Datasheet

KSZ8081MNX/RNB
DS00002202C-page 28 2016-2018 Microchip Technology Inc.
3.9 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.
3.9.1 USAGE
The following is a sample procedure for using LinkMD with Registers 1Dh and 1Fh:
1. Disable auto MDI/MDI-X by writing a ‘1’ to Register 1Fh, bit [13].
2. Start cable diagnostic test by writing a ‘1’ to Register 1Dh, bit [15]. This enable bit is self-clearing.
3. Wait (poll) for Register 1Dh, bit [15] to return a ‘0’, and indicating cable diagnostic test is completed.
4. 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, since 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.
5. 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:
D (distance to cable fault) = 0.38 x (Register 1Dh, bits [8:0])
D (distance to cable fault) is expressed in meters.
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 propaga-
tion that varies significantly from the norm.
3.10 NAND Tree Support
The KSZ8081MNX/RNB 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 KSZ8081MNX/RNB digital I/O (NAND tree input) pin is an
input to one NAND gate along the chain. At the end of the chain, the TXD1 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 3-6 and Table 3-7 list the NAND tree pin orders for KSZ8081MNX and KSZ8081RNB, respectively.