zero wing firmware
hardware: https://easyeda.com/a1k0n/zerowing
requires sdcc for compiling: on linux, sudo apt install sdcc; on MacOS, brew install sdcc
make to compile, make flash to program.
flashing requires stm8flash, an ST-Link V2 or equivalent, and a header to connect to the SWIM/NRST/GND/+5V pins on the bottom of the board.