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About dp83849i

Nat Wong
Genius 3070 points
Other Parts Discussed in Thread: DP83849I

Dear Sirs

I have some iss about Ethernet PHY dp83849i.

Auto MDI/MDIX   and   non-auto MDI/MDIX

In my opinion:

Auto mode: TX and RX all transceiver data in the initial communication.

Non-auto:TX transceiver ,RX receiver data

Is my opinion right?

Could you give me some suggestion about Auto MDI/MDIX   and   non-auto MDI/MDIX?

Thank you.

BRS

Nat

  • 0
    TI__Mastermind 31600 points

    Nat,

    When Auto-MDIX is disabled (MDIX_EN = 0x0) and MDIX is not forced (FORCE_MDIX = 0x0), the DP83849 will transmit using the TPTDP / TPTDM pins and will receive using the TPRDP / TPRDM pins.  If the link partner on the other side of the cable is configured in the same way, it will be necessary to use a cable with the wires crossed in order to connect the transmit and receive pairs between the devices. 

    Auto-MDIX is a mechanism that is defined by the IEEE 802.3 specification and occurs when link is being established during the process of Auto-Negotiation.  Auto-MDIX allows the PHY to swap the transmit and receive pairs in order to determine the correct configuration for linking with the partner on the other end of the cable.

    For most applications, the recommendation would be to enable Auto-Negotiation and to enable Auto-MDIX.  This will allow the PHY to determine the best link speed (10M or 100M) and the configuration of the transmit and receive pins (straight or crossed) needed to link with the partner at the other end of the cable.

    Patrick

  • 0 in reply to Patrick OFarrell
    Genius 3070 points
    Dear Sirs

    The datasheet of dp83849 suggest RJ45 Transformer TCT and RCT Pull high resistor.
    But I do not set Transformer RCT and TCT pull high resistor, it still work.
    Does the Transformer RCT and TCT pull-high resistor necessary?
    Why?

    Thank you.
    BRS
    Nat
  • 0 in reply to Nat Wong
    TI__Mastermind 31600 points

    Nat,

    Both transformer center taps should be connected to 3.3V and de-coupled to ground using 0.1uF capacitors.  Each of the transmit and receive pins that connect to the transformer should have a 50 Ohm termination resistor to the 3.3V supply.  These connections are required due to the fact that the DP83849 has a current mode line driver.

    Patrick

  • 0 in reply to Patrick OFarrell
    Genius 3070 points

    Dear Patrick

    Why should transformer center taps be connected to 3.3V?

    Thnak you.

    BRS

    Nat

  • 0 in reply to Nat Wong
    TI__Mastermind 31600 points
    Nat,

    Connecting the transformer center tap to the supply is intended to provide a better common mode point for the transformer. This connection generally provides better performance.

    Patrick
  • 0 in reply to Patrick OFarrell
    Genius 3070 points
    Dear Patrick

    If I do NOT connect the transformer center tap to the supply,
    do you have any comment?

    BRS
    Nat
  • 0 in reply to Nat Wong
    TI__Mastermind 31600 points
    The circuit will still operate if the transformer center tap is not connected to the supply. I would still recommend that you connect the center tap to the supply if possible. If this connection is not possible, you should validate your application carefully to make sure it meets the needs of your system with this connection.

    Patrick