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y use capacitors before XTAL

Discussion in 'Microcontrollers' started by uogian, Aug 12, 2009.

  1. uogian

    uogian New Member

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    I m unable to understand that why we use capacitors with XTAL-oscillator.
    Friendz if anyone of u know its logic, plz reply me
    thanx


    Baber Mustafa
     
  2. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Because they are part of the oscillator circuit, and required to make it oscillate.
     
  3. einton

    einton New Member

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    because they make oscilator frequency more stable.
     
  4. dave

    Dave New Member

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  5. Speakerguy

    Speakerguy Active Member

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    The crystal looks inductive in a narrow region near its resonance (actually, between its series and parallel resonances). That's 90 degrees of phase shift. You need another 90 degrees (provided by either of the caps) to get 180 degrees phase shift between in and out, which is the main condition for oscillation. That's about as simple as I can describe it.
     
    Last edited: Aug 13, 2009
  6. Hero999

    Hero999 Banned

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    Not the capacitors are a requirement and aren't optional, although if you've got large PCB traces it is possible for it to oscillate without capacitors but it's a silly idea and not stable.
     

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