'Crystal for 16c84'
Despite my age, I'm a novice when it comes to certain things. I need
to construct a pic-based circuit and I've become hung up on the exact
oscillator to use for the clock. I need high stability to allow
accurate timing measurements if an input frequency, so a quartz
crystal seems in order (50-100ppm will be good) - a ceramic oscillator is
too sloppy. My problem is that I don't know which one of the myriad 4
Mhz crystals to get and exactly how to hook the required capacitors to
it and how to choose the value of the capacitors. I have the
application note on oscillator design, but I'm too dense to translate
that info into part numbers from Digikey.
Can anyone help me along a bit?
Stephen Brown, Ph.D.
Applied Research Associates
RR 1 Box 120A, Waterman Road
South Royalton, Vermont 05068
(802) 763-8348 voice
(802) 763-8283 fax
|Can't remember a part number, but Digikey (and probably dozen's of
others) sell xtals for about $3. or so. Microchip has a document on
their web site (an application note) that desribes all the stuff. But
basically, all you do is connect the xtal across the two osc pins on the
PIC. The connect a small (about 20 picofarads or so) capacitor from each
xtal pin to ground.
Now, if you want real accuracy, replace one of the caps with a variable
cap (a trimmer) of say 5-35 pf. Then if via a friendly neighbourhood ham
radio type you can get access to a receiver that covers 4 mhz (the top
end of the amateur 80 metre band), and assuming he can calibrate the
receiver against one of the standards such as WWV or WWVH, then you
should be able to set the xtal frequeny to withing a few hertz.
It's even easier with e 10mz xtal, since using a receiver, you can
calibrate directly against WWV or WWVH, rather than depend on the
calibration of the receiver.
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