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CC2651R3SIPA: How to set “Custom RTC XOSC Coefficients” in Sysconfg.

K.Z
Expert 2490 points
Part Number: CC2651R3SIPA
Other Parts Discussed in Thread: SYSCONFIG

Tool/software:

Hi All,

I would like to oscillate a crystal unit, input the oscillation signal, and use it for an RTC inside a microcontroller, using the circuit diagram as shown in the attached figure (crystal unit circuit).


In this case, there is an item “RTC XOSC LF Compensation” on the syscfg setting,
I believe this is to compensate the frequency of the external crystal oscillator.

By checking the “Custom RTC XOSC Coefficients” setting, you can set the coefficients required for the correction.
How do I set the value of this coefficient?

I looked at this document but could not understand it.

Crystal Oscillator and Crystal Selection for the CC13xx, CC26xx, and CC23xx Family of Wireless MCUs (Rev. K)

Best Regards,

Ito

  • +1
    TI__Guru**** 171094 points

    Hi Ito,

    K.Z said:
    I would like to oscillate a crystal unit, input the oscillation signal, and use it for an RTC inside a microcontroller, using the circuit diagram as shown in the attached figure (crystal unit circuit).

    The device already has an internal RTC, so it should be simply a matter of following the reference design to have this function working properly. Because our reference design uses external capacitors, the internal compensation can be disabled in the sysconfig. Your drawing shows two 15pF load capacitors, which is larger than our reference design, therefore this might have some negative effects as covered by the application note you mentioned (section 3.3). Ultimately the characteristics of the crystal that you are using in your design will be the factor that defines these values. 

    K.Z said:
    I believe this is to compensate the frequency of the external crystal oscillator.

    You are correct

    K.Z said:
    How do I set the value of this coefficient?

    Section 3.3 of the application note you sent mentions a method to evaluate the coefficients of this capacitor, which is present on section 6 of the application note below.

    https://www.ti.com/lit/pdf/SWRA640

    Hope this helps,

    Rafael