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From: nje@philabs.UUCP (Norman J. Elias)
Newsgroups: comp.lsi
Subject: Re: SPICE time-step too small
Message-ID: <6654@philabs.UUCP>
Date: Thu, 16-Apr-87 08:10:23 EST
Article-I.D.: philabs.6654
Posted: Thu Apr 16 08:10:23 1987
Date-Received: Sun, 19-Apr-87 10:17:24 EST
References: <8704151403.AA16347@decwrl.dec.com>
Organization: Philips Laboratories, Briarcliff Manor, NY.
Lines: 10
Summary: Another suggestion for SPICE non-convergence


You will also encounter convergence problems if the input driving signal
changes too rapidly, for example a PULSE input with rise-time less than
the stepsize specified in the .tran statement.   What happens is that SPICE
steps over the pulse edge and encounters an apparent discontinuity in the
input signal.   The SPICE time-step reduction algorithm merely jumps the
time-point back and forth around the discontinuity and the algorithm never
converges.   Try checking your input signal specifications and see if the
rise-time is consistent with the time-steps and circuit time constants.
A more gradual rise or fall will correct some convergence problems.