Subject: R_load for linear ramp

From: "Robert Goodrich" <[email-masked]>
Date: Tue, 24 Nov 1998 14:39:52 -0500

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

---

From: "D. C. Sessions" <[email-masked]>
Date: Tue, 24 Nov 1998 14:27:07 -0700

> the user), does it matter as long as the R_load is specified in the

You want to use an R_load heavy enough that you see the whole output-
current profile of the driver, and light enough that the Miller
effect is representative and you swing enough to get some precision.
Half of supply at typical is pretty good, and that's just what you
have in your example.  Sounds like 50 ohms works well for you.

BTW, [Ramp] is only supported for historical reasons; you really
should use the [Rising Waveform] and [Falling Waveform] tables,
with each developed for both a pullup and a pulldown (Four tables
altogether).  That way you capture both the turn--on and turn-off
behavior of both the pullup and pulldown paths.  Doing just the
turn-on transition may be adequate in unterminated environments,
but terminators make the turn-off transitions important.

-- 
D. C. Sessions
[email-masked]