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DP83TC817S-Q1: MII Termination Resistors

Mark Pieper
Expert 4650 points
Part Number: DP83TC817S-Q1

Tool/software:

Hi,

Do the integrated termination resistors apply to just MII mode or also to the other MAC interface selections?

Is this termination able to be enabled/disabled?

Customer's comment:

"I would think that RMII, which is all about reducing routing congestion, would want to eliminate termination resistors.

And I am actualy interested in RGMII which is what have to use.

Does that have the series termination resistors or do I have to add them?"

Thanks a lot,

Mark

  • 0
    TI__Mastermind 27082 points

    Hi Mark,

    Internal termination resistors apply to all MAC interface selections. No external termination resistors are required for RMII interface. 

    Thanks,

    David

  • 0 in reply to David Creger
    TI__Expert 4650 points

    Hi David,

    Thanks. Just to be clear, these termination resistors are always in place and are not able to be disabled. Is this correct?

    There is a "part B" to this question concerning the need to rout the signals with 50 Ohm traces. Am I correct in saying that if the trace is very short (around an inch or so) we don't need to worry about that because it is shorter than the 1/4 wavelength of any harmonic we care about. Does this sound right to you? This implementation is in a very small PCB so everything is very tight. Below is the engineers (conservative, IMO) analysis.

    Thanks,

    Mark

  • +1 in reply to Mark Pieper
    TI__Mastermind 27082 points

    Hi Mark,

    These termination resistors are always in place and are not able to be disabled. Is this correct?

    Correct.

    Does this sound right to you?

    I think it is best not to calculate/estimate the performance effect on paper. We should recommend to route the MAC interface traces as 50ohm, no matter the length, as recommended in the datasheet. We have IBIS models available if detailed analysis is absolutely required, but 50ohm guidance should be followed regardless. 

    Thanks,

    David