Re: gEDA-user: A couple o' questions

[John Doty <jpd@xxxxxxxxxxxxx>]

On Apr 7, 2006, at 1:34 PM, Carlos Nieves Ónega wrote:

> If a pin is an input, the current flows from outside to inside the IC
> through that pin. Usually in a 7805 the current flows out of the  
> GND pin
> (from the 7805 to your circuit), so I would say it is a pwr_out. At
> least can we agree it is a pwr pin...

Wrong.

Which way does current flow at the input of a TTL gate?

Input vs output vs power has nothing to do with the direction of  
current flow. It has to do with the designer's intended cause and  
effect. Inputs control outputs. Power pins provide energy. In the  
knotty cases, the classification doesn't depend on the part, but how  
the part is used in the circuit, and also just how the designer  
choses to think about what's going on. So, for the usual application  
of the 7805, the GND pin serves *three* functions, each of which  
would correspond to a *different* classification in your new proposed  
scheme.

1. It's connected to the return for the input power, so it's "pwr".

2. It's connected to the return for the output power, so it's "pwr_out".

3. Its voltage controls the voltage at OUT, so it's "in".

In my battery charger, the returns are elsewhere, so only the last  
applies. So if you demand I make a choice, I'd call it "in", and I  
suspect that would work OK with your DRC. But I think this example  
shows how messy it is trying to shoehorn reality into an arbitrary  
classification scheme.

John Doty              Noqsi Aerospace, Ltd.
jpd@xxxxxxxxxxxxx