[SI-LIST] IBIS model Tr accuracy
- From: "Peterson, James F (FL51)" <james.f.peterson@xxxxxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Wed, 19 Oct 2005 05:48:15 -0700
Dear IBIS experts,
This question is related to the accuracy of the IBIS model's VT curves (or
the dv/dt #s).
We had noticed in the lab that our measured Tr and Tf were a lot slower than
what the IBIS model was stating, so we decided to take a closer look. The
part was a 54AC245.
(You'll see below that Teraspeed was kind enough to make a version of the
model with the other 7 outputs toggling so that we could see the loaded
effect. our lab loading wasn't exactly the same, nor were our measurements
as accurate as Teraspeed's, so we have to keep the comparison general and
relative.)
note that all Tr and Tf measurements were made at 20%-80%...
IBIS model (a measured model from Teraspeed)
(w/ other 7 outputs not toggling) Tr=0.9ns Tf=0.6ns
(w/ other 7 outputs toggling) Tr=2.4ns Tf=1.0ns
Honeywell Lab measurement
(w/ other 7 outputs not loaded) Tr=1.4ns Tf=1.2ns
(w/ other 7 outputs loaded) Tr=2.5ns Tf=1.6ns
The take away message is that loading down other buffers will slow down the
Tr and Tf of the critical net you are analyzing. How much? Well it seems to
depend on how well your device delivers current to its buffers (AKA your
PDS). A 54AC245 is perhaps an exaggerated example, because of its 1990s-like
PDS.
We appreciated TeraSpeed's (Tom Dagostino and Dekker Graden) technical
insight and support in this investigation. And I should add that Tom Bingel
gets the credit, here at least in Honeywell, for first proposing this
explanation which initiated the measurements.
best regards,
Jim Peterson
Honeywell
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