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Original question:

LMK04610: LMK04610

LMK04610: LMK04610: using PLL2 without input reference clock

Roel
Prodigy 200 points
Part Number: LMK04610
Other Parts Discussed in Thread: TS5A21366

Please advice. I would like to use the PLL2, driven by the VCXO, but with a frozen control voltage. I have some usecases where I don't have stable CLKin0 or CLKin1, and still I would like to be able to generate CLKoutX clock signals. What is the best way to do it?

  • 0
    TI__Guru** 111890 points
    Hi Roel,

    So in your application, you don't need PLL1?
    If you are in PLL2 only mode, then PLL2 needs a stable reference clock. You can use a VCXO with fixed control voltage or you can use a XO or TCXO.
  • 0 in reply to Noel Fung
    Prodigy 200 points

    I need the PLL1 only in specifc use cases, e.g. when an external clock is available. If not, I want to be able to startup the PLL2 without an input reference clock. 

    I was looking in the datasheet for a way to bring the VCXO to be controlled by a fixed voltage. The holdover mode might be suited, but direct control would be better. However I could not find a way to manually control that voltage. Other PLL's like the LMK4806 have an option for this (MAN_DAC). I could not find it for this PLL. 

  • 0 in reply to Noel Fung
    Prodigy 200 points
    See my reply
  • 0 in reply to Roel
    TI__Guru** 111890 points
    Hi Roel,

    Right, unfortunately, LMK04610 does not provide manual DAC tuning.
    Since you know when PLL is available, maybe you can add an analog switch (TS5A21366) to the Vctl pin of the VCXO to mux between a fixed voltage and CTRL_VCXO (pin 44) of LMK04610.
  • 0 in reply to Noel Fung
    Prodigy 200 points
    "does not provide manual DAC tuning", that was my conclusion also from the datasheet (and hoped you would say there are some hidden registers to do so..). However, still I don't know if I need manual DAC tuning. What other way would allow bringing the VCXO to a fixed voltage, e.g. by driving it to holdover mode etc.?
  • 0 in reply to Roel
    TI__Guru** 111890 points
    Hi Roel,

    You can put it in holdover mode but the VCXO control voltage may not be deterministic. Depending on the device status before you put it in holdover mode, the control voltage may be the last (PLL1) locking voltage; the rails (if PLL1 was not locking); or some other voltages.
    If you want to have a fixed, deterministic voltage whenever PLL1 is not used, my previous suggestion seems to be the simplest method.
  • 0 in reply to Noel Fung
    Prodigy 200 points

    Thanks Noel. Is the option "PLL1_STARTUP_HOLDOVER_EN" not suitable for this ? It says "PLL1 Start-up in Holdover. When PLL1_HOLDOVER_FORCE is 1 PLL1 enters holdover mode immediately on start-up." The datasheet suggest "CRTL_VCXO =VDD/2" when enabled.

  • 0 in reply to Roel
    TI__Guru** 111890 points
    Hi Roel,

    This option applies to the very first Vcc power up. If you are already up and running, this option cannot be used.