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DP83867CS: Double checking need to strap unused inputs in SGMII mode...

Bob C
Intellectual 585 points
Part Number: DP83867CS

My DP83867CS design utilizes SGMII, so there are quite a few unused input pins associated with RGMII mode. 

Section 6.1 of DP83867CS datasheet states "Even though a device may have internal pullup or pulldown resistors,a good practice is to terminate unused inputs rather than allowing them to float."

In reviewing the TI SGMII EVM board, I see that the following are left floating: TX_D2, TX_D3, GTX_CLK, and TX_CTRL.

All these pins have internal pull downs. 

Also, you have a note on page 6: "Internal pullup and pulldown resistors on the IO pins are disabled when the device enters functional mode after power up." 

Are the pull downs on the RGMII pins really disabled when using the device in SGMII mode? 

Please clarify the need for pulling the unused RGMII I/O pins with an external resistor or why it wasn't necessary on the TI SGMII EVM board. 

Thank you

Bob

  • 0
    TI__Mastermind 19780 points
    Hi Bob,

    Sorry for the delay here. You can leave the unused RGMII connections floating as seen in the SGMII EVM. When in the SGMII mode, the RGMII interface is ignored so floating voltages will have no impact.

    Our suggestion to terminate unused is for the purpose of tying possible inputs to a known state. This is especially important for pins like RESET_N and JTAG. The TX pairs, which serve no purpose, are inconsequential to leave floating.

    Best Regards,