PCB design for [M]ouse project, a PS/2 to 1351 mouse adapter for Commodore 64. Simple design, THT components.
Programming ATmega:
Firmware: https://github.com/svofski/mouse1351/blob/master/boot/mouse%2Bboot.hex
Fuses: HFUSE: 0xDC LFUSE: 0xE4
Even the cheapest programmer like USBasp compatible will do.
PS/2 Mouse adapter for Commodore 64 (1351 mouse hardware emulation)
svofski, Damian Woroch and others