How do you terminate unused TX/RX pairs when using magnetics for only one pair?

For 10/100 operations only, the unused TX/RX pairs C and D can be left as "no connects". Also, after power-up/reset, the KSZ9031RNX should be restricted to 'auto-negotiate' and link-up to 10/100 Mbps speeds only by programming the following register settings:

  • Set Register 0h, Bit 6 to '0' to remove 1000 Mbps speed.
  • Set Register 9h, Bits 9:8 to '00' to remove the Auto-Negotiation Advertisements for 1000 Mbps full/half duplex.
  • Write a '1' to Register 0h, Bit 9, a self-clearing bit, to force a restart of Auto-Negotiation.

If not restricted, there could be link-up issues when the KSZ9031RNX is connected to a gigabit PHY link partner and could attempt to auto-negotiate to 1000 Mbps speed. Auto-negotiation uses only TX/RX pairs A and B, which are present. As a result, the common 1000 Mbps speed is detected between link partners, but 1000 Base-T link-up is not possible with TX/RX pairs C and D not connected.

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