Hi Tom, I find your reply very useful. Thank you for the information. There is one question regarding C_comp that I would like to ask. When I run simulation to extract V-T curves, I used an extracted RC netlist, which means it has taken into account parasitic R and C of the buffer. In that case, do I still need to specify another C_comp value? Would that mean double-calculating this C_comp? Kindly advice. Thank you. Best regards, Mei Yee IC Design Engineer Avago Technologies ________________________________ From: owner-ibis-users@server.eda.org [mailto:owner-ibis-users@server.eda.org] On Behalf Of Tom Dagostino Sent: Thursday, December 21, 2006 4:14 AM To: April.Hachenburg@smsc.com; ibis-users@server.eda.org Cc: akvarma@ncsu.edu Subject: RE: [IBIS-Users] Ramp vs. Rising/Falling waveforms, Cref missing ... April R_load is the load the buffer is placed under when the ramp data is acquired. 50 Ohms is the typical value and it is usually terminated to ground for the dV/dT rise and to power for the dV/dT fall. R_fixture is the load the buffer is placed under when the VT waveforms as acquired. This value is typically 50 Ohms and is terminated to ground for both a rising and falling waveform and then terminated to power for the other set of rising and falling waveforms. C_fixture would be any capacitance associated with the R_fixture. But its not a good idea to have any C_fixture when extracting VT waveforms. I've seen simulator give very wrong answers when C_fixture is present. Rref is the manufacturer's timing test load's resistive component. This can be any value (within reason) and can be terminated to any reasonable voltage Vref the manufacturer deems appropriate for the buffer technology. It is usually terminated to the typical value the buffer would see in an actual application. If the buffer was an ECL output then 50 Ohms to Vcc-2Volts would be the load. Most CMOS devices have just a Cref, a capacitor terminated to ground. C_comp is the capacitance associated with the I/O buffer. It includes all the capacitance from metal, active or passive devices on the silicon. It forms the majority of the buffer's impedance reactive component. R_fixture should match the load the buffer is seeing in an actual application. Most circuit boards trace impedance runs between 40 and 70 Ohms or there about. Most buffers are characterized into a 50 Ohm load which is close to the board impedance. Since the model developer cannot predict every application 50 Ohms is a good compromise for most models. There are some technologies that are better served with a non 50 Ohm load such as RAMBUS, which is specified into 27 Ohms or very weak CMOS drivers that cannot drive a 50 Ohm load to Vinl or Vinh. I'd caution against using a 500 Ohm load in your model making unless there is a very good reason to do so. The Ramp key word is a legacy key word that was used before the VT waveforms were added to IBIS. It is still required to be present in the model. If R_fixture is not specified it defaults to 50 Ohm in the model. Tom Dagostino Teraspeed(R) Labs 13610 SW Harness Lane Beaverton, OR 97008 503-430-1065 tom@teraspeed.com www.teraspeed.com Teraspeed Consulting Group LLC 121 North River Drive Narragansett, RI 02882 401-284-1827 -----Original Message----- From: owner-ibis-users@server.eda.org [mailto:owner-ibis-users@server.eda.org] On Behalf Of April.Hachenburg@smsc.com Sent: Wednesday, December 20, 2006 9:39 AM To: ibis-users@server.eda.org Cc: akvarma@ncsu.edu Subject: [IBIS-Users] Ramp vs. Rising/Falling waveforms, Cref missing ... IBIS Users, I wondered if anyone could clarify for me the usage of Rload vs. Rref vs. Rfixture and similarly Cref vs. Cfixture. I am using s2ibis3 and discovered that the Cref parameter specified in my .s2i file does not appear in my .ibs model file. After looking at the netlists and reading the IBIS documentation, it appears Cref is not used for any simulations. What is the purpose of Cref? And why isn't it showing up in the .ibs file? Also, I read that C_comp is taken into account for the rising/falling V-T waveforms though I cannot find that paramater in any of the corresponding netlists. How is C_comp used? As for Rref vs. Rfixture, I thought I read somewhere that for best results Rref should match Rfixture. Is that true? In my model, I am using the keywords Rising/Falling Waveform. I found that if I do not specify Rload even though I am not specifying the [Ramp] keyword, then 50ohms is used in one of the transient simulations despite Rref and Rfixture being set to 500 ohms. Should I be specifying the [Ramp] keyword along with the [Rising/Falling Waveform] keywords? If so, how do they work together? - April --------------------------------------------------- April Hachenburg Core Technology Design Engineer SMSC -------------------------------------------------------------------- |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 Wed Dec 20 15:25:56 2006
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