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DP83TG720S-Q1: RGMII cannot communicate

Part Number: DP83TG720S-Q1

Hi team,

Here is a question one of our customers is experiencing, could you please provide some troubleshooting suggestions?

When debugging DP83TG720S-Q1, I configured the PHY to RGMII mode and completed the initialization sequence. After the initialization is completed, the relevant registers are dumped, and the working mode and Link Up status of the PHY chip are confirmed:

But when I communicated through RGMII, it's failed. I guess it's the timing of RGMII. I caught the waveform of RGMII through the oscilloscope, but I couldn't analyze it.

    I want to confirm whether it is a RGMII timing problem? If it is the RGMII timing, how can I adjust it?

This user says the signal voltage amplitude is correct and the picture above shows incorrect is an oscilloscope setting issue. 

Best Regards,


  • Hi Amy,

    It's possible this is an RGMII timing problem between the MAC and PHY.

    Please try testing the device in RGMII shift mode by strapping RX_D2 and RX_D1 high.

    Shift mode can also be enabled by programming register 0x602[1:0] = '11', and the shift delay can be tuned using register 0x430[11:4].

    This FAQ is a useful reference for selecting the correct RGMII delay mode.

    Thank you,


  • Hi Evan,

    Thank you for your assistance. The following is the customer feedback, could you please give your comments.

    I tried this advice:Enable shift mode by programming register 0x602 [1:0] = '11'; and all TX delay values were tried on register 0x430[11:8].

    (Why did I not change the RX delay because there are many ways to combine the two delays and I intend to switch on the tx link first.)

    My test equipment is "VECTOR VN5620". I haven't seen the data coming from VN5620.I would like to ask if this method of only looking at TX data and only adjusting TX delay is correct?

    The following figure shows the waveform on the TX link:

    Below I dump the relevant registers for use with reference:

    This data goes from MAC to PHY. Is there any way to check whether the phy is receiving data correctly?

    My MAC terminal uses SJA1110, have you tried this combination?



  • Hi Amy,

    If the TX/RX traces between the MAC and PHY are length-matched, I recommend testing with the same delay applied to both TX and RX in shift mode.

    To check if the PHY is receiving the data from the MAC:

    Enable RGMII loopback with reg<0x0000> = 0x4140, and probe the RGMII RX pins to see if the same data from the MAC is looped back. Register 0x063C[15:0] reports the total received packets, this can be compared to the number of packets sent from the MAC.

    Thank you,


  • Hi Evan,

    Thank you for your support over this time. The problem has been resolved, It's because of VDDIO level mismatch.

    Thank you.