This is a super high level (and probably incomplete in some places) guide and list of resources related to using a Raspberry Pi’s GPIO pins to flash USBaspLoader onto an ATmega328P microcontroller unit (MCU) and flashing QMK onto it over USB. I wrote it up after learning a bit about circuit design and MCU programming, and I hope it’s helpful as a resource to others.

Software prerequisites

You need to have avrdude and avr-gcc installed on a Raspberry Pi.

Flashing USBaspLoader

These two sites were very helpful in wiring up the ATmega328P to the GPIO pins on the Raspberry Pi:

The pin numbers in avrdude.conf are the GPIO numbers, not the physical pin numbers (so with the pinout diagram here 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

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 here.

In set:

F_CPU = 16000000
DEVICE = atmega328p

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 or the fuses, or the bootloader didn’t flash correctly.

Flashing QMK

I don’t have in-depth instructions for this, but the QMK hand-wiring guide is helpful. All of the changes mentioned are in this commit.

Clone the QMK repo here. Define the matrix rows and columns. Comment out soft serial pin. Add your layout to handwired2x2.h. Edit

MCU = atmega328p

Comment out BOOTLOADER.


PROGRAM_CMD = avrdude -c usbasp -p m328p -v -U flash:w:$(BUILD_DIR)/$(TARGET).hex


Compile and flash from the QMK repo directory:

make handwired2x2:default:program