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LAUNCHXL-CC26X2R1: accuracy of RTC on cc26x2r

kazola
Guru 18605 points
Part Number: LAUNCHXL-CC26X2R1


Dear all,

We want to use the CC26x2r RTC in a BLE project. We already have our tests for SET and GET time and seems to work. We only have two questions.

  1. How accurate is this RTC? How off will be after one day / week / month? 
  2. I say a couple threads where keeping time with this RTC is stated as "not recommended". Why is that?

Thanks!

  • +1
    TI__Guru**** 218777 points

    Hi kazola,

    1. The RTC is derived from the SCLK_LF which can be clocked from the internal 32 kHz RC oscillator or the external 32 kHz crystal oscillator.  The accuracy of the RTC can be improved by measuring RCOSC_LF relative to XOSC_HF and compensating for the RTC tick speed, functionality which is available through the TI-provided Power driver.  The  RCOSC_LF has an accuracy of ± 500 ppm (varies by part) with a temperature coefficient of 50 ppm/°C.  500(accuracy)x24(hours)x60(minutes)x60(seconds)/1000000(pmm)=43.2 seconds shift per day at maximum RCOSC_LF (in)accuracy without compensating for temperature.  Thus RTC accuracy will depend on oscillator source, clock compensation, and temperature variance.
    2. The RTC is not active during Shutdown or power-off states, and it can drift widely as shown above.  Thus a dedicated RTC component would be ideal.

    Regards,
    Ryan

  • 0 in reply to Ryan Brown1
    Expert 2160 points

    Hi Kazola and Ryan,

    I would say:

    1. For accurate RTC timing source SCLK_LF from the 32 kHz crystal oscillator. The accuracy will then be exactly the same as the crystal oscillator which is fully in your control. If even more accuracy is needed SCLK_LF can also be sourced from a 32 kHz TCXO through an IO pin.
    2. If Standby mode is the lowest power mode used the RTC is running continuously. 

    /F