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LMK04828: a question about external SYNC signal asynchronous to LMK04828B

shen jie
Prodigy 245 points
Part Number: LMK04828
Other Parts Discussed in Thread: LMK04832

Hi, 

I check the datasheet of LMK04828B, and find external SYNC signal will be sampled one time by VCO as below figure shown. When I want to synchronize output of  two LMK04828Bs on the different board, if SYNC signal is asynchronous to LMK04828B but have alignment edge,  does non-deterministic sample ( potential first time sample don't satisfy the setup and hold time requirement) make different arrive time of SYNC signal to internal divider, right? Is it will cause sometimes output clock phase of  two LMK04828Bs on the different board not be aligned(one vco period offset), or sometims will be aligned fully?

Thanks in advance!

Best regards!

Jason

  • +1
    TI__Expert 8421 points

    Jason,

    Exactly, you are correct. Because the SYNC signal is asynchronous the first sample will not satisfy the setup and hold time requirements.  This will lead to the output clock not being totally aligned as it is dependent on the VCO phase at the moment of the SYNC signal.

    Regards,

    Will

  • 0 in reply to William Pfennigwerth
    Prodigy 245 points

    Hi, William

    Very nice to see you on this thread.!

    Comparing to LMK04828, LMK04832 has not this D flipflop to sample input asynchronous SYNC signal. Does it mean alignment SYNC signal will arrive to internal divider of respective LMK04832 on the different board simultaneously, and lead to the output clock being totally aligned?

    Thanks in advance!

    Best regards!

    Jason

  • 0 in reply to shen jie
    TI__Expert 8421 points

    Jason,

    No, this flip flop is used only when re-clocking.  Re-clocking will align the sync pulse with the phase of the VCO generated SYSREF signal.  If the generated SYSREF phases of both devices are both in phase with each other (which could be the case in 0-delay mode) then re-clocking would lead to the output clocks being made in SYNC.  As you can see this functionality is also present on the LMK04828. 

    Regards,

    Will

  • 0 in reply to William Pfennigwerth
    Prodigy 245 points

    Hi, William:

    Two questions:

    1. As your said in above post, I'm very curious why the re-clocking function is mentioned very rarely in the datasheet? In the datasheet,  SYSREF_MUX is almost selected as Normal SYNC in the many examples, not Re-clocked. Is this re-clocking  only used for special application as below figure.2 shown?

    Figure.1

    Figure.2

    2. Another question: Maybe you don't get my meaning in my above post. There is no this D flipflop in the LMK04832 comparing to LMK04828 if SYSREF_MUX is selected as Normal SYNC, that mean  input asynchronous SYNC signal will arrive at internal divider directly and reset them. If only the SYNC signal to the different LMK04832s are aligned ,  output clock of two LMK04832 will be totally aligned with each other?

    Thanks in advance!

    Best regards!

    Jason

  • 0 in reply to shen jie
    TI__Expert 8421 points

    Jason,

    I will get back to you by the end of the week. 

    Regards,

    Will

  • 0 in reply to William Pfennigwerth
    TI__Expert 8421 points

    Jason,

    Sorry for the delay.

    1. Re-clocking is not used very often as it is only needed for specific multi-device synchronization scenarios.  Reclocking would be used to retime the SYNC across many devices, or when using SYREF in Zero Delay Mode.  This would be because retiming against the SYSREF in Zero Delay Mode, there would be an established phase relationship between SYSREF and the input and synchronizing the dividers with this signal would be easier than with the GHz level VCO which could violate the setup and hold time requirements.  

    2. The LMK04832 datasheet is incorrect, there is actually another flip-flop present for re-clocking just as in the LMK04828.

    Regards,

    Will

  • 0 in reply to William Pfennigwerth
    Prodigy 245 points

    Hi, William:

    As below figure shown, according to my application which need to synchronize two LMK04828s on the different board, should I select Re-clocking to replace Normal SYNC for SYSREF_MUX?

    Thanks in advance!

    Best regards!

    Jason

  • +1 in reply to shen jie
    TI__Expert 8421 points

    Jason,

    Re-clocking is especially useful for when you need to SYNC your outputs with your SYSREF pulse, which in turn would be related to the input signal in ZDM.  So you have more than just SYSREF signals being generated from your LMK04828 boards then yes, you should use re-clocking, otherwise it is not necessary.  

    Regards,

    Will