Subject: Programmable buffers

From: yoram@taux01 (Yoram Avigdor)
Date: Sun, 16 Jul 95 09:02:10 IDT

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>From [email-masked] Sat Jul 15 03:56:39 1995
Date: Fri, 14 Jul 95 14:38:00 PDT
From: Arpad Muranyi <[email-masked]>
To: [email-masked]
Subject: Programmable buffers

Hi IBIS folks,

I am writing this EMAIL to generate a discussion and a possible future 
EGG and BIRD on how to deal with programmable buffers in IBIS.

Problem:

There are buffers out there which have programmable strength, in other 
words, buffers which have not just a single set of I-V and edge rate 
description per pullup or pulldown structure.  Currently there are no 
provisions in the IBIS specification for such buffers.

The work around for such buffers is to create independent buffer models 
for the various buffer modes and include them as separate models in the 
IBIS file.  In the notes section, one can make a comment about the use 
of these additional models for the programmable buffers.

This method is possible in IBIS, since it is not illegal to have more 
buffers in an IBIS file than what the pin list section calls for.  The 
area where IBIS is lacking is that there are no nice ways defined to 
help the user to switch between the various modes of a buffer.

Suggestions:

One thing that came to my mind was to allow the listing of several 
buffers for one pin name, something like this (forgive me the bogus pin 
and buffer names):

1     reset   74ALS244
1     reset   74F244
1     reset   74HCT244  | or without repeating the pin and signal names:
2     AD0     74ALS241
              74F241
              74HCT241
3     AD1     74ALS241  ..... and so on.

The simulation tool could then give the user the option to select 
between the buffers when simulating with that pin.

One problem with this method is that the pin list can grow fairly large 
if there are lot of programmable pins in it.

Another idea would be to allow buffer models to contain more than just 
one set of I-V and edge rate data section.  If this method is used, we 
might need to invent another keyword, or some mechanism to be able to 
select between these sections.  This method would be more compact (at 
least in terms of the pin list) but might be more complicated on the 
buffer model level.

This is what I came up with so far.  Does anyone care to add comments, 
ideas to this conversation starter?

Sincerely,

Arpad Muranyi
Intel Corporation

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