RE: [SI-LIST] : Hspice: Windows vs Unix

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From: Todd Westerhoff (twester@hhnetwk.com)
Date: Tue Apr 10 2001 - 15:01:05 PDT


"So I think the accuracy issue is illusory."

I like that. Truer words were never typed ;-).

The argument of IBIS vs. HSPICE is a recurring one. While I think there is
a lot of substance to it, I also think the whole issue is incredibly
over-hyped. We're talking about analog analysis after all; error is
inherent. It *cannot* be avoided, and therefore, the real issue is keeping
the "accuracy" of the analysis in perspective. It doesn't do you much good
to go after the last 1% of accuracy with your simulator algorithms when your
models are only +/- 5% to start with. However, understanding how models
correlate back to reality is difficult, at best.

The real problem, I suspect, is that "HSPICE is more accurate than IBIS"
makes for a good sound bite, and "you really have to understand what you're
modeling, and how" doesn't. After all, modeling isn't fun. Right?

I think there are lots of places where the "SPICE or not to SPICE" arguments
have merit. But I've also spent enough time trudging through models and
data where the most basic things were wrong, to know that until we have a
firm, common foundation, arguing about details doesn't make much sense. And
guess what - we're not there yet!

The disguised blessing with IBIS is that by standardizing the model format,
it made it easier for users to find problems with the models they use, and
to correlate those models against test load conditions and datasheets.
HSPICE models, in contrast, are often encrypted and have unique interface
requirements (control pins, voltages and slew rates). Bottom line, an IBIS
model is a lot easier to check and use than a corresponding HSPICE model.

If you're analyzing phenomena that only a SPICE model can represent, then
there's no choice. But I'd use IBIS as the "first line of defense" in any
situation where I could, and only back that analysis up with SPICE when
needed.

My $0.01 (only worth half of a typical opinion).

Todd.

-----Original Message-----
From: owner-si-list@silab.eng.sun.com
[mailto:owner-si-list@silab.eng.sun.com]On Behalf Of Kim Helliwell
Sent: Tuesday, April 10, 2001 4:19 PM
To: Anthony Davidson
Cc: 'Dunbar, Tony'; 'si-list@silab.eng.sun.com'
Subject: Re: [SI-LIST] : Hspice: Windows vs Unix

Actually, accuracy isn't really the issue, or at least not in the
sense your management probably means it, Anthony.

SpecctraQuest uses a spice-like simulator, TLSIM, to do the work.
This simulator is a derivative of SPICE (I don't know which flavor),
and therefore has all the usual accuracy plusses and minuses of
any SPICE, including HSPICE.

In addition, TLSIM has a coupled transmission line model, and the
diode and transistor models have been removed, and the whole simulator
has been optimized for the problem it's intended to solve.

The question of whether TLSIM's coupled transmission line is more
or less accurate than HSPICE's W-element is one of the issues, and
I cannot quantify it, except that I've never seen any reason to
distrust either one. From that I conclude that they are probably
equally good.

The real issue you face is the classical conundrum of SPICE: that accuracy
of results depends on accuracy of the models. So the issue is: what's
more accurate: the manufacturer's original BSIM models of their buffers,
or their IBIS models? The answer is obvious, since presumably the IBIS
models derive from the SPICE buffer models (almost no one creates IBIS
models from lab measurements, you see). But IBIS models can be very
close to the buffer models they derive from, and it's possible to lose
very little accuracy in using them. Whereas you might not be able to even
get the buffer model. And then you are forced to use HSPICE's IBIS buffer
model, at which point the accuracy of the two is on an even footing, and
it's *MUCH* harder to use HSPICE in this way than to use SpecctraQuest.

So I think the accuracy issue is illusory. If your management has enough
confidence in you, you have a chance to educate him/her/them as to the
realities of the situation.

Personally, I've used SpecctraQuest a lot in the last 2 years, and HSPICE
only
occasionally. I use it for 2 things: as a field solver when the problem is
not easily expressible in terms that SQ understands, and perhaps to create
an IBIS model when the vendor provides an HSPICE buffer model but no IBIS
model.
A third possibility is when IBIS doesn't accommodate a particular type of
buffer.

So I think Tony has a good question, and it's also not clear to me what
value-added HSPICE provides in your management's view. There is some, but
perhaps not where they are looking for it.

Kim

Anthony Davidson wrote:
>
> Perhaps that's where I have seen your name.
>
> I am a new user to Hspice, and SpecctraQuest for that matter. But the
> opinions of my team leaders is that the tools that are able to do analysis
> on complete boards and board-to-board interconnects are not as accurate as
> Hspice. And Hspice is more accurate, however, the analysis of complex
(many
> connections) boards is very difficult.
>
> Note that the "less accurate" and "more accurate" statements are the
> opinions of others and not necessarily quantifiable.
>
> Anthony Davidson
>
> -----Original Message-----
> From: Dunbar, Tony [mailto:tony_dunbar@mentorg.com]
> Sent: Tuesday, April 10, 2001 11:18 AM
> To: 'Anthony Davidson'
> Subject: RE: [SI-LIST] : Hspice: Windows vs Unix
>
> Hi Anthony,
>
> No, neither Nortel, nor Univ of Western Ontario. Maybe you've seen my name
> on the list or something.
>
> The reasoning behind my question is that the platform on which you're
> running the other tools might give some pointers to the H-SPICE platform
> choice. Actually, since you're going with SPECCTRAQuest, I don't really
see
> the need for H-SPICE for the so-called "fewer, critical nets". On these
> nets, what is it you're looking for SPICE to provide that SPECCTRAQuest
> can't? I'm not saying there is never room for co-existance, because there
> often is. It's just the "more accurate" thing that intrigues me.
>
> Regards,
> Tony
>

--
Kim Helliwell
Senior CAE Engineer
Acuson Corporation
Phone: 650 694 5030  FAX: 650 943 7260

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