Re: Interpretation of I/O data

Text item:

John,
I will attempt to answer some of your questions.

1) You can add the Power and GND I-V curves together to get the "full" picture.
However, you must be aware of the fact that the Power clamp curve is expressed
in Vcc relative terms. (That is 0V in the table is where Vcc is, i.e. 5V).
This will include pullup and/or pulldown resistors. However, bus-hold circuits
might be expressed in the normal pullup or pulldown I-V curve tables, and might
not be part of the clamp curves. (This depends on the vendor who makes the
model, and I have not seen one yet with such circuits). As far as FAST-type
input currents are concerned, there are some dynamic currents there which happen
on a falling transition which we cannot directly model in IBIS yet. (Remember,
I-V curves are static).

2) I don't understand your question. Are you referring to simulations with
IBIS models, or measurements in the lab? For the second, I saw differences in
the clamping characteristics depending on whether the device was powered up, so
I would not measure anything without applying power to the device. In
simulation it might be possible, but why do you want to do it without power?
You can just power the device up and sweep it above the power supply voltage,
and you will see right away whether it clamps or not.

3) Hysteresis (this is the correct spelling) is not necessary in IBIS, as I see
it now. We are not concerned what is going on with the signal in the receiver,
we just want to know what the effects of the receiver is to the outside world.
Unless there are drastic changes in the input currents due to hysteresis, I
don't think is needs to be modeled. However, if there is, we cannot model it
today, since this is another one of those dynamic things. If it is important,
we need to address this issue in the Open Forum meetings.

4) Again, I am not clear on this question. What transition are you referring
to? Are you talking about a voltage change on the input node or the transition
of the signal from point A to B in the input structure? (Part of this might
have been answered already in the previous point).

Sincerely,
Arpad Muranyi
Intel Corporation
================================================================================
Hello,

(I'm rather new to IBIS, so if these topic has been covered, please
point me in the right direction!)

I've been reading up on IBIS, but certain aspects are not clear to
me yet. I would be grateful if someone could help me on the
following points:

1) Input characteristics:

   Is it correct to assume that if I fix the values of Power and Ground
   clamp references, the full input VI characteristic can be obtained by
   adding together the [Power clamp] and [GND clamp] tables, and that
   this will include the effects of internal pull-up and pull-down
   resistors, bus-hold circuits, FAST-type input current, etc ?

2) [Power clamp] when Vcc=0:

   Is it safe to use the [Power clamp] data, with Vcc=0, to
   deduce the presence/abscence of a real clamping diode? (Very useful
   information in designs with multiple power supplies!)

3) Hysterises

   Am I correct to conclude that in version 2.1, there is no way
   of knowing if an input has hysteresis? (If so, are there any plans
   to include this information. Perhaps with a "Vh" attribute to the
   [Model] keyword to indicate the minimum hysterises value?)

4) Input transition time

   Am I correct to conclude that in version 2.1, there is no way
   of finding out the maximum transition time allowed for a
   signal applied to an input? (If so, are there any plans
   to include this information. Perhaps with "Tinr","Tinf" attributes
   to the [Model] keyword?)

Thanks in advance,

John

--
John Fitzpatrick   <John.Fitzpatrick@ln.cit.alcatel.fr>
Alcatel CIT, 4 rue de Broglie, 22304 Lannion, France
Tel: (+33)96.04.79.33  Fax: (+33)96.04.85.09
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Subject: Interpretation of I/O data
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From: FITZPATRICK John <John.Fitzpatrick@ln.cit.alcatel.fr>
Date: Wed, 13 Dec 1995 16:44:27 +0100
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