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CC2640R2F: 32K Crystal not work on CC2640R2F

Yunhui Li
Prodigy 30 points

Part Number: CC2640R2F

Hi everyone,

I have a custom board which uses CC2640R2F as main MCU. The problem is that I can see the device advertising but cannot connect. When clicking Connect, the connection was lost and the device was advertising again. After digging in for a while, I found the 32K Crystal was not running. I couldn't see sine wave on oscilloscope, only DC bias voltage. And I could see sine wave on my Launchpad. Here is my test condition.

  • I am using Simple Peripheral as my starting point.
  • The circuit of CC2640R2F is referred to CC2640R2F LAUNCHPAD design.
  • XOSC_LF_EN bit of STAT0 is 1

Any suggestion is helpful.

Yunhui

 

  • 0
    TI__Guru** 106705 points
    Hi Yunhui,

    Good debugging so far!

    Can you provide the specs of the crystal you are using as well as the schematic and layout (only the crystal section is sufficient)?

    Are you running exactly the same SW on the LaunchPad as on your HW?

    Cheers,
    Fredrik
  • 0 in reply to Fredrik K
    Prodigy 30 points

    Hi Fredrik,

    Thanks for reply.

    The crystal we are using is EPSON's FC-135 32.768KHz ±20ppm 12.5pF and load capacitors are 12pF, which I think is the upper limit of CC2640R2F.

    The schematic is the same as Launchpad. Only difference is the value of load capacitors.

    For layout, it is about 4mm between CC2640R2F and 32K Crystal.

    For SW, they are the same on the Launchpad and my HW. Both are Simple Peripheral (simplelink_cc2640r2_sdk_1_50_00_58)

    Best,

    Yunhui

  • 0 in reply to Yunhui Li
    TI__Guru** 106705 points
    Hi Yunhui,

    The layout looks good, no issues there as far as I can see.

    The load capacitors are incorrect though. Since the crystal expects 12 pF your load caps should be approximately twice. This is because from the crystal point of view the caps are placed in series. Try changing the value to 22 pF.

    Cheers,
    Fredrik
  • 0 in reply to Fredrik K
    Prodigy 30 points
    Hi Fredrik,

    Thanks for your suggestion. Will try it later since I don't have proper caps in hand.

    Best,
    Yunhui