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Original question:

LMK05318B: The allowed frequencies for the XO input.

LMK05318B: EVM kit - XO frequency

Bruno B
Expert 1290 points

Part Number: LMK05318B

Tool/software:

Hi community,

We noticed that when using the 48.0048MHz XO the wizard did not set the XO doubler, while using X0=32.768MHz the internal doubler was used.

.Is there a reason to this ? I previously understood that the greater frequency, the better . therefore why not use the doubler for 48.0048 use case ?

Since 48.0048MHz may not be the most available value, Is it still OK to use the more standard 32.768 MHZ ?

With best regards,

Bruno

  • 0
    TI__Genius 17025 points

    Hi Bruno,

    It is true that a higher phase detector frequency results in lower APLL noise. The wizard does not enable the XO double for the 48 MHz frequency because the maximum phase detector frequency is 80 MHz per datasheet specification.

    It is okay to use the 32.768 MHz and you can enable the doubler with that frequency (PFD with doubler = 65.53600 MHz).

    Alternatively, a 48 MHz XO input frequency with the doubler disabled can be used as well. This is a commonly tested and recommended XO frequency for the LMK05318B.

    Also, are you enabling the DPLL (which means the PRIREF/SECREF inputs are used) in your design?

    Regards,

    Jennifer

  • 0 in reply to Jennifer Bernal
    Expert 1290 points

    Hi Jennifer,

    Thank you for your answers.

    • Yes we are going to use DPLL with 10MHz  PRIREF input, aiming to produce 100MHZ, and 156.25 MHz outputs

    I also have another question regarding PRIREF/SECREF input : is there any problem if the input contains some duty cycle modulation ?

    With best regards,

    Bruno

  • 0 in reply to Bruno B
    TI__Genius 17025 points

    Hi Bruno,

    Can you please specify the duty cycle percentage?

    Regards,

    Jennifer

  • 0 in reply to Jennifer Bernal
    Expert 1290 points

    Hi Jennifer,

    The DC is either 50%, 25% or 75%

    Regards

  • 0 in reply to Bruno B
    TI__Genius 17025 points

    Hi Bruno,

    That is fine. The LMK05318B has an amplitude detector as well as early and late window detectors to validate the PRIREF/SECREF input.

    Make sure the PRIREF/SECREF inputs meet the amplitude and slew rate specification listed in the datasheet.

    Regards,

    Jennifer