This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

DP83TC811EVM: Custom ethernet board - disconnecting issues

Part Number: DP83TC811EVM


i made a custom board that uses the same chips (DP83822 , DP83TC811) as the DP83TC811EVM. I have automotive ethernet (DP83TC811) connected to the control unit and ethernet (DP83822) to the PC. I read messages on the PC using the terminal. The problem is that the connection is unstable and disconnects and reconnects.

Can you please advise me where to look for a problem? On the board in the hardware or in the settings of the DP83822 and DP83TC811 chip registers? I did not set the registers in any way and they are in the default state.

Thank you


  • Hi Pavel,

    I have a few questions or things for you to check:

    1. On which PHY are you seeing the link instability? Is it the DP83TC811 or the DP83822?
    2. Are the MDI traces 100 ohms differential impedance?
    3. Can you check the jitter on the MDI lines and see if they are within spec?
    4. What cable length are you using? For automotive ethernet (DP83TC811), the max cable length is 25m per the standard. For normal ethernet (DP83822), the max cable length is 100m per the standard.
    5. Make sure the cable used for the DP83TC811 is rated for automotive ethernet.


    Adrian Kam

  • Hello Adrian,

    thank you for your questions/tips.

    1) Can you please advise me how to measure instability on each chip (DP83TC811 and DP83822) separately?

    2) MDI traces are routed as differential but their impedance is not 100 ohms.

    3) I measured jitter on MDI lines and it seems to meet spec.

    4) I am using very short cables. Their length is 1 meter or less.

    5) I am using cable for automotive ethernet.

    best regards,


  • Hi Pavel,

    You can probe the LED0 pin (which indicates link status) of each device and see whether it is the DP83TC811 or the DP83822 that has an unstable link.


    Adrian Kam