The firmware is written in C for the Raspberry Pi Pico. If you instead prefer a more standard toolkit, check out the experimental QMK config for Chrumm.
The firmware implements two USB HID interfaces. Interface 0 acts as a standard Boot Keyboard. Interface 1 sends HID Consumer codes for application-specific controls, including multimedia.
Two layers are available. Hold down the Fn key to momentarily
activate the alternative layer. Double tap Fn to stay on the
alternative layer until Fn is tapped again.
Optionally, the rotation signals of an EC11 encoder can be mapped to virtual key presses.
The special kBOOT keycode (Fn+Z) can be tapped three times
to put the keyboard into bootloader mode without the need to
unplug the USB cable.
For debugging purposes, the on-board LED of the Pico indicates non-default lock states. When Caps Lock is on, the LED is on. When Num Lock is off, the LED blinks once every 2 seconds.
The default UF2 binary is available on the Releases page.
To compile a custom version, install the Pico SDK and Arm GNU Toolchain.
Set the PICO_SDK_PATH environment variable to the SDK directory.
Append the Arm bin directory to the PATH environment variable.
The firmware can then be prepared and compiled with:
cmake -B build
cmake --build build
Hold down the BOOTSEL button on the Pico while plugging in
the USB cable. It should show up as a USB storage device.
After the first install, you can keep the keyboard plugged in
and instead tap Fn+Z three times.
Drag-and-drop the UF2 binary from the build directory onto the device. The Pico should automatically restart and run the firmware. Alternatively, install picotool and use:
picotool load build/chrumm.uf2 --force
picotool reboot