Calculadora de Capacitor de Carga del Cristal
Selección de capacitores externos para oscilador
Required Parameters
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Quick Answer
CL_ext = 2 × (CL_crystal - CL_stray). For a crystal with CL=12pF and 3pF stray: CL_ext = 2×(12-3) = 18pF per pin.
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.
Knowledge Base
¿Cómo calculo los capacitores de carga?
C_ext = 2 × (CL - C_parásita). CL viene del datasheet del cristal (típico 8-20pF). C_parásita incluye pines del IC y trazas PCB (2-5pF).
¿Qué pasa con capacitores incorrectos?
Demasiada capacitancia: frecuencia baja, arranque lento. Muy poca: frecuencia alta, inestabilidad, posible fallo de arranque.
¿Qué cristal elegir para mi MCU?
8 MHz: STM32, AVR general. 12 MHz: USB full-speed. 16 MHz: Arduino. 32.768 kHz: RTC. 11.0592 MHz: elimina error de UART.
¿Cristal vs oscilador MEMS?
Cristal: menor costo, requiere caps externos. MEMS: sin componentes externos, arranque rápido (<1ms), resistente a vibración, más caro.
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