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DS100DF410: Is the 25MHz reference clock optional?

Casey Morrison
Intellectual 1985 points
Other Parts Discussed in Thread: DS100DF410

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Question: For the DS100DF410, is the use of the 25MHz reference clock (REFCLK_IN, pin 19) mandatory or is it optional to reduce locking time? Is it required to have the best jitter performance?

Casey Morrison

  • 0
    TI__Intellectual 1985 points

    Answer: The DS100DF410 can be used in a referenceless mode (i.e. Ref_mode=0) or in a 25MHz reference clock mode (i.e. Ref_mode=3). TI recommends using Ref_mode=3 because the main advantage of using a 25MHz reference clock is that it greatly speeds up the channel locking process. For 1GbE applications, if operating in referenceless mode (i.e. Ref_mode=0), it is required to bypass the CDR by setting the override bit 5 of register 0x09 to 1, and set the data mux bits [7:5] to 3'b000 of register 0x1E. The CDR will lock to the 10GbE rate even in Ref_mode=0. In Ref_mode=3, it is not required to bypass the CDR, as the CDR will lock to both the 1GbE rate as well as the 10GbE rate. The jitter performance will be the same between Ref_mode=0 and Ref_mode=3, since the 25MHz reference clock is not used to clock the datapath in any way, it is only used to calibrate the VCO frequency. This also means that the phase noise of the 25MHz reference clock is not critical.

    Casey Morrison