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LMK05318: Questions of LMK05318

Jimmy Lai
Expert 6845 points
Part Number: LMK05318

Dear Team,

Please confirm whether the OCXO is connected to PRIREF_P/N、SECREF_P/N or XO_P/N。

In my opinion, the 1588 function OCXO is provided for DPLL use, not for APLL use, so it should be connected to PRIREF_P/N or SECREF_P/N. I don't know if my understanding is correct.

Many Thanks,

Jimmy

  • 0
    TI__Genius 14445 points

    Hi Jimmy,

    Generally any kind of XO/TCXO/OCXO is supposed to be connected to XO_P/N. In a typical application, PRIREF/SECREF are incoming clock signals that are references to the DPLL circuit and the crystal oscillator is the reference to the APLL circuits. I have not seen any situation with a crystal on PRIREF/SECREF yet. The datasheet has more explanations of how the device is supposed to be operated.

    Best,

    Evan Su

  • 0 in reply to Evan Su
    TI__Expert 6845 points

    Hi Evan,

    Thanks for your great support.

    I know.In generally,obsrefclk is reference to DPLL.

    But if the synce recovery clock obsrefclk is gone, or the line is disconnected and obsrefclk cannot be recovered, which clock is referred to by dpll?

     

    In my opinion, 1588 synchronization should be maintained for some time if there is an abnormal line disconnection, and the DPLL needs to refer to a local high-precision clock, usually provided by the OCXO.

    This is also what I consulted microchip and IDT to confirm.

    Many Thanks,

    Jimmy

  • 0 in reply to Jimmy Lai
    TI__Genius 14445 points

    Hi Jimmy,

    The 05318 has a holdover function to help maintain the output when the DPLL reference clocks (PRIREF/SECREF) are gone. In this state the device looks at the XO/TCXO/OCXO attached at XO_P/N, so you are correct that it looks at a local high-precision clock from a crystal. It's just that this crystal is attached to XO_P/N and not PRIREF/SECREF.

    Best,

    Evan Su

  • 0 in reply to Evan Su
    TI__Expert 6845 points

    Hi Evan,

    We appreciated your great suppport.

    whether the OCXO clock input from XO_P is used by DPLL？

    Many Thanks,

    Jimmy

  • 0 in reply to Jimmy Lai
    TI__Genius 14445 points

    Hi Jimmy,

    Jimmy Lai said:
    whether the OCXO clock input from XO_P is used by DPLL？

    In normal operation the DPLL circuit follows PRIREF/SECREF and OCXO is only used by the APLL circuits. In holdover I think the DPLL looks at the OCXO, but I am not super familiar with how holdover works on a circuit level so I'm not 100% sure.

    I think the general idea for the customer to remember is that

    • Device always needs some kind of crystal attached at XO_P/N for the APLL circuits to work
      • Basic XO is cheap
      • TCXO, OCXO are more expensive
    • When you are using the DPLL, it is supposed to follow PRIREF/SECREF, which are reference clock signals
      • In this normal operating mode, using XO vs TCXO/OCXO does not make a big difference
    • If PRIREF/SECREF are lost, device enters holdover mode
      • In holdover mode the quality of the crystal is important
      • Best holdover performance with TCXO/OCXO

    Best,

    Evan Su