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AFE7950EVM: cannot archive deterministic latency between two boards

Sebastian Held
Prodigy 20 points
Part Number: AFE7950EVM
Other Parts Discussed in Thread: LMK04828, AFE7950

Tool/software:

Our setup:

  • AMD ZCU102
  • two TI AFE7950EVM
  • external clock

Both LMK04828 are fed from the same clock and get synchronised. This results in a synchronized REFCLK and SYSREF between both AFE7950EVM. SYSREF is on permanently.

We use the options

  • useSpiSysref  =  False

  • continuousSysref  =  True

According to the manual, this should ensure deterministic latency, but we observe changing phase relationships between the carrier frequencies (currently 2.2 GHz), if one AFE7950 is reconfigured. We expected all PLL dividers to be reset due to the synchronized SYSREF.

Is it possible to get deterministic phase relationship between the carriers with this setup? What's the problem here?

  • 0
    TI__Genius 17831 points

    Hi Sebastian,

    Can you share the AFE configuration that you are using for this setup? When using a continuous sysref and a NCO that is not an integer multiple of sysref you would need to synchronize the NCOs on both AFEs using a common external signal. 

    One way to test this is to update your script such that the NCO frequency is an integer multiple of sysref and see if the phase delta becomes deterministic. 

    Another thing that could be part of the issue is the JESD link. The RBD setting for both the ADC and DAC links should be set to the same value for deterministic latency. Can you confirm if this is being done? 

    Regards,

    David Chaparro 

  • 0 in reply to David Chaparro
    Prodigy 20 points

    Hi David,

    We switched from continuous Sysref to single Sysref Mode. According to SBAA417 section 8.6: "Both modes maintain deterministic latency". With single Sysref, we can archieve the required phase relationship between the carriers of all four AFE7950EVM. The documentation seems to be wrong about continuous Sysref mode.

    My issue is resolved, but you should consider updating the manuals with your remarks. Thanks.