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From: Marshall.wbst@XEROX.COM.UUCP
Newsgroups: comp.lsi
Subject: Re: Internal time step error in SPICE2G6 ?
Message-ID: <870410-135237-1055@Xerox>
Date: Fri, 10-Apr-87 15:52:00 EST
Article-I.D.: Xerox.870410-135237-1055
Posted: Fri Apr 10 15:52:00 1987
Date-Received: Sat, 11-Apr-87 18:23:33 EST
Sender: daemon@ucbvax.BERKELEY.EDU
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Spice does not choose pivots when it is solving the circuit equations
but rather blithely assumes that the prechosen pivots will be good. That
is why all of spice is assembled with implicit double precision. This
can cause it to choose a set of very bad pivots because the resistance
of a MOSFET can vary several orders of magnitude. The error 
"INTERNAL TIME STEP TO SMALL" means that the integration step was
reduced to 10^-20 and the integration procedure still did not converge
and probably won't no matter how small a step it tries to take. This
error has nothing to do with the integration but is a symptom of
problems with solving the circuit equations. The solution is to try to
make the circuit better behaved by:

1) Changing the circuit in any way so that the order of rows in the
circuit matrix is different. This can be done by changing the order in
which circuit elements and nodes are defined or by adding elements for
other purposes.

2) Adding a low conductance (high resistance) or low capacitance between
nodes of a FET that can change in resistance by a wide margin such as
the source and drain. If spice ever pivots on the source-drain
resistance then the small additional conductance will change a zero
pivot to a finite pivot and you may get answers that are accurate
enough.

Hope some of the above helps.

--Sidney Marshall