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Original question:

DP83822HF: Transformerless operation

DP83822HF: Why are the Tx+ and Tx- signals changing the behavior randomly?

Josep Castro1
Prodigy 65 points
Part Number: DP83822HF

Hello

I'm trying to implement a transformerless operation with the DP83822HF device following this thread. I successfully changed the register 0x404 from 0x20 to 0x24 value. 

I expect that phy drives the Tx+ and Tx- signals negated and the combination follows MLT-3 encoding:

Blue -> Tx+

Red -> Tx-

Black ->  Blue - Red

Sometimes the application changes the behavior:

The application is also working, and the differential signal seems MLT3 encoding but the pair Tx- and Tx+ is not the same as before.

Best regards and thank you in advance, Josep

  • 0
    TI__Mastermind 19250 points

    Hi Josep,

    Thank you for getting in touch with us.

    Can you please share the following details?

    1. What is the overall setup of the PHYs? Do we have cap coupling on both the PHYs? What is the cable length?
    2. What is the probe point used? Are we using high impedance probes?

    Can you please measure the signal with the following changes in the setup?

    1. Short the earths of Board A RJ45 and Board B RJ45. Connect them to the earth of the oscilloscope used for probing.
    2. Disable the transmit from Board B. This can be done by programming reg<0x0401>= 0x0080 on PHYB

    --
    Regards,
    Gokul.

  • +1 in reply to Gokul Koraganji
    Prodigy 65 points

    Hello Gokul,

    Thanks for the answer, finally we can get the error reason, and it was related to the auto MDIX feature of the phys, the negotiation was switching the transmission and reception pins.