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CC2652RB: The crystal clock signal has a 50-Hz disturbance

Cherry Zhou
Mastermind 22235 points
Part Number: CC2652RB

Hi team,

Here's an issue from the customer may need your help:

In lab tests of board frequency deviation, a 50-Hz line frequency disturbance was found in the 32.768-kHz crystal, as shown in the following figure:

PCB layout is as the following figure:

1) Is the case because the RF link and crystal are too close together? Is there any other way to solve this problem than increasing the distance between the two? How much does the issue affect Bluetooth communication?

2) CC2652RB has Baw inside, but in Bluetooth configuration, it's not able to divide by Baw, only RCOSC_LF is the clock source. Is it possible to switch to Baw divide?

Thanks.

Best Regards,

Cherry

  • 0
    TI__Guru**** 202106 points

    Hi Cherry,

    Our HW experts will investigate the first point.  For the second, you can change the LF Clock Source to be Derived from HF XOSC which uses the Internal High Precision Oscillator as the HF Clock Source.  You can review the CCFG registers for more details.

    Regards,
    Ryan

  • 0 in reply to Ryan Brown1
    TI__Guru**** 168357 points

    Cherry,

    It is difficult to evaluate how the design is done via a photograph. We usually recommend customers to follow both the datasheet guidelines as well as the recommendations in the application note SWRA640.

    Crystals have spurious signals which are highly dependent on the crystal itself. I haven't seen a 50Hz spurious caused by the radio itself, therefore can your customer be sure the probing itself is not causing excessive loading on the crystal? That might change its behaviour and characteristics.

    If there are no identifiable characteristics, the customer may want to submit a design review request at: https://www.ti.com/tool/SIMPLELINK-2-4GHZ-DESIGN-REVIEWS

    I will think of any additional aspects and report back in case I find anything.

    Best regards,

    Rafael