RE: [SI-LIST] : IBIS or SPICE examples

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From: Reid, Chris (chris_reid@mentorg.com)
Date: Thu May 03 2001 - 09:34:56 PDT


Mark,

I like your summary of the issues between using IBIS or SPICE,
although I might have spun it a litte differently.

As you may be aware, the IBIS committee is considering a new
specification called IBIS-X that hopefully will address some
of your concerns. IBIS-X is fundamentally a language based
approach to modeling.

However, the problem of modeling the power and ground
distribution circuits is really a matter of obtaining from
somewhere a realistic description of the parasitics on the IC
and the PCB (and the connection between them) so that some
kind of simulator can then produce an accurate result. An
IBIS model implies certain currents through the pull-up,
pull-down, power-clamp, and ground-clamp elements. These
currents come from the power and ground distribution busses.
If the IBIS model is properly attached to these busses then
the effect of these current draws will be seen at the output
of the IBIS device. Are these currents close to what the
actual device would draw? Well, IBIS was not designed to
model those currents, so the answer seems to be a qualified
maybe. There is interesting work that has been done in this
direction.

If you are intested please visit the IBIS web site:

http://www.eigroup.org/ibis/ibis.htm

In particular the articles list:

http://www.eigroup.org/ibis/articles.htm

is very helpful. Specifically on currents drawn from power
and gound you might want to look at the paper by
Bernhard Unger (Siemens AG):

http://www.eda.org/pub/ibis/summits/jan00/unger.zip

Thanks,

Chris Reid

-----Original Message-----
From: Mark Alexander [mailto:Mark.Alexander@xilinx.com]
Sent: Thursday, May 03, 2001 8:48 AM
To: Todd Westerhoff
Cc: si-list@silab.eng.sun.com
Subject: [SI-LIST] : IBIS or SPICE examples

Todd et al,

Many of us are familiar with the fundamental differences between SPICE and
IBIS
-- one is difficult and highly accurate, the other is simple and moderately
accurate. What might be more useful in this discussion is information from
personal design experience about when you would use one versus the other.

For example, up until recently my department used IBIS exclusively for our
board
design and simulation. These boards involved single-ended signals running
from
0 to 200 Mhz and differential signals up to 840 Mhz. However as we move
into
the world of high-speed serial channels at 3, 5 and 10 Ghz, the ice on the
IBIS
pond gets a bit thin.

Right now we're running both SPICE and IBIS simulations of our systems in
order
to make sure we're seeing everything we need to. Our concerns stem both
from
the general aspect of IBIS's approximate nature, as well as the specific
aspect
of package modeling. Detailed package modeling within the confines of the
IBIS
spec is difficult. We see differences in our simulation results (between
SPICE
and IBIS), though we're just beginning to draw conclusions as to what's
causing
them and how we can improve the IBIS model.

Others have commented that in order to model systems with a non-ideal power
plane, you have to use SPICE because no one yet makes an IBIS simulator that
can
handle non-ideal power planes. I'm not very deep in this realm... can
anyone
comment?

-mark

Todd Westerhoff wrote:

> The answer to your question is: it depends.
>
> IBIS is actually a language for characterizing I/O behavior that has a
> defined structure and syntax. That's the long way of saying it models a
> defined subset of all possible I/O behavior. SPICE, on the other hand,
can
> model anything. Want to model viscous fluid flow through a pipe? Sure -
> come up with the correct electrical equivalent model, and SPICE can handle
> it for you.
>
> So, "it depends", means that you have to understand the interconnect and
I/O
> you want to analyze, and whether or not behavioral analog simulation will
> model enough of the effects properly to give you a reasonable answer. If
> IBIS provides an adequate answer, then by all means, use it. And, if IBIS
> won't give you the detailed answer you want, you still may be better off
> running IBIS up-front to get you in the ballpark, before you run a SPICE
> analysis. This is especially true if you're running pre-route analysis
and
> looking at a number of different scenarios. We regularly analyze hundreds
> and thousands of "variations" at a time using IBIS models, receiving
answers
> within minutes. You can imagine what the turnaround time would be if we
> were doing things differently.
>
> But for I/O structures that IBIS cannot address, SPICE may be your only
> choice. And for critical applications, it's almost always worth the
effort
> to look at structures with both forms of simulation, to make sure the
> answers correlate.
>
> I view simulators as tools. They help you get a job done, but you have to
> understand what their limitations are, how to use them, and to which
> problems they are best applied. Please don't expect a simulator or SI
tool
> to give you an answer - because it won't. Its purpose is to give you data
-
> on which you can base design decisions.
>
> Todd.

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