[SI-LIST] Flight time measurements and post-route analysis

Question for the group:

Most commercial SI tools seem to measure flight times to the receiver's
input thresholds.  Thus, for a given transition, they report a "min" and
"max" flight time based on the threshold settings.

However, there is a school of thought in SI that says measuring flight times
to Vil/Vih is too conservative.  It's based on the observation that
setup/hold specs for a part are measured the same way Tco is - with
reference to a specific threshold.  Therefore, the reasoning goes, you can
measure flight times to the point where the receiver crosses the reference
threshold, and, as long as the signal is within certain quality (i.e.
non-monotonic) and slew-rate limits, the flight time measurement is valid.
The required slew-rate is (or should be) part of the receiving device's
input spec.  If the input signal does NOT meet the quality/slew rate
requirements, well, then you have to use Vil/Vih method, or in some cases, a
derating formula.

My question is: what techniques are people predominantly using: the Vil/Vih
method, the reference voltage, or something in-between?

For those who use the reference voltage method, I'm curious to know how
post-route analysis is performed.  You can adapt any post-route SI tool to
measure at a single voltage by changing the models (if necessary), but - do
any of the tools report slew rate at the receiver inputs during post route
analysis?

Your comments, both on and off the list, are greatly appreciated.

Todd.

Todd Westerhoff
SI Engineer - Hammerhead Networks
5 Federal Street - Billerica, MA - 01821
email:twester@xxxxxxxxxxx - ph: 978-671-5084
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