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Watch crystal with SX28 — Parallax Forums

Watch crystal with SX28

Henry CassonHenry Casson Posts: 12
edited 2007-03-10 20:49 in General Discussion
I'd like to put a watch type 32 Khz crystal outboard. It comes with a a value of 12.5 pf for the load cap. OK, 2 of those to ground. The diagram in fig 27 of the SXkey manual shows a resistor across osc1 & osc2 labelled Rp. is this needed? and what value?

Comments

  • PJAllenPJAllen Banned Posts: 5,065
    edited 2007-03-10 12:59
    "...the SXkey manual shows a resistor across osc1 & osc2 labelled Rp."

    is this needed? -- Yes
    and what value? -- try 1Megohm
  • Tracy AllenTracy Allen Posts: 6,667
    edited 2007-03-10 20:49
    32khz watch crystals are rather delicate tuning fork structures and are usually used with the additional resistor in series that is shown as Rs in the SX documentation. The resistor limits the power drive to the crystal. If it is too low resistance, the crystal will be unstable or even damaged. Too high a resistance and it won't oscillate. The capacitors also enter the equation, as larger capacitors lower the drive level to the crystal. Good starting values are 1 Mohm for feedback and 100 Kohm for series.

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    Tracy Allen
    www.emesystems.com
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