Underclocking m68k CPUs
ajp166 at bellatlantic.net
Wed Apr 11 12:23:35 CDT 2007
>Subject: Re: Underclocking m68k CPUs
> From: "Ethan Dicks" <ethan.dicks at gmail.com>
> Date: Wed, 11 Apr 2007 13:00:29 -0400
> To: "General Discussion: On-Topic and Off-Topic Posts" <cctalk at classiccmp.org>
>On 4/11/07, Jules Richardson <julesrichardsonuk at yahoo.co.uk> wrote:
>> As subject... do 68k CPUs cope with being underclocked? I've got a surplus
>> 8-MHz part from an Amiga, and at the museum we've got a 68000 microprocessor
>> trainer which is short its CPU. I'm not sure what speed CPU the trainer
>> expects though (because I haven't looked in any detail :-) but it'll certainly
>> be 8Mhz or less.
>Section 10.7 of the Users Manual
>(http://iteso.mx/~temoc/micros/MC68000UM.pdf) indicates that 4MHz is
>the minimum clock speed for an 8MHz-rated part. I've seen 4MHz-badged
>parts, but it was in a 4MHz trainer from about 1982. I don't know
>where you'd find performance data on a part that old.
Motorola databook would have it and they weren't rare either.
I've run them down to 100khz, the timing of the signals moves a bit as
at those slow speeds the propagation delay is not significant anymore
relative to the cpu timing. I did that once with a 68k with apparent
normal (and very slow) behavour.
With rare expection most CPU underclock to the extreme and most will
overclock by some. The exceptions are any of the 8080/8085/z80 and
similar cpus that use dynamic memory cells and are NOT static. Though
I've taken them to very low clock speeds as well though not to full
stop. I'ts rare to see a data sheet specify a minimum clock that
isn't very slow or at least the vendors testing minima.
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