arduino-on-atmega8.md

arduino on atmega8

• purpose
• requirements
• howto
  • install boards into arduino
  • setup fuses
  • recover locked fuses
  • upload a sketch
  • wiring notes
• pin mapping
• serial communication
• oled SSD1306
• reset

purpose

Reuse of standalone atmega8 chips for small projects ( flash is 8kb vs 32kb of atmega328 standard for arduino ). In order to program these chip using arduino and its libraries there is no need to flash a bootloader if you use an external usbasp programmer connected to miso,mosi,sck,reset of target atmega.

requirements

• usbasp programmer
• atmega8 chip
• breadbord
• dupont cables

to test

• 1 led
• 1k resistor

howto

install boards into arduino

thanks to this project its easy to enable arduino to talk to atmega8 processor even at 8Mhz internal rc oscillator speed, to install just go to File/Preferences and in Additional Boards Manager URLS insert the json file https://raw.githubusercontent.com/carlosefr/atmega/master/package_carlosefr_atmega_index.json. now you'll see in tools boards ATmega Microcontrollers that include also 8/168/328 models

setup fuses

avrdude -c usbasp -p m8 -B 10.0 -U lfuse:w:0xa4:m -U hfuse:w:0xcd:m

• references
  • avr fuse calculator
  • for other models and/or different clock sources see boards.txt

Follows table of 8Mhz internal for various models

model	low fuse	high fuse	extended fuse
m8	0xa4	0xcd
m8 (*bod disabled)	0x64	0xcd
m168/168p	0xe2	0xdd	0xf9
m328/328p	0xe2	0xdb	0xff

(*) to disable Brown-out Detection on atmega8 need to set by fuse, while atmega328 support set at runtime

recover locked fuses

if inadvertently set a 16mhz clock usbasp could not connect atmega without proper crystal anymore, to recover you need to connect a crystal this way:

• XTAL1 -> crystal end1 -> capacitor 22pF to GND
• XTAL2 -> crystal end2 -> capacitor 22pF to GND

then issue a restore of fuse to use eg. 8mhz internal if want to go without requiring a crystal.

upload a sketch

• Tools

  • Board/ATmega8
  • Processor/ATmega8
  • Programmer/USBasp

• File/Examples/Basics/Led

• Sketch/Upload

wiring notes

• refer to official datasheet for an overview of pinout for atmega8
• connect GND,VCC,RESET,SCK,MISO,MOSI from usbasp to atmega
• if want to use 16Mhz you need external crystal plus capacitors and use different fuses ( low:0xdf )
• LED_BUILTIN for atmega8 is PB5

pin mapping

refer to this for atmega8 arduino pin mapping

serial communication

• connect a USB to TTL UART converter ( GND, RX-to-TX, TX-to-RX )
• setup speed if needed through Serial.begin(speed) in setup() then use Serial.println(x) as usual ( I tested it actually works even at 112500baud )

oled SSD1306

• it works even with 8mhz internal rc at 3.3v
• use https://github.com/olikraus/u8g2/ ( Sketch/Include/Manage libraries then filter for u8g2 )
• connect oled i2c display ( GND, VCC, SDA, SCL ) and uncomment U8G2_SSD1306_128X64_NONAME_F_SW_I2C constructor from the u8g2/FullBuffer/HelloWorld sample

reset

• just connect a push button between reset and gnd to restart program