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DAC3484 NCO frequency update without SiF

David Kelley
Prodigy 50 points

Other Parts Discussed in Thread: DAC3484

I have a DAC3484 on a Pentek XMC module.  In trying to update the NCO frequency without the SiF sync I do not see the frequency shift.  I have the following bits asserted in the syncsel_mixerAB and syncsel_mixerCD fields of the config31 register: auto-sync from register; sync; sif_sync.  So far I have been testing the CD data path.  I write to phase_addCD (config22 and config23) followed by writing to phase_offsetCD (config19).  When I do this I'm expecting to see the analog signal change immediately.  However, I find that no change occurs until after I toggle the sif_sync field in config31.

I appreciate any help you can give.

  • 0
    Prodigy 50 points

    From what I've seen by experimentation, I need to issue an NCO sync to get the mixer frequency to change even when I have syncsel_mixerCD set to auto-sync from register and write to phase_offsetCD after changing phase_addCD.  Is this the only way to do this?  Is there a what to change the frequency of one mixer without syncing both NCOs?  I don't want to reset the phase of the other NCO to zero when I update the NCO freq.  What is the required register setup for this?

    Thanks

  • 0 in reply to David Kelley
    TI__Guru* 82905 points

    David,

    You are correct that once you have updated the NCO double buffered registers (syncsel_mixer), you will need to initialize the NCO accumulator (syncsel_NCO) in order for the fine mixers to start counting to the new frequency. The choice for NCO sync sources are SIF_SYNC, SYNC, OSTR, and ISTR.

    The NCO accumulators of both data path are tied to same synchronization path. This is to ensure that the phase offset of the two NCOs can be properly created at the rising edge of the synchronziation source.

    It sounds like you need to create difference phase offset between the two NCOs. To create phase offset between the two NCOs, you may need to program different phase offset registers between the two NCO paths.

    -KH