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LMK04828: OPT_REG_1 / OPT_REG_2 values with VCO0

Douglas Deboys
Prodigy 40 points
Part Number: LMK04828
Other Parts Discussed in Thread: LMK04826
We are currently replacing the LMK04826 with LMK04828 in our design as we are unable to get hold of any of the former part. We use a VCO frequency of 2.50 GHz so will be using VCO0 on the LMK04828 instead of VCO1 on the LMK04826.
My understanding from the datasheet is that OPT_REG_1 (0x17C) and OPT_REG_2 (0x17D) are not used when using VCO0. Does it matter at all what value is written to these registers, if only VCO0 is used? Specifically, if these registers are written to the values that we already use for the LMK04826 (as per the datasheet) will that cause any problems on the LMK04828 (when only using VCO0)?
Anything else we should be aware of in replacing the LMK04826 with LMK04828?
Many thanks for any help.
  • 0
    TI__Mastermind 33790 points

    Douglas,

    If you're not using VCO1 on LMK04828, OPT1 and OPT2 registers can be ignored, and there will be no harm in writing the "wrong" value (or just skipping those registers entirely). If it's not easy to change OPT1 and OPT2, don't bother unless you're using VCO1. Even if for some reason you do need to use VCO1, you can actually write all the registers with incorrect OPT values the first time, and later overwrite OPT1 and OPT2 to the correct values; you just need to also rewrite the LSB of the N-divider after modifying OPT registers to re-trigger the calibration routine for the VCO.

    The only difference between LMK04826 and LMK04828 is the VCO range (and the impacted OPT registers, which only affects VCO1 usage). Aside from changing the VCO_MUX (and optionally the OPT registers), the programming for both parts in this case will be identical.

    Regards,

    Derek Payne

  • 0
    Prodigy 40 points

    Great, thanks for your help!