On Tue, Apr 13, 2004 at 09:45:10PM +0100, Matt Marsh wrote:
> Hi,
>
> I've seen a number of circuits where a capacitor is placed between
> the VSS and VDD pins of the PIC.

Of course.

> Is this something that should always be done?

Of course! ;-)

> What is the purpose of this capacitor?

Bypass capacitor. Some quick cap theory. In short it blocks fixed DC signals
and allows variable signals to pass across the cap. Clearly the VSS/VDD
connection should be as DC as it gets. But in fact there's a lot of noise
on power supply lines that can affect circuit operation. So one purpose of
the bypass cap is to create a low impeadance path to eliminate noise on the
power supply lines.

The second item is that many modern circuits that utilize CMOS technology will
draw significant spikes of power when switching. So at each clock edge you'll
get a bit of a voltage dip. Bypass caps can provide additional power in those
dips.

So in the end bypass caps stabilize your power supply to your chips reducing
the amount of noise on those power supply lines.

Sprinkle liberally.

BAJ

--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.