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68Ω Impedance on MII lines

LarryT LarryT
Prodigy 40 points
Other Parts Discussed in Thread: DP83848I-MAU-EK

Hi Everyone,

The AN-1469 PHYTER Design & Layout Guide says that the MII IEEE spec states that the bus should be 68 Ohms.  In the next section in the guide goes on to recommend series termination resistors of 50Ω on the PHY to MAC MII lines.  Wouldn't the optimal resistor be 68Ω for these termination resistors?  Also, since the design guide calls out 50Ω resistors, is it implying that 50Ω transmission lines are acceptable.  With MII running over a short distance to the MAC, say a few inches, wouldn't the transmission lines effects on the digital lines(excluding clock lines) be irrelevant to proper operation since the signals would all settle before any sampling occurred?  Since the clock lines are going from the PHY to the MAC, then if the transmission line to MAC interface are matched, would that work adequately too (ignoring the output impedance on the actual PHY)?

  • 0
    TI__Mastermind 31600 points

    There are a few notable aspects here:

    The IEEE specification does specify 68 Ohms impedance for the MII signals, and specifically notes this requirement for "electrically long transmission paths or path segments".  Note that the IEEE specification is intended in part to address connections across the MII via MII cables.  In this case, the impedance of the PHY MII I/Os should match the 68 Ohm impedance of these cables and remote MACs. 

    For a short point to point connection between a MAC and a PHY on the same PCB, it is more common to use 50 Ohm traces.  If any series terminations are implemented on the MII lines, they will need to take into account the impedance of the MII I/Os themselves.

    The DP836x0 devices include 50 Ohm integrated terminations so no additional series termination may be required.  For the DP83848 devices, it is common to use 33 Ohm series terminations (as seen in the DP83848I-MAU-EK evaluation board schematic, for example).  In either case, the corresponding IBIS model for the device can be used to simulate prior to board manufacture and the results should be verified in the lab on the final board.

    Patrick