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DS80PCI800 in Gen3 mode

tarun kakar1
Prodigy 20 points
Other Parts Discussed in Thread: DS80PCI800

I am using the chip for Gen3 connection in "pin mode". I can see the link up but with different lengths of QSFP cables, i see errors on the link.

Is it possible to control VOD and De-emphassis in pin mode using DEM pins for Gen3? I read in eval manual that DEMA/B are available only in Gen1/2 mode. For Gen3, it adjusts VOD based on Rx.

  • 0
    TI__Mastermind 29655 points

    Hi Tarun,

    In Gen3, you will still be able to notice amplitude changes in the output signal when setting VOD while in pin mode. However, in Gen3, the VOD output is determined by the amplitude of incoming VID, i.e. setting VOD = 1.2Vpp while the input swing is significantly less than 1.2Vpp will mean that your final VOD output will be less than 1.2Vpp as well. 

    VOD will track with the input until the VID gets to a point where the desired VOD output can be reached. This linear tracking is why we detail the following in the datasheet:

    Conversely, your DEM should not be affected by this.

    Are the QSFP cables on the incoming or outgoing end of the repeater? If you have significant line length before the repeater input, you may also need to adjust the EQ in addition to setting VOD/DEM controls.

    Thanks,

    Michael

  • 0 in reply to Michael Lu (Santa Clara)
    Prodigy 20 points

    Thanks Michael for reply.

    The usage block diagram is as below.

    So i guess i need more EQ_B for longer QSFP cables on the redriver Rx side(RX_B).

    For PCB side, being fixed loss, Can EQ_A be fixed? or it need to be  variable depending upon the QSFP cable length on TX_A side.

    In other words, Does the EQ on Rx side has impact on controlling VOD/DEM on it respective Tx side and thus controlling the link quality on QSFP side?

    Thanks

    Tarun 7762.BD.docx

  • 0 in reply to tarun kakar1
    TI__Mastermind 29655 points

    Hi Tarun,

    For clarity, I am going to use the convention I believe you are describing below:

    Yes, I would recommend experimenting with the various Redriver settings on the redriver RX_B side. Likewise, it is good to determine the maximum cable length the DS80PCI800 can drive by experimenting with the redriver TX_A. The shorter of the two maximum lengths determined by optimizing EQ, DEM, and VOD for RX_B and TX_A  will determine the maximum distance you can place between redriver and the QSFP endpoint.

    In general, the EQ effects apply to redriver RX input, and DEM/VOD effects apply to driver TX output. However, since your endpoints at the QSFP end (RX after TX_A) and the PCB end (RX after TX_B) are seeing the overall combination of EQ, DEM, and VOD, your choice of EQ may impact the choice of VOD/DEM in order to obtain the best overall eye.

    However, to keep things simple, I agree with your approach to keep EQ at RX_A and DEM/VOD at TX_B fixed. You can fix the EQ or TX on the PCB 10" trace side so that they are optimized, and then isolate your changes only to controlling VOD/DEM on TX_A and EQ on RX_B.

    Michael