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DP83TC811S-Q1: How to Distribute 50MHz clock to Multiple PHYs using RMII

Sven Wagner
Prodigy 20 points
Part Number: DP83TC811S-Q1

Tool/software:

When using multiples of this PHY in slave mode using RMII, can the 50MHz oscillator be daisy-chained from one PHY to another and then terminate at the MAC, or does each PHY require its own 50MHz oscillator which must also be connected to the MAC? What is the best topology when implementing multiple PHYs on the same board?

  • 0
    TI__Mastermind 26752 points

    Hi Sven,

    50Mhz RMII reference clock cannot be daisy chained. In RMII slave mode, the PHY does not output 50Mhz. Please share a block diagram of your design, I would like to see how multiple PHYs are needed on the same board.

    Thanks,

    David

  • 0 in reply to David Creger
    Prodigy 20 points

    Hi David, thanks for the quick reply. We are currently working on a project that requires multiple 100Base-T1 transceivers. As I have read in reference designs and App. Notes, when these are used in slave mode with RMII they need to share a 50MHz ref. Clock with the MAC. My question is, can one clock be shared between all the PHYs and the MAC, or does each PHY need its own oscillator?

  • 0 in reply to Sven Wagner
    TI__Mastermind 26752 points

    Hi Sven,

    Does the MAC have 4x RMII ports? Is there an RMII reference clock input for each port or just a single common one? 

    I don't see a problem with this implementation, but reflections and signal integrity of the 50Mhz clock will be a concern. Recommend to use a 5:1 buffer in this case to fan out the 50Mhz clock to each destination. 

    Please ensure that the input clock requirements given in the datasheet are met for each PHY. Please see below.

    Please also check with the MAC team that they can support this implementation.

    Thanks,

    David