RE: [IBIS-Users] A question on how to handle NA in the min/max columns

Arpad:

You have provided a good analysis and scratched the surface
of the problem.

I do not think IBIS should make it illegal to have some,
but not all corner data missing (e.g. the min and max
columns for some tables are NA).  There are pathological
cases where such a models are still accurate and useful.

E.g, ideal 50 ohm source resistors for the pullup and pulldown
tables to  the defined pulldown and pullup reference
voltages for all corners with only the typ table is given and
the min/max table corners have NA entries.  The intent might
be to model a fixed 50 ohm impedance for all pullup and pulldown
table corners and vary the V-T data.)  Or just providing an
ideal 50 ohm termination for all corners could be done
with just the typical data only, and that can be used with
other models with all numerical corner data given.

However, the user should be suspicious of and
probably reject models for real buffers with NAs in the
corners because they are incomplete or contain uncorrelated
information that would make corner analysis inaccurate.

Bob



-----Original Message-----
From: owner-ibis-users@eda.org [mailto:owner-ibis-users@eda.org] On Behalf
Of Muranyi, Arpad
Sent: Friday, July 11, 2014 10:45 AM
To: IBIS-users
Subject: [IBIS-Users] A question on how to handle NA in the min/max columns

Hello,

I would like to discuss the question of how to handle the situations
when min/max data is not available in certain IBIS keywords.  The IBIS
specification has several occurrences of the following statement (with
some variations here and there):


If minimum and/or maximum values are not available, the reserved word "NA"
must be used indicating the typical value by default.


(This one can be found on pg. 36 at the end of the 1st paragraph of the
Usage Rules on the top of the page).

The interesting thing is that this statement does not appear under every
single keyword, so my first question is whether this was intentional or
sloppiness.  For example, pg. 53 where the I-V tables are described, we
can read the following:


All four columns are required under these keywords. However, data is only
required in the typical column. If minimum and/or maximum current values are
not available, the reserved word "NA" must be used. "NA" can be used for
currents in the typical column, but numeric values MUST be specified for the
first and last voltage points on any I-V table. Each I-V table must have at
least 2, but not more than 100, rows.


This section does not state what the EDA tool should do when min/max
data is not available.  My guess is that the expectation was that the
EDA tool should use the typical data, but this is not stated.  The same
observation can also be made for the waveform tables on pg. 69, [Composite
Current] on pg. 71/72 and the [ISSO ***] keywords on pg. 56.

The story gets more interesting when we consider how the I-V tables and
the waveform voltages are related to each other by Ohm's law using the
R_fixture load resistor value.  One might argue that if the min/max data
is missing from the waveform tables, the typical waveform data might be
"adjusted" (scaled) relatively easily so that they would be in agreement
by Ohm's law.   One could even do the reverse when the min/max data is
available in the waveform tables but are missing in the I-V tables.  But
things will get more complicated or perhaps impossible with the [ISSO ***]
and/or [Composite Current] tables.  Imagine a certain waveform which
includes typ/min/max, but the [Composite Current] following it has only
typ data (or the other way around).

So the question I would like to clarify is whether the IBIS specification
should define what to do in these cases, or perhaps impose some requirements
on how the model maker should provide the data for these various keywords
(i.e. require all of them to have or not have min/max data).

Note that not all keywords fall into this category.  For example, C_comp
and its variants are relatively independent from I-V and waveform related
keywords, so these would not have to be "matched" with having or not
having min/max data.

Questions, comments welcome...

Thanks,

Arpad
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