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TM4C1294NCPDT: Ethernet connection to transformer - center taps

Part Number: TM4C1294NCPDT
Other Parts Discussed in Thread: TLK110

Tiva team,

The TM4C1294NCPDT datasheet shows the proper connection to an Ethernet transformer:

Specifically, on the HX1188FNL, the center taps '2' and '7' on the PHY side are connected to 3V3.  The question is why is this necessary?

I tried researching this on the web.  From what I could find, the center tap to Vdd (3V3) connection is made when the TX and RX signals are differential pairs (open drain pins).  The 3V3 connections of the center taps are needed to bias the transformer.  This is a bit too high level of an explanation though.

A customer has apparently connected the center taps to ground, and the Ethernet seems to be working fine.  I think this needs to be changed, but I cannot explain electrically what having the center taps connected to 3V3 does and why it is necessary.

Thank you and regards,

David