Kiran, and Chris, While it is true that most tools will not make use of multiple sets of Vt tables, there are some which do. To avoid upsetting anyone I will not mention which tools they are. The problem is that the tools which don't, usually take the first tables they find in the IBIS file and ignore the rest. For this reason it is extremely important to put the Vt tables in the right order in the IBIS file. The general rule I use in my work is to put the most important Vt tables first in a [Model]. I didn't experiment enough to know the details, but I wonder how much intelligence tool vendors put into their parser to find the appropriate Vt tables. For example, let's consider a CMOS buffer which normally uses four Vt tables. For the sake of the example, the IBIS model has two sets, one with 50 Ohm Rfixture, and another with 60 Ohm Rfixture. What happens if the order is such that there are two 50 Ohm rising waveforms and then two 60 Ohm rising waveforms, and then two 50 Ohm falling and two 60 Ohm falling edges in the file? Are tools smart enough to pick the first two 50 Ohm rising waveforms and then look for the falling waveforms and find the first two 50 Ohm falling tables? Or are they dumb enough to just take the first four tables, even if they are all rising edges and make no sense for simulations? To be on the safe side, I would use the following order: Four 50 Ohm waveforms, and four 60 Ohm waveforms. The same argument will also hold when the Vfixture is the variable. Chris, I would argue your statement on the over determined system. It has been shown that there are situations when four waveforms are not enough to adequately describe the buffer's behavior. See pg. 14-17 in the following presentation: http://www.eda.org/pub/ibis/summits/jun03b/muranyi1.pdf Arpad ----------------------------------------------------------- ________________________________ From: owner-ibis@server.eda.org [mailto:owner-ibis@server.eda.org] On Behalf Of Chris Reid Sent: Wednesday, May 03, 2006 8:53 AM To: seshadri.kirankumar@wipro.com; ibis-users@server.eda.org; ibis@server.eda.org Subject: [IBIS] Re: [IBIS-Users] Question regarding V-T data of IBIS model Kiran, The V(t) tables under different load conditions are all supposed to be used to generate a single model, so no choice is required. The nominal model is a pullup transistor, a pulldown transistor, a power clamp, a ground clamp, and capacitance to ground. The V(t) curves make it possible for the simulation tool to estimate the switching characteristics of the two transistors. Two V(t) curves at different loading conditions are required to compute the two transistor characteristics. If only one curve is supplied then the tool must assume that the switching of the two transistors is complimentary. If more than two V(t) curves are supplied the system is over determined. Most tools ignore the additional curves. Ideally a least-squares fitting should be used to generate a model that best matches all supplied V(t) values. I don't think any tools do that today. Christopher E. Reid Chief Scientist and Founder Pressando SI 9315 SW Lake Street Tigard, OR 97223-6034 503-869-6703 (cell) 503-624-8159 (home) ----- Original Message ----- From: seshadri.kirankumar@wipro.com To: ibis-users@server.eda.org ; ibis@server.eda.org Sent: Wednesday, May 03, 2006 6:46 AM Subject: [IBIS-Users] Question regarding V-T data of IBIS model Hi Experts, I have a doubt on IBIS modeling regarding to V-T data using a Signal Integrity tool. Suppose if IBIS model contains V-T data generated for different R_fixtures . how SI engineer is able to make the tool to use required R_fixture data from all V-T tables? As an example if I am generating V-T data(2 Risings and 2 Fallings) against 2 Rloads i.e RLOAD1 and RLOAD2 both are less than 100 Ohms, then s2ibis3 will put V-T will be in the below following order based on assumption RLOAD1 > RLOAD2 Rising connected VSS using RLOAD1 Rising connected VDDS using RLOAD1 Rising connected VSS using RLOAD2 Rising connected VDSS using RLOAD2 Falling connected VSS using RLOAD1 Falling connected VDDS using RLOAD1 Falling connected VSS using RLOAD2 Falling connected VDSS using RLOAD2 So how a person using SI tool will chose the required V-T table data based on RLOAD1 and RLOAD2 ? or will these be taken care automatically by the tool? Thanks for providing information on this Best Regards Kiran Kumar The information contained in this electronic message and any attachments to this message are intended for the exclusive use of the addressee(s) and may contain proprietary, confidential or privileged information. If you are not the intended recipient, you should not disseminate, distribute or copy this e-mail. Please notify the sender immediately and destroy all copies of this message and any attachments. WARNING: Computer viruses can be transmitted via email. The recipient should check this email and any attachments for the presence of viruses. 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