Subject: Why no variable Vref in 3.2?

From: "Mirmak, Michael" <[email-masked]>
Date: Mon, 20 Mar 2000 10:28:39 -0800

Fellow IBIS-fans,

We are seeing increasing numbers of buffers which require, per their
interface specifications, delay fixtures with variable reference voltages
(the pullup/pulldown voltage "Vref," not the measurement voltage "Vmeas").
A notable example is PCI, which has several recommended buffer delay
fixtures for timings generation, each with a resistor pullup voltage which
varies as the supply varies (in other words, with buffer corner).

I have two questions related to variable Vref:

1) IBIS 3.2 only supports [Model Spec] variation of input thresholds,
overshoots, pulse immunity and Vmeas; Vref cannot be varied using [Model
Spec].  Short of using [Model Selector], is their any convenient way to
specify Vref variation in an IBIS 3.2 model?  

2) I believe that a BIRD was proposed some time ago which added Vref, as
well as Cref and Rref, to the [Model Spec] keyword.  The BIRD was not
approved.  What was the reasoning behind keeping Vref, Cref and Rref out of
[Model Spec]?

Thanks in advance!

- Michael Mirmak
  Intel Corp.
  (916) 356-4261
  [email-masked]

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From: Bob Ross <[email-masked]>
Date: Mon, 20 Mar 2000 14:00:33 -0800

Michael:

BIRD66 has not been rejected.  It is still pending for
an IBIS Version 4.0 release.

We have not had time yet to consider it and some implications
of conflicting definitions and applications of timing measurement,
and also some possiblity of additional extensions in this area
(e.g., separate loads for rising and falling edges.)

I expect that some full discussions to be conducted
in the next teleconference meeting and possibly on the
IBIS reflectors.

Best Regards,
Bob Ross
Mentor Graphics

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From: [email-masked]
Date: Mon, 20 Mar 2000 15:15:16 -0800 (PST)

> approved.  What was the reasoning behind keeping Vref, Cref and Rref out of

I was not involved in that decision but ....

One of the problems we are facing with the growing ibis standard is
consistency.  Originally, there were some parameters, then [Model Spec]
was added.  Some parameters under it are new.  Some are repackaging of
some original parameters, with min and max added.  Now, we have some
twice.

Why can't they all have typ,min,max?  Just extend it on the same line
where it is.  Since we have [Model Spec], why not move them all there? and
obsolete the ones not under [Model Spec]?  Why do we need the duplication?

If all parameters were handled consistently, that would make it easier for
everyone, even if some of them have no real use.