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 Question:

oscillator - Why use capacitors with crystals?
  • AKR

    Whenever I come across crystals used in any of MCU or Processor, whether it be 32.768 kHz responsible for Real Time Clock or 25 MHz crystal required for clocking different interfaces connected with the MCU or processor. There are always two capacitors connected with the Quartz Crystal as shown in the fig.Capacitor with crystal

    I have few questions here:

    1. What will happen if don't connect capacitors with the crystal?
    2. How is the value of these capacitors decided?
    3. While checking crystals datasheetI will come across a specification like stability +/-5ppm or +/-10ppm. what is the significance of this term?
    4. And what are 2nd overtone and 3 overtone crystals?
  •  Answers:

  • Scott Seidman

    You are actually building an oscillator circuit, using a crystal, some capacitors, and the internal circuitry of your microcontroller. If you don't use the caps, your clocking won't work. The values are calculated based on the properties of the crystal, as described in http://ww1.microchip.com/downloads/en/appnotes/00826a.pdf, page 9

    Essentially, the load capacitance of the crystal, \$C_L\$, is set equal to the combination of the capacitors you're adding and the shunt capacitance of the crystal, so

    $$ C_L = \frac{C^2}{2C} + C_{shunt} $$

    The ppm figures specify the (in)accuracy of the crystal (when properly used): 5 ppm is 5 parts per million.

    Second and third overtones involve driving the crystal into resonance at a harmonic. I suggest not trying this.