Thursday, 29 August 2019

Impedance Matching Resistors on MII Ethernet Lines


Microchip is taking forever to activate my account so I will ask this here...


I am using the KSZ8081 PHY for MII Ethernet.


https://www.mouser.com/ProductDetail/Microchip-Technology-Micrel/KSZ8081MNXCA?qs=sGAEpiMZZMvdy8WAlGWLcC4eczfQr1zYEy39QIyqxLk%3d


The evaluation board schematic has 33 Ohm impedance matching resistors on the RX MII signal lines but NOT the TX MII signal lines.


In past experience with other PHY chips (TI DP83848 specifically), there are series resistors on both the TX[0:3] and RX[0:3] lines.


Why does KSZ8081 not have them on the TX[0:3] MII lines? Would it be a good idea to include them or should I follow the evaluation board exactly?


Here is a screenshot of the KSZ8081 eval board schematic...


enter image description here




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