Re: [SI-LIST] : internal timestep too small in transient analysis

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From: Kim Helliwell (khelliwe@acuson.com)
Date: Mon Mar 26 2001 - 09:07:21 PST


> Elias Lozano wrote:
>
> I have a question,
>
> I am having a lot of trouble with this error when doing signal integrity simulations of some I/O buffers with package
> models and T models for tranmission lines as well as some loading. I am using HSPICE2000 version. I will change to U
> and then W models for more accuracy.
>
> I would like to get a fresh look at what are the best options to use when having this problem.
> In the past, I have had the same problem with TSMC 0.18um models due to some negative-mos conductances?
> Is there anyone out there experiencing the same problem and if so any ideas on how to find the right set of options so
> that I don't get these erros?
>
> Once again this is what I am simulating:
>
>
> I/O buffer + Package model + Tmodel + capacitive loading
>
> I appreciate any comments.
>
> Regards
>
> Elias

If you are getting TSTS errors with the IBIS buffers, this probably
means there is a problem with the buffer model. TSTS errors are mostly
associated with MOS models because of the large dynamic range of
conductances and the difficulty in insuring that the capacitance models
are continuous across the piecewise boundaries. Negative capacitances
might do it, too!

Here are the steps I would take: I would get in touch with the HSPICE hotline
(800.346.5953) and tell them of the difficulties you are experiencing, and
get them to take a look at your circuit; they very well may find a bug in
the IBIS model code as a result.

Meanwhile, the options that help (about 30% of the time) in solving TSTS
problems are:

Increase ITL4 to 100 (if it isn't already there)
Increase RELTOL to .01 (no larger than that!)
Set BYPASS=0 (don't allow bypassing)

A perusal of Chapter 8 of the HSPICE manual might turn up other options
that would help, but the ones above are what I'm familiar with from other
simulators. The person answering the phone at the hotline might be able
to suggest other options to try.

-- 
Kim Helliwell
Senior CAE Engineer
Acuson Corporation
Phone: 650 694 5030  FAX: 650 943 7260

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