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DP83867ERGZ-S-EVM: Capacitive Coupling Assistance

Charles Harris
Prodigy 210 points
Part Number: DP83867ERGZ-S-EVM

We have attempted to capacitively couple one of our DP83867ERGZ-S-EVM boards by following the instructions listed in Section 2.8 of the User Guide, which I have listed below for reference. However, this board does not work for us after following those instructions. After adding the Capacitors, we powered this board on and plugged in an Ethernet cable to a PC and the activity lights did not turn on. Further, we are not able to ping through this capacitively coupled board as we can with a magnetically coupled board. Are those instructions listed in Section 2.8 correct, and are the C values and R values shown in the Schematic correct? The schematic shows these C values at 33pF and the R values at 0 ohms. Also, the Resistors were already populated on the board, so we did not have to place those on the board. Are we instead supposed to remove R27-R34?

 

2.8 Capacitive Coupling

The EVM can be operated in capacitive coupling mode instead of the magnetic coupling mode. The following changes are required for capacitive coupling.

  • Populate C22, C23, C25, C27, C28, C30, C31 C33, R27-R34.
  • Remove T1.
  • 0
    TI__Genius 9920 points

    Hi Charles,

    The values and instructions should be correct. You should not be removing R27-R34. What speed are you selecting for the PHY? Capacitive coupling is not supported on 1 Gb speed 

    Thanks,

    Cecilia

  • 0 in reply to Cecilia Reyes
    Prodigy 210 points

    Cecilia,

    Thanks for your quick response. We aren't configuring the speeds for the PHY. We're under the impression that they auto-negotiate to a speed up to 1 Gbps. If the devices can't run at 1 Gbps, they should auto-negotiate to a slower speed that they can run at. 

    We still can't get the PHY eval board to function properly after the Capacitive Coupling mod. We expected to at least see the activity LED when an Ethernet cable is connected from the device to a PC, but that is not the case. We also can't ping a target hw device through the modified PHY eval board like we can with an unmodified board.

    We have modified three other boards by taking off the magnetics and jumping the magnetic lines with straight 30 gauge wire and these boards function properly as well. We are expecting the capacitive coupling mod to function better than this jumping method, once we get the capacitive coupling to work correctly. Have y'all had success following the capacitive coupling instructions for this device in the past? Any additional insight on the capacitive coupling mod would be greatly appreciated.

    Thanks,

    Charlie

  • 0 in reply to Charles Harris
    TI__Genius 9920 points

    Hi Charlie,

    Did you modify that straps on the EVM? Are they strapped to 1G or 100M? I'm wondering if you strap it to a lower speed that it would work in forced mode. 

    Thanks,

    Cecilia

  • 0 in reply to Cecilia Reyes
    Prodigy 210 points

    Hey Cecilia,

    Thanks for the follow-up. I meant to post on this thread sooner. It turned out we placed the incorrect value capacitors on the board. After replacing them with capacitors close to 33pF (22pF), we were able to get the capacitive coupling working.

    Take care,

    Charlie