Re: ZIPGS 2-pin Crystal

[Wayne Stewart <waynes@intergate.ca>]

"Michael J. Mahon" wrote:
> 
> Wayne Stewart replied:

> >The main theory revolved around the second bank being a lot further
> >from the processor than the first bank and with those skinny leads.
> 
> The timing differential attributable to the longer leads is less than
> 0.1 ns., so unless your SRAMs are _really_ marginal (unlikely,
> since narrow ones are usually _very_ fast), timing should not
> be an issue.  This level of timing variation is less than the usual
> differences between similarly spec'd chips.  More likely, the
> additional loading of another chip degrades some critical edge.
> 
> Inadequate decoupling near the sockets could be an issue, but
> I don't recall the exact configuration of the decoupling caps.
> 
> It could also be that there is another timing issue in the
> custom logic which comes into play with the larger cache.

I can't say, this was Tony Diaz's theory
 
> >The cheap sockets Zip used didn't help either. I originally did my
> >upgrading by soldering a half socket in the middle of the original
> >socket. Later I started wondering about the contact so I removed the
> >half sockets and replaced them with machined pin sockets. The two
> >ZipGS I tried it on were previously stable at 14.5 mhz but not 15.
> >After I was able to run them at 15mhz. Not spectacular but maybe
> >indicative of something.
> 
> While poor contacts with sockets is always a problem, even
> inexpensive sockets usually make excellent contact with ICs
> unless they or the IC leads are dirty or physically damaged.
> All that is required is clean, springy contacts.
> 
> Machined pin sockets are the most expensive sockets, with
> tightly controlled tolerances and better wear characteristics,
> but that factor should not be decisive for normal applications.
> They are definitely preferred if chips will be changed frequently.
> (Of course, if they're changed frequently enough, then a ZIF
> socket is a good idea--but they have poorer high frequency
> characteristics than a simpler socket.)

I'm normally not too concerned about inexpensive sockets in most
situations. However if I'm trying to push the limits on something 
then I figure why not spend the little extra for higher end parts

> My conclusion is that something else changed to make your
> card more stable.  Like swapping two nominally identical chips
> which are different in actual speed, or getting a less "noisy"
> contact between a chip and a socket which was either damaged
> or had some dirt in it.

Possible but I removed the SRAM on one card, changed the sockets 
and put the same SRAM back in and it went slightly faster. Then 
I did the same on the second card and it went slightly faster. 
 
> When we push the speed of a digital system to the point where
> it begins to fail, its behavior becomes more like the analog
> system it actually is, rather than the digital abstraction we like
> to think it is.  Small power supply variations, temperature effects,
> induced noise from adjacent electronics, etc., will all conspire
> to give Murphy a field day.
> 
> This is why "best practices" call for "schmooing" a circuit by
> finding the limits of correct operation in several dimensions:
> frequency, voltage(s), and temperature, then operating it
> in such a way that the normal range of variations in those
> parameters will never get too near the conditions under which
> it becomes unreliable.
> 
> This also accounts for the usual ability to "push" a circuit
> beyond its "nominal" speed, and sometimes the "schmoo"
> plot is sufficiently wide that reliablilty is not compromised.
> But this is not always the case--and hobbyists are rarely
> willing to go to the trouble of characterizing the operation
> of a modified device to determing how close they are to
> its limits.
> 
> Good advice, then, is to try pushing the speed until you find
> the fastest point where it is still reliable, then back off 10%,
> to allow for variations in the parameters you are not controlling.

I usually suggest that too but I never do it unless it's mission 
critical. I always like pushing things. 

	Wayne