Jaume;

As a general practice, try to layout ground as solidly as possible.  Thin 10 mil
traces that weave all over a board are fatality.  One large solid track through the
processor section is great.  Also, try to ground the supply, regulator, and
processor as a single solid run.  Then, bring any I/O (the ones that act as
antennas.. positive AND 0V wires) back to a safe place, such as the power supply
point, where the caps are.

On the processor, you might try pulling up the MCLR a bit more, with say 1K.

Isolate any input circuits, say digital inputs, from the processor.  NEVER run an
external I/O line to a processor pin.  Run it into the base of a transistor, NPN
with a plus supply switch, or PNP with a ground switch.  Resistors where
appropriate, of course.  Then the pulled up/down collector goes to the processor.

Then try the ferrite beads, mu-metal, and magic potions.

Jaume Aragay Badia wrote:

> Hi,
>         I'm using a 16f84 to control a welding machine. The high frequency spike
> used by the TIG machine to start welding RESETS my PIC ONLY if the distance
> between the electrode and the metal part to be welded is "big" (3mm); if the
> distance is "right" (2mm) there is no problem at all.
>
> Any idea on how to isolate the PIC from this high freq. spike?
>
> TIA.
>
> Jaume.
>
> ----------------------
>   Jaume Aragay Badia
>    aragay@email.com
> ----------------------