Crystal Load Capacitance Calculator
This tool calculates the required external capacitance based on the crystal’s specified Load Capacitance (C_L) and the inevitable Stray Capacitance (C_stray) introduced by the PCB traces and MCU pins.
Use this tool to:
- Balance the Load: Correctly match the crystal’s internal requirements to your physical board layout.
- Account for Parasitics: Factor in the 3–7 pF of “hidden” capacitance caused by PCB copper and IC packages.
- Optimize Frequency: Ensure your system clock stays within the ppm (parts per million) tolerance specified by the manufacturer.
Technical Breakdown
- The Load Formula: The crystal sees C_1 and C_2 in series. If C_1 = C_2, their combined series capacitance is half of a single capacitor’s value. The formula accounts for this by doubling the difference between the target C_L and the stray C_s.
- Stray Capacitance (C_stray): This is the sum of the capacitance from the MCU’s internal pin circuitry and the copper traces on your PCB. Keeping traces short and symmetrical is key to minimizing this variable.
- Standard Values: Capacitors are manufactured in specific “E-series” values. If your calculated result is 26 pF, you should typically choose the nearest standard value, such as 27 pF.
Crystal Specifications
*Typical C_stray for a tight layout is 3-7 pF.
Required Capacitors
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Use the closest standard capacitor value (e.g., 22pF, 27pF).
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