[SI-LIST] : IBIS or SPICE examples

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From: Mark Alexander (Mark.Alexander@xilinx.com)
Date: Thu May 03 2001 - 08:47:55 PDT


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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