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Original question:

CC2530: Problem with an external 32.768 kHz crystal oscillator

TM4C129ENCPDT: Problem with an external 32.768 kHz crystal oscillator

user4973896
Expert 1555 points
Part Number: TM4C129ENCPDT

As pic below ，I use an external 32.768khz  crystal。but when i  measure the xosc1 pin， the scope  is  below ，there is  a drop  on  the  scope，why？

Then i change another crystal ,change new 20pf load cap.even i change another tm4c129 ,i  find  the scope on  XOSC1  is good 

but the scope on xosc0   is  not  good as below

 you can see the Inconsistent amplitude，w'hy? can somebody smart answer me  quickly？ I AM SO HURRIED!

  • 0
    TI__Guru*** 133705 points
    Hello,

    What scope probes are you using the measure the signals? You need to be using special low impedance probes or else the probes may end up affecting the signal.

    The other possible cause of issue could be crosstalk, you mention a custom board and showed an image, but two things I can't see from that is where the two capacitors are, and if there may be any other signals on a different layer from top which could contribute to crosstalk inference.
  • 0 in reply to Ralph Jacobi
    Expert 1555 points

      top layer

     bottom layer

      GND LAYER 

      3.3VCC  LAYER

    as pic above ，the two caps are on bottom layer。there is no other high  speed  signal  around  the crystal ，there is only a reset signal  and  a vbat power signal 。so i don't know why？

    the probe  i use  as the pic below

    there is another doubt I  find when i  set the probe 10x ,as pic below ,i can detect the signal 。

    but when i use 1x， whatever i do，i can't detect the scope,as pic below.

    When  I set the probe  1M Ω  ,I can detect the scope  ，see pic below

    but When  I set the probe  50 Ω  ,I can not detect the scope  ，see pic below

  • 0 in reply to user4973896
    TI__Guru*** 133705 points
    Hello,

    Those look like standard issue scope probes, so you may have some difficulty getting fully accurate measurements from them due to the capacitance it will place on the crystal.

    Also that you have a 4-layer board is a bit of a concern for having the capacitors on the underside. 2-layer wouldn't be a problem, but having the GND and Vcc planes between could result in issues.

    I want to take a step back and also ask, are you just trying to confirm crystal operation, or are you having an issue with your application you think is caused by the crystal?

    Also I found a very thorough resource from the MSP430 team who very often deals with 32kHz crystal, Section 4 is about how to test a crystal oscillator in general and would be a good read: www.ti.com/.../slaa322d.pdf
  • 0 in reply to Ralph Jacobi
    Expert 1555 points

    I use DPO 3032 test the scope  of  32.768k  crystal，I find  when under all bandwidth  ，there is a  lot  of  noise  on the scope ，when  the bandwidth gets lower ，the noise  will  get  less。  so  i want to  know  is there  a  standard  that specify  the   bandwidth  when  test  the 32.768k scope？

  • 0 in reply to user4973896
    TI__Guru*** 133705 points

    Hello,

    user4973896 said:
    so  i want to  know  is there  a  standard  that specify  the   bandwidth  when  test  the 32.768k scope？

    I am not aware of any such standard, but I am not an expert with crystals.

    Honestly this thread as a whole is a bit difficult for us device experts to help with as it isn't clear what is causing the issue and there are doubts that the scope probe used is adding capacitance which is affecting the measurements.

    Have you tried reaching out to the crystal manufacturer to request their comments on your measurement techniques as well as the issue you are seeing? They might be able to very quickly resolve your issue in comparison.