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DS32kHz interesting phenomenon

Discussion in 'Electronic Design' started by Chris Carlen, Dec 12, 2003.

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  1. Chris Carlen

    Chris Carlen Guest

    Greetings:

    I am using this TXCO in my Nixie clock. I was monitoring the current
    flow from the battery backup on my newly completed PCB, when I happened
    upon very infrequent large current pulse events.

    Usually, the current load is about 10-15uA from the VL2320 battery when
    the TCXO and AVR CPU are running from the battery (AVR in power-save
    sleep mode except for 2Hz interrupts taking about 60us to service from
    asynchronous timer2 compare match used to maintain the time).

    But I found that every 64.1 seconds there is a 400uA pulse lasting
    almost exactly 100ms. At first I thought the period was exactly 64.0s,
    but then on a closer look, it appears to be 64.1. Not exactly sure
    though, as the scope can't do delayed trigger beyond 50s, so I can only
    get a cursor measurement, which on a single shot 100s capture only has
    0.1s resolution.

    The 64s originally made me think some strange program bug was occurring,
    like the another interrupt waking the CPU. But I also suspected that
    maybe something in the TCXO might be responsible. I went back to the
    breadboard so I could measure the TCXO vs. CPU currents separately, and
    discovered the CPU was hovering around 11uA average like it should, with
    no current pulses every 64s.

    Sure enough, it was the DS32kHz. It appears to do something every 64s
    or so, that uses 400uA, then it goes back to its usual 4uA draw from the
    Vbat terminal. It is most likely the temperature compensating
    mechanism, as the datasheet states "...periodically measures the
    temperature and adjusts the crystal load to compensate."

    Well, that period is about 64s and the important thing is the TCXO pulls
    400uA for 100ms.

    Just a heads up to DS32kHz users. I suppose some battery backup
    circuits designed with the expectation of 4uA continuous, could hiccup
    on the 400uA pulses. It also means an extra 0.625uA of average current
    is really present which one might not see if measuring with a meter
    only. Fortunately none of this is a problem in my circuit.

    Good day!
     
  2. Tim Shoppa

    Tim Shoppa Guest

    Right... the DS32kHz is a fancy-pants TCXO that adjusts the crystal load
    according to a calibration curve burned into it at the factory.
    I've used it in several projects but never noticed the calibration surge
    (probably the on-chip microprocessor firing up the temperature sensor
    etc.) myself.

    Tim.
     
  3. Ben Bradley

    Ben Bradley Guest

    In sci.electronics.design, Chris Carlen
    I'm behind on my Standard s.e.d projects. I got some big nixie
    tubes around here somewhere...
    You mean the datasheet says absolutely nothing about it other than
    that sentence? ISTR this property (sudden surge of current pull) of
    this chip has been discussed here before, apparently because it's not
    well documented. Search for the part at groups.google.com.
     
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