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VLO stability

Mike Kaelin
Intellectual 430 points

Hi MSP430 Experts! I need to implement a real time clock in an MSP430G2xxx part without using a crystal. I'm not trying to get to PPM accuracy, but need to implement a 1-sec wakeup and implement an hour-counter (maybe +/- 10%). I'm thinking about using the MCLK factory calibration, and using that to measure the VLF frequency, and go from there. How stable is the VLF frequency? The specs show drift with temperature and voltage, but is it stable otherwise? Have any of you tried this? If so, what kind of accuracy did you achieve?

 

  • 0
    Guru 227245 points

    microcontroller said:
    he specs show drift with temperature and voltage, but is it stable otherwise?

    With the large drift over voltage and temperature (which is internal die temperature and highly depends on the MSPs internal workign and energy consumption) and the extremely large uncertainity of its base frequency, I don't think it makes any sense to talk about otherwise stability.
    I guess, nobody who was content with the large variation in the frequency, was also interested in stability or accuracy. So I don't think there are any experiences or data in this field.
    If the VLO would be by any means stable enough to make any prediction about its accuracy, I think, TI would have included calibration data for it.
    But I may be wrong (wouldn't be the first or last time) and you actually can get some stable, predictable, useful clock for them VLO. Well, someone is always the first one - go ahead :)

  • 0 in reply to Jens-Michael Gross
    Guru 12050 points

    A feature of most 2xx devices is the very low oscillator (VLO). This oscillator is designed to give a

    MSP430 the ability to periodically wake up from LPM3 without using external hardware such as a crystal.

    For many applications in which accuracy is not as critical as power consumption, simply using the VLO as

    a source for a timer is a perfect solution.

    http://focus.ti.com/lit/an/slaa340a/slaa340a.pdf

     

  • 0 in reply to Tony Philipsson
    Guru 227245 points

    Tony Philipsson said:
    For many applications in which accuracy is not as critical as power consumption, simply using the VLO as a source for a timer is a perfect solution.

    That was already known. The quesiton was whether the VLO is suitable if there is some concern for accuracy (even if it is not that high).
    My opinion is: no. But that's just an opinion and depends on how low you set your expectations. However, I cannot remember any thread here that has discussed this before.

     

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