Over the years, I've had ongoing problems with high frequency ceramic
resonators. The PIC is pretty finicky about board layout, resonator
losses, internal capacitors, etc. That's why I really like the clock
multiplier in the 18F series. I've never had a problem with 10MHz
resonators with internal capacitors. I'd have problems now and then with
16MHz and 20MHz. The "ultimate cure" I've used is to have muRata
characterize a resonator for use with your particular circuit. It's a
pretty slow process. You send them a couple of your boards. They vary a
bunch of resonator parameters and come up with a part number that will
work best in your board.

On the 10M resistor, I don't believe you need that. It's the DC feedback
resistor (forcing the inverter in the PIC into linear operation) and the
PIC has it inside. Perhaps your additional resistor is reducing the gain
enough to keep it from oscillating.

I really like use the three pin resonators, even with the occasional
problem. I'm now starting to use the internal RC oscillator instead,
though. It's amazing to me that they got an internal RC to be as precise
as they have.

Harold

> Hi, I was wondering if anyone could provide some oscillator guidance.  I
> am
> trying to run a 16f690 off a 20 mgherz ceramic resonator with built in
> capacitors.
>
> I think i have all my my programming correct, but when I go to turn it on,
> it does not work.  The oscillator does not start up.  However, if I try to
> measure the voltage with my multimeter and touch the Clock input pin, the
> PIC starts to work.  There is about 2.3V on the clock output and input pin
> .
>
> Right now i have a 1 mega Ohm resistor across the input and output leads
> of
> the resonator like the datasheet said for it to have. I am running the
> resonator in HS mode.
>
> My guess is that my programming is correct but there is not enough drive
> to
> get the oscillator going.  Unfortunately, I have no idea about how to fix
> the problem.  This is my first oscillator circuit which I am using for a
> school project.
>
> Anyone have any ideas?
>
> Thanks,
> Adam Stambler
> --
> http://www.piclist.com PIC/SX FAQ & list archive
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist
>


-- 
FCC Rules Updated Daily at http://www.hallikainen.com - Advertising
opportunities available!
-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist