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From: daveh@cbmvax.commodore.com (Dave Haynie)
Newsgroups: comp.sys.amiga.tech,comp.sys.amiga,comp.sys.amiga.hardware
Subject: Re: 25 mhz 68020 for A2620
Keywords: 68020, A2620
Message-ID: <15494@cbmvax.commodore.com>
Date: 31 Oct 90 18:06:43 GMT
References: <858@ccd700.UUCP> <0eqRR4w163w@valnet>
Reply-To: daveh@cbmvax.commodore.com (Dave Haynie)
Organization: Commodore, West Chester, PA
Lines: 46

In article <0eqRR4w163w@valnet> joseph@valnet.UUCP (Joseph P. Hillenburg) writes:
>mike@ccd700.UUCP (M. Greer) writes:
>
>> Will a 25 mhz 68020 work on an A2620? 

Well, an MC68020-25 will WORK in the A2620, but there's no way to clock it
at 25MHz, which is no doubt what you're after.

>>This seems like such a simple question, but I can't seem to get a reliable 
>>answer. 

You have it here on good authority; I designed the thing.

>>not qurantee that it would clock at 25 mhz (assuming of course that
>>a 25 mhz clock crystal and a 25 mhz 68881 or 82 were also added), the
>>other said that it would work and run at 25 mhz. 

The 68881/2 can certainly run at 25MHz (in fact, one of our guys tested an
A2630 with a 50MHz math chip, and in that the A2620 uses the same math
chip logic, I would expect a 50MHz FPU to work OK).  The CPU and MMU,
however, get their 14.3MHz clock from the A2000 motherboard.  These parts
work as one, really, so you couldn't even think of making an A2620 go 
faster than it does without the corresponding increase in MMU speed.  The
main problem, though, is that the A2620 interface logic counts on being
locked to the A2000 clocks, and would require some redesign to work with
an arbitrary asynchronous clock.

>Yes. From what I heard from both Commodore and AmigaWhirl/World 
>(FluffMag) you can speed up the board just by adding the new chip(s) and 
>the oscillator.

The board in the AmigaWorld "tips" column was the A2630, which is designed
for asynchronous operation.  They upped the clock from 25MHz to 28MHz, not
exactly a giant step.  And I would expect most, though perhaps not all,
A2630s to run that fast, at least at room temperature.  There are, after all,
timing margins in the A2630 to account for worst case chips and temperatures,
neither of which you typically encounter.  With some modifications and proper
CPU/FPU speeds, it could be possible to up the clock speed to 33MHz on the
A2630, but the A2620 clock speed is fixed by design.

>-Joseph Hillenburg

-- 
Dave Haynie Commodore-Amiga (Amiga 3000) "The Crew That Never Rests"
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	Standing on the shoulders of giants leaves me cold	-REM