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Published on Feb 2, 2013
Motorola 6800 processor running on a replica Apple 1 (Mimeo 1) computer.
The Apple 1 computer originally used a MOS 6502 microprocessor. The design by Steve Wozniak also made allowances to use a Motorola 6800 (or MOS 6501) processor.
Eric Smith recently wrote a version of the Apple 1 monitor program for the 6800 which runs on the MESS emulator (See http://whats.all.this.brouhaha.com/20...), but to my knowledge, noone has been successful in getting it to run on real hardware...Until now!
In theory, to use a 6800, the clock generator section needs to be fitted with components, a 7404 fitted to position C1, and the two "6502" solder links opened on the PCB.
However, I found that the 6800 won't work on the Apple 1 as is. Firstly using a 7404 in position C1 is too fast to properly generate the two-phase clock. However, I found that using a 74L04 works (because of a longer propagation delay). Secondly, pin 39 on the processor is left floating which causes unreliable operation. Pulling this to ground with a 10k resistor fixes that issue.