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Crystal load capacitor calculator

A crystal oscillates at its marked frequency only when it sees the load capacitance (CL) from its datasheet. The two capacitors from the crystal pins to ground give most of it, and the chip’s pins and the board add the rest. A wrong load shifts the frequency, which matters for radio and for clocks.

MCUXOUTXINC1C2YCstrayCstray

Crystal

CL, C0 and the motional capacitance Cm (often written C1) are in the crystal’s datasheet. The stray capacitance is the chip’s pin capacitance plus the board, usually 2–5 pF. Fill in C0 and Cm to see the frequency error.

Results

Each capacitor (C1 = C2)
– pF
SeriesC1 = C2Load the crystal seesFrequency error
E12– pF– pF–
E24– pF– pF–

How it’s calculated

The two capacitors are in series as the crystal sees them, and the stray capacitance adds to them. With two equal capacitors, each one is twice the load minus the stray capacitance. The frequency error comes from the crystal’s shunt capacitance C0 and motional capacitance C1 (written Cm below, to keep it apart from the load capacitor).

CL      = C1 · C2 / (C1 + C2) + Cstray
C1 = C2 = 2 · (CL − Cstray)

Δf / f  ≈ Cm / 2 · (1 / (C0 + CL,actual) − 1 / (C0 + CL,datasheet))

Keep its limits in mind.

  • The stray capacitance is a guess until you measure it. Start with 3–5 pF, then adjust after measuring the frequency on a real board.
  • Some chips have built-in load capacitors set in firmware, for example Nordic’s nRF5340. Check the datasheet before adding external ones.
  • The frequency error is the shift from the load alone. The crystal’s own tolerance and its drift with temperature come on top.
  • For 32.768 kHz crystals, the chip’s drive level and start-up margin matter too. Follow the chip maker’s guide for them.