Re: IBIS BIRD59 MODEL SPEC DIAGRAMS

I still find the descriptions to be confusing.

The [Model Spec] group seems to have the purpose of describing how the
analog representation of a waveform is interpreted as digital,
evaluating its logic state and quality.

My original interpretation of the old (pre model spec) Vinl and Vinh
is as follows:

The logic value switches from a "logic low" to a "logic high" when a
rising signal crosses through Vinh. It switches from "logic high" to
"logic low" when a falling signal crosses through Vinl.

In a static sense, a voltage below Vinl indicates a "logic low" state,
above Vinh indicates a "logic high" state, and between is unknown, in
transition.

On further reading, I believe this may not be the proper
interpretation.

Other parts of the spec lead me to believe that the intended
interpretation is:

The logic value switches from a "logic low" to a "logic high" when a
rising signal crosses through Vinl. It switches from "logic high" to
"logic low" when a falling signal crosses through Vinh.

Note interchange of Vinl and Vinh, compared to above. This seems
illogical to me, but clarifies (sort of) some of the other
parameters, and fits with the note on how to "mimic a hysteresis
effect" using only Vinl and Vinh.

Another interpretation could be:

The logic value switches from a logic low to "rising" when a
rising signal crosses through Vinl, then finally switches to logic
high on crossing Vinh. It switches from logic high to "falling" when
a falling signal crosses through Vinh, then switched to "logic low" on
crossing Vinl.

This adds the notion of an in-transition state, when the logic state
is neither high nor low, and possibly creates a use for the new
hysteresis parameters.

The composite (with the new hysteresis parameters) could be
interpreted as ......

The logic value switches from a "logic low" to "rising" when a rising
signal crosses through Vinl

Now, add "[Model Spec]" ....

It, too, has a Vinh and Vinl, this time with a min and max. I wonder
why it is necessary to have it twice. Why not just add optional min
and max fields to the original? or move the existing parameters to be
under ModelSpec, for consistency? The same applies to Vmeas.

It is still not clear what the hysteresis parameters do. It appears
to me that hysteresis can be adequately described without the new
parameters. What actually happens at Vinh+, Vinh- ? How does this +
and - differ from the min and max columns? This is still not clear.

--------------------

On the "overshoot" parameters ......

Add:

The purpose of the "overshoot" and "pulse" groups of parameters is to
determine the quality of an incoming signal.

A signal is considered to be "bad quality" if:

1. V ever exceeds D_overshoot_high
2. V ever drops below D_overshoot_low
3. V exceeds S_overshoot_high longer than D_overshoot_time
4. V drops below S_overshoot_low longer than D_overshoot_time

--------------

the "pulse" parameters ....

The wording is still unclear. The words and picture conflict with
each other, probably because of the confused interpretation of Vinh
and Vinl.
Received on Fri Aug 6 10:54:23 1999