Re: gEDA-user: Basic questions from a gEDA & Linux noob

[John Doty <jpd@xxxxxxxxxxxxx>]

On Aug 13, 2007, at 11:45 PM, Felix Fujishiro wrote:

> On Mon, 13 Aug 2007 07:43:42PM -0700, Ben Jackson wrote:
>
> >On Mon, Aug 13, 2007 at 07:33:40PM -0700, Felix Fujishiro wrote:
> >>
> >> I tried to do this by adding the 'net=GND:4' attribute, and that  
> works, but
> >> only if pin #4 is visible.
> >
> >I'm not sure what you mean.  My Altera FLEX board used a chip-shaped
> >symbol with all the power and gnd hidden using net= attributes.   
> There
> >are no pins for those anywhere.
>
> I guess I'm not being clear.... I'm using gschem for custom digital  
> IC design, perhaps there's a difference in the way the tool treats  
> MOS components and PCB components?  If I remove pin #4 (the  
> substrate/well connection) from the MOS symbol, gnetlist spits out  
> a three-terminal device instead of a  four-terminal device (which I  
> want).
>

Which back end are you using? Perhaps spice-sdb? If so, I think I  
understand what's going on.

Spice-sdb assigns pins by their pinseq= attributes, while the net=  
attribute works on pin number. Without a pin, you have no place to  
hang a pinseq= attribute, so spice-sdb won't understand that it needs  
to make a connection.

If you're not adjusting transistor parameters much you could make  
three terminal .SUBCKT models, each containing a single transistor,  
with the substrate connected to a global node. Use model-name= (and  
maybe file=, depending on how you organize things) to associate the  
symbol with the subcircuit. The problem here for me would be that I  
do mixed signal VLSI and I'm often tweaking individual transistor  
parameters (L, W, M). In the subcircuit approach, you need a  
different subcircuit for every geometry.

So for my work I just use the four terminal symbols and connect the  
substrate node explicitly. I have very small VSS and VDD symbols for  
this purpose. And I generally just box select and copy components in  
preference to the dialog, so I can copy the transistor+substrate  
combo together.

John Doty              Noqsi Aerospace, Ltd.
http://www.noqsi.com/
jpd@xxxxxxxxx




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