Aubrey, thanks for response - good thought on having the weak corner cover this. This model didn't have min or max corners, but hey, with a little effort I could make a weak corner out of the loaded slower model and add it as the min corner to the unloaded model. I will also bounce this question over to the IBIS List and report back anything interesting. Jim -----Original Message----- From: Aubrey_Sparkman@xxxxxxxx [mailto:Aubrey_Sparkman@xxxxxxxx] Sent: Wednesday, October 19, 2005 11:53 AM To: james.f.peterson@xxxxxxxxxxxxx; si-list@xxxxxxxxxxxxx Subject: RE: [SI-LIST] IBIS model Tr accuracy OK, disclaimer up front, one of the reasons I almost didn't respond to this is because I don't consider myself to be an IBIS expert. But to support your analysis, many IC suppliers know that supply sag due to package PDS will slow down the buffer switching speed. As a user of IBIS models, I expect that slowdown to be modeled in the "min" set of curves as the weak buffer corner case along with temperature effects and minimum supply voltage. The only, very minor, quibble I have is with your statement "54AC245 is perhaps an exaggerated example." Older technologies have a bigger difference between min and max corners. PCI-X buffers, for instance, are allowed to have a wide switching speed range, on the order of 0.36nS to 1.8nS for Tr and Tf. And I've seen many models (and components) with fall times that were faster than the rise times. Did your IBIS model include min, max, and typ curves? Due to the large variation in quality of the models I've acquired, I have gotten a wide range of correlation to lab measurements (from None to excellent). Aubrey Sparkman Enterprise Engineering Signal Integrity Team Dell, Inc. Aubrey_Sparkman@xxxxxxxx (512) 723-3592 -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Peterson, James F (FL51) Sent: Wednesday, October 19, 2005 7:48 AM To: si-list@xxxxxxxxxxxxx Subject: [SI-LIST] IBIS model Tr accuracy 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 ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List FAQ wiki page is located at: http://si-list.org/wiki/wiki.pl?Si-List_FAQ List technical documents are available at: http://www.si-list.org List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List FAQ wiki page is located at: http://si-list.org/wiki/wiki.pl?Si-List_FAQ List technical documents are available at: http://www.si-list.org List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu