capacitors

[Bilal]

Hi i have a problem with the values of Capacitors with 20MHz. i am using 24pF but i think its speed is varied with change of physical position of my board...
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[Ttelmah]

Of course it will.
It'll also depend on what your crystal actually wants.
A crystal is specified for a particular load capacitace to give the right frequency, and gain behaviour.

Cl = load capacitance the crystal requires. Typically something between 8pF, and 30pF - find from the data sheet of the crystal

Cs = stray capacitance of the board and pins. The pin capacitance is in the PIC data sheet, and the board capacitance will depend on your design. Things to avoid - long leads, running ground planes right round the capacitor legs, etc. etc..
A well designed board, with the crystal close to the PIC, will have Cs, at perhaps 4 to 5pF. A badly designed board with a ground plane close round the crystal legs, and long leads to the chip, can easily go up over 20pF.

Then:

Creqd = (Cl-Cs)*2

The '*2', is because the crystal sees the capacitances _in series_.

So for a 16pF crystal, with 5pF of stray capacitance, the required capacitors become:

(16-5)*2 = 22pF

However double your stray capacitance, and the value drops to just 12pF.

So board layout is _critical_ if you want good accuracy, and reliability from the oscillator...

Best Wishes

[Bilal]

means I should use less then 22pF of Caps...
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[Bilal]

thanks Ttelmah it will help..
thanks a lot..
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[SherpaDoug]

It also means you must be very careful of moving conductive surfaces near the crystal. These will cause the stray capacitance to change and the frequency to change. You may have to put a grounded metal "can" around the crystal circuitry, so that any movement is outside the can and shielded from the crystal.
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The search for better is endless. Instead simply find very good and get the job done.

[Bilal]

OOo thanks SherpaDoug..
this point is more important i believe..
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[Bilal]

15pF will work with 20MHz Crystal with few precautions??
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