ZX Spectrum Z80 Keeps Resetting

[Maciej W. Rozycki]

On Wed, 3 Jan 2018, Rob Jarratt via cctalk wrote:

> The reason I say this is that when I set up the logic analyser to *stop* 
> on a zero ROM address, it didn't. Which means it isn't resetting in the 
> way I thought it was. When I traced the instruction addresses during 
> boot up I could see lots of resets back to address zero, and assumed 
> that somehow it was getting further into the sequence to show the 
> copyright screen, but then resetting again, because the screen would 
> display the "snow" after showing the copyright screen.

 As I recall with the ZX Spectrum there wasn't any "snow" on TV with a 
subsequent reset (as when pressing the reset button on the Plus 
variation).

 Moreover DRAM refresh continued to work (presumably being driven by the R 
register of the Z80 CPU according to its machine cycles, although at this 
point I'd have to double check the details as my brain actually seems to 
be missing the R register altogether), so unlike with a power-up reset, 
where you'd initially get random rubbish on the screen, with a subsequent 
reset you'd see a black rectangle (with thin vertical stripes IIRC, 
meaning a pattern with a single bit set) instead as the part of memory 
used for video data gets cleared in the DRAM initialisation process, 
followed by the copyright notice you mention.

 That you see "snow" instead is I believe indeed a strong indication that 
something else goes wrong beyond (or instead of) just an intermittent 
reset.  And given that the ULA produces video output right away after 
power-up with no need to program any device registers is I think an 
indication that either the chip is broken or something else causes it to 
malfunction, just as you have concluded.

 FWIW,

  Maciej