6

The Apple-1 motherboard was designed in such a way as to make it possible for the hobbyist to remove the 6502 processor and use a Motorola 6800 as the CPU instead. This was not a trivial operation, as the 6502 and the 6800 were not pin-compatible (the earlier 6501 was pin-compatible, but was withdrawn after Motorola sued MOS Technology). However, some other hardware would need to be added, and the software needed to operate it would be completely different.

https://apple2history.org/history/ah02/#03

The picture below confirms that there was an area in the upper-left labeled (6800 ONLY). What did these changes specifically do? For example, which 6800 signals had been moved to other pins on the 6502? What new signals would need to be generated for the 6800?

Apple 1 board

Related: Was there ever any real use for a 6800-based Apple 1?

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The 6502 works with a single square wave clock signal that is connected to Phi0 pin 37. The chip has a built-in clock generator to use this clock input to internally generate two anti-phase non-overlapping clocks and will output these on two clock output pins, Phi1 (3) and Phi2 (39).

The 6800 also needs two anti-phase non-overlapping clocks for internal operation, but as it does not have a built-in clock generator, it is not able to generate these internally from a single phase clock. So these two clocks must be externally generated for the two 6800 clock inputs Phi1 and Phi2, pins 3 and 37.

The circuitry labeled "6800 only" is the external clock generator for the 6800. The circuitry uses logic chips to take in a single square wave clock signal and convert it into two non-overlapping anti-phase clock signals. Then there is a buffer stage made of discrete transistors to drive the clocks into the 6800.

As a side note, the 6501 would have been pin-compatible with 6800, but it was discontinued due to legal pressure.

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  • So the only "pin-compatibility" between the 6502 and the 6800 were the addition of those two clocks to the 6800 on unassigned pins? (Or the removal for the need of those two clocks to the 6502, not sure which direction things went ...)
    – davidbak
    Feb 27 at 16:09

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