Subject: R_load for linear ramp

From: [email-masked]
Date: Wed, 25 Nov 1998 14:46:48 -0600

Hello Robert,

Welcome to IBIS.  I think you'll find the people on this reflector to be
fairly responsive to requests for assistance in making a good IBIS
datasheet, although as a group we're lacking a bit in organized education.

D.C.'s comments about covering the IV space and Miller capacitance were
well said.  Going back to the semiconductor electronics is always important
in behavioral modeling.  I would add that I have had success in accurately
modeling an I/O buffer without VT tables (i.e. Rising Waveform and Falling
Waveform IBIS keywords).  In my opinion, your inclusion of a keyword or
subparameter should be based on whether or not you need it to accurately
capture the behavior of your particular I/O buffer.  I think it was
Einstein who said that we should strive to make things as simple as
possible - but no simpler!  Of course, you probably won't go wrong by
adding the extra keywords, either.  This is just my personal philosophical
bent.

The most important thing from a user's perspective is that the developer of
an IBIS datasheet correlate behavioral simulations of that I/O buffer
driving various loads with SPICE and lab waveforms.  That's what the "IBIS
Accuracy Specification" is all about.
(http://www.vhdl.org/pub/ibis/accuracy/)  It's a document that attempts to
define a means for communicating behavioral vs. SPICE and behavioral vs.
lab correlation data between the IC vendor and the user.  Bear in mind,
this is a work-in-progress.  Constructive criticism is welcome.  One area
that needs work is the question of whose simulator you use to do the
correlation work.  Your users will likely span the simulator product space,
and each simulator uses a different solution algorithm.  It's unrealistic
to think that an IC vendor can qualify his or her IBIS datasheet on all
simulators.  This issue has yet to be resolved.

There is one thing we can all agree on:  it's vitally important to document
correlation work!  That's the only way a user knows if an IBIS datasheet is
accurate enough for the application at hand.  And if a user doesn't feel
confident in the margins on a given net because the IBIS datasheet is in
question, then the user can't order the component.

(descend soapbox)

Greg Edlund
Advisory Engineer, Critical Net Analysis
IBM
3650 Hwy. 52 N, Dept. HDC
Rochester, MN 55901
[email-masked]

---------------------- Forwarded by Gregory R Edlund/Rochester/IBM on
11/25/98 12:44 PM ---------------------------

"Robert Goodrich" <[email-masked]> on 11/24/98 01:39:52 PM

To:   "[email-masked]" <[email-masked]>
cc:    (bcc: Gregory R Edlund/Rochester/IBM)
Subject:  R_load for linear ramp

Hi all:

In reading the "IBIS Forum I/O Buffer Modeling Cookbook" as well as the
IBIS v2.1 spec. on the [Ramp] keyword, the R_load  subparameter is said
to be a preferred 50 Ohm load.  The Cookbook states that "if the device
does not have enough drive capability to make a "significant" output
transition than a higher value of load resistance may be used."  My
question is - what constitutes a "significant" output transition?  The
particular 3.3V I/O I am trying to model (and I am very new to IBIS) can
only drive around a 1.5V transition with a 50 Ohm load.  Increasing
R_load increases the output transition and significantly changes the
[Ramp] dV/dt_r and dV/dt_f ratio's.  Knowing nothing about the IBIS
model users concerns (that is - what constitutes a good IBIS model to
the user), does it matter as long as the R_load is specified in the
model?

Regards,
Robert Goodrich
Motorola, SPS