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AdI found this formula to calculate the value of the capacitors for a crystal, C(1,2)=2(CL-Cstray). C1 and C2 will use the same values. How do one choose the Cstray? I read that it ranges between 3pF~5pF.

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You don't choose \$C_{stray}\$.

\$C_{stray}\$ represents parasitic capacitance, mainly between the signal traces connecting the crystal to the rest of the circuit and ground (or whatever other low-impedance nodes are nearby).

The trick is, you generally don't know \$C_{stray}\$ very accurately. You have to estimate it from your trace lengths and geometry, and possibly adjust your added capacitors based on measurements after building your first prototype boards.

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