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DS125BR401: redriver does not reliably enumerate with PCI gen 2 end point

AnBer
Mastermind 29675 points
Part Number: DS125BR401

Team,

Customer is trying to tune in the downstream direction (PC to EP, as described in section 9.5.1 of datasheet SNLS419D) for a PCI system.
The DS125BR401 sits on a custom PCI add-on card in a PC and is connected through Molex connector via a 50cm cable to a PCI gen 2 end point .

The add-on card is very simple. The 3.3V is coming from the PCI bus. The schematics and PCB guideline from datasheet have been followed.

Enumeration works ok for PCI gen 1, but it is not stable for PCI Gen 2.

SNLS419D Section 9.5.1 mentions PCI Gen 3.
-Is a different configuration/programming required for PCI Gen 2?

-What status register should be read in order to understand better the issue?
-How to debug further this issue?

Thanks in advance,

Anber

  • 0
    TI__Expert 7606 points

    Hi Anber,

    Hope you are doing well. I have some questions about your system which may be able to help in your debug:

    • What is your MODE pin set at? It's possible that if the device's mode pin is tied 1k ohm to ground (set to PCIe 1/2) the Root Complex will over-equalize for Gen 2 SSDs because of de-emphasis. To avoid this problem, try setting the MODE pin to FLOAT = AUTO Rate Select (for PCIe) or Tie 20 kΩ to GND = PCIe Gen-3 without De-emphasis to ensure that there isn't too much de-emphasis in the system. This will establish a baseline for future testing.
    • What is your RX DET pin set at?
    • What is your SD_TH pin set at?

    Please set the MODE pin to the settings in the first bullet point and let me know the results.

    Best wishes,

    Nick

  • 0 in reply to Nick Peabody
    Prodigy 20 points

    Hi Nick,

    Thank you for your help.

    The MODE pin is set to FLOAT. But I tried tying it to 1K GND. When left at FLOAT I was able to reach Gen. 2 but very unstable with many retrainings and speed changes between Gen. 1 and Gen. 2.

    RX_DET is FLOAT, SD_TH is FLOAT, too.

    Do I understand you right that I should try tying it to 20K GND to set it to Gen. 3 mode even when I want to reach Gen. 2?

    Thanks

    Best Regards

    Marvin

  • 0 in reply to Marvin Helmling
    TI__Expert 7606 points

    Hi Marvin,

    Yes, you understand me right. It is worth a shot because the root complex and the endpoint will be able to negotiate to Gen2 even if the mode pin on the redriver is Gen 3.

    Give it a try and let me know how it goes.

    Best regards,

    Nick P.