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

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From: Muranyi, Arpad (arpad.muranyi@intel.com)
Date: Thu Nov 18 1999 - 11:52:04 PST


On Will Hobb's (of Intel Corporation) request I am forwarding
his EMAIL to the list.

Arpad Muranyi
Intel Corporation
================================================================

-----Original Message-----
From: Hobbs, Will
Sent: Thursday, November 18, 1999 11:42 AM
To: Peters, Stephen; Muranyi, Arpad
Subject: FW: [SI-LIST] : IBIS datasheets for PCI and DDR

Stephen, Arpad,

I've tried twice to post this to si-list, but it didn't get through. Could
you forward this for me? Thanks.

Will

-----Original Message-----
From: Hobbs, Will
Sent: Thursday, November 18, 1999 9:23 AM
To: 'si-list@silab.eng.sun.com'
Subject: Re: [SI-LIST] : IBIS datasheets for PCI and DDR

Jim,

It is true that one advantage from the semiconductor vendor's point of view
is IP protection. When we introduced IBIS, the reality was that very few
models existed at all, due largely to IP concerns. IBIS isn't ideal, but it
has lots of advantages that have been enumerated in the mail thread,
including speed and the potential to be accurate enough for most uses.

To me, though, the largest benefit the industry has gained from IBIS is that
models are now widely available. Imagine trying to do your high speed
designs without them. IBIS removed the log jam that prevented engineers from
getting models at all. And it has evolved to attempt to keep up with
emerging needs. It trails, and will probably always trail, the leading edge
of our needs, but it sure beats the alternative (no models at all), and it
is still improving.

Will

-----Original Message-----
From: Jim Freeman [mailto:freeman@broadcom.com]
Sent: Tuesday, November 16, 1999 4:51 PM
To: si-list@silab.eng.sun.com
Subject: Re: [SI-LIST] : IBIS datasheets for PCI and DDR

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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