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RMII mode with DP83848

fabricepeden
Intellectual 545 points

Dear all,

We're working in RMII mode with a DP83848.

As it's mentionned in the datasheet (page 15 ; table 2-7), the pin 39 (RX_DV/MII_MODE) must be tied to high and pin 6 (SNI_MODE/TX3) must be  tied to low to be in RMII mode, I have done this.

Moreover, pin 40 (CRS_DV) is directly connected to my microcontoller.

With that, my Ethernet connexion doesn't work properly. But When I connect pin 40 to pin 39 (so pin 40 is tied to high), it's works perfectly, I have Ethernet communication, my modbus TCP works fine.

So, I don't really why it works in this case.

If somebody can help me to understand.

Thank you,

Regards,

Fabrice

  • 0
    TI__Mastermind 18425 points


    Hello Fabrice,

    Pin 40 is a strap pin on the DP83848. Some micro-controllers have a internal pull resistances. If these resistances are weak, they might set a voltage which falls outside the normal range for that strap pin & force the PHY in an undefined mode. Can you check if the micro-controller has internal resistance and what is the value for the internal resistance?

    -Regards,

    Aniruddha 

  • 0 in reply to Aniruddha Khadye
    Intellectual 545 points

    Dear Anirudda,

    Thank you for your answer. I'm using a STm32F207IG and I just checked the data sheet. The internal pull-up resistor is 40kohms and by default, I didn't activate it, so it can be the reason ?

    Thank you in advance,

    Regards,

    Fabrice

  • 0 in reply to fabricepeden
    TI__Mastermind 18425 points

    Hi Fabrice,

    40Kohm is a weak pull-up resistor and can potentially cause problems. Although you have not chosen to use the internal resistor of the MCU, the resistor is still included in the signal path. The way around this problem is to strap the DP83848 in a known state using external resistors. The datasheet recommendation for DP83848 is to connect external 2.2Kohm pull-up or pull-down resistors on the pin depending on what fucntionality is required.

    Please refer to the datasheet for more information about the strapping pins.

    -Regards,

    Aniruddha