On 30 Nov 2003 at 16:11, William Chops Westfield wrote:

This will be bottom posted

> On Sunday, Nov 30, 2003, at 14:41 US/Pacific, Bruce Partridge wrote:
>
> > On various parts, sometimes power is called VCC, and on others it is
> > VDD.
> > The same thing happens with ground.  Sometimes it is VSS and sometimes
> > it is
> > GND.  Is there any significance to this?
>
> Not really.  Not anymore, anyway.  Vdd is the voltage for Mosfet Drains,
> and Vss for the sources, and it might have been relevant back in the
> days
> of nMOS and pMOS chips with multiple supplies.  now, usage is
> historical.
> (Vcc is collector voltage for bipolar technology, etc.)
>
>
> > The reason I ask, is that in doing ERC's on shematics, it sometimes
> > complains like this:
> > "WARNING: Sheet 1/1: POWER Pin U$1 VSS connected to GND "
> > "WARNING: Sheet 1/1: POWER Pin U$1 VDD connected to VCC."
> >
> > Are there some standard ways of dealing with this.  I am using Eagle
> > CAD,
> > but I suspect this comes up whatever package you are using.
>
> This happens because the parts libraries you are using have the pins
> "internally" connected to the power supply.  There are people who think
> this a bad thing, and have their own libraries without the implicit
> connections.  That's one solution.
>
> The "standard" solution seems to be to ignore the ERC warning.  Or
> rather,
> to read the ERC warnings carefully to make sure that each warning is
> actually something that can be ignored.
>
> BillW
>

I don't use Eagle, but when I get those warnings, I usually find a way to shut them up
by creating nodes and labels or just connecting a pwr connector called Vcc to one
called Vdd and changing the properties of the part in the design cache so it connects
to Vdd. Something like that. Probably not doable in Eagle.

If you find some work around along those lines, just have to watch out that you don't
connect separate supplies together and/or do something that fouls up the PCB
layout. I have to watch that. regardles of whether I auto-route, I like the rats nest to
be correct. I'll route critical traces, draw some keepouts, and give the autorouter a
chance.

BRs,
Mike

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