LAN9252VSLASHML datasheet(137/329 Pages) MICROCHIP

137 / 329 page

 2015 Microchip Technology Inc.

DS00001909A-page 137

LAN9252

The TDR operates by transmitting pulses on the selected twisted pair within the Ethernet cable (TX in MDI mode, RX

in MDIX mode). If the pair being tested is open or shorted, the resulting impedance discontinuity results in a reflected

signal that can be detected by the PHY. The PHY measures the time between the transmitted signal and received reflec-

tion and indicates the results in the TDR Channel Length field of the PHY x TDR Control/Status Register (PHY_TDR_-

CONTROL_STAT_x). The TDR Channel Length field indicates the “electrical” length of the cable, and can be multiplied

by the appropriate propagation constant in Table 11-5 to determine the approximate physical distance to the fault.

FIGURE 11-5:

TDR USAGE FLOW DIAGRAM

Note:

The TDR function is typically used when the link is inoperable. However, an active link will drop when oper-

ating the TDR.

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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