You need to have avrdude and avr-gcc installed on a Raspberry Pi.
These two sites were very helpful in wiring up the ATmega328P to the GPIO pins on the Raspberry Pi:
- http://ozzmaker.com/program-avr-using-raspberry-pi-gpio/
- https://learn.adafruit.com/program-an-avr-or-arduino-using-raspberry-pi-gpio-pins/configuration
The pin numbers in avrdude.conf are the GPIO numbers, not the physical pin numbers (so with the pinout diagram at https://www.jameco.com/Jameco/workshop/circuitnotes/raspberry-pi-circuit-note.html you use the gp* numbers in avrdude.conf, not the numbers on the pins themselves).
Once everything is hooked up, make sure avrdude can see the MCU. This is assuming you named your custom programmer in avrdude.conf "pi".
avrdude -c pi -p m328p -v
Assuming that was successful, we can erase the MCU to start fresh:
avrdude -c pi -p m328p -v -e
You need to set various fuses on the MCU. This is a super helpful beginner-level tutorial: https://medium.com/@MasonWallerQVJ/setting-up-an-external-crystal-clock-source-with-fuse-bits-for-avr-atmega-microcontrollers-31b2e79ae09a
We'll set fuses for a 16 MHz crystal oscillator, unprogram CKDIV8 (divide clock speed by 8), disable brown-out detection, and probably some other things I'm forgetting about:
sudo avrdude -c pi -p m328p -v -u -U lfuse:w:0xFF:m
sudo avrdude -c pi -p m328p -v -u -U hfuse:w:0xD8:m
sudo avrdude -c pi -p m328p -v -u -U efuse:w:0xFF:m
Download USBaspLoader from https://github.com/baerwolf/USBaspLoader.
In Makefile.inc set:
F_CPU = 16000000
DEVICE = atmega328p
FLASHADDRESS = 0x7000
Compile and flash USBaspLoader:
make clean
make firmware
sudo make flash
Physically transfer the ATmega328P to another breadboard that's wired similar to the with-zener schematic from the V-USB repo (you can open this with an older free version of EAGLE; version 6.6.0 on macOS works) or the schematic for the plaid keyboard (you can open this with KiCad).
To test that the bootloader is working, hold down the switch that's wired to the JUMPER_BIT defined in USBaspLoader's bootloaderconfig.h, press the switch that's wired to the reset pin (pin 1 or PC6), release the reset switch, and then reset the jumper switch. This should boot the MCU into the bootloader that you just flashed, and you should be able to see the chip with avrdude:
avrdude -c usbasp -p m328p -v
If this doesn't work, something is probably wrong with your wiring, the fuses, or the bootloader didn't flash correctly.
I don't have in-depth instructions for this, but the QMK hand-wiring guide is helfpul. All of the changes mentioned are in this commit.
Clone the QMK repo from https://github.com/qmk/qmk_firmware. Define the matrix rows and columns. Comment out soft serial pin. Add your layout to handwired2x2.h. Edit rules.mk:
MCU = atmega328p
ARCH = AVR
Comment out BOOTLOADER.
Add:
PROGRAM_CMD = avrdude -c usbasp -p m328p -v -U flash:w:$(BUILD_DIR)/$(TARGET).hex
Set BOOTLOADER_SIZE to 2048.
Compile and flash from the QMK repo directory:
make handwired2x2:default:program