[IBIS] Inappropriate I-V tables in open_drain/source model types

At today's IBIS teleconference, I raised the issue of whether a bug
report should be logged against ibischk for its treatment of I-V tables
inappropriate to the model type.  Specifically, there should be no
[Pullup] table, or at least no non-zero currents for the [Pullup] table
in a model using open drain/sink topology, and there should be no
non-zero [Pulldown] table in a model using open source topology. 

If an inappropriate I-V table is present (with non-zero currents),
ibischk issues a warning message.  However, it goes on to check I-V/V-t
endpoint agreement using all the model's I-V tables.

Should ibischk instead ignore the I-V table incompatible with the
model_type?  A motivation for doing this is that sometimes model makers
mistakenly put internal bias characteristics for open_* parts into the
[Pullup] and [Pulldown] tables.  Ignoring inappropriate I-V tables
during I-V/V-t checking would expose this by causing I-V/V-t mismatches.


However, if using the inappropriate I-V table causes an I-V/V-t
mismatch, the current parser behavior exposes the problem.

So it may be valuable to report both when ignoring the table causes a
problem and when using the table causes a problem.  I can report that at
least one popular SI tool does not ignore the inappropriate table.  The
existing parser behavior has in fact encouraged this policy.  It is not
safe for the SI tool to ignore one of the I-V tables the parser depended
on to "bless" the model, unless it does the alternative endpoint
checking itself.

Would it be worth adding a separate I-V/V-t endpoint check under the
assumption of open_* topology to highlight models that depend on
significant currents in the inappropriate table?  Would it equivalently
be worth elevating the inappropriate-table warning to an error?  It
seems unsafe to drop the existing I-V/V-t test policy for open_* models.

John Angulo
Software Development Engineer
Hyperlynx Products
Mentor Graphics Corp.

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