Mikkel says, “The C64 USB keyboard connectos to a PC and is detected as a USB HID device. It uses Objective Development’s AVR-USB firmware only driver on an Atmel ATmega8. The microcontroller takes care of scanning the keyboard matrix of an old Commodore 64 “breadbox” and debouncing and converting keypresses to USB HID reports. Should be easily modifiable for creating a USB keyboard in a different form factor. Full source code and hardware documentation available.” Link.
C64 USB keyboard