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HOWTO Determine Unknown XTAL Freq

Discussion in 'Electronic Basics' started by news.cogeco.ca, Apr 20, 2007.

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  1. Trust me... don't go there. >:)
     
  2. I thought some microprocessors like that were meant to be very cheap,
    designed to allow a realtime clock as well as the stuff for processing
    timebased information. One of those might be ideal. Just tap the realtime
    clock circuit for the reference.
     
  3. Typo, meant 0.3%, even 0.1%, certainly better than 0.58% should be had
    from a freq/volt converter and multimeter...
     
  4. Some micros have two oscillators built in, so a reference xtal and a
    DUT could be connected directly. But I can buy a lot of new
    guaranteed-accurate crystals for the cost of the engineering to make
    such an instrument.
    Yup. It might make more sense to just use the micro as an oscillator
    and measure the frequency with a frequency counter! But even that
    requires some programming (or configuration, at least) on many modern
    micros.


    Best regards,
    Spehro Pefhany
     
  5. Typically that's the intended purpose of the alternate oscillator. You
    have to provide the external crystal and perhaps some additional
    parts.

    You don't necessarily have access to the internal nodes to make it do
    what you want, it might take some external gates or it might not even
    be possible to make it work well, depending on the particular
    processor and peripheral implementation. For example, you might have
    to live with jitter in the interrupt latency affecting the gate time
    if the right hardware isn't in place internally.


    Best regards,
    Spehro Pefhany
     
  6. jasen

    jasen Guest

    the yearly (or weekly) average is extremely good, but minute to minute
    it's not all that hot - a cheap crystal keeps better time.
    most people would run it through a step-down transformer first :)
    pics are so cheap: why bother?

    maybe you could make it so it gets its comparison time from a serial port

    send it a pulse on the serial port (0 byte - 9 bit times worth of "+12V" on
    an rs232) and it responds with the duration of the pulse in crystal cycles.

    this way you compare the unknown crystal against the crystal in the serial
    port.

    Bye.
    Jasen
     
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