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

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