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LMK03318: Clock input auto select

Graeme Parker
Expert 1265 points
Part Number: LMK03318


Hi,

We have a design that uses multiple LMK03318.

There is a TXC 7M48072001 crystal connected to the SECREF pins, and an LVDS clock source (currently disabled) connected to the PRIREF pins.

If the LMK is set to look only at SECREF then the output clocks are as expected.

If the LMK is set to automatic mode where it checks each input in turn, then it won't stay on the SECREF crystal source. It will output the correct clocks for a short period, then it will switch to the PRIREF input, and then back again to SECREF. This cycle repeats.

What would cause the LMK03318 not to settle on the SECREF as a clock source?

  • 0
    TI__Mastermind 18800 points

    Hi Graeme,

    According to the datasheet, the Automatic mode prefers the PRIREF input over SECREF. My understanding is that it will only use SECREF if PRIREF is detected as unavailable, and will switch to PRIREF as soon as possible. So in principle it's not supposed to settle on SECREF. However am I understanding correctly that PRIREF is completely off during your tests? In that case it shouldn't be switching away from SECREF, you can give us some more details and we will look into it.

    Best,

    Evan Su

  • 0 in reply to Evan Su
    Expert 1265 points

    Thanks for the reply. That was my understanding as well, that if there was a PRIREF clock, then it would settle on that.

    The PRIREF, in my case, is driven by a clock distribution buffer, 8P34S1102N. The clock buffer output is ac coupled to the PRIREF. The input to the clock buffer is ac coupled from another source. This differential source does not have a clock on its output, but has static high & low values.

  • 0 in reply to Graeme Parker
    TI__Mastermind 18800 points

    Hi Graeme,

    Thanks for the info. I looked into the 8P34S1102N and it doesn't seem to have a pin option to enable or disable the outputs. Is that device powered down completely, or alternatively is the high/low differential source powered off? I'm trying to think of ways that there could be unintended activity on PRIREF that would get the LMK03318 to switch.

    Best,

    Evan Su

  • 0 in reply to Evan Su
    Expert 1265 points

    Hi. The 8P34S1102 isn't powered off, but the device that is driving it is. So my supposition is that the diff clock inputs to the 8P34S1102 are both at the VREF voltage. This in turn may cause some activity on the outputs and hence some signal may be seen at PRIREF.

  • 0 in reply to Graeme Parker
    TI__Mastermind 18800 points

    Hi Graeme,

    That could be a cause of the problem, You could try powering down the 8P34S1102 (I expect that if there's no power to the output driver, there would be no possible signal) and see if the LMK03318 still tries to switch to PRIREF. The 8P34S1102 apparently has fail-safe operation, so it can handle up to 4.6 V of DC voltage on the input pins when it's off in case you can't turn off the VREF at its inputs. Let me know how it goes or if you have other ideas.

    Best,

    Evan Su