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Tuning 32.768KHz RTC for cC2541

Peter Borenstein
Mastermind 8695 points

I am attempting to use an oscilloscope with 2 Giga-samples per second to tune my 32.768KHz real time clock and ensure it is within +/-20ppm. The clock signal is buffered through a GPIO by using the PMUX register. The crystal is an EPSON MC-306; one of the recommended crystals from the TI wiki.

The problem is that every clock period seems to different. Some clock period are 30.507us and others are 30.523us. The crystal should have +/-20 ppm accuracy. The period should be between 30.5182us and 30.05170us I can't observe this accuracy because the periods vary too much. 

Is it normal for clock periods to vary so much? If so, what tools should be used to observe the frequency?

  • 0
    TI__Guru** 106705 points
    Hi Peter,

    I would recommend using a frequency counter to measure the 32 kHz clock.

    Cheers,
    Fredrik
  • 0 in reply to Fredrik K
    TI__Guru 56590 points
    Peter,
    I agree with Fredrik.
    Although the crystal is specified at 20 PPM accuracy, the jitter (flank to flank variation) might be larger than the rated frequency tolerance. By measuring with a frequency counter you will find the average period which should be within the tolerance given.
  • 0 in reply to Eirik V
    Mastermind 8695 points

    I received this reply from EPSON (manufacturer of the MC-306) "MC-306 is a passive device so jitter doesn’t apply. Jitter depends on his oscillation circuit or IC he is using the MC-306 to clock."

    I don't have a frequency counter so I used the watch dog timer which is driven by the 32.7K clock to toggle a GPIO at 2Hz. This means 16,384 clock cycles were in every period so I assume the jitter of every period cancels out..