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SX28AC/DP Oscillator — Parallax Forums

SX28AC/DP Oscillator

MacGeek117MacGeek117 Posts: 747
edited 2005-07-28 19:57 in General Discussion
Do I need to connect a resistor across the pins of the Murata resonator?
bugg

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Comments

  • KenMKenM Posts: 657
    edited 2005-07-26 03:35
    Yuuuupp, you do.

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    Ken
    Resonator Resistor.jpg
    855 x 427 - 79K
  • ForrestForrest Posts: 1,341
    edited 2005-07-26 13:37
    Hmm, the schematic of the BS2SX OEM (which uses the SX28AC/DP and a 50 MHz Murata Ceramic Resonator) does NOT show a resistor across the pins of the resonator. In fact the BS2SX OEM schematic (and the picture of the an assembled board) shows a 5 pF capacitor between OSC2 and Ground.

    The SX Tech board doesn't have a location to attach a resistor across the resonator.

    Can someone from Parallax please comment.
  • Guenther DaubachGuenther Daubach Posts: 1,321
    edited 2005-07-26 14:29
    To Resist(or) or not to Resist(or)...

    Well, the information in the table, Ken has provided is the "official" one, contained in the SX datasheets published by Ubicom. It clearly states that parallel resistors are required for each resonator type, and caps to ground for those types w/o internal caps. To my knowledge, the resonators sold by Parallax all have internal caps, so we need not to worry about them.

    On the other hand, you can find many SX designs without parallel resistors, including the Parallax products, and others. When prototyping an SX application, I usually do not add a parallel resistor either, and so far never had problems because of this. IOW the clock-generation always worked w/o the resitor. I have never tested a "resistor-less" SX assembly across the full temperature and supply voltage range but possibly, the parallel resistor becomes important under extreme environmental conditions, like very low temperatures around -40°C.

    Therefore, in my "production designs", I always provide a parallel resitor for safety reasons - I think this is worth the extra penny for the resistor.

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    Greetings from Germany,

    Günther
  • MacGeek117MacGeek117 Posts: 747
    edited 2005-07-26 17:06
    Thanks for the quick replies! Also, what is the max temperature for the SX28AC/DP?
    bugg

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  • Ken GraceyKen Gracey Posts: 7,401
    edited 2005-07-26 17:15
    Bugg,

    Take a look at the datasheet.

    - Ken
  • KenMKenM Posts: 657
    edited 2005-07-26 19:17
    Adding to Guenthers comments:
    In my contest entry http://www.parallax.com/sx/contest/contest_comfort_meter.asp
    The SX18 and previous version with SX28 did not use a parallel resistor with the resonator.
    The SX IC was subjected to starting up under extreeme temperatures (-30 C to 70 C). It was reported to me that intermittenly the diplay did not work, but the analog output did work. This is an indication that the SX was not properly running. This would occur ONLY when starting in the extreeme temps mentioned.
    Unfortuanately I was never able to make any measurements during the failure mode.
    I have since altered the design using the internal 32k hz osc and have not had a startup failure since (both cold and hot conditions).
    I have to believe (guess) that the problem may very well be due to the lack of· a parallel resistor with the resonator.
    Guenther Daubach said...
    To Resist(or) or not to Resist(or)...
    I have never tested a "resistor-less" SX assembly across the full temperature and supply voltage range but possibly, the parallel resistor becomes important under extreme environmental conditions, like very low temperatures around -40°C.

    Therefore, in my "production designs", I always provide a parallel resitor for safety reasons - I think this is worth the extra penny for the resistor.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Ken
  • Guenther DaubachGuenther Daubach Posts: 1,321
    edited 2005-07-26 23:50
    Bugg,

    in case you don't have the datasheet handy, it specifies "Ambient temperature under bias" as -40°C to +85C - I hope that I'll never have to work under that bias smile.gif .

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    Greetings from Germany,

    Günther
  • MacGeek117MacGeek117 Posts: 747
    edited 2005-07-27 02:39
    Me neither, Guenther! Thanks for all the help, guys! Man I love Parallax Forums!
    bugg

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  • MacGeek117MacGeek117 Posts: 747
    edited 2005-07-28 14:13
    What value of resistor do I use for 50MHz resonator?
    bugg

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  • KenMKenM Posts: 657
    edited 2005-07-28 14:46
    10k ohms
    And here is a clue....http://forums.parallax.com/forums/attach.aspx?a=3266

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    Ken
    pdf
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    sx20 & sx 28.pdf
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  • MacGeek117MacGeek117 Posts: 747
    edited 2005-07-28 19:57
    Thanks, Ken!
    bugg

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