Datasheet
Table Of Contents
- Features
- Pin Configurations
- Overview
- Resources
- Data Retention
- About Code Examples
- Atmel AVR CPU Core
- AVR ATmega8 Memories
- System Clock and Clock Options
- Power Management and Sleep Modes
- System Control and Reset
- Interrupts
- I/O Ports
- Introduction
- Ports as General Digital I/O
- Alternate Port Functions
- 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
- External Interrupts
- 8-bit Timer/Counter0
- Timer/Counter0 and Timer/Counter1 Prescalers
- 16-bit Timer/Counter1
- Overview
- Accessing 16-bit Registers
- Timer/Counter Clock Sources
- Counter Unit
- Input Capture Unit
- Output Compare Units
- Compare Match Output Unit
- Modes of Operation
- Timer/Counter Timing Diagrams
- 16-bit Timer/Counter Register Description
- Timer/Counter 1 Control Register A – TCCR1A
- Timer/Counter 1 Control Register B – TCCR1B
- Timer/Counter 1 – TCNT1H and TCNT1L
- Output Compare Register 1 A – OCR1AH and OCR1AL
- Output Compare Register 1 B – OCR1BH and OCR1BL
- Input Capture Register 1 – ICR1H and ICR1L
- Timer/Counter Interrupt Mask Register – TIMSK(1)
- Timer/Counter Interrupt Flag Register – TIFR(1)
- 8-bit Timer/Counter2 with PWM and Asynchronous Operation
- Serial Peripheral Interface – SPI
- USART
- Two-wire Serial Interface
- Analog Comparator
- Analog-to- Digital Converter
- Boot Loader Support – Read- While-Write Self- Programming
- Boot Loader Features
- Application and Boot Loader Flash Sections
- Read-While-Write and No Read- While-Write Flash Sections
- Boot Loader Lock Bits
- Entering the Boot Loader Program
- Addressing the Flash During Self- Programming
- Self-Programming the Flash
- Performing Page Erase by SPM
- Filling the Temporary Buffer (Page Loading)
- Performing a Page Write
- Using the SPM Interrupt
- Consideration While Updating BLS
- Prevent Reading the RWW Section During Self-Programming
- Setting the Boot Loader Lock Bits by SPM
- EEPROM Write Prevents Writing to SPMCR
- Reading the Fuse and Lock Bits from Software
- Preventing Flash Corruption
- Programming Time for Flash when using SPM
- Simple Assembly Code Example for a Boot Loader
- ATmega8 Boot Loader Parameters
- Memory Programming
- Program And Data Memory Lock Bits
- Fuse Bits
- Signature Bytes
- Calibration Byte
- Page Size
- Parallel Programming Parameters, Pin Mapping, and Commands
- Parallel Programming
- Enter Programming Mode
- Considerations for Efficient Programming
- Chip Erase
- Programming the Flash
- Programming the EEPROM
- Reading the Flash
- Reading the EEPROM
- Programming the Fuse Low Bits
- Programming the Fuse High Bits
- Programming the Lock Bits
- Reading the Fuse and Lock Bits
- Reading the Signature Bytes
- Reading the Calibration Byte
- Parallel Programming Characteristics
- Serial Downloading
- Serial Programming Pin Mapping
- Electrical Characteristics – TA = -40°C to 85°C
- Electrical Characteristics – TA = -40°C to 105°C
- ATmega8 Typical Characteristics – TA = -40°C to 85°C
- Active Supply Current
- Idle Supply Current
- Power-down Supply Current
- Power-save Supply Current
- Standby Supply Current
- Pin Pull-up
- Pin Driver Strength
- Pin Thresholds and Hysteresis
- Bod Thresholds and Analog Comparator Offset
- Internal Oscillator Speed
- Current Consumption of Peripheral Units
- Current Consumption in Reset and Reset Pulsewidth
- ATmega8 Typical Characteristics – TA = -40°C to 105°C
- Register Summary
- Instruction Set Summary
- Ordering Information
- Packaging Information
- Errata
- Datasheet Revision History
- Changes from Rev. 2486Z- 02/11 to Rev. 2486AA- 02/2013
- Changes from Rev. 2486Y- 10/10 to Rev. 2486Z- 02/11
- Changes from Rev. 2486X- 06/10 to Rev. 2486Y- 10/10
- Changes from Rev. 2486W- 02/10 to Rev. 2486X- 06/10
- Changes from Rev. 2486V- 05/09 to Rev. 2486W- 02/10
- Changes from Rev. 2486U- 08/08 to Rev. 2486V- 05/09
- Changes from Rev. 2486T- 05/08 to Rev. 2486U- 08/08
- Changes from Rev. 2486S- 08/07 to Rev. 2486T- 05/08
- Changes from Rev. 2486R- 07/07 to Rev. 2486S- 08/07
- Changes from Rev. 2486Q- 10/06 to Rev. 2486R- 07/07
- Changes from Rev. 2486P- 02/06 to Rev. 2486Q- 10/06
- Changes from Rev. 2486O-10/04 to Rev. 2486P- 02/06
- Changes from Rev. 2486N-09/04 to Rev. 2486O-10/04
- Changes from Rev. 2486M-12/03 to Rev. 2486N-09/04
- Changes from Rev. 2486L-10/03 to Rev. 2486M-12/03
- Changes from Rev. 2486K-08/03 to Rev. 2486L-10/03
- Changes from Rev. 2486J-02/03 to Rev. 2486K-08/03
- Changes from Rev. 2486I-12/02 to Rev. 2486J-02/03
- Changes from Rev. 2486H-09/02 to Rev. 2486I-12/02
- Changes from Rev. 2486G-09/02 to Rev. 2486H-09/02
- Changes from Rev. 2486F-07/02 to Rev. 2486G-09/02
- Changes from Rev. 2486E-06/02 to Rev. 2486F-07/02
- Changes from Rev. 2486D-03/02 to Rev. 2486E-06/02
- Changes from Rev. 2486C-03/02 to Rev. 2486D-03/02
- Changes from Rev. 2486B-12/01 to Rev. 2486C-03/02
- Table of Contents
37
2486AA–AVR–02/2013
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 (V
POT
)
• 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 V
CC
is below the Brown-out
Reset threshold (V
BOT
) and the Brown-out Detector is enabled