Re: [SI-LIST] : IBIS datasheets for PCI and DDR

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From: Jim Freeman (freeman@broadcom.com)
Date: Tue Nov 16 1999 - 16:50:44 PST


Hi All,
    The discussion below seems to forget that the main reason IBIS models were invented was to prevent Chip vendors from giving out the c"crown" jewels of Spice parameters. The First vendor to do so was Intel because they are the
most paranoid about secrecy.

Jim Freeman

Muranyi, Arpad" wrote:

> Fred,
>
> Very good comments, and I would like to put an emphasis on the
> assumption you make, IF the models are correct (big IF). It is
> also true that SPICE models can or usually have a lot more detail
> so they can be more accurate (if done correctly). But detail does
> not equal accuracy. And, it is very true that they both have their
> appropriate places for use. A circuit designer will not get anything
> done with IBIS models, but a system designer will probably never get
> his/her work done with SPICE models either.
>
> I want to point out another thing that kind of hit a nerve. You
> mentioned the number of BIRDs (64 and counting) in the IBIS case.
> To be fair, you should have also mentioned the number of SPICE
> flavors that exist. These exist mostly because each vendor has some
> (unfortunately incompatible) "value added" feature. In addition, look
> at the number of "levels" each SPICE has for the various models, some
> of which are, again, incompatible. BSIM alone has at least three levels,
> and BSIM3 of one tool does not necessarily work with another tool
> supporting BSIM3. You don't even have to use different flavors of SPICE,
> just think about one of the comments someone made about the scaling factors.
> You simply can't simulate with two SPICE models, if one of them uses
> certain scaling factors that another model doesn't need unless you rewrite
> the model (good luck). (Of course this is only a problem for those who do
> system level simulations where you must have more than just your own
> design's SPICE model).
>
> I don't claim that IBIS is perfect, it is an evolving standard, mostly
> done by volunteer efforts. I also wish that it would have a lot more
> badly needed features, that it would be more consistent and more general.
> We have serous talks now about these issues among the "IBIS officials".
> We want to fix the shortcomings. However, if we point these problems
> out for IBIS as negatives or drawbacks, we should also be honest and
> admit that SPICE DOES have similar problems, even if it may seems to be
> a more stable language...
>
> Arpad Muranyi
> Intel Corporation
> ======================================================================
>
> I agree strongly with most of the statements but disagree with the
> accuracy issue. Assuming that the SPICE model is correct and the IBIS
> model is correct, there is no way that an IBIS model can be more
> accurate than a SPICE model. This is regardless of where the IBIS
> model came from. Keep in mind that MOST IBIS models are produced from
> the SPICE model using s2ibis. That means by default MOST IBIS models
> cannot be more accurate than the source. For one thing SPICE is a
> different animal and contains much more information regarding both the
> topology and process of the design. Secondly there have been and
> continue to be accuracy as well as other issues with IBIS. If this were
> not the case there would not be 64 birds to date and counting.
>
> Both the IBIS and SPICE model are prone to incorrect data. Being not
> correct is NOT the same as model accuracy. In my view both SPICE and
> IBIS models have their application. There is some overlap in the SI
> community. You probably should not use SPICE models to simulate a
> whole board. But accuracy is a different issue. For most SI applications
> IBIS will do. But there are cases where one may need the SPICE model.
>
> Best Regards,
> --
> Fred Balistreri
> fred@apsimtech.com
>
> http://www.apsimtech.com
>
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