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Linux/BQ32002: RTC calibration fix

Deepanraj Anbarasan
Expert 1140 points
Part Number: BQ32002

Tool/software: Linux

Hi Team,

We are using TI BQ32002  RTC controller  in our design. Actually in our implementation we are getting a time lag when the device is powered off and turned ON after 24 hrs.

For this we calibrated the RTC by setting the register and found out the time drift was not observed after calibration.

Is it possible to compensate the calibration setting done through hardware ?

Thank you,

Deepanraj

  • 0
    TI__Genius 16845 points
    Hi Deepanraj,

    I don't fully understand your question but I hope I can provide some assistance here.

    In table 6.5 of the datasheet the pre-calibration accuracy of the RTC is given as +/-35ppm.
    35ppm * 3600sec/hr * 24hr = +/-3.03 seconds

    This corresponds to a daily time lag if you are using the same crystal, board, and capacitive loading. This gives an idea for the approximate daily time lag you may expect with this device prior to calibration. It is also important to recognize that each crystal is cut with some error that will likely need to be calibrated out. The crystal frequency is also impacted by temperature.

    To otherwise minimize calibration error on a hardware level mainly involves optimizing board design:
    -Take care to select a crystal that meets the datasheet requirements of 12pF capacitive loading. Additional loading can slow down the oscillator.
    -Minimize extraneous capacitance on the crystal pins by minimizing traces to OSCI, OSCO pins.
    -Board stackup can contribute to stray capacitance. Consider using a PCB cutout area underneath the crystal.

    Kind regards,
    Lane
  • 0 in reply to Lane Boyd
    Expert 1140 points

    Hi Lane,

    Actually what we wanted to understand is when we do calibration in the RTC there is no time drift.

    By calibration we are actually changing the ppm value of the crystal.

    So if we change the crystal to the corresponding ppm value will the drift be fixed ?

    Thank you,

    Deepanraj

  • 0 in reply to Deepanraj Anbarasan
    TI__Genius 16845 points
    Hi Deepanraj,

    Theoretically you are correct, but in practice you have to also take into consideration some other factors. Firstly, all crystals will have some frequency tolerance at the rated temperature. For example, KDS DMX-26S is the crystal used in the datasheet with a frequency tolerance of +/-20 ppm. Frequency will also be impacted by temperature and loading. Finally, the BQ32002 device itself will have some contribution to frequency error.

    Based on these factors, it isn't practical to compensate for drift without calibration.

    Kind regards,
    Lane
  • 0 in reply to Lane Boyd
    Expert 1140 points

    Hi Lane,

    According to DMX-26S the typical value of drive level is 1 uW. But in the crystal which we have used typical drive level is 0.1 uW.

    Will this difference cause any issue related to time drift ?

    Thank you,

    Deepanraj

  • 0 in reply to Deepanraj Anbarasan
    TI__Genius 16845 points
    Hi Deepanraj,

    You can measure the crystal if this is a concern. Measured frequency variation that exceeds the limitation of the datasheet may indicate that the selected crystal is being driven past the maximum drive level, which can cause reliability issues for a crystal.

    Kind regards,
    Lane
  • 0 in reply to Lane Boyd
    Expert 1140 points

    Hi Lane,

    Actually we have done a experiment by removing the load capacitance and checked the RTC time after 24hrs.

    It RTC time is proper and there is no drift.

    Please let us know what is the reason for this behavior. Is this because the crystal which we used is already having a internal load capacitance ?

    Thank you,

    Deepanraj

  • 0 in reply to Deepanraj Anbarasan
    TI__Genius 16845 points
    Hi Deepanraj,

    What external load capacitance was removed? What crystal are you using?

    A crystal will have an internal load capacitance specified in the datasheet. You can compensate with external load capacitance to match the required crystal load capacitance stated in the datasheet. Closely matching the total crystal load capacitance to the datasheet will minimize frequency error.

    Kind regards,
    Lane
  • 0 in reply to Lane Boyd
    Expert 1140 points

    Hi Lane,

    Thanks for the support.

    We have connected the load capacitance to the crystal and In our crystal datasheet Load capacitance has been mentioned as 12.5pF.

    Since this load capacitance is the required load for BQ32002 RTC. Is it proper to understand that no other load capacitance is required for the RTC ?

    Thank you,

    Deepanraj

  • 0 in reply to Deepanraj Anbarasan
    TI__Genius 16845 points

    Hi Deepanraj,

    No external load capacitance is require because the crystal load capacitance matches the crystal load capacitance specified in the datasheet.

    Kind regards,
    Lane