Subject: RE: Non Monatomic waveform, practically, how important?

From: "Hobbs, Will" <[email-masked]>
Date: Tue, 10 Nov 1998 08:52:00 -0800

Nick,

IBIS doesn't disallow non-monotonic curves, but the parser issues a 
warning. The discussion (back in '93) that led to this parser 
feature was based on the observation that such curves frequently 
are in error-- most I/V behavior on real parts is monotonic. Note 
that this is a warning only, and meant to spur the engineer to take 
a second look to be sure the curve is accurate.

Will Hobbs
Intel Corp.

Has anybody run test cases against popular simulators to determine 
how important/sensitive monotonicity is to:

1) Convergence
2) Accuracy

We are frequently running into models with non-monotomic features.  The 
simulator
vendors recommend that models be monotomic and I believe the IBIS 
spec demands it.  Screening these features out is time consuming and
testing the resultant model is even more time consuming.  IBIS simulators 
generally do not have the knobs that SPICE simulators do.  Is there a 
mathematical reason for this, or is it more a customer issue?

Anybody have a few pearls of wisdom on this?  What is the history behind it?

Nick Krull
StorageTek
11/10/98

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From: "fabrizio zanella" <fabrizio=[email-masked]>
Date: Tue, 10 Nov 98 11:59:17 -0500

Nick, it's not true that the IBIS spec demands monotonicity on models.  If 
you run one of the free IBIS model viewers available on a model with 
non-monotonic curves, you will get warnings, not errors.  I know of two SI 
simulators which accept non-monotonic IBIS models.  Non-monotonicity is a 
reality in some devices, as I've learned in talking to engineers at 
semiconductor vendors.  

Regards, 
Fabrizio Zanella
EMC corporation
[email-masked] 
-------------
Original Text
From: "Krull, Nick J" <[email-masked]>, on 11/10/98 11:29 
AM:
To: smtp@Eng@EMCHOP1["[email-masked]'" <[email-masked]>]

1) Convergence
2) Accuracy

Anybody have a few pearls of wisdom on this?  What is the history behind 
it?

Nick Krull
StorageTek
11/10/98

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From: "Muranyi, Arpad" <[email-masked]>
Date: Tue, 10 Nov 1998 09:35:00 -0800

Also, don't forget that some of these non-monotonic curves come from the 
numerical separation of the pulldown/up and clamp curves which will 
become monotonic again after they are added back together.  The problem 
is that the checker checks for non-monotonocity without adding them up.

Arpad
=======================================================================

The classic reason we argued against allowing non-monotonic IV curves 
was because the non-monotonic behavior requires a feedback path (and 
thus a delay) that we have never been able to properly define. If you 
don't know the feedback path delay then you do not know the inherent 
non-stable point for the driver. We have tried to work with IC companies 
that have this problem (and I think we did get a few non-real examples 
from some people) but the actual resolution of the feedback delay has 
never been completely addressed.

I think that the current IBIS simulators take one of two approaches: 
1) Allow the non-monotonic behavior and assume 0 delay.
2) Filter out the non-monotonic behavior automatically.

Which one is right? Depends on the driver and the feedback delay.

regards,
Jon Powell
Manager, High Speed Consulting Services, Viewlogic Systems.

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From: Andrew Ingraham <[email-masked]>
Date: Tue, 10 Nov 1998 17:34:36 -0500

The other problem with feedback (as in a "bus-hold" circuit) is that the
I/V curve has hysteresis.  There is some range of voltages over which
there are two possible current values.  Which one you get depends on
previous history (i.e., whether you ramp up or down).

A non-monotonic I/V curve means the device is capable of being an
amplifier within that region, and depending on its load, may oscillate.
Sometimes it doesn't actually oscillate but causes the simulator to
become unstable or otherwise run into numeric difficulties.

Regards,
Andy Ingraham