For a dollar or two, you can be free of all of these issues with a  
little powered clock.  Four pins, Vdd, GND, CLKOUT, and something  
else (probably an enable-like function) and you are good to go.   
40MHz, no problem.  Just set your clock config flag to EC (external  
clock).  Available in through-hole or SMT (actually the surface mount  
version is smaller or at least similar to the SMT crystals I used to  
use).

I've spent way too much time already in my young life fighting with  
crystal oscillator circuits.  Anything of mine in the future that  
isn't intended for mass mass production (and maybe stuff that does)  
will have a powered clock, unless there is some reason not to that  
I'm not aware of.

-n.


On Dec 10, 2006, at 8:11 PM, Bob Axtell wrote:

> Adam stambler wrote:
>> 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
>>
> I have never been able to get reliable startups with the 20M seramic
> resonators. The fastest reliable speed
> for me was 16M. And I love these critters dearly.
>
> I believe the phase-shifting caps do not quite match the Microchip
> oscillator. Drop back to a lower speed
> or tinker with cap values. Incidentally, the ones by Murata may run  
> at a
> slightly faster rate.
>
> --Bob
> -- 
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