LAN8740AI-EN datasheet(46/136 Pages) MICROCHIP

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LAN8740A/LAN8740Ai

DS00001987A-page 46

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3.8.11

CABLE DIAGNOSTICS

The LAN8740A/LAN8740Ai provides cable diagnostics which allow for open/short and length detection of the Ethernet

cable. The cable diagnostics consist of two primary modes of operation:

• Time Domain Reflectometry (TDR) Cable Diagnostics

TDR cable diagnostics enable the detection of open or shorted cabling on the TX or RX pair, as well as cable

length estimation to the open/short fault.

• Matched Cable Diagnostics

Matched cable diagnostics enable cable length estimation on 100 Mbps-linked cables.

Refer to the following sub-sections for details on proper operation of each cable diagnostics mode.

3.8.11.1

Time Domain Reflectometry (TDR) Cable Diagnostics

The LAN8740A/LAN8740Ai provides TDR cable diagnostics which enable the detection of open or shorted cabling on

the TX or RX pair, as well as cable length estimation to the open/short fault. To utilize the TDR cable diagnostics, Auto-

MDIX and Auto Negotiation must be disabled, and the LAN8740A/LAN8740Ai device must be forced to 100 Mb full-

duplex mode. These actions must be performed before setting the TDR Enable bit in the TDR Control/Status Register.

With Auto-MDIX disabled, the TDR will test the TX or RX pair selected by register bit 27.15 (AMDIXCTRL). Proper cable

testing should include a test of each pair. When TDR testing is complete, prior register settings may be restored. Figure

3-13 provides a flow diagram of proper TDR usage.

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