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DP83867IR: Max trace length of the MII and the MDI

reto hermann
Intellectual 740 points

Part Number: DP83867IR

Dear support team

In the past, we prioritized placing the PHY as close as possible to the MAC (MCU or FPGA) to minimize trace length on the MII interface. This design strategy was based on the assumption that single-ended - in our case 3.3V DDR (Double Data Rate) MII signals - are more susceptible to signal integrity issues than the differential MDI signal pairs. 

Now we have read the application report from Lysny Woodahl "Ethernet PHY PCB Design Layout Checklist " that recommends the opposite: Ethernet PHY PCB Design Layout Checklist

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My question: What is the rationale behind the 2-inch maximum trace length requirement for MDI (Medium Dependent Interface) traces between the PHY and magnetics? This constraint seems particularly strict given that:

  1. The cable on the other side of the magnetics can be up to 100 meters long

  2. The 1:1 transformer ratio means the magnetics don't fundamentally alter the signal characteristics

What makes the pre-magnetics interface so critical compared to the post-magnetics cable?

  • 0
    TI__Mastermind 35855 points

    Hi Reto,

    The reason for keeping a shorter MDI trace relative to MII trace is because of the long cable connection on the MDI and the variable from the link partner's MDI trace routing.

    • The MII lines are something of a closed-system where a designer often controls the trace length, connectors, and so on. For xMII lines, 6" is what we've seen to work in terms of trace length. 
    • On the other hand, the MDI will pass through your board traces, up to 100m of cable, and through the link partner's board traces. Keeping the MDI traces short (<2in) gives margin for whatever may be connected on the cable or link partner side. 
    reto hermann said:
    What makes the pre-magnetics interface so critical compared to the post-magnetics cable?

    The 2in requirement is for the entire MDI trace between the PHY and the RJ-45 connector (both pre and post magnetics), or if your magnetics are integrated into your RJ-45 it would be from the PHY to the magnetics (pre-magnetics). The idea with keeping MDI traces short is to give margin for whatever may be connected to the RJ-45. Shorter traces are less prone to noise coupling and will incur less insertion loss.

    Best,

    Shane