Subject: Assumptions BIRD

From: Will Hobbs <[email-masked]>
Date: Tue, 10 May 94 22:48:11 PST

Text item: Text_1

BIRD ID#:      15
ISSUE TITLE:   Clarification on the usage of the V/I tables. 
REQUESTER:     Arpad Muranyi, Intel Corporation

DATE SUBMITTED:                   May 10, 1994
DATE ACCEPTED BY IBIS OPEN FORUM: Pending

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STATEMENT OF THE ISSUE:

There is room for different interpretations regarding the usage of
the various V/I tables in the current IBIS specification because it
does not clearly explain all of the initial assumptions.  If models
are generated with one assumption and a simulation tool is based on
another, inaccurate results may occur.

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STATEMENT OF THE RESOLVED SPECIFICATIONS:

A new section must be added to the IBIS specification, in which the
assumptions governing model generation and usage is clearly spelled
out.  This section should be placed at the end of the "Usage Rules" of
the [Pulldown], [Pullup], [GND_clamp], [POWER_clamp] keyword section
in the IBIS specification and should say the following:

| Other Notes:   ...
|
| Assumptions:   It is assumed that the simulator sums the clamp
|                curves together with the appropriate pullup or
|                pulldown curve when a buffer is driving high or low,
|                respectively.  From this assumption and the nature
|                of 3-statable buffers it follows that the data in
|                the clamping curve sections are handled as
|                constantly present curves and the pullup and
|                pulldown curves are used only when needed in the
|                simulation.
|
|                The clamp curves of an input or I/O buffer can be
|                measured directly with a curve tracer, the I/O
|                buffer being 3-stated.  However, sweeping enabled
|                buffers results in curves that are the sum of the
|                clamping curves and the output structures.  Based on
|                the assumption outlined above, the pullup and
|                pulldown curves of an IBIS model must represent the
|                difference of the 3-stated and the enabled buffer's
|                curves.  (Note that the resulting difference curve
|                may demonstrate a non-monotonic shape).  This allows
|                the simulator to sum the curves, without the danger
|                of double counting, and arrive at an accurate model
|                in both the 3-stated and enabled conditions.
|
|                Since in the case of a non 3-statable buffer this
|                difference curve cannot be generated through lab
|                measurements (because the clamping curves can not be
|                measured alone), the pullup and pulldown curves of
|                an IBIS model may contain the sum of the clamping
|                characteristics and the output structure.  In this
|                case the clamping curves must contain all zeroes, or
|                the keywords must be omitted.
|
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EXAMPLES:

N/A

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ANALYSIS PATH/DATA THAT LED TO SPECIFICATION:

Confusion raised in the IBIS open forum meetings during April 1994 
led to the generation of this clarification for the IBIS 
specification.

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ANY OTHER BACKGROUND INFORMATION:

N/A

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From: [email-masked] (Jon Powell)
Date: Wed, 11 May 94 10:40:37 PDT

I disagree with bird 15. I believe that the possition taken, though teniable,
is not necessarily the best approach and should NOT be required.

jonp