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oscillator startup time

Discussion in 'Electronic Design' started by Octavio, Feb 25, 2007.

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  1. Octavio

    Octavio Guest

    I'm using a 74HC4060(A) oscillator/divider with a 15MHz crystal in the
    oscillator section. That oscillator must be able to turn on/off (using the
    reset pin) with a 43uS ON / 8uS OFF duty cycle.

    I'm not familiar with oscillator startup time stuff so I ask:
    The oscillator must start oscillating within 1uS to give a reasonable number
    of counts, say about 7 million cycles at the end of that microsecond.

    Is this possible?

    Thanks for any input!

    Octavio
     
  2. I don't understand your 7 million cycles at the end of a microsecond
    question. Do you mean 7000GHz? Or do you mean 7MHz? ???

    Anyway, quartz crystals have a very high Q, more than 10^4 or 10^5.
    Their startup time is on the order of Q cycles, or possibly more than
    5ms for a 15MHz crystal. That doesn't fit with your 43us / 8us
    timing requirement.
     
  3. John Fields

    John Fields Guest

     
  4. PeteS

    PeteS Guest

    Or an oscillator that can be gated on and off (they are available with
    enable pins) although that is virtually guaranteed to generate runt
    pulses at some time.


    Cheers

    PeteS
     
  5. Jamie

    Jamie Guest

    Don't do that, keep it running always, use Logic gates to switch it
    on/off .. That way, you can precisely control it via count of
    pulses if you wish using a counters. This will give you a perfect
    timing on the edges.
    something like 645 counts for the 43 us and 120 ct's for the 8Us.

    i think i have the math correct there.
    i could be off by 1 decimal point.
    as far as the rest of your statement, i think you should read that
    again.
     
  6. Octavio

    Octavio Guest

    I could. I just wanted to save one chip.
    Reading what you and other people answered, I believe it's the only way.

    Thanks!
     
  7. Octavio

    Octavio Guest

    I meant that, even if the oscillator is slow to start up, I could at least
    get 7M cycles for the built-in counter to count by the end of the first
    microsecond. This would have given a minimal error at start up for my
    project.
    *But* as far as I see, from you and other people who answered my question,
    it is not possible. Startup time is much longer. I will use a separate
    oscillator with a gate as a switch.

    Thanks!

    Octavio
     
  8. Jamie

    Jamie Guest

    Look at the High speed low pin count AVR's or PIC's, with careful
    coding, you can get your self a Xbase clock with the scaling generated
    signals you need from a single chip with the Xstyle
    Of course, don' expect that uC to do any other processing because
    it will up set the timing of course unless you use something like an
    AVR that can be calculated easy to account for the CPU usage of the clock.
    In any case, many of those uC have programmable counters that can
    output directly to a pin with on/off duty cycle programming and clock
    them self's directly from the base osc.
     
  9. PeteS

    PeteS Guest

    For best results (not to say avoiding metastability) make sure it
    switches the signal on and off synchronously.

    Cheers

    PeteS
     
  10. Octavio

    Octavio Guest

    The project is more or less like an ansynchronous receiver with a clock 16
    times the input data rate. So the clock doens't need to be synchronous as
    the error will be only few percent or 1/16.

    Regards!
    Octavio
     
  11. That's the problem: 7M means 7,000,000 cycles, and a 15MHz signal has
    only 15 cycles in one microsecond. Or did you mean at least 7 cycles
    in the first us, which would be like an immediate 7MHz signal?

    A crystal oscillator starts up when the amplifier noise at 15MHz is
    captured by the crystal and saved as resonate energy. This saved
    signal is amplified and then again presented to the crystal, so its
    signal builds up. But until the amplitude becomes high enough for
    logic circuits to properly respond there is no output. Also, when
    the logic output first begins, after the long no-output delay, it
    has poor edge risetimes and isn't suitable to use for flip flops,
    counters, or microprocessor circuitry. That's why microprocessors
    have a long forced-reset interval. Otherwise they could perform
    incorrect operations, or crash, from the defective clock signal.
    Many engineers use a gated oscillator circuit, which can start
    instantly, and is trimmable to a reasonable frequency accuracy,
    at least over a short time frame, like 43us (only 645 cycles).

    If you need we can point you to such a circuit, or present one
    as an ASCII drawing.
     
  12. John Fields

    John Fields Guest

    ---
    "Resonate energy"???

    Not true. When the crystal is hit with noise which causes its
    physical dimensions to change, it replies by "ringing" at its
    resonant frequency and generating a voltage when it relaxes. If the
    amplifier to which it's connected is biased around noise and has
    enough gain, then that voltage will be enough to cause a transition
    in the amplifier's output which will, if it's timed correctly, hit
    the crystal again, squeezing it and, once that edge is gone, causing
    the crystal to generate a voltage which will cause the cycle to
    begin anew.
     
  13. You don't think of that as the resonant energy building up in
    the crystal? The crystal's increasing voltage appears across
    it's resonate capacitance, with E = 0.5 C V^2. On the other
    half cycle this energy is stored as current in the crystal's
    inductance, E = 0.5 L I^2. That's increasing resonate energy,
    driving the increasing voltage appearing across the crystal.
    We're speaking, of course, of the very well-known electrical
    resonate LCR models of the crystal's piezo-mechanical action.
     
  14. Robert Baer

    Robert Baer Guest

    It is possible to get one-cycle turn-on times, *but* you then must
    accept a square-wave oscillator (well, as square as the IC will allow)
    and, in this case an overdriven crystal.
    I suggest you use a comparitor, using one input as the oscillator) gate.
     
  15. Robert Baer

    Robert Baer Guest

    One "runt" pulse at the end of the gate time, at worst.
     
  16. John Fields

    John Fields Guest

     
  17. Guest

    Or if the precision doesn't need to be perfect, use a RC oscillator.
    It will start in one cycle. Alternatively add a crystal to calibrate
    the RC oscillator

    Regards

    Klaus
     
  18. Rich Grise

    Rich Grise Guest

    I think they mean that the gate edge should be in sync with the clock so
    that you don't get fractional pulses, which could confuse the counter.

    Good Luck!
    Rich
     
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