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

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From: Fred Balistreri (fred@apsimtech.com)
Date: Wed Jan 19 2000 - 16:20:14 PST


In some SPICE programs there is an iteration limit of 10.
Refering to Dmitri's comments below.... an iteration limit of
10 means the simulator will cut the time step 10 times before
giving up and printing the time step to small message. One
can increase the number to say 100 for non-linear circuits that
are difficult to converge. Also keep in mind that the convergence
criterial for SPICE may be to small for Digital applications. Generic
SPICE from Berekely has absolute current acceptance criteria of
1pA and voltage criteria of 1uV. If this kind of accuracy is not needed
then this can be relaxed. On a side note: if one uses Behavioral or
IBIS models in SPICE then most if not all of the convergence problems
disappear. SURPRISE!!!

Best Regards,

Dmitri Kuznetsov wrote:

> Interestingly, to get rid of the "internal timestep too small" error,
> one needs to make the time step SMALLER.
>
> The error is caused by nonconvergence due to previous time steps being
> too large. SPICE tries to "backtrack" and redo the current time
> iteration with subsequently smaller time steps, and when it fails, the
> above error is generated.
>
> I am sure you knew about that. But many novice SPICE users try to
> increase the time step when they see the "timestep too small" error,
> which makes the problem worse.
>
> Dmitri Kuznetsov
>
>
> Jim Freeman wrote:
> >
> > Haven't we all
> >
> > Jim Freeman
> >
> > "Mellitz, Richard" wrote:
> >
> > > I've got one thing to say about spice.
> > >
> > > I've grown to hate the following line:
> > >
> > > **error**: internal timestep too small in transient analysis
> > >
> > > Rich Mellitz
> > > Intel
> > >
>
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--
Fred Balistreri
fred@apsimtech.com

http://www.apsimtech.com

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