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mali005
Visitor II
October 11, 2022
Question

Why is there not STM32MP157F-DK2 RGMII TX termination resistor?

  • October 11, 2022
  • 1 reply
  • 845 views

I check STM32MP157F-DK2 schematic. Transmit Channels(ETH_TXD0..ETH_TXD3) isn't have termination resistor. Receive Channels (ETH_RXD0..ETH_RXD3 ) is have termination resistor. But you add termination resistor on Transmit Channels in your  application note(AN5031 page 74 ). I confused about transmit channels termination resistor.

Please explain this.

Screenshots

0693W00000Unpu3QAB.jpg0693W00000Unpu8QAB.jpg0693W00000UnpuDQAR.jpg

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    1 reply

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    PatrickF
    Technical Moderator
    October 12, 2022

    HI @Community member Gürbüz​ 

    This is not termination resistors, but rather source damping resistor (or impedance matching) to improve signal integrity. Those resistors should be put close to driver side.

    Usually, it could be skipped if lines are assumed to be short and with minor impedance discontinuities (e.g. vias) or if signal integrity analysis show they are not needed.

    I guess that on STM32157F-DK2 board, decision come that as we could change the TX drive level, the resistors could be skipped. Whereas on RX path, it is more difficult to control the output impedance from PHY side.

    Regards.

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