From: Jim Freeman ([email protected])
Date: Thu Nov 18 1999 - 12:31:58 PST
Hi Will,
I have been performing Hspice simulations for board level analysis for 10 years and have had no trouble receiving or using a vendor's spice models.
Your statement below basically says that we should be happy to get any models at all. In the past I have had to respond by not using Intel's parts. All of the simulators that use IBIS are designed to ensure signal integrity
within the bounds of the capability of the models. The sso question goes largely unanswered by the use of IBIS and makes bypass calculations basically a swag. IBIS models do not project the variability of output impedance for the
driver correctly. A transistor operates much differently than an ideal current source as is projected by IBIS usage. The IBIS builds on the industry formed using mainly bi-polar parts for drivers which can be more accurately
portrayed as a constant current source over a wide variety of collector-emitter voltages. We are now in the CMOS era and this assumption is no longer true. Most of the board designers have grown up with the bipolar concept of
constant output impedance and the IBIS feeds that mentality however misguided.
There are now three major versions of IBIS models that have been proposed and most of the simulators are at least a major rev behind. This makes it difficult to really use the innovations that are coming out.
To my mind, all that IBIS has done is muddy the waters sufficiently to satisfy the needs of semiconductor vendors and let the designers handle the board issues with inaccurate models that foster rampant overdesign while the
semiconductor companies enter the board business with faster and better solutions and gain market share.
Jim Freeman
"Muranyi, Arpad" wrote:
> 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: '[email protected]'
> 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:[email protected]]
> Sent: Tuesday, November 16, 1999 4:51 PM
> To: [email protected]
> 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
> > [email protected]
> >
> > http://www.apsimtech.com
> >
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