Choosing Crystal Capacitors

Here is a typical crystal oscillator circuit for a microcontroller:

But what value should the capacitors be?

Every crystal will have a published load capacitance (Cload). The values of C1 and C2 are related to this figure by the formula:

Cstray is the stray capacitance in the circuit made up of contributions from component leads, pcb tracks, etc.  A typical value for this might be around 5-10pF.

So, if for example the datasheet for your crystal specifies a load capacitance of 20pf, a value of 22pF would be a suitable value for C1 and C2 since (22x22/(22+22)) + 10 = 21 - close enough to the specified 20.

If you are looking for a crystal to include in your project, check out the Bitsbox crystal & resonator section.