RE: [SI-LIST] : **error**: internal timestep too small

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From: Zabinski, Patrick (zabinski.patrick@mayo.edu)
Date: Thu Jan 20 2000 - 12:46:45 PST


> 1) Increase RELV, and/or RELI if it doesn't hurt your accuracy needs.
> 2) Use .IC to initialize nodes. Nodes that are (almost) floating
> can solve to about anything and setting them to something helps
> a great deal.
> 3) I observed this with the U-element in HSPICE (anyone still using
> it?). If you connect more than two of them to the same node it
> will blow up (with kVolt differences on the two ends of the same
> element). However, a small resistor (yes, resistor) between the
> U-elements and the common joint will allow it to converge. Anyone
> knows why?
>

I cannot claim to know why this works, but if RELV, RELI, IC,
or NODESET helps convergence or if there are problems
with any form of transmission line (T, U, or W), I
simply insert 500K ohm resistors from various nodes to
node 0 (ground). Generally, the value is high enough
to not effect the results, but I've found it not only helps SPICE
find its way, but it can shave off 50-80% off run-time
as well (depending upon the circuit).

For example, I have an SSN simulation setup right now where
I have three layers of W-elements between the driver
and the board (bond wires, package, connectors). When
I originally set up the circuit, SPICE would not converge.
I then inserted these resistors at various points, and
the runs completed successfully. I then added two more
resistors, and the runs produced the sames results in
half the time.

I don't have any easy to describe rules for where
to insert these resistors, but I've picked up a pretty
good 'feeling' for it over the years. Next time
you run into the same problem, I suggest giving it
a shot.

Good luck,
Pat

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