Perry, The IBIS reference load is supposed to represent the standard load into which the componet is tested and it's AC spec's generated from. Simulate the driver into the reference load and then simulate the driver into your load. You will get two delay values; Tref and Tactual. What you will notice is that the value Tref will not equal the Spec AC Tco of your device. This is because the IBIS model does not model the delay thru the device, only it's IO characteristics (In most cases, the SPICE models also do not model the full output path delay). By subtracting Tref from Tactual, you will obtain the interconnect delay with respect to the reference load Twire = (Tactual -Tref). So now you have 2 pieces of data: 1) Delay of device into standard load (Tco) 2) Interconnect delay with respect to standard load (Twire) To compute your timing delay, add the Twire value to the spec Tco of your device. Douglas Burns Chief Consultant Signal Integrity Software Inc. 6 Clock Tower Place Maynard, MA 01754 978-461-0449 x14 www.sisoft.com dburns@xxxxxxxxxx -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Perry Qu Sent: Thursday, October 02, 2003 3:03 PM To: si-list Subject: [SI-LIST] negative propagation delay Hi, I'm using ICX for SI/timing analysis. One thing I noticed, if a driver IBIS specified a large reference load (e.g., 50pF for uP), I will always end up with a negative propagation delay since my actual loading is much smaller compared with reference load. Would like to understand how I should correct the number to reflect my actual loading for timing analysis. I would think we can get the internal logic delay of driver through: (Tco - time_to_Vmeas@reference load); But then, how shall I add buffer delay for a real load and propagation delay for a real load--taking into account all the reflection effect (e.g., a daisy-chained bi-directional data bus). Thanks Perry -- Perry Qu Design & Qualification | 600 March Road Alcatel Canada | Ottawa, ON K2K 2E6, Canada DID: (613) 7846720 | FAX: (613) 5993642 ------------------------------------------------------------------ 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 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 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