RE: [IBIS-Users] Variety of Aproaches on IBIS Modeling on Differential I/O Buffers (with and without Pre-/De-Emphasis)

All,

I am sensing a little confusion regarding NA-s, IBIS simulators,
and Verilog-AMS here, so I will attempt to set the records straight
with this message.

While it is true that IBIS allows NA-s in the tables, and IBIS
simulators must support those, this is NOT what the original
message was talking about.  We don't seem to be having any
problems with IBIS simulators and NA-s in IBIS files these days.

The original email made a suggestion that the $table_model keyword
of Verilog-AMS should also support NA-s in the tables.

Verilog-AMS and IBIS are two different animals.  Verilog-AMS is a
hardware description (modeling) language, which is basically a
specialized programming language geared towards hardware modeling.
It has a keyword called $table_model which allows the programmer
to reference a data file (NOT an IBIS FILE) which contains tables
of data and read it into the model.  This is all done inside the
Verilog-AMS environment, has nothing to do with IBIS or IBIS
simulators.  This situation is very much like the PWL sources in
HSPICE, for example.

The request of the original email was to add support for NA-s
in the $table_model keyword of Verilog-AMS.  This may be a
reasonable request considering that there may be situations
when the $table_model is used to read tables that come from
IBIS files.  However, the problem is that Verilog-AMS's file
reading and parsing capabilities are quite limited.  There
is no IBIS file reading and parsing built into the $table_model
keyword.  It has its own file format requirements which are
described in the Verilog-AMS language reference manual.  (I would
even dare to extend the original request of supporting NA-s in
$table_model to ask for a complete IBIS file parser in it).

Because of the very limited file I/O capabilities of Verilog-AMS,
it is very difficult if not impossible to write an IBIS file parser
in Verilog-AMS, and we are much better off if we wrote that file
parser with another program or script, such as Perl.

Since we are currently converting from IBIS to the Verilog-AMS
syntax with an external program, it is not too hard to implement
the proper handling of NA-s in that program as well, so that the
$table_model of Verilog-AMS can read a file that is formatted
to its liking.

Now, the additional complication or confusion may have come from
the IBIS external language linkages which includes Verilog-AMS.
The IBIS links to these external languages does not automatically
mean that the external models written in these languages will have
to read data tables out of IBIS files.  The external models described
with these languages can do anything the model writer wants to do.
It is certainly possible, though, to make use of IBIS I-V and V-t
tables in an *-AMS model, as it has been shown in the IBIS Macromodel
Library.  However, as I said it above, these models rely on another
program to read and parse the IBIS file and extract and convert the
syntax of the I-V and V-t tables that they need.

This technique, however, does not imply that an IBIS simulator, which
supports the external languages will have to impose the IBIS rules of
supporting NA-s in the tables on the keywords of the external language.
This is actually impossible, and nonsense, because it would practically
mean that the IBIS rules are valid and applicable to the *-AMS
languages.

I hope that this helped to clarify the NA rules in IBIS and the
capabilities
of the $table_model in Verilog-AMS.

Thanks,

Arpad
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