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OPT3101: Frequency calibration related

dongming cui
Prodigy 160 points
Part Number: OPT3101

Hi Alex,

I have a few questions for external clock frequency calibration, as follows:

1、After using an external clock to perform frequency calibration, how many ppm will the frequency error be? Does the selected external clock frequency affect the calibration effect? If you want to make the calibration effect good, is there a recommended frequency?

2、The software development of the host computer is based on the SDK( version is 0.8.). Codes related to frequency calibration have been added to the initialization code（OPT3101_configuration.cpp）. code show as below:

Note: The external reference frequency is 1MHZ

this->reg.en_cont_fcalib = 1; // //Enables continuous frequency calibration
this->reg.start_freq_calib = 1; // //Starts the frequency calibration block
this->reg.en_floop = 1; // //Enables the frequency correction loop
this->reg.en_auto_freq_count = 1; // //Enables automatic frequency count
this->reg.en_freq_corr = 1; // //Enables digital frequency correction
this->reg.sys_clk_divider = 5; // //Divider for system clock
this->reg.ref_count_limit = 20480; // //Counter limit
this->reg.gpio2_obuf_en = 0; // //Disables output buffer on GPIO2
this->reg.gpio2_ibuf_en = 1; // //Enables ref clock input of GPIO2

Best,

Dongming.cui

  • 0
    TI__Mastermind 49956 points

    Hello Dongming,

    I wanted to let you know that the device expert is out of the office, he should be able to get back to you in a couple of days.

  • 0 in reply to Praveen Aroul
    Prodigy 160 points

    Hi,Praveen Aroul

    Ok,thank you for your notification!

    Best,

    Dongming.cui

  • 0 in reply to dongming cui
    TI__Mastermind 25895 points

    Hi Dongming,

    1. This will be negligible. As long as you can bring the SYS_CLK_DIVIDER value close to the reference frequency this will work well. For example with the default of 10, system clock = 40 MHz / 2^10 = 39.0625 kHz to bring close to 32.768 kHz. This is basically setting your LSB resolution for the correction so it does not matter which frequency you choose to use.

    2. This looks good.

    Best,

    Alex

  • 0 in reply to Alex Bhandari-Young
    Prodigy 160 points

    Hi Alex,

    Thank you very much for your reply.

    1、Use the same external clock (frequency 1M) to perform frequency calibration on the two OPT3101, and observe the modulation waveforms of the two OPT3101 TX terminals through an oscilloscope. From the modulation waveforms, it can be seen that the frequencies of the two waveforms are inconsistent, about 3%. One of the OPT3101's TX waveforms has a lower modulation frequency than 10M. Is this situation normal? If it is too normal, what is the cause and whether it will affect the ranging accuracy. If it's not normal, can I analyze where my mistake is.

    Note: I am worried that there is a problem with the external clock source used for the test, and the measurement is fine. In addition, I adjusted the frequency of the external clock to 32768HZ, 60000HZ, 300000HZ, 1M, 2M. The modulation frequency of the TX end of each OPT3101 differs by about 3%.
    2、Summarize question 1, whether the internal clock of OPT3101 can be accurately calibrated to 40M by external clock calibration.
    Best,
    Dongming.cui
  • 0 in reply to Alex Bhandari-Young
    Prodigy 160 points

    Hi  Alex,

    Hope to get a reply from Alex. Thank you!

    Best,

    Dongming.cui

  • 0 in reply to dongming cui
    TI__Mastermind 25895 points

    Hi Dongming,

    The OPT3101 does not update its clock frequency based on the external input so that you observe 3% variation on the tx frequency is okay. The internal clock frequency is compared with the reference and the outputted data is corrected based on any error due to drift of the internal clock with reference to the external clock. What you would really want to be checking is the accuracy of the output data of the OPT3101. Why so much focus on checking the frequency compensation here? Is the data output from the OPT3101 having some error or was this a check to make sure the compensation was applied?

    Best,

    Alex

  • 0 in reply to Alex Bhandari-Young
    Prodigy 160 points

    Hi Alex,

    Thanks a lot for Alex's reply, and answered my doubts in using OPT3101!

    The reason why I asked the previous question is to understand the OPT3101 more deeply so that the calibration can be done better in the future.

    Best,

    Dongming.cui

  • 0 in reply to dongming cui
    TI__Mastermind 25895 points

    Hi Dongming,

    Understood, glad I could help!

    Best,

    Alex