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TM4C1231H6PGE: Oscillator issue

Janet Shen
Expert 7795 points
Part Number: TM4C1231H6PGE

Dear Champs,

The problem is the same with below E2E post: 

https://e2e.ti.com/support/microcontrollers/tiva_arm/f/908/p/536594/1954975#1954975

If customer want to know the voltage level spec on oscillator, is it mean VIHE?

Because I try to measure the Y1 input voltage level on DK-TM4C123G by runing hibernation projectg, it just is 485mV and not match VIHE. 

If you have any suggestion, please feel free to let me know.

Thanks a lot.

  • 0
    TI__Guru**** 183226 points
    Hi Janet,
    Which pin XOSC0 or XOSC1 are you measuring?
  • 0 in reply to Charles Tsai
    Guru 117855 points
    *** LIKE ***
    Two "Likes" awarded if the Oscillator's "Output Pin" would be noted. (that's the one which (should) be probed...)
  • 0 in reply to Charles Tsai
    TI__Expert 7795 points

    Dear Charles,

    We measure XOSC0.

    I also measure XOSC0 on DK-TM4C123G Y1.

    Here is measurement waveform on voltage of XODC0. Is it correct?

    If you have any suggestion, please feel free to let me know.

    Thanks a lot.

  • 0 in reply to Janet Shen
    TI__Guru**** 183226 points
    Hi Janet,
    Please measure the XOSC1 instead.
  • 0 in reply to Charles Tsai
    TI__Expert 7795 points
    Dear Charles,

    I have exchanged to measure XOSC1 pin, too. But it's the similar result.
    Thanks a lot.
  • 0 in reply to Janet Shen
    TI__Guru**** 183226 points
    Hi Janet,
    Reading the Vih parameter description again, it is for CMOS input as if you are applying a clock input via a function generator. The parameter value is similar to Vih for ordinary CMOS input. The parameter is not for condition with external crystal connected.
  • 0 in reply to Charles Tsai
    Guru 117855 points

    Hi Charles,

    Most such oscillator input circuits are highly sensitive - the act of "imposing external measurement probes" - if done inexpertly - can be expected to impact the oscillator.

    Note too - poster does not provide "justification" for such "oscillator examination."

    Might a safer and preferred method be to instead, "Output a replica of the MCU's System Clock" and use that as a (superior) means to confirm oscillator's operation & accuracy? (i.e. there is NO disturbance to the oscillator's critical input circuit via this method and the "performance" of the oscillator is revealed!)

  • 0 in reply to cb1_mobile
    TI__Guru**** 183226 points
    Hi cb1,
    Thanks. The passive probe itself would have put some capacitance to affect the measurement. The recommendation will be to use active probe for such type of measurements. I think Janet's original question is not about the frequency of the hibernate oscillator but about the input voltage level requirement with the "external crystal" connected for which the table for hibernate oscillator input characteristic did not specify.
  • 0 in reply to Charles Tsai
    TI__Expert 7795 points
    Dear Charles,

    Yes, my problem is is that "the input voltage level requirement with the "external crystal" connected for which the table for hibernate oscillator input characteristic did not specify".

    And as you said, VIH is the input voltage when using external clock generator, not external crystal, right?

    So that, is this problem actually a concern? Or user do not care about the input voltage level with external crystal?
    Because the function is working on hibernation mode, but customer need our confirmation for input voltage level range of external crystal.

    Please feel free to let me know any suggestion.
    Thanks a lot.
  • 0 in reply to Janet Shen
    TI__Guru**** 183226 points
    Hi Janet,
    Yes, my interpretation that the specified parameter is not for input with connected crystal. With external crystal it just needs a bit of energy to excite the opamp to kick start and feedback the oscillation.
  • 0 in reply to Charles Tsai
    TI__Expert 7795 points
    Dear Charles,

    Understood.
    So that, we do not have any limitation or need any input voltage on external crystal, right?
    Customer only look at ESR/C1 & C1 on external crystal, and we do not have other more limitation when using external crystal. "It just needs a bit of energy to excite the opamp to kick start and feedback the oscillation" and I think this drive energy that need to ask crystal vendor. How about you think?

    If this summary is correct, I will explain it to customer and update their feedback if they have more concern.
    Thanks a lot.
  • 0 in reply to Janet Shen
    TI__Guru**** 183226 points
    Hi Janet,
    Normally there is no Vih specified for external crystal. The stability, startup time over temp/voltage is very dependent on what load capacitors are used. I will encourage each customer to consult the crystal vendors for validation.
  • 0 in reply to Charles Tsai
    TI__Expert 7795 points
    Dear Charles,

    Thanks for your help.
    If customer still have other concern, will update the status. Or this issue will be closed.