Michael, Your response is helpful. I too am doing my first differential buffer so on a slightly different note .... Does the s2ibis3 script handle generation of differential buffers? If not, is there a similar script available? - April --------------------------------------------------- April Hachenburg Core Technology Design Engineer SMSC - Phoenix "Mirmak, Michael" <michael.mirmak@intel.com> Sent by: owner-ibis-users@server.eda.org 08/24/2007 05:10 PM To "Chitiz Mathema" <cit@cypress.com>, <ibis-users@server.eda.org> cc Subject RE: [IBIS-Users] IBIS question on C_comp Chitiz, Thanks for your questions on IBIS. Here are some hopefully useful answers... 1,2) The purpose of C_comp is to describe the capacitance of the buffer at the pad, as it appears "looking in." You are correct that the V-t tables and ramp information already take C_comp into account, for drivers. However, C_comp is still needed in order to describe the capacitive load the buffer presents to waveforms (reflections) coming back into the driver. In general, tools ensure that C_comp as a discrete number and its effects in the table and ramp data are not double-counted. This may be implemented by simply "backing out" or derating the V-t tables for the effects of C_comp, then adding an actual capacitor to the driver circuit in simulation. In that way, the derated-table-plus-capacitor waveforms will match the original IBIS driver data, but the system will "see" the capacitance of the buffer as a load. Specific tools may have different ways to approach the problem. 3) C_comp should exclude the effects of the package, but include all the buffer's on-die parasitic capacitances -- pad, die routing, etc. 4) Information on extraction methods for C_comp, as well as differential treatments, can be found in the IBIS Cookbook: http://www.eda.org/ibis/cookbook/cookbook-v4.pdf In addition, several presentations on C_comp issues are available in the IBIS Summit archive: http://www.eda.org/ibis/summits/index-bytitle.htm To address your concern, I would not say that IBIS per se is not properly addressing high-frequency effects. However, C_comp in traditional table-driven IBIS is a single value. Capacitance for a buffer is frequency- and voltage-dependent. Therefore, the method of extraction and the frequency at which the extracted value applies are critical when correlating the resulting IBIS model. In the case you describe, I would check to make sure that the C_comp value was not extracted at a frequency below 400 MHz. A more comprehensive treatment of buffer capacitance is possible using advanced IBIS techniques, including multi-lingual methods (IBIS 4.1, 4.2). I hope this helps! - Michael Mirmak Intel Corp. Chair, EIA IBIS Open Forum -----Original Message----- From: owner-ibis-users@server.eda.org [mailto:owner-ibis-users@server.eda.org] On Behalf Of Chitiz Mathema Sent: Friday, August 24, 2007 4:31 PM To: ibis-users@server.eda.org Subject: [IBIS-Users] IBIS question on C_comp Hello, I have some questions about C_Comp parameter in IBIS models. I am trying to resolve some issues for IBIS models of high frequency differential output buffers. I am trying to compare my SPICE model transient simulations with the IBIS model I generated for a differential output buffer. At low frequencies (eg ~ 100MHz), the SPICE and IBIS model output simulation waveforms correlate closely. But at higher frequencies (>400MHz), the SPICE and IBIS model simulations don't match as well. I am trying to see if the mismatch is due to the C_comp parameter, or if this is just a limitation of IBIS models not being able to capture high frequency effects. 1. How IBIS simulators use the C_comp parameter during simulations? 2. Don't the V-T tables and ramp information already take into account the effect of C_comp? 3. What does the C_comp parameter exactly represent? -should it include die routes, pad capacitance, etc? 4. How should it be measured? 5. Should C_comp be measured in a special manner for differential buffers like LVDS buffers? I would appreciate your response. Thank you, -Chitiz Mathema Cypress Semiconductor -- 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 1993 -- 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 1993 -- 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 Mon Aug 27 08:13:38 2007
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