Quinn Dunki did something like that when building her "Veronica" 68k computer. IIRC the VGA output remains AVR-based. Seems like a good way to go, provided you

Quinn Dunki did something like that when building her "Veronica" 68k computer. IIRC the VGA output remains AVR-based. Seems like a good way to go, provided you don't get bogged down in timing constraints. If you just need to test a unit (and don't need full functionality), a nice logic analyzer works well... I have an HP 16700A with pattern generator, which I can use to simulate just about anything (more or less.. sometimes it's a bit cumbersome to use.)