[IBIS] Re: IV curve sweep range

I've spoken out about this before, and I still feel the same now.  Here are
my (STRONGLY RECOMMENDED) rule suggestions.

I think it's OK to restrict the required sweep range, but ONLY in the case
where the device (and all other drivers that are likely to be connected to
that net) are designed to be less than rail-to-rail.  Such as GTL or ECL.

To make that happen, I think IBIS needs to define nominal I/O or Input
voltage parameters, which are not necessarily tied to the supply rails; and
these new parameters are to be used instead of Vcc in the -Vcc to 2*Vcc
calculation.

I don't like restricting the sweep range just because the currents get
enormous.  So what if they get enormous.  They are enormous either because
they were derived from faulty SPICE models, or because the device currents
really are huge.  It's not the IBIS model that's broken.

Even though terminated transmission lines may never reach -Vcc or 2Vcc, the
simulator might try those points when the incident wave reaches the end of
the line and it doesn't yet know how much current goes into the terminators
or the clamps.  I don't expect the simulator to converge on a solution
near -Vcc or 2Vcc, but it might TRY values near there.  For that reason, I
think the IBIS data must require tables that go out that far; or far enough
to include the doubling at the end of the transmission line for reduced
voltage swing technologies.

Of course the data points don't need to be equally spaced.  I don't have a
problem with a table having 98 accurate data points that cover all
reasonable signal conditions, and two more on the ends, if the shape is
reasonably well behaved.  And I don't buy the argument that you have to
throw away two good data points to add the ones on the ends.

It may be that the endpoints aren't as accurate as the rest of the tables,
especially if the chip goes into meltdown or convergence problems prevent
getting real data there.  Maybe there should be a way to indicate that
certain datapoints are estimates and have reduced accuracy compared to the
rest of the table?

Regards,
Andy


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