I made a first prototype with a STM32F411 that I had in stock, and it happened to work fairly well and was quick to test thanks to the STM32 framework.
Then I also ordered some CH554 boards, and evaluated the possibility to use them for a USB mouse to Atari adapter. See on the picture the poor Electrodragon’s board with its female USB on top of the original male one, and the Subd-9 connector on the side…
I then quickly drawn a PCB around the CH554G , easier to solder and still small enough to fit under the USB plug. It all fits perfectly under the Atari Falcon keyboard :
Source Code : add to examples folder in ch554_sdcc (https://github.com/Blinkinlabs/ch554_sdcc)