Re: ZIP GS Questions...

[dempson@atlantis.actrix.gen.nz (David Empson)]

In article <49iodi$1de4@hearst.cac.psu.edu>,
Jason Jeffries <JJeffries@psu.edu> wrote:
> I have been playing with my Zip GS v.1.02. I need to know some info:
> 
> My Zip doesn't seem stable at 12mhz. The 65816 is rated at 10, and I'm
> using 25ns cache that is stable to at least 10mhz.

As others have stated, you are driving the processor above its rated
speed, which is why it is unreliable.

You may be able to increase its potential speed by installing the
higher voltage supply circuitry that the ZIP supports (it generates
about 5.5V from the 12V rail), assuming it isn't there already.
Raising the voltage increases the maximum operating speed of the
processor somewhat.

I don't know exactly what is required here.  My original ZIP doesn't
have this circuitry, and my new one does, but I don't want to remove
it.  You can look to see if it is already installed by locating "Q2"
near the middle of the bottom of the card.  There are several
components in that area which form the higher voltage supply.  If
there are several empty component positions, then it hasn't been
installed.

> I haven't changed anything else--do I need to change whatever little
> logic chip for use over 10mhz?

There is supposed to be a fault in the ASIC which prevents operation
above about 12.5 MHz unless some modifications are made to the board
to bypass part of it.  I don't know the details on this.

It may be necessary to change the 74F00 chip to a 74HC00 in some
cases, but I think this really only applies at higher speeds (e.g. 14
MHz and up).

> When I set the Zip to run at a slower speed then full--ex, 9.9mhz
> with a 12mhz setup--how does it actually run at a slower speed? 
> Does it "really" run the processor at that speed, or does it do
> something strange to simulate a slower speed?

It might drop CPU cycles (stopping the clock for a few cycles,
effectively stretching the high or low part of the clock cycle),
stretch both halves of the cycle, or use the RDY input (not likely).

Slowing down the operating speed of the ZIP will only affect the
reliability if the second case applies (the ZIP slows down both halves
of the clock cycle).
-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P.O. Box 27-103, Wellington, New Zealand