Z80 /WAIT signal question

Alexis Kotlowy thrashbarg at kaput.homeunix.org
Fri Apr 22 04:15:16 CDT 2016

On 22/04/2016 3:06 AM, Eric Smith wrote:
> A friend building a Z80 system asked me about whether the Z80 /WAIT
> signal has any effect during machine cycles that aren't
> memory/IO/intack cycles (i.e., neither /MREQ and /IORQ asserted). The
> user manual only describes the use of /WAIT for adding wait states,
> so I expect it probably only affects mem/IO/intack cycles, but I
> can't find anything definitive in the user manual.
> I'm hoping someone can save me the time of hooking up a logic
> analyzer and running the experiment.
> Thanks! Eric


Looking at this Z80 datasheet (page 20 of the PDF) the timing diagrams
say the /WAIT signal is sampled on the falling edge of clock state T2.
If it is active, additional cycles are introdced between T2 and T3. This
only occurs on op-code fetch, memory reference, input-output and
interrupt acknowledge cycles.


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