Re: [IBIS-Users] Monotonicity problem in IBIS models

Non-monotonic warnings are common from the Pullup and Pulldown tables in I/O
models.  They are warnings, not errors, and they can usually be ignored
(after looking at them to make sure there is nothing actually wrong).

They commonly happen because of the way Pullup and Pulldown tables are made.
Pullup and Pulldown tables are made by subtracting the clamp currents
(measured with the I/O buffer tri-stated) from the drive currents (measured
with the I/O buffers enabled).  Where the clamp currents are significant,
the subtraction becomes a small difference between two large numbers, and
can be non-monotonic.

Also, on many buffers the enabled and tri-stated curves converge towards one
another at larger voltages where the clamp current is quite large, meaning
that the pullup or pulldown transistor is not contributing any additional
current not already going into the clamp.  So the Pullup or Pulldown table
becomes non-monotonic.

This isn't really a problem for simulation, because the simulator should
ONLY be working with the summed currents (Pullup or Pulldown plus Clamps),
and THOSE currents should be monotonic.  But the IBIS checker doesn't (and
can't easily) sum the tables together before checking for non-monotonicity,
so the result is all those warnings you are seeing.

As long as the clamp currents, and the total currents, are monotonic, your
device model should be OK.

The other likely way to get non-monotonic tables, is if you are constructing
an IBIS model from measurements.  Noise in the measurements can result in
weakly non-monotonic behavior.  You can smooth out the data to avoid this.

Regards,
Andy

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