HJ Simpson wrote:

> I have a project using up to 20 16F84's on one board.
> I am thinking of clocking all from one source, rather than a xtal on
> each.

You asked this last week, didn't you? I assume no answer. OK, I'll
give it a shot.

You can use either a buffer, or daisy chain it from PIC to PIC. It
really depends on your needs. Are you trying to make each PIC
sync'ed up to the other, or just trying to save some money?

Daisy chaining it would be cheapest (going from pic to pic). I believe
there is a app note out on this also, I believe they suggest a small
resistor (220 ohm?) between each PIC. Your first PIC would have
the crystal on it. You'd take the output from it (pin 15) and feed it
via the resistor to pin 16 of the next PIC. Then do the same down
the line. The downsides of this approach, as compared to buffering,
is your going to get some phase delay on the oscillator between each
PIC. Also, if one PIC fails, you could lose all clock signals downstream
of that PIC.

On buffering, you can get the phase of each clock fairly close (with
proper PCB design), and you have the added benefit of if one PIC
fails, you'll not lose everything downstream of that PIC. One of
those TTL DIP pack oscillators should work fine. I'd still isolate
each PIC with a small value resistor. The buffer/resistor would feed
pin 16 of each PIC.

Now, I have no idea if you could get each PIC sync'ed up to the other
(and even if that is what you want). I'd think it would be hard to impossible
due to the varying delays of the POR and PUT  timers.

Mind if I ask what you need 20 PIC's on the same board for?

-Jeff