Hi! First off, let me thank you all in advance for your help. I have found this forum, and many of the places linked from it including Garth's 6502 Primer is be absolutely invaluable. I have no doubt it will continue to be so.
To my current design:
I want to enable single-stepping on an instruction-by-instruction basis, using the RDY and SYNC signals on the WDC 65C02. I have designed a circuit (see attachment), which was in part inspired by
Thrashbarg's design and the circuit suggested on page 12 of
The Apple I manual.
There's nothing wrong with Thrashbarg's design in itself, but I wanted to reduce the number of different types of component, and the 74HC123's datasheet has some omninous warnings about the dangers of the timing capacitor ruining things under certain circumstances. The Apple circuit seemed to me a much simpler solution, though I don't think its a complete design as it doesn't appear to actually work as drawn and doesn't have any debounce in place. As a bonus, I had a free flip-flop I used for an NMI button, which I'll use like a software 'Abort' button, similar to Garth's.
I'm hoping that someone might kindly look over my circuit for any potential pitfalls I've missed, and also perhaps help me further reduce the component count. In particular, I would love to be able to use actual momentary SPST switches for the Step and NMI buttons rather than SPDT, and I have a sneaking suspicion that not all of the flip-flops are strictly necessary.
Thank you.
P.S. The attachment is an excerpt from the larger computer design, hence the high part numbers. I can provide the whole design if necessary.
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Want to design a PCB for your project? I strongly recommend
KiCad. Its free, its multiplatform, and its easy to learn!
Also, I maintain KiCad libraries of
Retro Computing and
Arduino components you might find useful.