crystal loading capacitor

[treitmey]

I feel a little silly asking this question.
I would like to use this crystal in my design.
CM309S18.432MABJTR with a loading cap. of 18pF
http://rocky.digikey.com/WebLib/Citizen/Web%20Data/Citizen%20Crystal%20Part%20No%20System.pdf
What cap do I connect to ground on the osc1 and osc2?

Is it simply the load value? a 18pF on each side?

or do I need to consider the loading in the input of the PIC pins?

or is it some series/parallel cap calculation?

[treitmey]

Got it.
http://forum.microchip.com/tm.aspx?m=49934
http://ww1.microchip.com/downloads/en/AppNotes/FACT001.PDF
start at 2x the load. Caps are in series thus effective cap is 1/2 of an individual.

ie if loading is rated at 18pF start with 2 caps each rated at 36pF.

[PCM programmer]

Here is a FAQ from a crystal manufacturer.
http://www.foxonline.com/techfaqs_cry.htm#a4

The problem with all the FAQs and Tech notes offered by crystal
manufacturers is that they never show the equation "solved" for
the capacitor value. They show it "solved" for the crystals' load
capacitance (CL), which in fact we already know. I'll solve it
for the cap value:

Let's assume that both caps will be the same value (This is the
standard practice). Then the equation shown in the link becomes:

CL = ((C * C) / 2C ) + Cs

Reducing this down, we get:

CL = C/2 + Cs

Then we can easily solve it for C (the value of each capacitor):

C = (CL - Cs) * 2

Now let's plug in your numbers into it. Let's assume the stray
capacitance (Cs) is 5 pf. (Typical Cs values are from 3 to 5 pf).

C = (18 - 5) * 2

C = 13 * 2

C = 26 pF

You could use 27 pf for that caps, since that's a standard value.

In actual practice, I have used 22 pf for all crystals from 4 MHz to 20 MHz
and it always works. However, if the capacitors are not the proper
value for the circuit, the frequency may be pulled off of the nominal value.