Pic16F88 and a 20mhz Xtal need advice.

Thread Starter

Geoffr67

Joined Dec 5, 2011
69
Ok im setting up a circuit and while ive used the ceramic osc in 20mhz id like to use a 20mhz crystal this time to provide more accurate timing of my PWM software i wrote. This will be the first time i am useing a 20Mhz Xtal, can anyone with pic experience give me some recommendations as to what crystal to use and a ballpark of Capacitor values to go along with it to establish oscillation?
Thank you!
 

joeyd999

Joined Jun 6, 2011
4,271
Ok im setting up a circuit and while ive used the ceramic osc in 20mhz id like to use a 20mhz crystal this time to provide more accurate timing of my PWM software i wrote. This will be the first time i am useing a 20Mhz Xtal, can anyone with pic experience give me some recommendations as to what crystal to use and a ballpark of Capacitor values to go along with it to establish oscillation?
Thank you!
You need a parallel resonant crystal and a 15-18pf ceramic cap on each lead to ground. It really is that simple.
 

thatoneguy

Joined Feb 19, 2009
6,359
If you have a frequency counter, use on 18pf cap on one side, and a trimmer cap on the other until you are at exactly 20Mhz. Make a simple program that toggles an output each time through the main loop and you should get a sold 5Mhz square wave from a PIC if you want to use a scope. If it is off, tweak the trimmer cap a bit until it is on.

You may want to do this where the device will actually be located, and avoid as much thermal change as possible. If there will be a wide temperature working band, look at an oven controlled crystal instead, for more $$.
 

Thread Starter

Geoffr67

Joined Dec 5, 2011
69
You need a parallel resonant crystal and a 15-18pf ceramic cap on each lead to ground. It really is that simple.
im on mouser looking for one that fits the bill, but many of the datasheets do not definitively specify if its series or parallel cut crystal.do you have an example of a suitable part?
Thank you!
 

Markd77

Joined Sep 7, 2009
2,806
That datasheet doesn't give much to go on.
I think that if you look at the mouser page, and if you see a capacitance value (eg. 20pF) in the item description, then it should be a parallel cut crystal.
If you accidentally get a series one it will most likely still work, but the frequency will be slightly off.

http://qooljaq.com/CrystalOscillator.htm
 
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