Re: gEDA-user: Taking advantage of internally connected pins

[Dave N6NZ <n6nz@xxxxxxxx>]

On Jan 30, 2010, at 4:00 PM, Ben Jackson wrote:

> I need to make a key matrix.  I'm considering selecting a switch which
> has 4 leads (two leads per internal net):
> 
>  o--+--o
>     '
>     /
>     |
>  o--+--o
> 
> This appears to be a boon for the grid-style routing I need for the
> switch matrix.  The rows can hop the columns inside the part, making
> everything fit on a single layer.  (obviously I wouldn't do this with
> an IC, but it seems harmless for a switch)
> 
> Anyway, if I give all pins different numbers, PCB doesn't know about
> the internal structure.  If I give the electrically connected pins the
> SAME numbers then PCB thinks it needs to connect them with copper itself.
> If I connect them on a 'fake' layer then the autorouter works but the
> toporouter spins and I have to kill pcb.
> 
> Any better ideas?

I have no better ideas, and in fact have this exact problem on a design I am doing right now.  Since PCB can't differentiate between "like numbered pins must all be connected" and "one of the group of like numbered pins is sufficient", I think we are stuck with PCB doing the wrong thing on some designs.  At least until the new "pin swapping syntax" becomes reality -- although I'm not sure how the current spec would handle this situation.

Thinking about it, is PCB making the correct choice here?  On IC's where several pins must be connected together, the IC typically has different pin numbers -- I'm thinking here of parts with multiple ground pins, for instance.  What is the case where multiple footprint pins with the same pin number must be connected together in the PCB?

-dave





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