> I've downloaded the free 'student' version, and I have a question.  I 
> managed
> to suss out how to model a switch opening or closing after a period of 
> time,
> but is it possible to model a 'noisy' switch change at all?
>
> Also, PSPICE to me so far seems very good for modelling individual little
> modules, but I'm not sure how I'd go about drawing the schematic for my 
> whole
> project, ready to feed in to something to lay out a PCB - how can I put in
> place a dumb 16/18pin IC to represent a PIC or any other IC not in the
> library?  or a connector strip, ICP programming header?
>
> Am I using the wrong tool for what I need?
>
> Regards,
> Mark
>
>

You can model your "noisy" switch a few different ways:

1) Add additional resistance to the model (I'm assuming you are using the 
voltage controlled switch or current controlled switch).  You can use the 
built-in parameter or add your own to the net list to simulate cross-talk.
2) Add external capacitance to the net list, also inductance to simulate 
leads, etc.
3) Drive the switch with a pulse or custom waveform to make it "chatter" (ie 
contact bounce of  a mechanical switch or relay)
4) Some versions of spice include a "noise" waveform, don't know about the 
student edition of Pspice.  That could be "injected" on either side of the 
switch through a capacitor.
5) Some versions of spice include "noise" analysis where the Johnson noise 
of resistors is included.

As far as having a tool for doing schematics and small layouts, Eagle has a 
"free" version and various levels of "paid" versions depending on what you 
need.

There have been numerous threads on PicList about CAD software so perhaps 
the archive can disgorge some information for you.  Otherwise you could 
start a weekend CAD-religion war and see what kind of responses you get... 
:-)

Rob Young
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