The firmware is based on this USB HID examples. I will post the code within a few days, just need to clean it a little bit.

It's nothing super fancy, just code for handling the keyboard matrix. Just don't forget to disable the PD7 NRST functionality using the './minichlink -D' command.

Another interesting thing I encountered is that each USB device should have a unique combination of Vendor ID and Product ID. So, I invested a couple of bucks to buy it for PikoKey from MCS electronics (VID=16D0, PID=13B9).