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Original question:

DS280BR820EVM: KR to SFI conversion

DS280DF810: DS280DF810 and KR<->SFI Conversion

Calvin McBride
Prodigy 20 points
Part Number: DS280DF810
Other Parts Discussed in Thread: DS280BR820EVM

Tool/software:

I'm developing a board that takes 100G-KR backplane Ethernet (across four lanes) to a host-side QSFP28 assembly (in other words, the cage and receptacle). I have my eye on the DS280DF810 as a retimer for the signals. There's several points of confusion that I can't find definitive answers on.

  1. Does the DS280DF810 support conversion from KR to SFI?
    1. If so, what do I need to do with the DS280DF810 to have it output the correct SFI signal for use with the QSFP28 module?
  2. What, if anything, do I need to do with the DS280DF810 to allow the QSFP28 assembly to support both direct-attach copper and fiber modules?

I've tried to do my due diligence but it just results in contradictory or inconclusive claims. There's another thread that's similar ("DS280BR820EVM: KR to SFI conversion"), but it simply states "You might also consider DS280DF810" without mentioning to what extent it supports this use case.

Any guidance is appreciated.

Thanks,

Calvin

  • +1
    TI__Mastermind 36373 points

    Hi Calvin,

    The DS280DF810 does not fully support -KR signaling.

    KR is very similar to other Ethernet signaling, but has two differentiating features:

    1. Auto-negotiation
      Auto-negotiation involves communicating the capabilities of each link partner at a low speed (I believe 312.5 Mbps) prior to establishing a link a full speed.  Our retimer is unable to retime this data since it's outside of the CDR lock range, but we can support passing the data through using CDR bypass.  We do not expect issues using auto-negotiation with DS280DF810.
    2. Link training
      Link training involves a feedback loop between the link partners in order to establish the best TX FIR settings for signal integrity.  In order to participate in this sequence, a retimer device would have to be protocol aware.  Our retimers are not protocol aware, so we're unable to support link training.

    If you are implementing a system with a fixed backplane connection, we expect that link training is not strict requirement for a successful design.  If you are able to disable link training and auto-negotiation on the link partner, the DS280DF810 would be a good part for this design.

    The retimer has adaptive RX EQ, which can help enable to support both DAC and fiber.  One thing to note is that the retimer has a high minimum CTLE boost.  In order to avoid over-equalization of signals coming from optical modules, we recommend ensuring at least 3 dB insertion loss between the module and the retimer.

    Thanks,

    Drew

  • 0 in reply to Drew Miller1
    Prodigy 20 points

    Thanks Drew, I'll reach out to our customer and see if disabling auto-negotiation and link training is a possibility. Should that not be the case, does TI have another part that would support SFI/QSFP28 and KR signalling for this application?

    Thanks,

    Calvin

  • 0 in reply to Calvin McBride
    TI__Mastermind 36373 points

    Hi Calvin,

    No, there is not another part TI has supporting 25GbE and -KR signaling.

    Thanks,

    Drew