1. In an open drain buffer if you were to tie your "pullup" resistor to ground how much of a transition would you get? Ohms law states that the current through a resistor is proportional to the voltage across it. So, a open drain buffer who's source is tied to ground has a resistor on its drain tied to ground will in the off state have zero current and in the on state will have zero current. 2. Since the voltage swing is going to be different with different termination voltages then it would be wise to model your output at the various voltages it may operate at. 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: NG,MEI-YEE [mailto:mei-yee.ng@avagotech.com] Sent: Tuesday, December 05, 2006 3:47 PM To: tom@teraspeed.com; Lynne D. Green; ibis-users@server.eda.org Subject: RE: [IBIS-Users] IBIS load Hi all, Thank you for clarifying the difference between the R_fixture and the Rref for my buffer. I have 2 more questions regarding an open drain buffer: *According to cookbook pg 19, for open drain buffer, the recommended load circuit is 1R+1F into manufacturer's suggested pullup resistor termination and voltage. 1) Therefore, when I generate the VT tables for Rise and Fall, my resistor is always tied to the termination voltage (eg. VDD)? Does this imply that I am simulating for the case where PCB traces (load) are always tied to high? 2) The buffer is supposed to work with pullup voltage of VDD AND 1.8V. Does this mean my VT tables will have termination voltages of VDD and 1.8V? Thank you very much for the useful feedback. Best regards, Mei Yee SID, Avago Technologies (Malaysia) Tel no.:604-6108515 -----Original Message----- From: Tom Dagostino [mailto:tom@teraspeed.com] Sent: Tuesday, December 05, 2006 4:12 PM To: NG,MEI-YEE; 'Lynne D. Green'; ibis-users@server.eda.org Subject: RE: [IBIS-Users] IBIS load The load that is seen by a driver is first of all the trace on the circuit board. Usually trace impedance is in the 40 to 65 Ohm range. Thus the IBIS model should characterize the buffer driving the load that is expected to be seen by the driver. In your case the waveforms are relatively slow, even when the buffer is terminated in 50 Ohms. I assume your 10 nsec rise and fall times were measured without any capacitance at the load. If you simulated with a C load then you will likely have simulations issues in actual circuits. If the buffer is really 10 nsec, then you will start seeing SI issues only after about 2 nsec trace lengths or about 12 inches on the board. The only parts we see these days with such slow edge speeds are RS232 typ drivers. In this case the manufacturer has characterized the prop delay of the buffer when it is driving a 1.5K load terminated to Vdd. The timing is measured at Vmeas. If there was any C associated with the timing test load it would be included along with the timing test resistor. Also, in a real application of this buffer with a 1.5K pullup there will be relatively low currents flowing and slow edges at the load but the transition at the output of the buffer will be faster. 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 NG,MEI-YEE Sent: Monday, December 04, 2006 9:55 PM To: tom@teraspeed.com; Lynne D. Green; ibis-users@server.eda.org Subject: RE: [IBIS-Users] IBIS load Hi Tom, Thanks for clarifying this. One more question, what is the difference between the manufacturer's suggested pullup load resistor used in V-T values extraction and the Rref value specified? In my case, the suggested pullup is 1.5KOhm, so I only need a 50 Ohm V-T table and my Rref = 1.5Kohm? Thank you very much. Best regards, Mei Yee SID, Avago Technologies (Malaysia) Tel no.:604-6108515 -----Original Message----- From: Tom Dagostino [mailto:tom@teraspeed.com] Sent: Tuesday, December 05, 2006 12:17 PM To: 'Lynne D. Green'; NG,MEI-YEE; ibis-users@server.eda.org Subject: RE: [IBIS-Users] IBIS load Lynne The 50 Ohm pullup does the trick for "biasing control" of the buffer. There is no need for the 1.5K. 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 Lynne D. Green Sent: Monday, December 04, 2006 7:56 PM To: 'NG,MEI-YEE'; ibis-users@server.eda.org Subject: RE: [IBIS-Users] IBIS load Hello, Mei-Yee, The 1.5K resistor is needed for bias control for an open-source/open-sink buffer. In addition to that resistor, one needs a V-t load (the 50 ohms you refer to). The 50 ohm load is described in the IBIS Cookbook, http://www.vhdl.org/pub/ibis/cookbook/cookbook-v4.pdf. This means your simulation will have both resistors: the 1.5K and the 50 ohms. Once you have completed your model, please feel welcome to submit your model to the IBIS Model Review Committee, http://www.eigroup.org/ibis/support.htm. Best regards, Lynne "IBIS training when you need it, where you need it." Dr. Lynne Green Green Streak Programs http://www.greenstreakprograms.com 425-788-0412 lgreen22@mindspring.com ________________________________ From: owner-ibis-users@eda.org [mailto:owner-ibis-users@eda.org] On Behalf Of NG,MEI-YEE Sent: Monday, December 04, 2006 5:28 PM To: ibis-users@eda.org; ibis-users@eda.org Subject: [IBIS-Users] IBIS load Hi experts, According to the cookbook, when extracting V-T data, it is recommended to use a load circuit of 1R + 1F into manufacturer's suggested pullup resistor termination and voltage. However, when I use the suggested pullup resistor (1.5K), the rising waveform takes up to 100ns to stabilize, but the falling waveform requires less than 10ns. This creates inaccuracy due to the resolution of the points (when I use 100ns simulation time). In the IBIS model, I also included a waveform for 50Ohm pullup resistor (as recommended by cookbook). The rising waveform for this is also within 10ns. I wonder if I can generate IBIS model with ONLY 50 Ohm pullup because the resolution is more appropriate in this case? I have verified this model and the simulation shows acceptable waveforms. Kindly advice whether it is a MUST to have a 1.5K pullup resistor V-T table. Thank you. Best regards, Mei Yee Ng IC Design Engineer Illumination and Color Management Solid-state Illumination Division Avago Technologies Malaysia Tel no: +604-6108515 -------------------------------------------------------------------- |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 -------------------------------------------------------------------- |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 -------------------------------------------------------------------- |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 Dec 5 16:00:13 2006
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