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DS280DF810: Inter-Pair Length Matching for QSFP28

Gireesh Nair P
Prodigy 40 points
Part Number: DS280DF810

Tool/software:

Hi Ti Team,

We are using the DS280DF810ABVT on our card to mux/demux the eCPRI interface to two different QSFP28 connectors. Please find the attached block diagram for your reference. The QSFP28 uses a CAUI-4 C2M interface.

We understand that the inter-pair (pair-to-pair) length matching requirement is determined by the processor and the QSFP28 module, Kindly confirm this. As per the datasheet, DS280DF810ABVT introduces a maximum channel-to-channel inter-pair skew of less than 30 ps.

However, we would like to confirm or clarify the specific length matching requirements for the QSFP28 (CAUI-4 C2M) when used through a DS280DF810ABVT IC

1. Is inter-pair (pair-to-pair) length matching required for the CAUI-4 C2M interface type?

2. Please let us know any additional details that need to be taken care on the layout of DS280DF810

Your insights on this matter will be greatly appreciated. Please let us know if additional details are required.

Looking forward to your response.

Regards,
Gireesh

  • 0
    TI__Mastermind 20901 points

    Hello Gireesh,

    Due to the US Christmas and New Year holidays, the response to your question will be delayed. We have assigned an engineer to this E2E thread and will look into your question when the engineers return (which I expect early next week). Sorry for the wait and any inconvenience it may cause.

    Best,
    David

  • 0 in reply to David Waier
    TI__Mastermind 36153 points

    Hi Gireesh,

    I just got back from holiday and am looking into your questions.  I expect to get back to you within the next couple days.

    Thanks,

    Drew

  • 0 in reply to Drew Miller1
    TI__Mastermind 36153 points

    Hi Gireesh,

    Thanks for reaching out regarding your design.

    Gireesh Nair P said:
    We understand that the inter-pair (pair-to-pair) length matching requirement is determined by the processor and the QSFP28 module, Kindly confirm this. As per the datasheet, DS280DF810ABVT introduces a maximum channel-to-channel inter-pair skew of less than 30 ps.

    IEEE802.3 section 86.3.2 defines maximum channel-to-channel inter-pair skew.  However, it's also worth reviewing the processor and module specifications to ensure these are consistent with CAUI-4 specification.  This will also help you understand the skew margin you have in your system.

    Gireesh Nair P said:
    1. Is inter-pair (pair-to-pair) length matching required for the CAUI-4 C2M interface type?

    Yes, please see section 86.3.2 of the IEEE802.3 specification.  As defined in the specification, SP2 (host TX) should have skew less than 43ns, and SP5 (host RX) should have skew less than 145ns.

    Based on this, the length matching requirement will be dependent on your host's skew, retimer skew (<30 ps), and the propagation speed of the signal through your board.

    Gireesh Nair P said:
    2. Please let us know any additional details that need to be taken care on the layout of DS280DF810

    The DS280DF810 has a high minimum CTLE boost.  In order to avoid over equalization, we recommend having a minimum of at least 3 dB of insertion loss prior between a host TX and the DS280DF810 RX.

    Beyond this, please review the layout guidelines and layout example in the data sheet.

    Please let us know if you have any additional questions.

    Thanks,

    Drew