Why no variable Vref in 3.2?

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
  michael.mirmak@intel.com
Received on Mon Mar 20 10:30:10 2000