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In <1992Aug27.135703.26001@zip.eecs.umich.edu> johnwitt@cdrom.engin.umich.edu (John P Wittkoski ) writes:



>For a 33MHz 68030, there is a minimum clock speed listed for it in
>the databooks. I think it's somewhere around 25Mhz for a 33MHz version, but I
>don't have my databook handy.

>My question is: why is there a minimum?
>One possibility I have heard is that the on-chip registers and/or cache
>are dynamic in nature, and since the chip is geared for 33MHz, it divides
>this down to whatever rate is needed for the refresh circuitry. Therefore,
>if you go below the minimum, the refresh rate slows to below the minimum
>needed for the refresh.

Yup, you're about right. Sometimes the circuitry is a standard dymanic
RAM cell, othertimes slightly different. Apart from the minimun clock
speed required one can tell if the CPU is 'dynamic' by the single step
circuitry (usually shown in data sheets) the 68ks have a halt pin
or something similar to single step the processor. You maybe able to use
this as a bodge to get a slower processor, but you will still need to
satisfy the minimum clock requirement.

Incidently the Z80 was static so nice and easy to debug, just have a manual
clock.

>Now, is there someone out there who knows about this? I'm interested
>specifically in the 68xxx CPUs, but any general info would be helpful.
>For example:
>1. Is the guess I made above accurate, or is there some other reason?
>2. If the reason above is correct, does it apply to the registers, the
>   cache, or both (or neither?)
>3. If it's just a testing parameter, how far could I crank the clock speed
>   down? 20Mhz? 16,8,2,1???

> Just curious.

Well thats what makes this group intersting, discussions.

Raggy

-- 
Raggy                 | God smiles upon the little children, the alcoholics, and
u9dmlb@ohm.york.ac.uk | the permanantly stoned. S.King.