Andres,

You asked:
> Most PICs are rated to being able to source up to 20mA and to sink up to
> 25mA at any pin. I understand that this is an Absolute Maximum Rating that
> one cannot stress.
>
> Now, keeping into the safe side, say that I want to source 15mA from ALL
> output pins of a 16F84...
...snip...
>So the question is: How do I compute
> the maximum current a given chip can source at a time?

There are also absolute max specs for each Port (or group of them),
and for Max current drawn from Vdd or sunk to Vss.

There is also an absolute max spec for power dissipation on the
chip, the data sheet shows how this is specified.

Make sure you have the complete data sheet (www.microchip.com).

Notice that the Output High Voltage and Low Voltages are specified
at a much lower than that maximum current (+8.5 mA, -3.0mA for
my PIC)  No guarantee, but I'm guessing that this specification point
is the designers' expected maximum for normal operation when
many Port pins are used.

>I guess that if I find that it is too much for the
> PIC I'd have to use transistors for driving my output charges.

Yes, and that's sometimes desirable, to protect the PIC pins from
the big bad world anyhow.

------------
Barry King, KA1NLH
Engineering Manager
NRG Systems "Measuring the Wind's Energy"
Hinesburg, Vermont, USA
www.nrgsystems.com