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MSP430F5335: RTC_B RTCCTL2 Register setting value

user5344581
Prodigy 160 points
Part Number: MSP430F5335

Hi,

Would you please teaching me about  RTCCTL2 Register setting value.

There is the explanation about RTCCALS & RTCCAL in slau208q.pdf.

Q1. If setting value is RTCCALS = 0 and RTCCAL=1, Is the result -6ppm(=-4ppm + (-2ppm))?

      -4ppm: The minimal calibration by setting RTCCALS = 0 

      -2ppm : the decreasing value by RTCCAL= 1

       My understanding is that there is offset -4ppm. Similarly, in the case RTCCALS = 1, my understanding is that  there is offset +8ppm.

       Is the my understanding correctly?

 

Q2. If absolute error (ppm) = |106 (fMEASURED – fRTCCLK)/fRTCCLK| = 0ppm, what value are setting value of RTCCALS & RTCCAL? 

       My understanding is that if the calibration is not necessary, it must prohibit setting to RTCCTL2 Register after power on reset. 

      Is the my understanding correctly?

 

 

Best Regards,

  • 0
    Prodigy 160 points

    Hi,

    There is the explanation about RTCCALS & RTCCAL in slau208q.pdf.

    23.2.5 Real-Time Clock Calibration

    Best Regards

  • 0 in reply to user5344581
    TI__Mastermind 32900 points

    As you have solved the problem, I will close this thread.

    Eason

  • 0 in reply to Eason Zhou
    Prodigy 160 points

    Hellow, Eason-san

    I have not solved My issue yet.

    I have no answer about Q1 and Q2.

    I need the answer about Q1 and Q2.

    Best Regards,

  • 0 in reply to user5344581
    TI__Mastermind 32900 points

    Hi，

    Please see the UG:

    Q1: RTCCALS = 0 and RTCCAL=1, Is the result should be -2.17ppm. But the see the note, it should be -4ppm

    Q2: After reset, RTCCALS = 0 and RTCCAL=0, the offset will be -4ppm.

    Normally, here is the performance from an crystal, it will have +-10ppm around full temperature. Consider the main application like e-meter, it require the PPM of RTC is 5.78ppm. RTC_B is just fit it.

    Eason

  • 0 in reply to Eason Zhou
    Prodigy 160 points

    Hi, Eason-san

    Thank you for your repling.

    I've more question.

    Q.3 : For example, assume that RTCCLK is configured to output at a frequency of 512 Hz. The measured
            RTCCLK is 511.9658 Hz. This frequency error is approximately 66.8 ppm too low. To increase the
            frequency by 66.8 ppm, RTCCALS would be set, and RTCCALx would be set to 15 (66.8 / 4.34).

             After calibration, How much(ppm) is the frequency error?

    Best Regards,

  • 0 in reply to user5344581
    TI__Mastermind 32900 points

    It is about 1.7ppm error.

    I think you must know that it is not a math question but an engineering problem. What I explain is all based on UG, which helps you to understand how to use RTC_B calibration and evaluate if RTC_B can meet your accuracy requirement. And see from UG, it says it is about 4.34 per bit and will change on different device, but I don't find the range on datasheet.

    I really advise you to adjust it according to a real project.

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