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DP83869HM: Interfacing with i.MX6 SOM

Ryan Thompson
Prodigy 50 points
Part Number: DP83869HM
Other Parts Discussed in Thread: DP83869, DP83869EVM

I have the DP83869 configured in media mode and generally working okay. When I run through magnetics and RJ45 jacks everything looks good. Unfortunately, when I try to connect to 1000 Base T signals directly on the board, I have no luck. The SOM already has conversion from RGMII to 1000 Base T, as shown below. The only thing I can think of that's different is the magnetics. I see that there are some magnetics specs in the DP83869 guide but it did not occur to me previously that it may not work correctly at all without them. Is there a chance I still need to drop a transformer between the below chip and the DP83869?

 

  • 0
    TI__Mastermind 27122 points

    Hi Ryan,

    Can you provide a block diagram of the setup here? I am a little confused what the intention is. The copper interface is not expected to function without a transformer. 

    Thanks,

    David

  • 0 in reply to David Creger
    Prodigy 50 points

    Okay, have attached some block diagrams in pdf. Basically, I need to get the DP83869 onto the same board as the AR8035. The DP83869 is in media converter mode. I unfortunately cannot use the RGMII to 1000 Base X mode because the AR8035 is already build into the SOM we are using and we do not have access to the RGMII signals, only the normal 1000 Base T differential pairs. Since my first post, I have also tried testing leaving the HX5008NL in the circuit. Have tried terminating the AR8035 side of the HX5008NL the same way I had the 0826-1X1T-32-F terminated, but no luck. The DP83869 side is terminated the same way as usual (and as recommended in DP83869 documentation), with the center-taps each having their own separate capacitor to ground. I tried looking in the AR8035 documentation for termination recommendations for magnetics but there doesn't seem to be anything. When successfully using the Magjack on that side, we have tied all center-taps together with single 1nF cap to GND. I am going to try to do separate caps this morning. If I don't update in the next few hours, you can assume that this also did not work.

    Block Diagrams.pdf

  • 0 in reply to Ryan Thompson
    Prodigy 50 points

    Indeed, splitting the center-taps did not work. Are you sure that it can't work without magnetics? I am seeing in the datasheet as well as on the routing of the DP83869EVM that there is an option for capacitive coupling. Looks like the caps on there are 33nF. Maybe I'll give that a shot. Any other thoughts you have are welcome.

  • 0 in reply to Ryan Thompson
    Prodigy 50 points

    For magnetics, it seems like I may be having issues due to the auto-transformer and CMC being there. The Magjack I was using does seem to have common-mode filtering on the PHY side, but may not have that extra autotransformer winding. Normally, this autotransformer and filter would be on the RJ45 side but since I have 2 PHY chips on the board close together, maybe I should be using a simpler transformer, such as TG1G-E010NZRL. This one uses "Circuit C" on the datasheet, which has no extra filtering. Maybe one of the transformers from Circuit A would be better since it seems like the filter side is closer to the Magjack I was using before with the SOM. In the meantime, I'll order a couple of each to mess around with.

  • 0 in reply to Ryan Thompson
    TI__Mastermind 27122 points

    Hi Ryan,

    The DP83869 does support capacitive coupling, yes. In that case 33pF capacitors should be used. Does the AR8035 also support capacitive coupling?

    Thanks,

    David