Support for using the ATmega8 as standalone board. More...
Detailed Description
Support for using the ATmega8 as standalone board.
Overview
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
| MCU | ATmega8 |
|---|---|
| Family | AVR/ATmega |
| Vendor | Microchip (previously Atmel) |
| RAM | 1KiB |
| Flash | 8KiB |
| EEPROM | 512B |
| Frequency | 1MHz/8MHz (up to 16MHz with external clock) |
| Timers | 3 (2x 8bit, 1x 16bit) |
| ADCs | 6 analog input pins |
| UARTs | 1 |
| SPIs | 1 |
| I2Cs | 1 (called TWI) |
| Vcc | 4.5V - 5.5V (ATmega8), 2.7V - 5.5V (ATmega8L) |
| Datasheet | Official datasheet |
Pinout
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 |
|---|---|
| MISO | 18/PB4/MISO |
| VCC | 7/VCC |
| SCK | 19/PB5/SCK |
| MOSI | 17/PB3/MOSI |
| RESET | 1/RESET |
| 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.
Files | |
| file | board.h |
| Board specific definitions for the standalone ATmega8 "board". | |
| file | periph_conf.h |
| Peripheral MCU configuration for the ATmega8 standalone "board". | |
