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                       Buffer Issue Resolution Document  (BIRD)

BIRD ID#:       82.2
ISSUE TITLE:    Clarification of Clamp Table Use
REQUESTOR:      Lynne Green, Cadence, and Robert Haller, SiSoft
DATE SUBMITTED: June 27, 2003, August 7, 2003, August 22, 2003
DATE ACCEPTED BY IBIS OPEN FORUM:  August 22, 2003

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STATEMENT OF THE ISSUE:

Double-counting of clamp currents is the result of confusion over how the
clamp currents are summed by the tools.  Model makers are often unsure if
clamps are always active, or whether the [Power Clamp] works only with the
[Pullup] and the [GND Clamp] only with the [Pulldown].  This is difficult,
if not impossible, to detect with a parser.

This issue can be resolved by clarification of the text under
|    Keywords:  [Pulldown], [Pullup], [GND Clamp], [POWER Clamp]

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STATEMENT OF THE RESOLVED SPECIFICATIONS:

Old text:
|         It is assumed that the simulator sums the clamp tables
|         together with the appropriate [Pullup] or [Pulldown] table
|         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 table sections are handled as
|         constantly present tables and the [Pullup] and [Pulldown]
|         tables are used only when needed in the simulation.
|

Proposed text:
|         It is intended that the [POWER Clamp] and [GND Clamp] tables are
|         summed together and then added to the appropriate [Pullup] or
|         [Pulldown] table 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 table sections are handled
|         as constantly present tables and the [Pullup] and [pulldown]
|         tables are used only when needed in the simulation.
|


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ANALYSIS PATH/DATA THAT LED TO SPECIFICATION:

Comments from members of the Quality committee, as well as others,
indicated that some models double-count clamp currents.


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ANY OTHER BACKGROUND INFORMATION:

Initially, it was suggested that this be detected by the parser, but
it was decided that this was too difficult.  This led to further
discussion of why double-counting occurred, and how clarification of
the spec could help address the issue.


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