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Crystal Load Capacitors

A crystal only hits its marked frequency when it sees its specified load capacitance CL. With two equal capacitors, C = 2(CL − C_stray) — and getting this wrong pulls the frequency off, which is why a clock drifts or USB fails to enumerate.

Crystal CL (pF)

Stray capacitance (pF)

Actual C1 fitted (pF)

Actual C2 fitted (pF)

Required C1 = C2

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Effective CL as fitted

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Deviation from spec

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Stray is the PCB trace plus the oscillator pin capacitance — typically 2–5 pF per side. Too much load makes the crystal run slow and may stop it starting; too little makes it run fast.

A helper, not a substitute for engineering judgment. Verify against the governing standard.