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Clock multiplexer with automatic switchover

Mike Twieg
Intellectual 960 points
Other Parts Discussed in Thread: CDCUN1208LP, LMK05028, LMK05318, TLC2932A, CDC7005, LMK04832, LMX2491

In my design I want to distribute an externally provided clock, but switch to a backup on-board clock (both are 10MHz) if the external clock fails. I'd like the mux to make this determination without external control, and I'd like the transition to be smooth (no missing or runt pulses). The ICS581-02 basically does this, but unfortunately it's obsolete.

Does TI offer anything with this functionality? CDCUN1208LP looked close, but it seems that it doesn't have a PLL inside, and therefore the output will stall during switchover. 

  • 0
    TI__Mastermind 18070 points

    Hi Mike,

    I'm afraid that the only devices we have that support hitless switching (meaning that phase at output is not disturbed by the switchover event) are the DPLL chips such as LMK05318 and LMK05028.

    Regards,
    Hao

  • 0 in reply to Hao Z
    Intellectual 960 points

    Hi Hao, thanks for the reply.

    Another option that occurred to me is to use a VCXO as the on-board clock along with a separate PLL. When the external reference is present, the VCXO+PLL would lock to it. But when the reference is absent, the VCXO would free free run (+/-100ppm of drift is fine when the external reference is absent). This would totally avoid issues with switching clock sources.

    For this, I just need a simple PLL (PFD+charge pump, no dividers or VCO necessary). I searched around and found the TLC2932A, and it seems like just using its PFD section might do the trick. However it also looks like the TLC2932A is NRND. I can't find any other parts with a separate PFD I can use with an external VCXO though (except for maybe the CDC7005, but that's huge overkill for my application).

    Any alternative parts for a VCXO solution?

  • 0 in reply to Mike Twieg
    TI__Mastermind 18070 points

    Hi Mike,

    If CDC7005 is an overkill then I'm afraid that the other devices that I was about to recommend are even more overkill. So on top of my head, devices that support external VCXO are LMK04832 and LMX2491.

    The TLC device that you mentioned is not part of our product line. Can you maybe post a new question starting with that part number? Then it'll lead you to the corresponding product line.

    Regards,
    Hao

  • 0 in reply to Hao Z
    Intellectual 960 points

    Ok will do, thanks for your help!