Re: gEDA-user: problem

[al davis <ad106@xxxxxxxxxxxxxxxx>]

On Sunday 05 March 2006 15:01, Stuart Brorson wrote:
> For PCB layout, there is no tool, at least as far as I know.
>  The reason is that the PCB fab process is less well
> characterized & more variable than the IC process.  Also,
> real models for the parts used don't exist.  Most op-amps,
> etc., provide only macromodels, which don't capture this kind
> of info.

There are several commercial tools.  I don't know of any free 
ones.  The commercial tools are quite sophisticated.  They do 
waveforms, essentially one trace at a time, with the neighbors 
are "aggressors".  They model waveforms, crosstalk, and 
radiation.

They are basically circuit simulators, not necessarily Spice 
based.  The best ones are not Spice based.  They model 
transmission lines very accurately, but are weak on other 
devices.  Mostly they use "IBIS" models for the drivers and 
receivers.  Most devices do have appropriate models published.

IBIS models define digital devices as analog behavioral models, 
using a pre-defined topology.  The model consists mainly of 
tables, showing things like waveforms and I/V characteristics.  
The tools for generating the models, based on simulations or 
measured data, are free/open-source.  Most of the models are 
generating using one particular tool, that generates a first 
cut model.  They usually require some manual tuning, but many 
modelers don't bother, which leads to some strange models.

Supporting IBIS in a simulator is much harder than would be 
expected.  One issue is that the waveforms are specified with 
real loads, like 50 ohms, 24 pf, to + 5 volts.  Usually two 
rising and two falling waveforms are supplied.  The simulator 
needs to figure out how to do it.  One commercial one that I 
know of (very well) does a multi-dimensional fit and will 
handle any number of tables with arbitrary loads. 

IBIS also models the passive parts of a device, including the 
package and connector.  There is also a standard for connector 
models.