Subject: Re: sample / hold times

From: [email-masked] (Bob Ward)
Date: Wed, 16 Mar 94 6:57:09 CST

Fellow ibis folk--

The context of my question about setup and hold time is this ... if we are
including high and low thresholds "for the purpose of timing analysis", we
are including only a small part of what is needed.  Yes, these data points
are needed to know where to measure times from and to, but we can do
nothing with them after measuring them without something to compare them
to.  So it seems that if ibis models are not to get into the timing arena
then we do not need high and low thresholds on input models, either.  That
data would be supplied from the data book just as would the setup and hold
times to compare against.  My point is that if we are going to put in
something for timing analysis, lets put in enough to make it truly useful,
or else leave it all out.

I appreciate the points about having enough to do to get SI simulation
right, but if the model we are building is to be useful also for timing
analysis, as put forth in Bird 2.1, then we should put in enough to make
it fully useful.  Otherwise we still have to get half of what we need from
another source.  I am well aware that there are still other things needed
to calculate the time of arrival of pulses at pins, not the least of which
is the transit time along traces, skew of pulse launching, etc. but that
is a function of a different model than ibis, and is simply something more
that must be supplied to the simulator.  Likewise there is more to a spice
model than transistors, but we try to capture all the data about the
transistors in a small set of models, all in one place.  I am seeking the
analogous situation here of enough data in one place to be usefulin its
own right.  I agree that carried to it logical extreme, this could go on
forever.  That is not what I am proposing.  But likewise lets not put a
tiny part of what's needed in and then claim to be able to do timing
analysis with it, either.
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From: [email-masked] (Jon Powell)
Date: Wed, 16 Mar 94 10:01:45 PST

re: usefull timing data.

Quad Design manufactures two main products, A transmission line
simulation tool (XNS) and a static timing verification tool (MOTIVE).
We have been in the business for about 7 years now of supplying 1) SI
information and 2) timing data. We have found that the information you
need in the SI tool inorder to provide usefull information to the
Timing tool are: Input Thresholds and VM delay correction.

The rest of the timing data is needed by the Timing tool but is not used in
any context by the SI tool, while the SI tool CANNOT PROVIDE ADEQUATE
data to the TIMING tool without information about input logic
thresholds.

my point, Input thresholds are necessary for SI models to be able to
provide the required pin-to-pin delay data to Timing.

VM correction is a really sticky wickett. This involves correcting for
the manner that the manufacturer measured and specified internal part
delay (usually driving a 50pf load). It is a nice thing to have but
can usually be approximated or calculated on the fly by the SI tool.

regards,

jon