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PCIe Gen-3 Application: DS125BR820 or DS80PCI800?

Mahesh KC03
Intellectual 1360 points
Other Parts Discussed in Thread: DS125BR820, DS80PCI800, DS80PCI810

Hi Team,

My Customer Seagate is evaluating DS125BR820 for the following PCI application.

The pre-channel loss is 20 dB. The DS125BR820 can only equalize up to 10 dB at Nyquist. I wanted to suggest DS80PCI800 which can equalize up to 32 dB of pre-channel loss.

The application involves link training. I read that DS80PCI800 can support link training but the next version DS80PCI810 / DS125BR820 has a more linear output and thus more suitable for link training. To equalize the 12 dB post channel loss, can I use DE from my repeater or just rely on the DE from the TX ASIC? Will it affect the link training?

Considering all this, can I suggest the DS80PCI800 here ?

If not, can I use the following configuration of 2 repeaters using the DS125BR820? Will it be able to provide the same/better performance in terms of  cleaning the DJ?

Please advise.

-Thank you

  • 0
    TI__Mastermind 43795 points

    Mahesh,

    The behavioral equalizer in the PCIe-3 specification calls out 12 dB of CTLE and a one-tap DFE.  In order for a PCIe receiver to work with the long calibration channels and run error free under stressed eye conditions it seems a better Rx equalization would be needed. 

    Link training is run by the Rx, this Rx doesn't really have much adjustment so I'm not sure how it could be effective with even a 20 dB channel loss.

    Is this ASIC Rx PCIe compliant? 

    I would avoid running series repeaters if at all possible. It is more difficult to implement.

    Regards,

    Lee

  • 0 in reply to Lee Sledjeski
    TI__Intellectual 1360 points
    Hi Lee,
    Thank you for the quick reply.

    I am meeting the customer tomorrow. I will suggest the DS80PCI800 for his application then. Can I get the AMI/IBIS model for this so that we can simulate the link?

    "Link training is run by the Rx, this Rx doesn't really have much adjustment so I'm not sure how it could be effective with even a 20 dB channel loss."
    I thought, in Link Training, the TX ASIC adjusted its pre and post cursors according to the channel trace following it. The receiver would just send an acknowledgement back through another channel. Is not how it works? Can you please explain more on this link training/ auto-negotiation? Also, it would be helpful if you can send me any document that explains it.

    Thanks,
    Mahesh K C
  • 0 in reply to Mahesh KC03
    TI__Mastermind 43795 points

    Mahesh,

    Please find out more about the ASIC Rx PCIe compliance.  PCIe Rx's we have seen in the field all have had 12 dB CTLE.

    The best location for the PCI800 in this application would be ~ 6-8 dB from the ASIC Rx.

    Here is a quick summary of the training sequence.  The details are all listed in PCIe specifications.  Very similar approaches have been used in 10G-KR and SAS3 as well.

    1. Rx requests Tx for a specific starting point 

    2. Rx works to optimize eye through CTLE/DFE.

    3. In some cases the Rx may get a good eye and declare the training finished.

    3a. In other cases the Rx will try to increase/decrease the FIR energy from the Tx.

    4. The Rx works to optimize the eye with the updated Tx FIR settings and decides if the result is better or worse.

    4a. This sequence can loop several times until the best solution is obtained, it will depend on the channel characteristics and training starting point.

    Regards,

    Lee