Subject: Re: FW: FW: Spice Model of IBIS

From: Jon Powell <[email-masked]>
Date: Tue, 31 Mar 1998 09:01:02 -0800

Well, I just couldn't resist.

I certainly wouldn't argue with anyone that SPICE (actually some SPICE
commercial derivative) is the premier circuit simulation technology that
is currently publically available. I also wouldn't argue with anyone who
said that IBIS has some problems that need to be addressed.

However, the market has shown that there is a need for a PCB SI
simulation technology that offers features that transcend the current
SPICE node based syntax. Think of the bigger problem. You need to
simulate the SI and Crosstalk effects of every net on a PCB. You need
MIN MAX timing from every driver to every receiver. You need SI and
Noise margin numbers for every net. And you need them quickly enough so
that you can use the data to turn the design a couple of times. I
believe these issues are difficult to solve using SPICE as it (they)
exists on the market today.

Of course, I use SPICE every day. I think it is great. I just believe in
using the right tool for the right job and I have more than one job to
do.

Here is a conceptual example:
you have a 32 wire bus. Each wire couples to it's neighbors. You need to
simulate every wire and the effects on that wire by it's immediate
neighbors. You do not want to waste simulation time by simulating
effects of the wires that are more than a couple of conductors away.

How would you solve this problem using SPICE?
Can it be solved automatically?
How would spice represent the names of the different nets? (ie. what is
the crosstalk on D13?).

As for accuracy:
Leave the argument of which approach is more accurate for the moment.
what is the accuracy of your inputs? (PCB stack-up, SPICE model data,
Dielectric constant, trace width).
If you are simulating to a higher accuracy than your data inputs then
you are wasting time and deceiving yourself. I think that a teacher told
me this in 5th grade but I never really believed it until I started
doing SI simulation.

---

From: Scott McMorrow <[email-masked]>
Date: Tue, 31 Mar 1998 10:02:22 -0800

Jon,

Well said!  I agree, let's use the right tools for the job.
All are appropriate within specific contexts.  And lets not
forget the comfort factor.  Once one becomes familiar with
a specific tool that does a given job, it is hard to overcome
the inertia and change to a tool that might be different, if
not better.

You're right.  If the pcb tolerance is +/- 10% and the part process
tolerance is +/- 30%, simulator differences are generally
inconsequential.

I and my customers value my time.  For my time, I generally choose
a tool based upon ease of use, speed, and accuracy.  In that order.

Scott McMorrow
SiQual

---

From: "John Synesiou" <[email-masked]>
Date: Tue, 31 Mar 1998 12:29:42 -0600

Jon,

You raise a good point. I raised this point originally to question the need
to develop a unique syntax for IBIS, when an existing syntax would prevail.
In my opinion, IBIS should describe a template using existing SPICE syntax
(or XSPICE syntax) so that the engineering community can choose the best
tool for their simulation and analysis, i.e. IBIS simulator or SPICE
simulator.

Pre PCB layout simulation could be simulated quite effectively in SPICE,
with the added advantage that more extensive analysis could be done if
required. IBIS simulators on the other hand have a particular purpose and
hence the analysis as I understand it is limited to time domain waveforms
(rise/fall time, ringing, reflections) and a simple spectral plot. For
instance, I'm not sure how you would calculate the power in a termination
resistor, a simple example, but you get the point.

Regarding crosstalk, I'm not sure how the IBIS simulators work, but I'm sure
that they don't consider crosstalk from one trace to all other traces, but
instead consider only the closest traces (one or two traces). Considering
that some EDA vendors IBIS simulators are actually SPICE simulators with an
IBIS to SPICE translator, I would expect that this problem could also be
solved in SPICE directly. I'm not sure I need to make this point, but I put
it in for further discussion.

My point really is that I may like to use SPICE to do more extensive
analysis than is available with an IBIS simulators. I can take any IBIS
model today and translate it into a SPICE model to examine various issues,
SI, EMC and others. I just find the translation process time consuming and
error prone. In essence, why does it have to be an either or, why can't
SPICE simulators be an extension of IBIS simulators?

Regards

John Synesiou			[email-masked]
U.S. Power, Inc			Phone (612)826-1111
6497 City West Parkway	Fax (612)826-1003
Eden Prairie			Date: 03/31/98
MN, 55344			Time: 11:45 AM

-----Original Message-----
From:	Jon Powell [mailto:[email-masked]]
Sent:	Tuesday, March 31, 1998 11:01 AM
To:	[email-masked]; [email-masked]
Subject:	Re: FW: FW: Spice Model of IBIS

As for accuracy:

---

From: Fred Balistreri <[email-masked]>
Date: Tue, 31 Mar 1998 11:07:15 -0800

> me this in 5th grade but I never really believed it until I started
Surprise! Every item you mention above has been sucessfully acomplished
by many of our customers and other Spice vendors as well. Using IBIS
or behavioral models a whole board can be done in reasonable times using
Spice. The problem of simulating wires by distance or number of
conductors
is related to the user interface control and the field solver not a 
simulator issue at all. This has been done by the way in our system
since
1992 and some other SPICE vendors a bit later. Node names are not an
issue
in modern Spice programs. Your dealing with 2.6 from Berekley circa
1968.
PCB stackups have nothing to do with Spice. It's an issue on integrating
a
good field solver with your simulation engine. Again its been done by 
several Spice vendors. You are either trying to confuse customers by
these
issues you raise or you truly don't understand modern day Spice tools.
Of course Spice is not perfect and there are plenty of problems. But
neither
is IBIS. It has its place but at the moment the capabilites are a bit
on the lite side...and there are plenty of problems as you yourself have
raised. We'll leave accuracy aside for the moment as you mentioned. How
about flexibility, capability? Can IBIS handle Analog? How about more
sophisticated output stages that dynamically change current
sourcing/sinking
capability? Where's the information about the power supply currents?
Heck
some IBIS data doesn't even contain switching information. How about
ground
bounce? Since there is no mention of the gate or base of the
transistors,
once noise is introduced via the power/gnd what's the voltage on the
base/
or gate of the I/O output transistors? Without that you don't even know 
which transistor is on or off. I'm for bringing out these issues and
solving
them using IBIS not covering them up or changing the argument. Spice is 
just a simulator. It will survive or die on its own merits. How about
improving IBIS? 

Best Regards, 
-- 
Fred Balistreri
[email-masked]

http://www.apsimtech.com