Crystal Oscillator Load Capacitor Calculator
Calculate the correct C1 and C2 load capacitors for a Pierce oscillator from your crystal's datasheet CL specification.
Crystal Parameters
Cstray is typically 2–5 pF from PCB traces and microcontroller pin capacitance.
Results
Pierce oscillator: C1 = C2 = 2 × (CL − Cstray)
Common Crystal Reference Values
| Frequency | CL (datasheet) | Recommended C1 = C2 |
|---|---|---|
| 32.768 kHz | 7 pF | 8 pF |
| 8 MHz | 12 pF | 18 pF |
| 12 MHz | 12 pF | 18 pF |
| 16 MHz | 12 pF | 18 pF |
| 20 MHz | 18 pF | 30 pF |
| 25 MHz | 15 pF | 24 pF |
* Assumes Cstray = 3 pF. Adjust based on your PCB layout.
Why Crystal Load Capacitance Matters
A crystal oscillates at its specified frequency only when it sees the correct load capacitance CL on its terminals. Using wrong capacitor values will shift the oscillation frequency, cause start-up problems, or prevent oscillation entirely.
In a Pierce oscillator, two equal capacitors C1 and C2 are placed from each crystal pin to ground. The series combination of C1 and C2, plus the stray capacitance, must equal the crystal's specified CL:
Load Capacitance Formula
CL = (C1 × C2) / (C1 + C2) + CstrayFor C1 = C2 (symmetric)
C1 = C2 = 2 × (CL − Cstray)Key Points
- CL is specified in the crystal datasheet — always check it
- Stray capacitance comes from PCB traces and IC pin capacitance (2–5 pF typical)
- Too low C1/C2: crystal may oscillate at incorrect overtone frequency
- Too high C1/C2: frequency pulls low, oscillator may not start
- Most microcontrollers also specify an acceptable CL range — follow both
- For 32.768 kHz RTC crystals, CL is often only 6–7 pF
Applications
- Microcontroller system clock (8, 16, 20 MHz)
- Real-time clock (32.768 kHz)
- USB clock generation (12, 48 MHz)
- RF reference oscillators (25, 26 MHz)