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Discussion in 'Electronic Design' started by [email protected], Jul 31, 2013.

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  1. Guest


    Crystal 1

    Crystal 2

    I am using the above mentioned Crystals with MSP430F5438AIPZ

    The micro used one of these crystals to generate 17 MHz clock. Is it a good idea to get that clock out of the micro on one of its pins and route it to a connector for oscilloscope probing reasons inorder to confirm its frequency.

    I do not have much knowledge about layout. But Can this cause noise issues or issues with EMC or interference problems.

  2. Guest

    The thing is that the micro will be on the bottom side of the printed circuit board. The tester will not be able to acess the micro or crystal pins using scope. So, I am planning to route that clock trace to a 2.54mm spacing single row three pin connector.

    The connector will allow the user to hook upi the scope to one of the pinsof the connector. There will addition two other signal avialable on the rest of the two pins.

    Is this a good idea? Will it cause any EMC problems during EMC testing or any other interference problems?

  3. Guest

    The printed circuit board will be mounted inside the box and its top side can be accessable to tester. The micro will be on the bottom side of the printed circuit board.
  4. I would not bring out the crystal frequency directly. It should be
    possible to program one of the micros timers to output a divided-down
    version of the clock on an I/O pin, on some signal trace that is already
    accessible. Perhaps as part of a special test mode. The frequency/period
    of this will scale directly and precisely with the crystal frequency.

    E.g. some of the real time clock chips have a "1 Hz" output that can be
    used in a similar way to calibrate the 32768Hz crystal.
  5. Joerg

    Joerg Guest

    That's the way to go. If the uC has to blink an LED let that come off a
    timer that provides an exact divider ratio. Then switch the frequency
    counter to period measurement and ... bingo.
  6. Joerg

    Joerg Guest

    Afraid I don't quite follow what you are saying. How can a divider
    inside the uC affect the crystal frequency?
  7. Joerg

    Joerg Guest

    Huh? If I have a 16MHz crystal on the uC, run it through a timer that is
    supposed to deliver 0.5Hz. When I then measure a stable 0.500000000Hz,
    how is that not verification that the crystal oscillator frequency is

    It is quite inconceivable that a crystal operates in a stable fashion on
    some unwanted frequency and it just so happens that I misprogrammed the
    timer register so that it exactly compensates this.
  8. Joerg

    Joerg Guest

    I have never seen any modulation by that. Ok, if you drive drive a
    coffee cup warmer with it and have no power plane, maybe :)

    A lot of old-style (not so integrated) PLLs from the 80's relied on the
    fact that such modulation would not happen, and in mine it didn't.
  9. Joerg

    Joerg Guest

    Yes, substrate can be nasty. Though on a well designed IC they should
    have taken care of "cocooning" the oscillator enough. That's what we do
    with pulse drive paths on ultrasound chips, got to do it. Sometimes we
    even sink off-phase dummy currents. Like they do with the balance shafts
    on luxury cars, to make them run smooth.

    But I doubt Jessica will care much if the clock is 17.000000MHz or
    17.000050MHz. If she does then I would never even think about using the
    on-board oscillator. That's where dedicated TCXO's come in, they don't
    cost much anymore these days.
  10. John S

    John S Guest

    How do you know?
  11. Think it depends a bit on the situation. During initial development of
    the system, bringing up new hardware and writing drivers, yes I would
    just probe the clock pins directly as a quick check that things were
    working. Not as a precision frequency measurement, just to see if it was
    oscillating properly.

    But if the purpose is to do a production calibration of the oscillator,
    (or compensation of a frequency error), then you need to stay well away
    from it. And using a divided-down version is ideal for that.

    And the last thing you want to be doing with a high frequency clock
    crystal signal is bringing it out any significant distance. They can be
    very susceptible to interference, and can dominate emissions too.
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