KSZ8081MLX datasheet(21/56 Pages) MICROCHIP

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DS00002264A-page 21

KSZ8081MLX

3.8

LinkMD® Cable Diagnostic

The LinkMD function uses time-domain reflectometry (TDR) to analyze the cabling plant for common cabling problems.

These include open circuits, short circuits, and impedance mismatches.

LinkMD works by sending a pulse of known amplitude and duration down the MDI or MDI-X pair, then analyzing the

shape of the reflected signal to determine the type of fault. The time duration for the reflected signal to return provides

the approximate distance to the cabling fault. The LinkMD function processes this TDR information and presents it as

a numerical value that can be translated to a cable distance.

LinkMD is initiated by accessing Register 1Dh, the LinkMD Control/Status register, in conjunction with Register 1Fh, the

PHY Control 2 register. The latter register is used to disable Auto MDI/MDI-X and to select either MDI or MDI-X as the

cable differential pair for testing.

3.8.1

USAGE

The following is a sample procedure for using LinkMD with Registers 1Dh and 1Fh:

Disable auto MDI/MDI-X by writing a ‘1’ to Register 1Fh, bit [13].

Start cable diagnostic test by writing a ‘1’ to Register 1Dh, bit [15]. This enable bit is self-clearing.

Wait (poll) for Register 1Dh, bit [15] to return a ‘0’, and indicating cable diagnostic test is completed.

Read cable diagnostic test results in Register 1Dh, bits [14:13]. The results are as follows:

00 = normal condition (valid test)

01 = open condition detected in cable (valid test)

10 = short condition detected in cable (valid test)

11 = cable diagnostic test failed (invalid test)

The ‘11’ case, invalid test, occurs when the device is unable to shut down the link partner. In this instance, the test is

not run because it would be impossible for the device to determine if the detected signal is a reflection of the signal

generated or a signal from another source.

Get distance to fault by concatenating Register 1Dh, bits [8:0] and multiplying the result by a constant of 0.38.

The distance to the cable fault can be determined by the following formula:

EQUATION 3-1:

Concatenated value of Registers 1Dh bits [8:0] should be converted to decimal before multiplying by 0.38.

The constant (0.38) may be calibrated for different cabling conditions, including cables with a velocity of propagation

that varies significantly from the norm.

3.9

NAND Tree Support

The KSZ8081MLX provides parametric NAND tree support for fault detection between chip I/Os and board. The NAND

tree is a chain of nested NAND gates in which each KSZ8081MLX digital I/O (NAND tree input) pin is an input to one

NAND gate along the chain. At the end of the chain, the CRS/CONFIG1 pin provides the output for the nested NAND

gates.

The NAND tree test process includes:

• Enabling NAND tree mode

• Pulling all NAND tree input pins high

• Driving each NAND tree input pin low, sequentially, according to the NAND tree pin order

• Checking the NAND tree output to make sure there is a toggle high-to-low or low-to-high for each NAND tree input

driven low

Table 3-5 lists the NAND tree pin order.

DDis

tan

to cable fault in meters



0.38





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