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DP83849C: Internal loopback Vs External Loopback: Bist Test

Rochus
Prodigy 150 points
Part Number: DP83849C

Dear DP83849C experts,

I am working with a solution that uses a DP83849C phy. I have configured the phy, and have a BIST test in internal loopback mode, and the BIST test passes.

I have made an ethernet loopback plug, and have re-run the test, albeit with internal loopback mode disabled. It fails. Am I correct in thinking that if the BIST test passes with the internal loopback, it should also pass with the external loopback, and that if any failure in the test  points to the section of the solution between the phy and the RJ45 pins ?

Kind regards,

Rocco

  • 0
    TI__Mastermind 18425 points
    Hi Rocco,

    When you use ethernet loop back cable, do you see link up? Internal loopback works on the MAC side and loopback cable is connected on the cable side. So it is possible that internal loopback might pass but testing with loopback cable might fail.

    -Regards
    Aniruddha
  • 0 in reply to Aniruddha Khadye
    Prodigy 150 points
    Hi Aniruddhu,

    Thanks for the reply. The link status bit of the Basic Mode Status Register, is set to '1', indicating a valid link. The description of the field states that the link validity is implementation specific. I am not sure what that means.

    Looking out of the RJ45, the right LED flashes green. The left LED is off.

    IF the internal BIST passes, and the external BIST fails, does that point to an issue in the chain between the Phy and the RJ45 ?

    Kind regards,

    Rocco
  • 0 in reply to Rochus
    TI__Mastermind 18425 points

    Hi Rocco,

    If the BIST with external loopback cable is failing then it does point to an issue on the cable side. Please check that the circuit connection, RBIAS resistor value, Magnetics specification, and crystal specification are as per the datasheet requirement.

    -Regards

    Aniruddha