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Crystal Load Capacitance Calculator - Online Pierce Oscillator Tool

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Crystal Load Capacitance Calculator

For Pierce Oscillator Circuits — Calculate exact load capacitance CL from your C1, C2 & stray capacitance.

Input Parameters
Board trace + pin capacitance, usually 2–5 pF.
Load Capacitance CL — pF
CL = (C1 × C2) / (C1 + C2) + Cs
✅ Matches typical crystal load specs (12–20 pF).
Target & Assumptions
If C1 is fixed, we solve for C2.
Recommended External Capacitors — pF each (C1 = C2)
C = 2 × (CL − Cs)
💡 Standard values: 15, 18, 22, 27, 33 pF.

Frequently Asked Questions

Load capacitance is the total external capacitance seen by the crystal in an oscillator circuit. It determines the oscillation frequency – if the actual CL differs from the crystal's specified load, the frequency will shift. The Pierce oscillator uses two capacitors (C1, C2) and stray capacitance from traces and pins to create this load.

The formula is: CL = (C1 × C2) / (C1 + C2) + Cs. Cs represents stray capacitance (typically 2–5 pF). For symmetric designs C1 = C2, this simplifies to CL = C/2 + Cs.

Stray capacitance includes parasitic capacitance from PCB traces, pads, and the microcontroller’s pin input capacitance. A safe estimate is 2–5 pF. For precise designs, measure your board or consult the MCU datasheet for pin capacitance.

Frequency drift often occurs because the actual load capacitance seen by the crystal does not match its rated CL. This tool lets you verify your network, or calculate the right C1/C2 for your crystal’s specified load, minimizing frequency error.

Yes, asymmetric capacitor values are possible and sometimes used to adjust drive level or startup margin. The general formula still applies. Switch to asymmetric mode in the calculator to solve for one capacitor when the other is fixed.

With Cs ≈ 3 pF, the required external capacitor becomes C = 2 × (12 − 3) = 18 pF. The nearest standard value is 18 pF or 22 pF. Always verify with the actual PCB and crystal datasheet.
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