what size load capacitor for crystal oscillator?

[mike wax]

OK so i have a homemade PCB, a uC, and an 8MHz crystal. According to the common formula, CL = (C1 * C2) / (C1 + C2) + Cstray. CL is supposed to be 18pF.
simple enough except for Cstray. most articles i've read say that Cstray is probably between 3 & 5 pF. is this a reasonable figure?
Here are some pics of my board, a standard FR4 with 1oz. copper. pardon the lousy camera:

 

[davenn]

so, did you google the datasheet for the chip ?
it will have component options​

 

[mike wax]

no the data sheet is pretty meagre. http://www.ecsxtal.com/store/pdf/ECS-3x10X 3x9X.pdf

 

[davenn]

no the data sheet is pretty meagre. http://www.ecsxtal.com/store/pdf/ECS-3x10X 3x9X.pdf

I said the chip, not the crystal, tho the crystal datasheet does give indications of the value

 

[mike wax]

I said the chip, not the crystal, tho the crystal datasheet does give indications of the value

oh... yeah that's a good idea, hold on.... yes it does have a suggested circuit:

so yeah if i could figure out that Rext. i guess i'd have to either scope it or find some kind of timing routine to measure it. but then i can't get a resistor with some specific precise value.

 

[mike wax]

and yeah the rated CL is 18pF

 

[BobK]

The capacitance is not critical. Use 27 or 33pF.

Bob

 

[mike wax]

The capacitance is not critical. Use 27 or 33pF.
Bob

well i wouldn't even use a crystal except it's supposed to be necessary for the frequency of the USB interface. so a pair of 33s is good enough for that?

 

[BobK]

Yes. Changing the temperature would have more effect than the capacitors being off by a little bit.

Bob

 

[mike wax]

gotcha thanx