“The project keeps the original Casio LCD but replaces the entire motherboard with a modern microcontroller and open firmware.”
The F-91W’s case and LCD stay. The guts become an open microcontroller with a thermometer, a stopwatch and sunrise times, configured by editing one C array. There’s even a WebAssembly emulator to test your watch faces in a browser. A cheap watch you can rewrite yourself beats an expensive one that phones home.