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Re: [Help-gnucap] Time step control

From: al davis
Subject: Re: [Help-gnucap] Time step control
Date: Tue, 19 Feb 2008 01:19:31 -0500
User-agent: KMail/1.9.7

On Monday 18 February 2008, a r wrote:
> On Feb 18, 2008 8:58 PM, al davis <address@hidden> 
> > Truncation error step control for Euler always gives very
> > small steps, significantly smaller than the steps used for
> > trap.
> Sure, truncation errors will be larger (that's Euler after
> all) but it is just a matter of relaxing truncation error
> requirements. What I need is a variable time step that adapts
> to the circuit "activity".

Truncation error is only loosely related to "activity".

If there are no storage elements, there is no truncation error, 
but there is still "activity", and step control is still 

For now, you can add some switches to your circuit, as activity 

For the future, there are plans to add node-voltage-change based 
time step control.

> > Euler will never give you ringing artifacts.
> But without time step control it may give you totally wrong
> results. 

There is time step control.  It is not based on truncation error 
of the charge calculation.

> Say we have a synchronous logic circuit - how gnucap 
> is going to recognize that a clock edge appeared before or
> after a transition of a sampled signal? For the simulation
> both of them may appear to have happened at the same time
> step of the transient simulation if the time step is too
> large.

That has nothing to do with truncation error.

That is why I suggested adding a switch, to recognize the edge 
and control the time step.

> There are only two ways to solve this problem:
> 1. decrease time step to such a value that it will precisely
> simulate fastest signals in the design.
> 2. make time step variable (i.e. use time step control) and
> depend on simulator to recognize circuit activity (within a
> specified precision) and slow down when necessary.

Truncation error is only loosely related to circuit activity, 
usually too little, too late.  That is why it is still possible 
to see trap ringing sometimes.

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