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cc430 different voltages leads to slightly different frequencies. why?

Dragos Ionescu
Intellectual 680 points

Hi,

I'm using two EM430F6137RF900 boards with crystals installed and powered with different voltages. The first one is battery powered (3v) and the second one is powered from a regulated power supply  with 2V.

The PMMCOREV  is set to 1

I mapped the Timer_A (sourced from ACLK->XT1) compare output to a port  where I can visualize it by using a scope. Same firmware is running in both devices.

By making a comparison between signals it is obvious that when the boards are powered with different voltages, the frequency of the timers becomes slightly different making very difficult to maintain a synchronization between them.

If I increase the voltage from the power supply the drift between devices decrease until becomes 0.

Is this a normal behavior? Why?

Thanks

  • 0
    Guru 46710 points

    Dragos Ionescu said:
    I mapped the Timer_A (sourced from ACLK->XT1)

    You mean XT1 -> ACLK? Do you use 32KHz crystal on XT1? If not then what you use?

    In case you don't use crystal then RC oscillators have voltage-dependent drift. Oscillator temperature and voltage drifts are specified in the datasheet.

  • 0 in reply to Ilmars
    Intellectual 680 points

    I'm using 32KHz crystals on both. sorry for the wrong arrow direction.

  • 0 in reply to Dragos Ionescu
    Guru 46710 points

    Maybe it's not XT1 oscillator but REFOCLK your ACLK is running from? Check oscillator fault flags, look how to start XT1 correctly and ensure that it's running. Also it's suggested to check ACLK frequency by routing it to ACLK output pin.

  • 0 in reply to Dragos Ionescu
    Guru 227245 points

    Dragos Ionescu said:
    I'm using 32KHz crystals on both.

    both?

    Well, does the crystal run properly, and if it does, did you clear the fault flags? if not (e.g. due to a fault in a different active oscillator), XT1 won't be used and the default fallback to REFO is active (which drifts with voltage). Be sure that you have cleared OFIFG (and it stays clear) so the crystal is not only running but also used.

    8hint: if you short the crystal pins during operation, you'll probably see no change in the output frequency, A clear sign that you were not using the crystal at all but REFO.

  • 0 in reply to Jens-Michael Gross
    Intellectual 680 points

    The problem dissapeared somehow by itself, I m sure that it was my mistake, not the crystals but something else.

    Thanks for the tip,

    Dragos

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