Bootloading ATmega 328 containing 16Mhz Arduino bootloader with 8Mhz Arduino bootloader

arduino_atmega_8mhz.md

Instructions how to turn ATmega chip having bootloader for 16Mhz Arduino to bootloader for 8Mhz Arduino

1. Alter boards.txt for Arduino IDE

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xx.name=328 8mhz external
xx.upload.tool=avrdude
xx.upload.protocol=arduino
xx.upload.maximum_size=30720
xx.upload.maximum_data_size=2048
xx.upload.speed=57600

xx.bootloader.tool=avrdude
xx.bootloader.unlock_bits=0x3F
xx.bootloader.lock_bits=0x0F
xx.bootloader.low_fuses=0xFF
xx.bootloader.high_fuses=0xDA
xx.bootloader.extended_fuses=0x05
xx.bootloader.file=atmega/ATmegaBOOT_168_atmega328_pro_8MHz.hex

xx.build.mcu=atmega328p
xx.build.f_cpu=8000000L
xx.build.core=arduino
xx.build.variant=standard

2. Put this chip in a board with 16Mhz crystal which supports usbasp protocol. In Arduino select board "328 8mhz external", select programmer and burn bootloader
3. Put this chip in a board with 8Mhz crystal, and now it can be programmed with TTL cable from Arduino or Platformio.
4. Configuration for platformio

platform = atmelavr
framework = arduino
board = uno
# board = pro8MHzatmega328
# board = 328p8m
upload_speed = 57600
# upload_protocol = usbasp
board_f_cpu = 8000000L