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Use of JESD204B SYSREF in normal operation

Greg Hammerquist
Prodigy 20 points
Other Parts Discussed in Thread: ADS52J90, AFE5818

I'm curious what to consider regarding SYSREF once initial alignment is complete and you've entered normal mode.  One article indicated:

• Once alignment is completed, future SYSREF pulses may be used to check the alignment of the local frame and multiframe clocks.

• Since a periodic SYSREF signal acts as a sub-harmonic clock of the converter sampling clock and may have spurious effect on the converter performance, it may be turned off during normal operation once synchronization has been achieved.

• If SYSREF is turned off during normal operation, TX and RX devices must have the ability to generate a “Generate SYSREF” request to the clock generator whenever a synchronization request is detected at the SYNC interface.

Is it prudent to turn on SYSREF occasionally? How often?  What factors affect this?

All I can think of is that temperature changes may affect timing so it would be smart to check alignment as often as your system architecture allows. Otherwise you would have to rely on frame alignment monitoring (via replacement characters) which doesn't seem like it is deterministic. 

  • TI__Guru* 91947 points

    Greg,

    Since a periodic SYSREF signal acts as a sub-harmonic clock of the converter sampling clock and may have spurious effect on the converter performance. This can very from one ADC to another. I have seen some devices were it caused many spurs and I have seen others were it was not even noticed. For DAC's, it is even less a problem. What devices are you looking into? On most of our JESD204B EVM's that come with configuration files, we normally issue 4-8 SYSREF pulses then disable the signal after synchronization is achieved. I have had many EVM's running over the weekend with SYSREF disabled and the link still running properly. To keep a system deterministic, the SYSREF source must go to every device.

    Regards,

    Jim  

  • in reply to jim s
    Prodigy 20 points
    Thanks for the reply. We're looking at using the AFE5818 but it essentially has the ADS52J90 inside it. It sounds like we'll have to see if it is even a problem but leaving some ability to control SYSREF would be a good idea.
  • in reply to jim s
    TI__Genius 9695 points
    Hi Greg,

    You can rely on the sync interface to determine when sysref is needed. Link alignment errors can be reliably detected inside the RX device (without sysref running continuously) and reported over the sync interface. If misalignment errors persists because of temperature drifts, the JESD204B link will hold the sync interface low until the link is realigned again with sysref.

    As Jim pointed out, keeping sysref running continuously can degrade data converter performance and is not recommended.

    Thanks,
    Eben.