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Original question:

CC2340r5 : internal 32.8 kHz RC Oscillator (LFOSC)

CC2340R5: Improve accuracy of LFCLK derived RTC when using LFOSC as source

Stefan Spuhler
Prodigy 35 points
Part Number: CC2340R5

Hello

We are using the internal 32 kHz RC Oscillator (LFOSC) as a source for the low frequency system clock (LFCLK).

In the datasheet I read, that the accuracy of the LFCLK - derived Real Time Clock can be improved using the TI-provided power driver.



Could you please help me understand, how this is done?

From the API of the CC2340R5 Power driver



I assumed, that it might be PowerLPF3_initHFXTCompensation / PowerLPF3_enableHFXTCompensation / PowerLPF3_disableHFXTCompensation.
In this case, I could enable the RF Temperature Compensation in the .syscfg file of my project.



But since the function descriptions do not mention any relation to the LFCLK, I suspect this is not correct?

Thank you very much in advance for any help you might be able to provide.

  • 0
    TI__Guru**** 217417 points

    Hello Stefan,

    I agree that the Datasheet footnote is not related to the *HFXTCompensation APIs currently in the Power TI Driver.  It could instead be alluding to CLK_01 errata (linked to the Q1 version since the CC2340R5 errata has not yet been updated) which as mentioned will be addressed in the v8.10 SDK which will release early Q2 of 2024.  I will ask the TI Drivers team for further comments

    Regards,
    Ryan

  • 0 in reply to Ryan Brown1
    Prodigy 35 points

    Hello Ryan

    Thank you for your answer.
    I am very glad about any comments that the TI Drivers team might be able to provide.

    As the footnote is not related to the *HFXTCompensation APIs, is there is currently anything we can do to improve the accuracy of the LFCLK-derived RTC? Or might there be functionality added to the TI-provided power driver sometime in the future that we then will be able to use?

    Thank you for providing the errata. Currently we plan to use peripheral role but might need central role in the future. If I understand the errata correctly, this will not be possible with the workaround and we might need to add an external crystal? We only need to act in central role very briefly and I am wondering, if it would be a solution to simply prevent the chip from sleep during this period?

    I still struggle to understand when and how exactly the LFOSCs inaccuracy is being compensated. Could you please tell me if there is any documentation on the subject that I failed to find?

    Thank you in advance and best regards
    Stefan

  • 0 in reply to Stefan Spuhler
    TI__Guru**** 217417 points

    Thank you for waiting.  For technical reasons it was found that improving the accuracy of the LFCLK-derived RTC, and thus the footnote will be removed from the datasheet.  It is recommended to use an external 32 kHz crystal (LFXT) as the source of the LFCLK for the most accurate RTC.

    The TI Driver team is providing a workaround different from that described in the footnote involving window widening, although this will only support BLE peripheral and not central when using LFOSC.  The details will be made available when the software solution is released.  I've only learned about this is the past day.

    Preventing the chip from sleeping is not a viable workaround for the central role.  I cannot determine from your description of central role usage/brevity whether it is a concern.  You may consider posting a separate thread which discusses your BLE needs in further detail.  Once again the external crystal is recommended for the most robust solution.  

    Regards,
    Ryan

  • 0 in reply to Ryan Brown1
    Prodigy 35 points

    Hello Ryan

    Thank you for your answer.

    So if I understood correctly, there is nothing we can/have to do software-wise about the accuracy of the LFCLK-derived RTC. I did not fully understand the workaround but I guess it will be clearer once the software solution is released.

    As of now we do not have an external crystal in our hardware design. We will do some measurements and if required open a new thread concerning the central role as you suggested.