Specific support for the STM32 hardware is available at the Arduino for STM32 forum and the repository for "Roger's core" (also referred to as LibMaple in the forum). There's also an "official" Arduino core with more support, but the USBComposite library (which drives the main functions of the keyboard) is not available for it, so we can't use it here.

How to build the firmware

First of all, you CAN'T compile this from DevTerm itself. The compiler (arm-none-eabi-gcc 4.8.3-2014q1) has no support for aarch64 as far as I know, but workarounds are possible (you can try using a newer version of arm-none-eabi-gcc).

Install Arduino CLI, create a configuration file and add the STM32 package url to the board manager:

arduino-cli config init
arduino-cli config add board_manager.additional_urls http://dan.drown.org/stm32duino/package_STM32duino_index.json

Install the core:

arduino-cli core update-index
arduino-cli core install stm32duino:STM32F1

(Optinal) Check which variant of the board you need to compile to (STM32F103R8 is the default):

arduino-cli board details --fqbn stm32duino:STM32F1:genericSTM32F103R

Add the DSP library (for the trackball) to the Arduino Library folder:

cd ~/Arduino/libraries
git clone https://github.com/dangpzanco/dsp

To compile, just run build_bin.sh. This script runs arduino-cli compile with the default options (change it according to your keyboard variant):

arduino-cli compile --fqbn stm32duino:STM32F1:genericSTM32F103R:device_variant=STM32F103R8,upload_method=DFUUploadMethod,cpu_speed=speed_72mhz,opt=osstd devterm_keyboard.ino --output-dir upload

The firmware devterm_keyboard.ino.bin will be saved to the upload directory.

Customizing the trackball

TO DO (see the comments in trackball.ino)

How to upload the firmware

Uploading directly via arduino-cli (or the Arduino IDE) might be possible, but I couldn't do it consistently.

One of the main problems when uploading the firmware is reliably resetting the keyboard to the DFU mode without pressing the reset button on the back of the board. If something goes wrong, you might need a paperclip to "press" it. The reset button is not actually soldered to the board, so you need to short it with some small metal thingy (I've been using the back of a SIM card ejector).

Setup device permissions

In order to have user access to serial port, you need to add this udev rule to 50-devterm_keyboard.rules:

SUBSYSTEM=="usb", ATTRS{idVendor}=="1eaf", ATTRS{idProduct}=="0024", SYMLINK+="ttyACM0", MODE="664", GROUP="uucp"
SUBSYSTEM=="usb", ATTRS{idVendor}=="1eaf", ATTRS{idProduct}=="0003", MODE="664", GROUP="uucp"

This is a bit of a trial and error, so also check the Arduino support for extra steps.

Uploading

The upload_bin.sh script inside the upload folder uses a Python script (dfumode.py) to reset the keyboard, which relies on the pySerial library to send the commands via serial. You should install it before running the script:

pip install pyserial

Just run upload_bin.sh and it should enter DFU mode and upload the firmware devterm_keyboard.ino.bin:

cd upload
./upload_bin.sh

If the upload fails because of permissions, you should still be able to upload with sudo, but you might need to install the package python-pyserial:

pamac install python-pyserial # on Manjaro
sudo apt install python3-serial # on Debian/Ubuntu
sudo ./upload_bin.sh

The original upload method is via upload-reset.c, so it's included as well.