My experience with simulators is consistent with IBIS' recommendations. If you put reactive loads in the VT curve load many times the simulations blow up. If you allow the SI simulator to simulate the actual load seen by the driver - a transmission line plus package plus any other parasitics, the simulations tend to match measurements. As a model maker you will never know what kinds of loads the end user places on the driver. So you cannot anticipate what reactive load to put in the model. Once you get on the board and assuming (and at times this is a big assumption) there is power/ground plane beneath the trace from the driver to the load, the load seen by the driver will be a transmission line load. If the end product's board looks like lumped RLCs there are going to be problems. Tom Dagostino Teraspeed(R) Labs 13610 SW Harness Lane Beaverton, OR 97008 503-430-1065 503-430-1285 FAX tom@teraspeed.com www.teraspeed.com Teraspeed Consulting Group LLC 121 North River Drive Narragansett, RI 02882 401-284-1827 From: Sudarshan H N [mailto:hn.sudarshan@gmail.com] Sent: Monday, August 18, 2008 11:27 PM To: tom@teraspeed.com Cc: Chetana Raghuwanshi; ibis@server.eda.org; ibis-users@server.eda.org; owner-ibis-users@server.eda.org Subject: Re: [IBIS-Users] Re: [IBIS] Different PMOS and NMOS driver impedance HI, I had a similar discussion about the value of the Rfixture some time back with the IBIS group, and that time the conclusion was, as you said we should select Rfixture close to the impedance seen by the driver on the board i.e, around 50ohm. But the exact value might be different and hence these V-t curves in IBIS will just give one set of values for given Rfixture, and board level simulation tools will use these V-t curves as a reference to get the actual V-t waveforms for a given load. Still my doubt is, the pad will see some value of RLC on the board, and IBIS discourages to use reactive loads when generating Vt curves. So how much accuracy these V-t curves will give , assuming its only generated with Rfixture and not with other reactive loads. Regards Sudarshan On Mon, Aug 18, 2008 at 10:08 PM, Tom Dagostino <tom@teraspeed.com> wrote: No, no and no. The load resistor used for extracting the VT waveforms should represent the load seen by the driver on the circuit board. Most reasonable circuit boards will have trace impedances between 40 and 75 Ohms and usually the design targets 50 Ohms. So the IBIS model should characterize the buffer into an impedance close to that. The generally accepted value is 50 Ohms. It has nothing at all to do with the output impedance of the driver. You are not trying to match load/output impedance or maximize power transfer, you are trying to get a macro model to be characterized near its operating conditions. There are no such things as IV VT curves. There are no loads associated with extracting IV curves. Extending the simulation time may or may not "fix" IV/VT curve mismatches. Extending simulations times will only fix mismatches if the original simulations did not allow the VT waveforms to reach their final value. If you have a problem understand the cause before jumping to a solution. Just jumping to a solution in most cases will not work. Tom Dagostino Teraspeed(R) Labs 13610 SW Harness Lane Beaverton, OR 97008 503-430-1065 503-430-1285 FAX tom@teraspeed.com www.teraspeed.com Teraspeed Consulting Group LLC 121 North River Drive Narragansett, RI 02882 401-284-1827 From: owner-ibis-users@server.eda.org [mailto:owner-ibis-users@server.eda.org] On Behalf Of Chetana Raghuwanshi Sent: Monday, August 18, 2008 6:51 AM To: Sudarshan H N Cc: ibis@server.eda.org; ibis-users@server.eda.org; owner-ibis-users@server.eda.org Subject: Re: [IBIS-Users] Re: [IBIS] Different PMOS and NMOS driver impedence Hi Sudarshan, You are right. My concern is IV VT mismatch error. How would it disappear by increasing the simulation time ? Best Regards Chetana -------------------------------------------------------- Chetana Raghuwanshi CTO / Process & Library Technology NXP Semiconductors India NXP Block C, 4th Floor MFAR Manyata Tech Park Nagavara, Bangalore-560045 Tel : +91 80 4024 7072 -------------------------------------------------------- Error! Filename not specified."Sudarshan H N" <hn.sudarshan@gmail.com> "Sudarshan H N" <hn.sudarshan@gmail.com> Sent by: owner-ibis-users@server.eda.org 2008-08-18 06:43 PM Error! Filename not specified. To Error! Filename not specified. "Chetana Raghuwanshi" <chetana.raghuwanshi@nxp.com> Error! Filename not specified. cc Error! Filename not specified. ibis-users@server.eda.org, ibis@server.eda.org Error! Filename not specified. Subject Error! Filename not specified. [IBIS-Users] Re: [IBIS] Different PMOS and NMOS driver impedence Error! Filename not specified. Error! Filename not specified. Hello Chetana, You can use any of the 2 values for Rfixture to generate Vt curves. There is no rule that , your Rfixture should match to the PMOS or NMOS driver resistance. But you should always use one value to generate the one set of rising and falling waveforms(experts, correct me if i am wrong). If you are seeing any errors with respect to IV and Vt curve mismatch , just simulate for more time so that it will reach the saturation. Regards Sudarshan On Mon, Aug 18, 2008 at 6:37 PM, Chetana Raghuwanshi < <mailto:chetana.raghuwanshi@nxp.com> chetana.raghuwanshi@nxp.com> wrote: Hello Experts, One of my IO cell has a buffer with different PMOS and NMOS impedences. PMOS impedence is approx 200 Ohms and that for NMOS is 96 Ohms. In this case what should be the value of Rfixture for calculating IV VT curves ? Would it be Rfixture1 for calculating pull down and falling waveforms and Rfixture2 for calculating pull up and rising waveforms ? If I do rise/pullup and fall/pulldown simulations with different Rfixture values, is it acceptable ? Best Regards Chetana -- This message has been scanned for viruses and dangerous content by <http://www.mailscanner.info/> MailScanner, and is believed to be clean. -- This message has been scanned for viruses and dangerous content by <http://www.mailscanner.info/> MailScanner, and is believed to be clean. -- This message has been scanned for viruses and dangerous content by <http://www.mailscanner.info/> MailScanner, and is believed to be clean. -- This message has been scanned for viruses and dangerous content by MailScanner, and is believed to be clean. -------------------------------------------------------------------- |For help or to subscribe/unsubscribe, e-mail majordomo@eda-stds.org |with the appropriate command message(s) in the body: | | help | subscribe ibis <optional e-mail address, if different> | subscribe ibis-users <optional e-mail address, if different> | unsubscribe ibis <optional e-mail address, if different> | unsubscribe ibis-users <optional e-mail address, if different> | |or e-mail a request to ibis-request@eda-stds.org. | |IBIS reflector archives exist under: | | http://www.eda-stds.org/pub/ibis/email_archive/ Recent | http://www.eda-stds.org/pub/ibis/users_archive/ Recent | http://www.eda-stds.org/pub/ibis/email/ E-mail since 1993Received on Tue Aug 19 00:35:26 2008
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