Re: gEDA-user: GND and Vcc pin on IC 74245

[Stephen Meier <smeier@xxxxxxxxxxxxxxxxxxx>]

I really like the idea of a virtual "gate" that has at least one of a 
devices power pins (Vcc would get one pin and Vdd a second pin) the 
other pins can be hidden nets. I like to have a page in my schematics 
for just the power and power filtering/decoupling. I would put all of 
these virtual gates on that page with the decoupling caps neted to them. 
I might even make "hidden nets" visible for easy reading.

I also like the idea of something more powerfull such that gnetlist 
could decide if such a virtual symbol was missing and generate an error. 
Also gnetlist should generate an error if the pins of the virtual device 
are unconnected to a net.

Well Ales what do you think? Can you have this completed by say Sunday 
at 3 pm? ;)

Steve Meier


Wilbert Knol wrote:

> On Friday 09 September 2005 23:45, David D. Hagood wrote:
>  
>
> > Wilbert Knol wrote:
> >    
> >
> > > Idea and I have always been totally intrigued why anyone would
> > > want to do this - I have yet to see a single, good argument in
> > > favour.
> > >      
> > OK, try this - there is not always a one-to-one mapping from
> > entities on the schematic to ICs. Consider a 7400 quad NAND gate
> > chip: this part is not rendered as a box with 4 NANDs in it, but as
> > four separate NAND gates "floating around" on the schematic. OK,
> > so, where do you put the power pins?
> > 1) On every device, and have the problem of somebody connecting
> > U1A's Vcc to the +5 digital net, and U1B's Vcc to the +5 analog
> > net? 
> >    
>
> Agreed. Unworkable.
>
>  
>
> > 2) On one device in the package? Now one device is "special" 
> > and has extra pins, so you cannot auto-number them.
> >    
>
> Again, I agree. 
>
> Incidentally, this is why the argument that 'gschem allows you to have 
> a choice' doesn't wash. There is no workable alternative to hidden 
> power nets.
>
>  
>
> > 3) On a hidden net so that the normal behavior is that the parts
> > "Just Work".
> > I agree - when you are dealing with a device for which there is a
> > one-to-one mapping between schematic and board hidden nets are not
> > really needed, but for some devices they really do make things
> > cleaner.
> >    
>
> This is stating two bad alternatives to (somehow) justify an even 
> worse solution - see my previous posting why I strongly dislike 
> hidden nets. 
>
> Suggestions:
>
> 1. Have the power pins show up only on a virtual, extra gate, or even 
> just a box to symbolise the package.
>
> 2. Or make them show up only on the slot (gate) of one's choice, and 
> attach an attribute to the gate. This attribute  - a list, similar to 
> the present 'slotdef' attribute - exempts the power pins from being 
> auto-numbered, and identifies them as being unique to this slot.
>
> Anyway, this is a minor gripe. Gschem is still a great tool.
>
>
> Wilbert.
>
>