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DP83849C: MAC to Phy loopback

Part Number: DP83849C

Dear DP83849C experts,

I am using the DP83849C phy. I understand that there are 3 types of loopback tests available with this phy. They are:

- Internal BIST test. A pseudo random pattern is generated in the phy and looped from the phy internal TX to the phy internal RX and compared.

- External BIST test. Same as internal BIST above, except the loopback is performed with a ethernet loopback plug that is plugged into the RJ45

- PMD test. A test which loops back data received from the physical layer back into the tx path and in turn back to the physical layer. I presume you would need some sort of device/application that can verify the integity of loopbacked data.

I have two questions in relation to this.

1) Is my description above correct ?

2) Is it not possible to loop back data in the phy that it received from the MAC, such that the MAC receives data it transmitted ?

Kind regards,

Rocco

  • Hi,

    1) Yes, your understanding is correct.

    2) You can use MII  loopback using register 0x0000 bit[14]

    Regards,
    Geet

  • HI Geet,

    I am trying to get MII loopback working. I have the following setting of the Basic Mode Control Register 0x6001.

      I am taking a logical analyser waveform.  From that waveform I do not see the MTXD0-3 data, but not the MRXD0-3. I have attempted to capture on both the MTXEN and MRXDV, as the spec says that in MII loopback synchronisation is lost,  and a dead time of 500us may occur.

    With the Basic Mode Control Register Setting of 0x6100 alone, should I be expect MII loopback to work? Is there any diagnostic data in the registers that gives a hint on why MII loopback is currently not working ?

    I have seen data on the MRXD once but I have failed to see any time since.

    Kind regards,

    Rocco

  • Hi Rocco,

    You can read the bit and reconfirm that it is intact set. We don't have any other status register to confirm.

    On TX pins, have you checked TX_EN also getting triggered ? Also, given the PHY has two MAC interface. you have to ensure your configuration is for the right port.

    Regards,

    Geet

  • Hi Geet,

    Thanks, from that I understand that other than setting the loopback setting for the target port, there is no other setting required.

    I got it working. I  had to add a delay to compensate for the dead time.

    Thanks,

    Rocco