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

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From: Willis, Ken ([email protected])
Date: Thu May 03 2001 - 13:03:03 PDT

Hi Mark,

I have run into a couple of chip suppliers that indicate things the
other way around, which I found interesting. In this particular
case, I had put in a request for a model, and eventually got to
the model developer. His opinion was that for the high speed
(2.5 GHz) data rates we were talking about, the driver HAD to be very
linear and symmetric by definition. His take on it was that when
looking at these kinds of speeds, the driver model actually became
simpler and the interconnect model becomes much more detailed and complex.
He felt the result would be "interconnect-dominant" rather than
"device-dominant". He gave me the IBIS data I asked for and some
details on the package. I was able to model all this up, run a quick
pseudo-random data pattern down there, and get some eye patterns that
were very close to what we had in the lab. We were able to track down
some stubbing problems inside the receiving package that were closing
the eyes down a bit as well. So it seems in certain applications you may
be OK using IBIS style models for some pretty high speed applications.

A couple of important caveats (of course). Power and ground problems did
not seem to be an issue in this application. And the loss tangent values
are very important to get proper results. Also it is critical to model
vias in detail, including the stubbing that has been discussed at length
here. I used the spice subcircuit capability of SPECCTRAQuest
quite a bit for modeling complex passive stuff like packages, vias, etc.
Your results will be very dependent on how well you can model these things.
So it wasn't push button "out-of-the-box" tool usage, there was some
stuff to do under the hood, but it worked pretty well. At any rate, there
is another data point for you on behavioral vs. structural device modeling.
I'm sure there are cases in which structural modeling is mandatory, but this
didn't seem to be one of them. As always it will be dependent on your
application, and your mileage will vary.


-----Original Message-----
From: Mark Alexander [mailto:[email protected]]
Sent: Thursday, May 03, 2001 11:48 AM
To: Todd Westerhoff
Cc: [email protected]
Subject: [SI-LIST] : IBIS or SPICE examples

Todd et al,

Many of us are familiar with the fundamental differences between SPICE and
-- 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
design and simulation. These boards involved single-ended signals running
0 to 200 Mhz and differential signals up to 840 Mhz. However as we move
the world of high-speed serial channels at 3, 5 and 10 Ghz, the ice on the
pond gets a bit thin.

Right now we're running both SPICE and IBIS simulations of our systems in
to make sure we're seeing everything we need to. Our concerns stem both
the general aspect of IBIS's approximate nature, as well as the specific
of package modeling. Detailed package modeling within the confines of the
spec is difficult. We see differences in our simulation results (between
and IBIS), though we're just beginning to draw conclusions as to what's
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
handle non-ideal power planes. I'm not very deep in this realm... can


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,
> 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
> 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
> looking at a number of different scenarios. We regularly analyze hundreds
> and thousands of "variations" at a time using IBIS models, receiving
> 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
> 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
> 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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