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Standalone ATmega8

Support for using the ATmega8 as standalone board. More...

Detailed Description

Support for using the ATmega8 as standalone board.


As the ATmega8 can run from the internal oscillator, placing it on a breadboard, connecting an USB-UART adapter and power is enough to run RIOT on it. (An ISP programmer will be needed to program it; or to program a bootloader to subsequently allow programming via UART.)


MCU ATmega8
Family AVR/ATmega
Vendor Microchip (previously Atmel)
Flash 8KiB
Frequency 1MHz/8MHz (up to 16MHz with external clock)
Timers 3 (2x 8bit, 1x 16bit)
ADCs 6 analog input pins
SPIs 1
I2Cs 1 (called TWI)
Vcc 4.5V - 5.5V (ATmega8), 2.7V - 5.5V (ATmega8L)
Datasheet Official datasheet


Pinout of the ATmega328p

All credit for above pinout image goes to https://github.com/MCUdude/MiniCore#pinout

Clock Frequency

The ATmega8 has an internal oscillators clocked at 1MHz that allow it to be operated without any external clock source or crystal. By default the fuses are configured to use the internal oscillator and an operating mode resulting in a clock speed of 1MHz. By setting the CKSEL fuses to 0100 the clock will operate at 8MHz without an external clock source. This can be done like this:

avrdude -c usbasp -p m8 -B 32 -U lfuse:w:0xe4:m

(Replace usbasp with the ISP programmer you are using. The -B 32 might be needed on some ISP programmers to communicate with slow ATmega MCUs. It will not be needed anymore after the clock device has been disabled.)

This "board" is configured to use 8MHz as core clock, so that the ATmega8 runs at the highest frequency possible without external clock sources.

By setting the environment variable ATMEGA8_CLOCK to a custom frequency in Hz (e.g. 1000000 for 1MHz), this core clock can be changed easily. Refer to the datasheet on how to configure the ATmega8 to use an external crystal, an external clock source or the clock divider.

Relation Between Supply Voltage, Clock Frequency and Power Consumption

A higher supply voltage results in a higher current drawn. Thus, lower power consumption can be achieved by using a lower supply voltage. However, higher clock frequencies require higher supply voltages for reliable operation.

The lowest possible supply voltage at 8 MHz is 2.7V (Atmega8L) or 4.5V (Atmega8).

Flashing the Device

In order to flash the ATmega8 without a bootloader, an ISP programmer is needed. Connect the programmer as follows:

ISP pin ATmega8 pin
Ground 22/GND

The tool avrdude needs to be installed. When using the usbasp running

make BOARD=atmega8 flash

will take care of everything. To use the programmer <FOOBAR> instead, run

make BOARD=atmega8 PROGRAMMER=<FOOBAR> flash

Serial Terminal

Connect a TTL adapter with pins 2/RXD and 3/TXD an run

make BOARD=atmega8 term

Please note that the supply voltage should be compatible with the logic level of the TTL adapter. Usually everything between 3.3 V and 5 V should work.


file  board.h
 Board specific definitions for the standalone ATmega8 "board".
file  periph_conf.h
 Peripheral MCU configuration for the ATmega8 standalone "board".