ZX Spectrum Z80 Keeps Resetting

Rob Jarratt robert.jarratt at ntlworld.com
Thu Jan 4 01:21:55 CST 2018

>  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
>  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
> I'd have to double check the details as my brain actually seems to be
> the R register altogether), so unlike with a power-up reset, where you'd
> get random rubbish on the screen, with a subsequent reset you'd see a
> 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
> And given that the ULA produces video output right away after power-up
> 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
> concluded.

Thanks Maciej. Since I never had Spectrum when they were new, I didn't know
that this was how it behaved on a reset. I am glad that my analysis seems to
be on the right track (given what I know at the moment). I am going to
concentrate on the ULA and the video output, but given that the keyboard
also doesn't seem to work the ULA has to be the prime suspect at the moment.

I am still hoping someone will know if I can try swapping the ULA with a
newer one from a later model I have. I may also look at getting a NebULA as
they are not expensive.



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