21 Jul
2010
21 Jul
'10
7:41 p.m.
On 21/07/2010 18:17, JP Hindin wrote:
Hey;
I was reading something the other day about being able to hand-clock a
Z80, that it was so stable (due to not using dynamic registers,
apparently) that with the appropriate debounce circuit you could literally
manually step it through instructions.
Is this as rare as it sounds?
Has anyone -tried- hand-clocking a Z80?
Yes, using a pair of NAND gates forming an RS flip-flop to debounce a
switch. I used to have a 7400 taped to a microswitch with three leads
coming off it (+5V, 0V, and "clock") for that very reason.
I think the last time I used it was to demonstrate to a student that it
was his Z80-to-memory interface that was broken, not my RAM test :-)
Single-stepping can be a useful tool. I remember being disgusted that a
6502 can't do that (it's dynamic so there's a minimum clock speed which
is a lot faster than I can click a microswitch - somewhere in the kHz
range IIRC). Of course on most microprocessors you can devise a circuit
using WAIT states to achieve a similar (but not identical) effect; you
just can't stop at arbitrary parts of the machine cycle. With a Z80,
you can stop in any T-state you want.
--
Pete Peter Turnbull
Network Manager
University of York