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DP83TG720R-Q1: RGMII alignment and bootstrap resistors

Paul Thomas
Prodigy 50 points
Part Number: DP83TG720R-Q1

1) The SNLU289 eval board has no pin strap resistors on RX_D0, RX_D1 & RX_D2. So these  lines would all be open at boot (MAC[0], MAC[1] & MAC[2] would all be 0). According to table 8-18 of the DP83TG720R-Q1 datasheet this is a reserved state ? Can you confirm it is acceptable to have no pin-strap resistors on these lines?

2) If these boot strap resistors are required, do you have any placement guidance - I am concerned these resistors would create a stub on a high frequency transmission line - which doesn't sound like a good idea.

3) Is there any best practice guidance on whether TX & RX RGMII alignment is done at the PHY, the MAC or at the PCB traces?

  • +1
    TI__Mastermind 19621 points

    Hi Paul,

    I understand your confusion and I hope my response below will clear things up for you.

    1. You are correct, on the EVM there are no strap resistors on RXD[0..2]
      1. For the R version of the 720, this is indeed a Reserved State.
      2. The EVM uses the S version of the 720, where this is not a reserved state (see Table 7-19 in the SNLS604E 720S Datasheet)
      3. The main difference between the S and R version of the 720 is that the S supports SGMII, where the R does not.
      4. If you are using the 720R in your design, please choose a different MAC interface (See section 8.4.8 MAC Interfaces in the SNLS603C 720R datasheet).
    2. We recommend you place any Strapping Resistors as close as possible to the Pin of the PHY
    3. It should be done at all levels. Firstly, PCB traces should be length matched. Any subsequent delays can be handled by the PHY and MAC.
      1. We do have a Layout checklist that I recommend reading and following
        1. https://www.ti.com/lit/an/snla387/snla387.pdf?ts=1690827398949&ref_url=https%253A%252F%252Fwww.google.com%252F

    Regards,

    Alvaro