I'd like to add a couple comments to what Michael said. I'm sure most of us already understand the concept of flight time, but it doesn't hurt to mention it once in a while, when such topics come up. =20 In the timing methodologies I'm familiar with the IBIS simulations are designed to capture flight time from the buffer output to receiver input. This flight time is obtained by subtracting the buffer delay into the test load, obtained from an IBIS simulation, from the overall delay seen in an IBIS simulation into the actual load topology, with both simulations launched in parallel at t0. The process of subtracting out IBIS buffer delay then normalizes the flight time for the random delay in the IBIS model V-T curves and leaves you with flight time.=20 The flight time then includes the prop delay from driver output to reciever, plus the delta in driver tCO between that defined in the driver characterization sims, generally done at transistor level, and that which is seen when driving into the actual load. You can then take this flight time and add it to the characterized tCO of the driver to calculate the overall delay from driver input to receiver. However, since most devices provide AC timing specs defined at the pad or pin, which already comprehend the tCO of the driver, this summation is generally not neccessary. Most timing analysis I am involved in uses only AC specs and flight times or skews to calcualte timing margins. In addition, IBIS models must be constructed with V-T curves that fully switch within the minimum switching period seen in simulation. For square wave stimulus this would be one-half the switching period. Otherwise you risk inconsistent results, due to the switching into an unfinished edge phenomenon. Something that is not handled consistently in all simulators. Therefore, trying to add a true tCO delay to the leading edge of the V-T curves just makes it that much more difficult to capture the transistion within the switching interval. On many interfaces, such as DDR2, its a challenge to capture these edges without distortion, due to the limited slew rate of teh drivers.=20 Whether this problem is addressed better in teh multi-lingual IBIS extensiosn is not something I have inquired about, but in traditional V-T curve based IBIS it is an importany consideration.=20 Brian P. Moran Intel Corporation=20 -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Mirmak, Michael Sent: Friday, November 17, 2006 10:54 AM To: si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: Buffer delay All, I would disagree somewhat with the conclusions regarding accuracy and IBIS vs. SPICE models. We must be very careful in how the words "buffer delay" are being used here. For system analyses, many tools define "buffer delay" as the time elapsed between the start of *simulation* and the crossing of a defined measurement voltage, using a defined measurement load. For a well-constructed IBIS model, even using traditional V-t and I-V tables, excellent correlation to SPICE transistor-level models may be obtained for buffer delay in this sense. As a result, IBIS could be -- and is -- used for system timing analysis where the timing equations use buffer delay. Timings for driver outputs relative to an incoming clock signal -- for example, "Tvalid" or "Tco" (Time clock to out) -- could not be directly simulated under traditional IBIS, as the format does not include the data path from the incoming clock node to the driver, to "latch" the signal out. =3D20 Advanced IBIS models, such as those under IBIS 4.1 and 4.2 using multi-lingual extensions, may contain these driver clock-to-out connections and paths, should the model author choose to include them. A direct comparison of clock-to-out waveforms between this kind of advanced IBIS model and a SPICE transistor model may then be obtained. I hope this helps! - Michael Mirmak Intel Corp. Chair, EIA IBIS Open Forum -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of raj singh Sent: Friday, November 17, 2006 01:34 To: yang@xxxxxxxxxx; si-list@xxxxxxxxxxxxx Cc: liuweidong@xxxxxxxxxx Subject: [SI-LIST] Re: Buffer delay Hspice delays should be accurate and in general more than ibis. Better to use ibis only for SI sims and not for timing. =3D20 regards Raj yan hang <yang@xxxxxxxxxx> wrote: =3D20 Hi All, Now I do some board timing simulations . I got the hspice model and ibis model for same buffer. Then I set a test fixture(Ex. Cref=3D3D15pf,Rref=3D3D1E6,Vref=3D3D3.3V),I contrast the simulation = result of =3D buffer delay using hspice model and ibis model. I found there is a little big diffrence between them. So when translate hspice model to ibis model,how to consider about the buffer deley issue?=3D20 Should I trust the buffer delay value from ibis model simulation? Best Regards. Yan Hang Huawei Technology 086-21=3D3DA3=3D3DAD68644808=3D3DA3=3D3DAD24043 ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List FAQ wiki page is located at: http://si-list.org/wiki/wiki.pl?Si-List_FAQ List technical documents are available at: http://www.si-list.org List archives are viewable at: =20 //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu =20 ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List FAQ wiki page is located at: http://si-list.org/wiki/wiki.pl?Si-List_FAQ List technical documents are available at: http://www.si-list.org List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu