Quarz-Lastkondensator-Rechner — MCU-Takt
External capacitor selection for crystal accuracy
Required Parameters
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Quick Answer
Crystal load caps: C_ext = 2 × (CL − C_stray). For CL=12 pF and 3 pF stray → 18 pF per pin. Wrong caps cause UART baud drift.
Crystal Load Capacitor Calculator — Clock Accuracy Guide
Use this crystal load capacitor calculator to size the two external caps that match your crystal's datasheet CL spec. Wrong load caps are the #1 cause of UART baud drift and USB timing failures.
Formula
C_ext = 2 × (CL_crystal − C_stray)
Where:
- CL_crystal — load capacitance from the crystal datasheet (typically 8–20 pF)
- C_stray — pin + trace + IC input capacitance per pin (usually 2–5 pF)
The two external capacitors appear in series to the crystal; each pin gets the same value C_ext.
Worked example — 16 MHz, CL = 12 pF
Assume C_stray = 3 pF per pin:
- C_ext = 2 × (12 − 3) = 18 pF per pin
- Use nearest standard: 18 pF or 22 pF C0G/NP0
- Verify frequency with a counter — expect ±10–50 ppm shift per ~5 pF error
Common MCU crystal choices
| Application | Frequency | Notes |
|---|---|---|
| General MCU | 8 / 16 MHz | Arduino Uno uses 16 MHz |
| USB full-speed | 12 MHz | Exact multiple required |
| UART-friendly | 11.0592 MHz | Zero baud error at 9600–115200 |
| RTC | 32.768 kHz | Divides to exactly 1 Hz |
Layout tips
- Keep crystal traces short (< 5 mm ideal)
- Place load caps adjacent to crystal pins
- Use C0G/NP0 for load caps — X7R drift affects ppm stability
Related tools
- Crystal Load Cap Calculator — interactive sizing
- SMD Capacitor Code Calculator — decode cap markings
- Component Derating Calculator — voltage margin on MLCC load caps
- UART Baud Rate Calculator — verify baud after clock selection
Design Notes
The crystal's load capacitance (CL) is specified by the manufacturer (typically 8-20pF). The two external caps appear in series to the crystal. Stray capacitance includes pin capacitance (~2pF), trace capacitance (~1-3pF), and IC input capacitance. Wrong load caps cause frequency deviation — too much cap = lower frequency, too little = higher.
Common Mistakes
- 1
Using the crystal's CL value directly as the cap value — you need to solve for the series combination.
- 2
Forgetting stray capacitance — it can be 3-7pF, significantly affecting small CL crystals.
- 3
Placing long traces between crystal and IC — adds stray capacitance and may pick up noise.
Engineering Handbox
1. CL from datasheet = 12 pF 2. Stray per pin = 3 pF 3. C_ext = 2 × (12 − 3) = 18 pF per pin
Knowledge Base
Wie berechne ich Quarz-Lastkondensatoren?
C_ext = 2 × (CL − C_stray). CL aus dem Quarz-Datenblatt (8–20 pF), C_stray typisch 2–5 pF pro Pin.
Was passiert bei falschen Lastkondensatoren?
Zu viel Kapazität: Frequenz sinkt, Startzeit steigt. Zu wenig: Frequenz steigt, Instabilität möglich. ±5 pF ≈ 10–50 ppm Abweichung.
Welche Quarzfrequenz für UART?
11,0592 MHz eliminiert Baudratenfehler bei 9600–115200. Alternativ: 8 MHz, 12 MHz (USB), 16 MHz (Arduino).
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