LAN9250 datasheet(225/421 Pages) MICROCHIP

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DS00001913A-page 225

LAN9250

12.2.14.1

Time Domain Reflectometry (TDR) Cable Diagnostics

The PHY 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 PHY must be forced to 100 Mbps full-duplex mode. These actions must be per-

formed before setting the TDR Enable bit in the PHY TDR Control/Status Register (PHY_TDR_CONTROL_STAT). With

Auto-MDIX disabled, the TDR will test the TX or RX pair selected by register bit 27.13 (Auto-MDIX State (AMDIX-

STATE)). Proper cable testing should include a test of each pair. TDR cable diagnostics is not appropriate for 100BASE-

FX mode. When TDR testing is complete, prior register settings may be restored. Figure 12-5 provides a flow diagram

of proper TDR usage.

FIGURE 12-5:

TDR USAGE FLOW DIAGRAM

Disable AMDIX and Force MDI (or MDIX)

Write PHY Reg 27: 0x8000 (MDI)

- OR -

Write PHY Reg 27: 0xA000 (MDIX)

TDR Channel Status Complete?

Disable ANEG and Force 100Mb Full-

Duplex

Write PHY Reg 0: 0x2100

Enable TDR

Write PHY Reg 25: 0x8000

Reg 25.8 == 0

YES

Reg 25.8 == 1

Check TDR Control/Status Register

Read PHY Reg 25

Save:

TDR Channel Type (Reg 25.10:9)

TDR Channel Length (Reg 25.7:0)

MDIX Case Tested?

YES

Repeat Testing

in MDIX Mode

Done

Start

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