Re: A question to David Empson on ZipGS Cache Memory Speeds

[marvinmiller@hotmail.com (Marvin Miller)]

Thanks David !

I appreciate the response. As far as the 3/F -30 for the tag ram, that
came from the original post. 

None of this is too critical though. The main concern I had was that
the gist of the data was correct, but more importantly, that there
_should_ be no problems using the fastest possible chips in the cache
side. 

With answers in hand, I can now research the fastest chips, dig up the
part nos, and order them. I'll post my results when the results are
in!

Best;
Marvin

On Mon, 13 Nov 2000 22:49:32 +1300, dempson@actrix.gen.nz (David
Empson) wrote:

>Marvin Miller <marvinmiller@hotmail.com> wrote:
>
>> Hi David !!
>
>Howdy.
>
>> Long, long ago in a galaxy far, far away you posted a message relating
>> to ZipGS memory speeds that I've attached below to 'jog the fog' so to
>> speak!
>
>Well, that was a long time ago, but I vaguely recall it upon rereading.
>
>> Back then, you made a nice table on ZipGS memory speed requirements &
>> oscillator speeds.
>
>> [1] In light of the time that's gone by (and probably increased
>> learning) would you say that this table still makes a good 'general'
>> guide?
>
>With a caveat: based on a theory of how the Zip cache architecture
>worked.  I have never had confirmation of this, so I can't be confident
>that the numbers are accurate.
>
>I expect that it is more or less correct, but now I'd be inclined to err
>on the side of caution, and get the RAM as fast as possible: preferably
>something like 2/F minus a constant.
>
>> [2] If a person bought the fastest possible cache chips (say for a
>> 20Mhz //GS) and than ran the Zip at half that speed (say 10Mhz) would
>> there be a problem or would it be advantageous to run the processor
>> Mhz as near as possible to the reccommended chips?
>
>I wouldn't expect any problems with the RAM being too fast.
>
>Having said that, we have encountered a problem in this area at work,
>with one of our older circuits: it works fine with a 120 ns or 150 ns
>static RAM, but fails if a 70 ns RAM is used.  The product is several
>years old and has been superseded twice, so we just avoid the problem
>and haven't investigated it in detail.
>
>In this case it is probably a race condition due to a multiplexed
>address and data bus, and a probable design fault that is only exposed
>if the RAM produces read data too quickly.
>
>> Here's a more complete table based on your long ago theories;
>> 
>> 
>>   CPU    Oscillator  ------- DATA RAM speed ----------TAG RAM Speed
>>  Freq.   Freq. (F)   min (2/F)  max (3/F)  est (2.5/F)     (3/F -30ns)
>> 
>>  7 MHz    28 MHz      71.4 ns     107 ns      89.2 ns     77.1ns
>>  8 MHz    32 MHz      62.5 ns     93.0 ns     78.1 ns     63.7ns
>>  9 MHz    36 MHz      55.5 ns     83.0 ns     69.4 ns     53.3ns
>> 10 MHz   40 MHz      50.0 ns     75.0 ns     62.5 ns     45.0ns
>> 11MHz    44 MHz      45.4 ns     68.1ns      56.8 ns     38.1ns
>> 12 MHz   48 MHz      41.6 ns     62.5 ns     52.0 ns     32.5ns
>> 13Mhz     52 MHz     38.4 ns      57.6 ns     48.0 ns     27.6ns
>> 14 MHz   56 MHz     35.7 ns      53.5 ns     44.6 ns     23.5ns
>> 15MHz    60 MHz     33.3 ns      50.0 ns     41.6 ns     20.0ns
>> 16 MHz   64 MHz     31.2 ns      46.8 ns     39.0 ns     16.8ns
>> 17MHz    68 MHz     29.4 ns      44.1 ns     36.7 ns     14.1ns
>> 18MHz    72 MHz     27.7 ns      41.6 ns     34.7 ns     11.6ns
>> 19MHz    76 MHz     26.3 ns      39.4 ns     32.8 ns      9.4ns
>> 20MHz    80 MHz     25.0 ns      37.5 ns     31.2 ns      7.5ns
>> 21MHz    84 MHz     23.8 ns      35.7 ns     29.7 ns      5.7ns
>> 22MHz    88 MHz     22.7 ns      34.0 ns     28.4 ns      4.0ns
>> 23MHz    92 MHz     21.7 ns      32.6 ns     27.1 ns      2.6ns
>> 24MHz    96 MHz     20.8 ns      31.2 ns     26.0 ns      1.2ns
>> 25MHz   100MHz     20.0 ns      30.0 ns     25.0 ns      0.0ns
>
>Of course anything above 20 MHz is pure speculation - the ASIC is only
>rated at 20 MHz, and the CPU is theoretically rated at 14 MHz (though
>most appear to have been able to operate as fast as 20 MHz).  Has anyone
>succeeded in getting a ZipGS working faster than 16 MHz?
>
>Where did you get the (3/F - 30 ns) formula for the tag RAM?  As noted
>in my original post, I've seen a 9 MHz ZipGS supplied with 70 ns tag and
>data RAM, which is way over your 53.3 ns figure (though it could have
>been hand-picked RAM which was actually faster than this).
>
>Just to confuse things, my 8 MHz ZipGS was supplied with 45 ns RAM.