
85
°C
25
°C
-40
°C
0
0.5
1
1.5
2
2.5
3
3.5
4
2.5 33.5 4 4.5 5 5.5
V
CC
(V)
Threshold (V)
85
°C
25
°C
-40
°C
263
2486Z–AVR–02/11
ATmega8(L)
Figure 159. Reset Pin as I/O – Input Threshold Voltage vs. VCC (VIL, I/O Pin Read as “0”)
Figure 160. Reset Pin as I/O – Pin Hysteresis vs. VCC
0
0.5
1
1.5
2
2.5
2.5 33.5 4 4.5 5 5.5
V
CC
(V)
Threshold (V)
85
°C
25
°C
-40
°C
0
0.5
1
1.5
2
2.5 33.5 4 4.5 5 5.5
V
CC
(V)
Input Hysteresis (V)
85
°C
25
°C
-40
°C
264
2486Z–AVR–02/11
ATmega8(L)
Figure 161. Reset Input Threshold Voltage vs. VCC (VIH, Reset Pin Read as “1”)
Figure 162. Reset Input Threshold Voltage vs. VCC (VIL, Reset Pin Read as “0”)
0
0.5
1
1.5
2
2.5
2.5 33.5 4 4.5 5 5.5
V
CC
(V)
Threshold (V)
85
°C
25
°C
-40
°C
0
0.5
1
1.5
2
2.5
2.5 33.5 4 4.5 5 5.5
V
CC
(V)
Threshold (V)
85
°C
25
°C
-40
°C
265
2486Z–AVR–02/11
ATmega8(L)
Figure 163. Reset Input Pin Hysteresis vs. VCC
Bod Thresholds and
Analog Comparator
Offset
Figure 164. BOD Thresholds vs. Temperature (BOD Level is 4.0V)
0
0.2
0.4
0.6
0.8
1
2 2.5 33.5 4 4.5 5 5.5
V
CC
(V)
Input Hysteresis (V)
85
°C
25
°C
-40
°C
3.7
3.8
3.9
4
4.1
4.2
4.3
-50 -40 -30 -20 -10 0 10 20 304050607080 90 100
Temperature (°C)
Threshold (V)
Rising VCC
Falling VCC
266
2486Z–AVR–02/11
ATmega8(L)
Figure 165. BOD Thresholds vs. Temperature (BOD Level is 2.7V)
Figure 166. Bandgap Voltage vs. VCC
2.4
2.5
2.6
2.7
2.8
-50 -40 -30 -20 -10 0 10 20 304050607080 90 100
Temperature (°C)
Threshold (V)
Rising V
CC
Falling V
CC
1.29
1.295
1.3
1.305
1.31
1.315
2.5 33.5 4 4.5 5 5.5
Vcc (V)
Bandgap Voltage (V)
-40°C
25°C
85°C
267
2486Z–AVR–02/11
ATmega8(L)
Figure 167. Analog Comparator Offset Voltage vs. Common Mode Voltage (VCC = 5V)
Figure 168. Analog Comparator Offset Voltage vs. Common Mode Voltage (VCC = 2.7V)
-0.006
-0.005
-0.004
-0.003
-0.002
-0.001
0
0.001
0.002
0.003
0 0.5 1 1.5 2 2.5 33.5 4 4.5 5
Common Mode Voltage (V)
Comparator Offset Voltage (V)
85°C
25°C
-40°C
-0.005
-0.004
-0.003
-0.002
-0.001
0
0.001
0.002
0.003
0 0.5 1 1.5 2 2.5 3
Common Mode Voltage (V)
Comparator Offset Voltage (V)
85°C
25°C
-40°C
268
2486Z–AVR–02/11
ATmega8(L)
Internal Oscillator
Speed
Figure 169. Watchdog Oscillator Frequency vs. VCC
Figure 170. Calibrated 8MHz RC Oscillator Frequency vs. Temperature
1100
1120
1140
1160
1180
1200
1220
1240
1260
2.5 33.5 4 4.5 5 5.5
V
CC
(V)
F
RC
(kHz)
85°
C
25°
C
-40°
C
6.5
6.7
6.9
7.1
7.3
7.5
7.7
7.9
8.1
8.3
8.5
-60 -40 -20 0 20 40 60 80 100
Temperature (°C)
F
RC
(MHz)
5.5V
2.7V
4.0V
269
2486Z–AVR–02/11
ATmega8(L)
Figure 171. Calibrated 8MHz RC Oscillator Frequency vs. VCC
Figure 172. Calibrated 8MHz RC Oscillator Frequency vs. Osccal Value
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
V
CC
(V)
F
RC
(MHz)
85
°
C
25
°
C
-40
°
C
4
6
8
10
12
14
16
016324864 80 96 112 128144 160 176 192 208224 240
OSCCAL VALUE
F
RC
(MHz)
270
2486Z–AVR–02/11
ATmega8(L)
Figure 173. Calibrated 4MHz RC Oscillator Frequency vs. Temperature
Figure 174. Calibrated 4MHz RC Oscillator Frequency vs. VCC
3.5
3.6
3.7
3.8
3.9
4
4.1
4.2
-60 -40 -20 0 20 40 60 80 100
Temperature (°C)
F
RC
(MHz)
5.5V
2.7V
4.0V
3.5
3.6
3.7
3.8
3.9
4
4.1
4.2
2.5 33.5 4 4.5 5 5.5
VCC (V)
F
RC
(MHz)
85
°
C
25
°
C
-40
°
C
271
2486Z–AVR–02/11
ATmega8(L)
Figure 175. Calibrated 4MHz RC Oscillator Frequency vs. Osccal Value
Figure 176. Calibrated 2MHz RC Oscillator Frequency vs. Temperature
2
3
4
5
6
7
8
016324864 80 96 112 128144 160 176 192 208224 240
OSCCAL VALUE
F
RC
(MHz)
1.8
1.85
1.9
1.95
2
2.05
2.1
-60 -40 -20 0 20 40 60 80 100
Temperature (°C)
F
RC
(MHz)
5.5V
2.7V
4.0V
272
2486Z–AVR–02/11
ATmega8(L)
Figure 177. Calibrated 2MHz RC Oscillator Frequency vs. VCC
Figure 178. Calibrated 2MHz RC Oscillator Frequency vs. Osccal Value
1.7
1.8
1.9
2
2.1
2.2
2.5 33.5 4 4.5 5 5.5
V
CC
(V)
F
RC
(MHz)
85
°C
25
°C
-40
°
C
0.8
1.3
1.8
2.3
2.8
3.3
3.8
016324864 80 96 112 128144 160 176 192 208224 240
OSCCAL VALUE
F
RC
(MHz)
273
2486Z–AVR–02/11
ATmega8(L)
Figure 179. Calibrated 1MHz RC Oscillator Frequency vs. Temperature
Figure 180. Calibrated 1MHz RC Oscillator Frequency vs. VCC
0.9
0.92
0.94
0.96
0.98
1
1.02
1.04
-60 -40 -20 0 20 40 60 80 100
Temperature (°C)
FRC (MHz)
5.5V
2.7V
4.0V
0.9
0.95
1
1.05
1.1
2.5 33.5 4 4.5 5 5.5
V
CC
(V)
F
RC
(MHz)
85
°
C
25
°
C
-40
°
C
274
2486Z–AVR–02/11
ATmega8(L)
Figure 181. Calibrated 1MHz RC Oscillator Frequency vs. Osccal Value
Current Consumption
of Peripheral Units
Figure 182. Brown-out Detector Current vs. VCC
0.5
0.7
0.9
1.1
1.3
1.5
1.7
1.9
016324864 80 96 112 128144 160 176 192 208224 240
OSCCAL VALUE
F
RC
(MHz)
0
5
10
15
20
25
30
2.5 33.5 4 4.5 5 5.5
V
CC
(V)
I
CC
(µA)
25
°C
85
°C
-40
°C
275
2486Z–AVR–02/11
ATmega8(L)
Figure 183. ADC Current vs. VCC (AREF = AVCC)
Figure 184. AREF External Reference Current vs. VCC
0
50
100
150
200
250
300
350
400
450
2.5 33.5 4 4.5 5 5.5
VCC (V)
ICC (µA)
85°C
25°
C
-40°C
0
50
100
150
200
250
2.5 33.5 4 4.5 5 5.5
V
CC
(V)
I
CC
(µA)
85
°
C
25
°C
-40
°C
276
2486Z–AVR–02/11
ATmega8(L)
Figure 185. 32kHz TOSC Current vs. VCC (Watchdog Timer Disabled)
Figure 186. Watchdog Timer Current vs. VCC
0
5
10
15
20
25
2.5 33.5 4 4.5 5 5.5
V
CC
(V)
I
CC
(µA)
25°C
0
10
20
30
40
50
60
70
80
22.533.544.555.5
V
CC
(V)
I
CC
(µA)
85°C
25°C
-40°C
277
2486Z–AVR–02/11
ATmega8(L)
Figure 187. Analog Comparator Current vs. VCC
Figure 188. Programming Current vs. VCC
0
10
20
30
40
50
60
70
80
90
100
2.5 33.5 4 4.5 5 5.5
V
CC
(V)
I
CC
(µA)
25
°
C
85
°
C
-40
°
C
0
1
2
3
4
5
6
7
2.5 33.5 4 4.5 5 5.5
V
CC
(V)
I
CC
(mA)
25
°C
85
°C
-40
°C
278
2486Z–AVR–02/11
ATmega8(L)
Current Consumption
in Reset and Reset
Pulsewidth
Figure 189. Reset Supply Current vs. VCC (0.1MHz - 1.0MHz, Excluding Current Through The
Reset Pull-up)
Figure 190. Reset Supply Current vs. VCC (1MHz - 20MHz, Excluding Current Through The
Reset Pull-up)
0
0.5
1
1.5
2
2.5
3
3.5
4
0 0.1 0.2 0.30.4 0.5 0.6 0.7 0.80.9 1
Frequency (MHz)
I
CC
(mA)
5.5V
5.0V
4.5V
3.3V
3.0V
2.7V
4.0V
0
5
10
15
20
25
0246810 12 14 16 1820
Frequency (MHz)
ICC (mA)
5.5V
5.0V
4.5V
3.3V
3.0V
2.7V
279
2486Z–AVR–02/11
ATmega8(L)
Figure 191. Reset Pulse Width vs. VCC
0
200
400
600
800
1000
1200
1400
2.5 33.5 4 4.5 5 5.5
V
CC
(V)
Pulsewidth (ns)
85
°C
25
°C
-40
°C
280
2486Z–AVR–02/11
ATmega8(L)
Register Summary
Address Name Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0 Page
0x3F (0x5F) SREG I T H S V N Z C 11
0x3E (0x5E) SPH – – – – – SP10 SP9 SP8 13
0x3D (0x5D) SPL SP7 SP6 SP5 SP4 SP3 SP2 SP1 SP0 13
0x3C (0x5C) Reserved
0x3B (0x5B) GICR INT1 INT0 –––– IVSEL IVCE 49, 67
0x3A (0x5A) GIFR INTF1 INTF0 – – – – – – 67
0x39 (0x59) TIMSK OCIE2 TOIE2 TICIE1 OCIE1A OCIE1B TOIE1 – TOIE0 72, 100, 119
0x38 (0x58) TIFR OCF2 TOV2 ICF1 OCF1A OCF1B TOV1 –TOV072, 101, 119
0x37 (0x57) SPMCR SPMIE RWWSB – RWWSRE BLBSET PGWRT PGERS SPMEN 206
0x36 (0x56) TWCR TWINT TWEA TWSTA TWSTO TWWC TWEN –TWIE 165
0x35 (0x55) MCUCR SE SM2 SM1 SM0 ISC11 ISC10 ISC01 ISC00 33, 66
0x34 (0x54) MCUCSR –––– WDRF BORF EXTRF PORF 41
0x33 (0x53) TCCR0 – – – – – CS02 CS01 CS00 71
0x32 (0x52) TCNT0 Timer/Counter0 (8 Bits) 72
0x31 (0x51) OSCCAL Oscillator Calibration Register 31
0x30 (0x50) SFIOR –––– ACME PUD PSR2 PSR10 58, 74, 120, 186
0x2F (0x4F) TCCR1A COM1A1 COM1A0 COM1B1 COM1B0 FOC1A FOC1B WGM11 WGM10 96
0x2E (0x4E) TCCR1B ICNC1 ICES1 – WGM13 WGM12 CS12 CS11 CS10 98
0x2D (0x4D) TCNT1H Timer/Counter1 – Counter Register High byte 99
0x2C (0x4C) TCNT1L Timer/Counter1 – Counter Register Low byte 99
0x2B (0x4B) OCR1AH Timer/Counter1 – Output Compare Register A High byte 99
0x2A (0x4A) OCR1AL Timer/Counter1 – Output Compare Register A Low byte 99
0x29 (0x49) OCR1BH Timer/Counter1 – Output Compare Register B High byte 99
0x28 (0x48) OCR1BL Timer/Counter1 – Output Compare Register B Low byte 99
0x27 (0x47) ICR1H Timer/Counter1 – Input Capture Register High byte 100
0x26 (0x46) ICR1L Timer/Counter1 – Input Capture Register Low byte 100
0x25 (0x45) TCCR2 FOC2 WGM20 COM21 COM20 WGM21 CS22 CS21 CS20 114
0x24 (0x44) TCNT2 Timer/Counter2 (8 Bits) 116
0x23 (0x43) OCR2 Timer/Counter2 Output Compare Register 116
0x22 (0x42) ASSR –––– AS2 TCN2UB OCR2UB TCR2UB 117
0x21 (0x41) WDTCR – – – WDCE WDE WDP2 WDP1 WDP0 43
0x20(1) (0x40)(1) UBRRH URSEL ––– UBRR[11:8] 152
UCSRC URSEL UMSEL UPM1 UPM0 USBS UCSZ1 UCSZ0 UCPOL 150
0x1F (0x3F) EEARH – – – – – – –EEAR8 20
0x1E (0x3E) EEARL EEAR7 EEAR6 EEAR5 EEAR4 EEAR3 EEAR2 EEAR1 EEAR0 20
0x1D (0x3D) EEDR EEPROM Data Register 20
0x1C (0x3C) EECR –––– EERIE EEMWE EEWE EERE 20
0x1B (0x3B) Reserved
0x1A (0x3A) Reserved
0x19 (0x39) Reserved
0x18 (0x38) PORTB PORTB7 PORTB6 PORTB5 PORTB4 PORTB3 PORTB2 PORTB1 PORTB0 65
0x17 (0x37) DDRB DDB7 DDB6 DDB5 DDB4 DDB3 DDB2 DDB1 DDB0 65
0x16 (0x36) PINB PINB7 PINB6 PINB5 PINB4 PINB3 PINB2 PINB1 PINB0 65
0x15 (0x35) PORTC – PORTC6 PORTC5 PORTC4 PORTC3 PORTC2 PORTC1 PORTC0 65
0x14 (0x34) DDRC – DDC6 DDC5 DDC4 DDC3 DDC2 DDC1 DDC0 65
0x13 (0x33) PINC – PINC6 PINC5 PINC4 PINC3 PINC2 PINC1 PINC0 65
0x12 (0x32) PORTD PORTD7 PORTD6 PORTD5 PORTD4 PORTD3 PORTD2 PORTD1 PORTD0 65
0x11 (0x31) DDRD DDD7 DDD6 DDD5 DDD4 DDD3 DDD2 DDD1 DDD0 65
0x10 (0x30) PIND PIND7 PIND6 PIND5 PIND4 PIND3 PIND2 PIND1 PIND0 65
0x0F (0x2F) SPDR SPI Data Register 127
0x0E (0x2E) SPSR SPIF WCOL – – – – –SPI2X 126
0x0D (0x2D) SPCR SPIE SPE DORD MSTR CPOL CPHA SPR1 SPR0 125
0x0C (0x2C) UDR USART I/O Data Register 148
0x0B (0x2B) UCSRA RXC TXC UDRE FE DOR PE U2X MPCM 148
0x0A (0x2A) UCSRB RXCIE TXCIE UDRIE RXEN TXEN UCSZ2 RXB8 TXB8 149
0x09 (0x29) UBRRL USART Baud Rate Register Low byte 152
0x08 (0x28) ACSR ACD ACBG ACO ACI ACIE ACIC ACIS1 ACIS0 186
0x07 (0x27) ADMUX REFS1 REFS0 ADLAR – MUX3 MUX2 MUX1 MUX0 199
0x06 (0x26) ADCSRA ADEN ADSC ADFR ADIF ADIE ADPS2 ADPS1 ADPS0 200
0x05 (0x25) ADCH ADC Data Register High byte 201
0x04 (0x24) ADCL ADC Data Register Low byte 201
0x03 (0x23) TWDR Two-wire Serial Interface Data Register 167
0x02 (0x22) TWAR TWA6 TWA5 TWA4 TWA3 TWA2 TWA1 TWA0 TWGCE 167
281
2486Z–AVR–02/11
ATmega8(L)
Notes: 1. Refer to the USART description (“USART” on page 129) for details on how to access UBRRH and UCSRC (“Accessing
UBRRH/UCSRC Registers” on page 146)
2. For compatibility with future devices, reserved bits should be written to zero if accessed. Reserved I/O memory addresses
should never be written
3. Some of the Status Flags are cleared by writing a logical one to them. Note that the CBI and SBI instructions will operate on
all bits in the I/O Register, writing a one back into any flag read as set, thus clearing the flag. The CBI and SBI instructions
work with registers 0x00 to 0x1F only
0x01 (0x21) TWSR TWS7 TWS6 TWS5 TWS4 TWS3 –TWPS1 TWPS0 166
0x00 (0x20) TWBR Two-wire Serial Interface Bit Rate Register 165
Register Summary (Continued)
Address Name Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0 Page
282
2486Z–AVR–02/11
ATmega8(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 ← 0xFF − Rd Z, C, N, V 1
NEG Rd Two’s Complement Rd ← 0x00 − Rd Z, C, N, V, H 1
SBR Rd,K Set Bit(s) in Register Rd ← Rd v K Z, N, V 1
CBR Rd,K Clear Bit(s) in Register Rd ← Rd • (0xFF - K) Z, N, V 1
INC Rd Increment Rd ← Rd + 1 Z, N, V 1
DEC Rd Decrement Rd ← Rd − 1 Z, N, V 1
TST Rd Test for Zero or Minus Rd ← Rd • Rd Z, N, V 1
CLR Rd Clear Register Rd ← Rd ⊕ Rd Z, N, V 1
SER Rd Set Register Rd ← 0xFF None 1
MUL Rd, Rr Multiply Unsigned R1:R0 ← Rd x Rr Z, C 2
MULS Rd, Rr Multiply Signed R1:R0 ← Rd x Rr Z, C 2
MULSU Rd, Rr Multiply Signed with Unsigned R1:R0 ← Rd x Rr Z, C 2
FMUL Rd, Rr Fractional Multiply Unsigned R1:R0 ← (Rd x Rr) << 1 Z, C 2
FMULS Rd, Rr Fractional Multiply Signed R1:R0 ← (Rd x Rr) << 1 Z, C 2
FMULSU Rd, Rr Fractional Multiply Signed with Unsigned R1:R0 ← (Rd x Rr) << 1 Z, C 2
BRANCH INSTRUCTIONS
RJMP k Relative Jump PC ← PC + k + 1 None 2
IJMP Indirect Jump to (Z) PC ← Z None 2
RCALL k Relative Subroutine Call PC ← PC + k + 1 None 3
ICALL Indirect Call to (Z) PC ← ZNone3
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
283
2486Z–AVR–02/11
ATmega8(L)
Mnemonics Operands Description Operation Flags #Clocks
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
Instruction Set Summary (Continued)
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Mnemonics Operands Description Operation Flags #Clocks
CLT Clear T in SREG T ← 0 T 1
SEH Set Half Carry Flag in SREG H ← 1H1
CLH Clear Half Carry Flag in SREG H ← 0 H 1
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
Instruction Set Summary (Continued)
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Ordering Information
Notes: 1. This device can also be supplied in wafer form. Please contact your local Atmel sales office for detailed ordering information
and minimum quantities
2. Pb-free packaging complies to the European Directive for Restriction of Hazardous Substances (RoHS directive). Also
Halide free and fully Green
3. Tape & Reel
Speed (MHz) Power Supply (V) Ordering Code(2) Package(1) Operation Range
8 2.7 - 5.5
ATmega8L-8AU
ATmega8L-8AUR(3)
ATmega8L-8PU
ATmega8L-8MU
ATmega8L-8MUR(3)
32A
32A
28P3
32M1-A
32M1-A Industrial
(-40°C to 85°C)
16 4.5 - 5.5
ATmega8-16AU
ATmega8-16AUR(3)
ATmega8-16PU
ATmega8-16MU
ATmega8-16MUR(3)
32A
32A
28P3
32M1-A
32M1-A
Package Type
32A 32-lead, Thin (1.0mm) Plastic Quad Flat Package (TQFP)
28P3 28-lead, 0.300” Wide, Plastic Dual Inline Package (PDIP)
32M1-A 32-pad, 5 × 5 × 1.0 body, Lead Pitch 0.50mm Quad Flat No-Lead/Micro Lead Frame Package (QFN/MLF)
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Packaging Information
32A
2325 Orchard Parkway
San Jose, CA 95131
TITLE DRAWING NO.
R
REV.
32A, 32-lead, 7 x 7 mm Body Size, 1.0 mm Body Thickness,
0.8 mm Lead Pitch, Thin Profile Plastic Quad Flat Package (TQFP) C
32A
2010-10-20
PIN 1 IDENTIFIER
0°~7°
PIN 1
L
C
A1 A2 A
D1
D
eE1 E
B
Notes:
1. This package conforms to JEDEC reference MS-026, Variation ABA.
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 8.75 9.00 9.25
D1 6.90 7.00 7.10 Note 2
E 8.75 9.00 9.25
E1 6.90 7.00 7.10 Note 2
B 0.30 – 0.45
C 0.09 – 0.20
L 0.45 – 0.75
e 0.80 TYP
COMMON DIMENSIONS
(Unit of Measure = mm)
SYMBOL MIN NOM MAX NOTE
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28P3
2325 Orchard Parkway
San Jose, CA 95131
TITLE DRAWING NO.
R
REV.
28P3, 28-lead (0.300"/7.62 mm Wide) Plastic Dual
Inline Package (PDIP) B
28P3
09/28/01
PIN
1
E1
A1
B
REF
E
B1
C
L
SEATING PLANE
A
0º ~ 15º
D
e
eB
B2
(4 PLACES)
COMMON DIMENSIONS
(Unit of Measure = mm)
SYMBOL MIN NOM MAX NOTE
A – – 4.5724
A1 0.508 – –
D 34.544 – 34.798 Note 1
E 7.620 – 8.255
E1 7.112 – 7.493 Note 1
B 0.381 – 0.533
B1 1.143 – 1.397
B2 0.762 – 1.143
L 3.175 – 3.429
C 0.203 – 0.356
eB – – 10.160
e 2.540 TYP
Note: 1. Dimensions D and E1 do not include mold Flash or Protrusion.
Mold Flash or Protrusion shall not exceed 0.25 mm (0.010").
288
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32M1-A
2325 Orchard Parkway
San Jose, CA 95131
TITLE DRAWING NO.
R
REV.
32M1-A, 32-pad, 5 x 5 x 1.0 mm Body, Lead Pitch 0.50 mm, E
32M1-A
5/25/06
3.10 mm Exposed Pad, Micro Lead Frame Package (MLF)
COMMON DIMENSIONS
(Unit of Measure = mm)
SYMBOL MIN NOM MAX NOTE
D1
D
E1 E
e
b
A3
A2
A1
A
D2
E2
0.08C
L
1
2
3
P
P
0
1
2
3
A 0.80 0.90 1.00
A1 – 0.02 0.05
A2 – 0.65 1.00
A3 0.20 REF
b 0.180.230.30
D
D1
D2 2.95 3.10 3.25
4.90 5.00 5.10
4.70 4.75 4.80
4.70 4.75 4.80
4.90 5.00 5.10
E
E1
E2 2.95 3.10 3.25
e 0.50 BSC
L 0.30 0.40 0.50
P – – 0.60
– – 12o
Note: JEDEC Standard MO-220, Fig. 2 (Anvil Singulation), VHHD-2.
TOP VIEW
SIDE VIEW
BOTTOM VIEW
0
Pin 1 ID
Pin #1 Notch
(0.20 R)
K 0.20 – –
K
K
289
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Errata The revision letter in this section refers to the revision of the ATmega8 device.
ATmega8
Rev. D to I, M
•First Analog Comparator conversion may be delayed
•Interrupts may be lost when writing the timer registers in the asynchronous timer
•Signature may be Erased in Serial Programming Mode
•CKOPT Does not Enable Internal Capacitors on XTALn/TOSCn Pins when 32KHz Oscillator is
Used to Clock the Asynchronous Timer/Counter2
•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 first 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 the asynchronous Timer/Counter register(TCNTx) is 0x00.
Problem Fix / Workaround
Always check that the asynchronous Timer/Counter register neither 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. Signature may be Erased in Serial Programming Mode
If the signature bytes are read before a chiperase command is completed, the signature may
be erased causing the device ID and calibration bytes to disappear. This is critical, espe-
cially, if the part is running on internal RC oscillator.
Problem Fix / Workaround:
Ensure that the chiperase command has exceeded before applying the next command.
4. CKOPT Does not Enable Internal Capacitors on XTALn/TOSCn Pins when 32KHz
Oscillator is Used to Clock the Asynchronous Timer/Counter2
When the internal RC Oscillator is used as the main clock source, it is possible to run the
Timer/Counter2 asynchronously by connecting a 32KHz Oscillator between XTAL1/TOSC1
and XTAL2/TOSC2. But when the internal RC Oscillator is selected as the main clock
source, the CKOPT Fuse does not control the internal capacitors on XTAL1/TOSC1 and
XTAL2/TOSC2. As long as there are no capacitors connected to XTAL1/TOSC1 and
XTAL2/TOSC2, safe operation of the Oscillator is not guaranteed.
Problem Fix / Workaround
Use external capacitors in the range of 20pF - 36pF on XTAL1/TOSC1 and XTAL2/TOSC2.
This will be fixed in ATmega8 Rev. G where the CKOPT Fuse will control internal capacitors
also when internal RC Oscillator is selected as main clock source. For ATmega8 Rev. G,
CKOPT = 0 (programmed) will enable the internal capacitors on XTAL1 and XTAL2. Cus-
tomers who want compatibility between Rev. G and older revisions, must ensure that
CKOPT is unprogrammed (CKOPT = 1).
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5. 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.
291
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Datasheet
Revision
History
Please note that the referring page numbers in this section are referred to this document. The
referring revision in this section are referring to the document revision.
Changes from Rev.
2486Y- 10/10 to
Rev. 2486Z- 02/11
1. Updated the datasheet according to the Atmel new Brand Style Guide.
2. Updated “Ordering Information” on page 285. Added Ording Information for
“Tape&Reel” devices
Changes from Rev.
2486X- 06/10 to
Rev. 2486Y- 10/10
1. Max Rise/Fall time in Table 102 on page 239 has been corrected from 1.6ns to 1600ns.
2. Note is added to “Performing Page Erase by SPM” on page 209.
3. Updated/corrected several short-cuts and added some new ones.
4. Updated last page according to new standard.
Changes from Rev.
2486W- 02/10 to
Rev. 2486X- 06/10
1. Updated “DC Characteristics” on page 235 with new VOL maximum value (0.9V and
0.6V).
Changes from Rev.
2486V- 05/09 to
Rev. 2486W- 02/10
1. Updated “ADC Characteristics” on page 241 with VINT maximum value (2.9V).
Changes from Rev.
2486U- 08/08 to
Rev. 2486V- 05/09
1. Updated “Errata” on page 289.
2. Updated the last page with Atmel’s new adresses.
Changes from Rev.
2486T- 05/08 to
Rev. 2486U- 08/08
1. Updated “DC Characteristics” on page 235 with ICC typical values.
Changes from Rev.
2486S- 08/07 to
Rev. 2486T- 05/08
1. Updated Table 98 on page 233.
2. Updated “Ordering Information” on page 285.
- Commercial Ordering Code removed.
- No Pb-free packaging option removed.
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ATmega8(L)
Changes from Rev.
2486R- 07/07 to
Rev. 2486S- 08/07
1. Updated “Features” on page 1.
2. Added “Data Retention” on page 7.
3. Updated “Errata” on page 289.
4. Updated “Slave Mode” on page 125.
Changes from Rev.
2486Q- 10/06 to
Rev. 2486R- 07/07
1. Added text to Table 81 on page 211.
2. Fixed typo in “Peripheral Features” on page 1.
3. Updated Table 16 on page 42.
4. Updated Table 75 on page 199.
5. Removed redundancy and updated typo in Notes section of “DC Characteristics” on
page 235.
Changes from Rev.
2486P- 02/06 to
Rev. 2486Q- 10/06
1. Updated “Timer/Counter Oscillator” on page 32.
2. Updated “Fast PWM Mode” on page 88.
3. Updated code example in “USART Initialization” on page 134.
4. Updated Table 37 on page 96, Table 39 on page 97, Table 42 on page 115, Table 44 on
page 115, and Table 98 on page 233.
5. Updated “Errata” on page 289.
Changes from Rev.
2486O-10/04 to
Rev. 2486P- 02/06
1. Added “Resources” on page 7.
2. Updated “External Clock” on page 32.
3. Updated “Serial Peripheral Interface – SPI” on page 121.
4. Updated Code Example in “USART Initialization” on page 134.
5. Updated Note in “Bit Rate Generator Unit” on page 164.
6. Updated Table 98 on page 233.
7. Updated Note in Table 103 on page 241.
8. Updated “Errata” on page 289.
Changes from Rev.
2486N-09/04 to
Rev. 2486O-10/04
1. Removed to instances of “analog ground”. Replaced by “ground”.
2. Updated Table 7 on page 29, Table 15 on page 38, and Table 100 on page 237.
3. Updated “Calibrated Internal RC Oscillator” on page 30 with the 1MHz default value.
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ATmega8(L)
4. Table 89 on page 218 and Table 90 on page 218 moved to new section “Page Size” on
page 218.
5. Updated descripton for bit 4 in “Store Program Memory Control Register – SPMCR”
on page 206.
6. Updated “Ordering Information” on page 285.
Changes from Rev.
2486M-12/03 to
Rev. 2486N-09/04
1. Added note to MLF package in “Pin Configurations” on page 2.
2. Updated “Internal Voltage Reference Characteristics” on page 42.
3. Updated “DC Characteristics” on page 235.
4. ADC4 and ADC5 support 10-bit accuracy. Document updated to reflect this.
Updated features in “Analog-to-Digital Converter” on page 189.
Updated “ADC Characteristics” on page 241.
5. Removed reference to “External RC Oscillator application note” from “External RC
Oscillator” on page 28.
Changes from Rev.
2486L-10/03 to
Rev. 2486M-12/03
1. Updated “Calibrated Internal RC Oscillator” on page 30.
Changes from Rev.
2486K-08/03 to
Rev. 2486L-10/03
1. Removed “Preliminary” and TBDs from the datasheet.
2. Renamed ICP to ICP1 in the datasheet.
3. Removed instructions CALL and JMP from the datasheet.
4. Updated tRST in Table 15 on page 38, VBG in Table 16 on page 42, Table 100 on page
237 and Table 102 on page 239.
5. Replaced text “XTAL1 and XTAL2 should be left unconnected (NC)” after Table 9 in
“Calibrated Internal RC Oscillator” on page 30. Added text regarding XTAL1/XTAL2
and CKOPT Fuse in “Timer/Counter Oscillator” on page 32.
6. Updated Watchdog Timer code examples in “Timed Sequences for Changing the
Configuration of the Watchdog Timer” on page 45.
7. Removed bit 4, ADHSM, from “Special Function IO Register – SFIOR” on page 58.
8. Added note 2 to Figure 103 on page 208.
9. Updated item 4 in the “Serial Programming Algorithm” on page 231.
10. Added tWD_FUSE to Table 97 on page 232 and updated Read Calibration Byte, Byte 3, in
Table 98 on page 233.
11. Updated Absolute Maximum Ratings* and DC Characteristics in “Electrical Character-
istics” on page 235.
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ATmega8(L)
Changes from Rev.
2486J-02/03 to
Rev. 2486K-08/03
1. Updated VBOT values in Table 15 on page 38.
2. Updated “ADC Characteristics” on page 241.
3. Updated “ATmega8 Typical Characteristics” on page 242.
4. Updated “Errata” on page 289.
Changes from Rev.
2486I-12/02 to Rev.
2486J-02/03
1. Improved the description of “Asynchronous Timer Clock – clkASY” on page 26.
2. Removed reference to the “Multipurpose Oscillator” application note and the “32kHz
Crystal Oscillator” application note, which do not exist.
3. Corrected OCn waveforms in Figure 38 on page 89.
4. Various minor Timer 1 corrections.
5. Various minor TWI corrections.
6. Added note under “Filling the Temporary Buffer (Page Loading)” on page 209 about
writing to the EEPROM during an SPM Page load.
7. Removed ADHSM completely.
8. Added section “EEPROM Write during Power-down Sleep Mode” on page 23.
9. Removed XTAL1 and XTAL2 description on page 5 because they were already
described as part of “Port B (PB7..PB0) XTAL1/XTAL2/TOSC1/TOSC2” on page 5.
10. Improved the table under “SPI Timing Characteristics” on page 239 and removed the
table under “SPI Serial Programming Characteristics” on page 234.
11. Corrected PC6 in “Alternate Functions of Port C” on page 61.
12. Corrected PB6 and PB7 in “Alternate Functions of Port B” on page 58.
13. Corrected 230.4 Mbps to 230.4 kbps under “Examples of Baud Rate Setting” on page
153.
14. Added information about PWM symmetry for Timer 2 in “Phase Correct PWM Mode”
on page 111.
15. Added thick lines around accessible registers in Figure 76 on page 163.
16. Changed “will be ignored” to “must be written to zero” for unused Z-pointer bits
under “Performing a Page Write” on page 209.
17. Added note for RSTDISBL Fuse in Table 87 on page 216.
18. Updated drawings in “Packaging Information” on page 286.
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ATmega8(L)
Changes from Rev.
2486H-09/02 to
Rev. 2486I-12/02
1. Added errata for Rev D, E, and F on page 289.
Changes from Rev.
2486G-09/02 to
Rev. 2486H-09/02
1. Changed the Endurance on the Flash to 10,000 Write/Erase Cycles.
Changes from Rev.
2486F-07/02 to
Rev. 2486G-09/02
1. Updated Table 103, “ADC Characteristics,” on page 241.
Changes from Rev.
2486E-06/02 to
Rev. 2486F-07/02
1. Changes in “Digital Input Enable and Sleep Modes” on page 55.
2. Addition of OCS2 in “MOSI/OC2 – Port B, Bit 3” on page 59.
3. The following tables have been updated:
Table 51, “CPOL and CPHA Functionality,” on page 127, Table 59, “UCPOL Bit Settings,”
on page 152, Table 72, “Analog Comparator Multiplexed Input(1),” on page 188, Table 73,
“ADC Conversion Time,” on page 193, Table 75, “Input Channel Selections,” on page 199,
and Table 84, “Explanation of Different Variables used in Figure 103 on page 208 and the
Mapping to the Z-pointer,” on page 214.
4. Changes in “Reading the Calibration Byte” on page 227.
5. Corrected Errors in Cross References.
Changes from Rev.
2486D-03/02 to
Rev. 2486E-06/02
1. Updated Some Preliminary Test Limits and Characterization Data
The following tables have been updated:
Table 15, “Reset Characteristics,” on page 38, Table 16, “Internal Voltage Reference Char-
acteristics,” on page 42, DC Characteristics on page 235, Table , “ADC Characteristics,” on
page 241.
2. Changes in External Clock Frequency
Added the description at the end of “External Clock” on page 32.
Added period changing data in Table 99, “External Clock Drive,” on page 237.
3. Updated TWI Chapter
More details regarding use of the TWI bit rate prescaler and a Table 65, “TWI Bit Rate Pres-
caler,” on page 167.
Changes from Rev.
2486C-03/02 to
Rev. 2486D-03/02
1. Updated Typical Start-up Times.
The following tables has been updated:
Table 5, “Start-up Times for the Crystal Oscillator Clock Selection,” on page 28, Table 6,
“Start-up Times for the Low-frequency Crystal Oscillator Clock Selection,” on page 28,
Table 8, “Start-up Times for the External RC Oscillator Clock Selection,” on page 29, and
Table 12, “Start-up Times for the External Clock Selection,” on page 32.
2. Added “ATmega8 Typical Characteristics” on page 242.
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ATmega8(L)
Changes from Rev.
2486B-12/01 to
Rev. 2486C-03/02
1. Updated TWI Chapter.
More details regarding use of the TWI Power-down operation and using the TWI as Master
with low TWBRR values are added into the datasheet.
Added the note at the end of the “Bit Rate Generator Unit” on page 164.
Added the description at the end of “Address Match Unit” on page 164.
2. Updated Description of OSCCAL Calibration Byte.
In the datasheet, it was not explained how to take advantage of the calibration bytes for 2, 4,
and 8MHz Oscillator selections. This is now added in the following sections:
Improved description of “Oscillator Calibration Register – OSCCAL” on page 31 and “Cali-
bration Byte” on page 218.
3. Added Some Preliminary Test Limits and Characterization Data.
Removed some of the TBD’s in the following tables and pages:
Table 3 on page 26, Table 15 on page 38, Table 16 on page 42, Table 17 on page 44, “TA =
-40°C to +85°C, VCC = 2.7V to 5.5V (unless otherwise noted)” on page 235, Table 99 on
page 237, and Table 102 on page 239.
4. Updated Programming Figures.
Figure 104 on page 219 and Figure 112 on page 230 are updated to also reflect that AVCC
must be connected during Programming mode.
5. Added a Description on how to Enter Parallel Programming Mode if RESET Pin is Dis-
abled or if External Oscillators are Selected.
Added a note in section “Enter Programming Mode” on page 221.
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2486Z–AVR–02/11
ATmega8(L)
Table of Contents
Features 1
Pin Configurations 2
Overview 3
Block Diagram 3
Disclaimer 4
Pin Descriptions 5
Resources 7
Data Retention 7
About Code Examples 8
Atmel AVR CPU Core 9
Introduction 9
Architectural Overview 9
Arithmetic Logic Unit – ALU 11
Status Register 11
General Purpose Register File 12
Stack Pointer 13
Instruction Execution Timing 13
Reset and Interrupt Handling 14
AVR ATmega8 Memories 17
In-System Reprogrammable Flash Program Memory 17
SRAM Data Memory 18
Data Memory Access Times 19
EEPROM Data Memory 19
I/O Memory 24
System Clock and Clock Options 25
Clock Systems and their Distribution 25
Clock Sources 26
Crystal Oscillator 27
Low-frequency Crystal Oscillator 28
External RC Oscillator 28
Calibrated Internal RC Oscillator 30
External Clock 32
Timer/Counter Oscillator 32
Power Management and Sleep Modes 33
Idle Mode 34
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ATmega8(L)
ADC Noise Reduction Mode 34
Power-down Mode 34
Power-save Mode 34
Standby Mode 35
Minimizing Power Consumption 35
System Control and Reset 37
Internal Voltage Reference 42
Watchdog Timer 43
Timed Sequences for Changing the Configuration of the Watchdog Timer 45
Interrupts 46
Interrupt Vectors in ATmega8 46
I/O Ports 51
Introduction 51
Ports as General Digital I/O 52
Alternate Port Functions 56
Register Description for I/O Ports 65
External Interrupts 66
8-bit Timer/Counter0 69
Overview 69
Timer/Counter Clock Sources 70
Counter Unit 70
Operation 70
Timer/Counter Timing Diagrams 70
8-bit Timer/Counter Register Description 71
Timer/Counter0 and Timer/Counter1 Prescalers 73
16-bit Timer/Counter1 75
Overview 75
Accessing 16-bit Registers 77
Timer/Counter Clock Sources 80
Counter Unit 80
Input Capture Unit 81
Output Compare Units 83
Compare Match Output Unit 85
Modes of Operation 87
Timer/Counter Timing Diagrams 94
16-bit Timer/Counter Register Description 96
8-bit Timer/Counter2 with PWM and Asynchronous Operation 102
Overview 102
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ATmega8(L)
Timer/Counter Clock Sources 103
Counter Unit 104
Output Compare Unit 105
Compare Match Output Unit 107
Modes of Operation 108
Timer/Counter Timing Diagrams 112
8-bit Timer/Counter Register Description 114
Asynchronous Operation of the Timer/Counter 117
Timer/Counter Prescaler 120
Serial Peripheral Interface – SPI 121
SS Pin Functionality 125
Data Modes 127
USART 129
Overview 129
Clock Generation 130
Frame Formats 133
USART Initialization 134
Data Transmission – The USART Transmitter 136
Data Reception – The USART Receiver 138
Asynchronous Data Reception 142
Multi-processor Communication Mode 145
Accessing UBRRH/UCSRC Registers 146
USART Register Description 148
Examples of Baud Rate Setting 153
Two-wire Serial Interface 157
Features 157
Two-wire Serial Interface Bus Definition 157
Data Transfer and Frame Format 158
Multi-master Bus Systems, Arbitration and Synchronization 161
Overview of the TWI Module 163
TWI Register Description 165
Using the TWI 168
Transmission Modes 171
Multi-master Systems and Arbitration 184
Analog Comparator 186
Analog Comparator Multiplexed Input 188
Analog-to-Digital Converter 189
Features 189
Starting a Conversion 191
Prescaling and Conversion Timing 191
Changing Channel or Reference Selection 194
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ATmega8(L)
ADC Noise Canceler 195
ADC Conversion Result 199
Boot Loader Support – Read-While-Write Self-Programming 202
Boot Loader Features 202
Application and Boot Loader Flash Sections 202
Read-While-Write and No Read-While-Write Flash Sections 202
Boot Loader Lock Bits 204
Entering the Boot Loader Program 205
Addressing the Flash During Self-Programming 207
Self-Programming the Flash 208
Memory Programming 215
Program And Data Memory Lock Bits 215
Fuse Bits 216
Signature Bytes 218
Calibration Byte 218
Page Size 218
Parallel Programming Parameters, Pin Mapping, and Commands 219
Parallel Programming 221
Serial Downloading 230
Serial Programming Pin Mapping 230
Electrical Characteristics 235
Absolute Maximum Ratings* 235
DC Characteristics 235
External Clock Drive Waveforms 237
External Clock Drive 237
Two-wire Serial Interface Characteristics 238
SPI Timing Characteristics 239
ADC Characteristics 241
ATmega8 Typical Characteristics 242
Register Summary 280
Instruction Set Summary 282
Ordering Information 285
Packaging Information 286
32A 286
28P3 287
32M1-A 288
Errata 289
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ATmega8(L)
ATmega8
Rev. D to I, M 289
Datasheet Revision History 291
Changes from Rev. 2486Y- 10/10 to Rev. 2486Z- 02/11 291
Changes from Rev. 2486X- 06/10 to Rev. 2486Y- 10/10 291
Changes from Rev. 2486W- 02/10 to Rev. 2486X- 06/10 291
Changes from Rev. 2486V- 05/09 to Rev. 2486W- 02/10 291
Changes from Rev. 2486U- 08/08 to Rev. 2486V- 05/09 291
Changes from Rev. 2486T- 05/08 to Rev. 2486U- 08/08 291
Changes from Rev. 2486S- 08/07 to Rev. 2486T- 05/08 291
Changes from Rev. 2486R- 07/07 to Rev. 2486S- 08/07 292
Changes from Rev. 2486Q- 10/06 to Rev. 2486R- 07/07 292
Changes from Rev. 2486P- 02/06 to Rev. 2486Q- 10/06 292
Changes from Rev. 2486O-10/04 to Rev. 2486P- 02/06 292
Changes from Rev. 2486N-09/04 to Rev. 2486O-10/04 292
Changes from Rev. 2486M-12/03 to Rev. 2486N-09/04 293
Changes from Rev. 2486L-10/03 to Rev. 2486M-12/03 293
Changes from Rev. 2486K-08/03 to Rev. 2486L-10/03 293
Changes from Rev. 2486J-02/03 to Rev. 2486K-08/03 294
Changes from Rev. 2486I-12/02 to Rev. 2486J-02/03 294
Changes from Rev. 2486H-09/02 to Rev. 2486I-12/02 295
Changes from Rev. 2486G-09/02 to Rev. 2486H-09/02 295
Changes from Rev. 2486F-07/02 to Rev. 2486G-09/02 295
Changes from Rev. 2486E-06/02 to Rev. 2486F-07/02 295
Changes from Rev. 2486D-03/02 to Rev. 2486E-06/02 295
Changes from Rev. 2486C-03/02 to Rev. 2486D-03/02 295
Changes from Rev. 2486B-12/01 to Rev. 2486C-03/02 296
Table of Contents i
2486Z–AVR–02/11
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