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I want to use the LAN9500AI controller in an application. This is a USB <-> Ethernet bridge with access to the MII interface. Since I just need the USB and MII interface, the ethernet interface (RXP, RXN, TXP, TXN) will be unconnected. But what's the "default" state of these pins if they are unused? Should I connect pull ups/downs to these pins? (I assume, this is not necessary for TXP, TXN).

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  • \$\begingroup\$ The typical ethernet connection for those pins require 50ohm pull-ups to 3.3V, for all four pins. Not sure what the MII interface requirements for it would be. Have you emailed Microchip/SMSC support? \$\endgroup\$ – Passerby Oct 11 '15 at 19:06
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In principle, the device should work damage free without anything connected.

But, since you can't be sure without direct input from Microchip or whatever SMSC is, just hook up resistors like on page 8 of this datasheet and leave out the inductor and other nodes that remain floating. Or, put differently: As Passerby comments all lines up to 3.3V with 50Ohm.

Unless you can obtain an app-note I failed to find explicitly saying otherwise, that seems to be the best idea.

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