Datasheet

 2016-2017 Microchip Technology Inc. DS00002112B-page 17
KSZ8795CLX
3.1.9 LINKMD
®
CABLE DIAGNOSTICS
The LinkMD feature utilizes time-domain reflectometry (TDR) to analyze the cabling plant for common cabling problems
such as open circuits, short circuits, and impedance mismatches.
LinkMD works by sending a pulse of known amplitude and duration down the MDI and MDI-X pairs and then analyzes
the shape of the reflected signal. Timing the pulse duration gives an indication of the distance to the cabling fault with
maximum distance of 200m and accuracy of ±2m. Internal circuitry displays the TDR information in a user-readable dig-
ital format.
Note: Cable diagnostics are only valid for copper connections only.
3.1.9.1 Access
LinkMD is initiated by accessing the PHY special control/status Registers 26, 42, 58, 74 and the LinkMD result Registers
27, 43, 59, and 75 for Ports 1, 2, 3, and 4 respectively; and in conjunction with the Port Control 10 Register for Ports 1,
2, 3, and 4 respectively to disable Auto-MDI/MDI-X.
Alternatively, the MIIM PHY Registers 0 and 1d can also be used for LinkMD access.
3.1.9.2 Usage
The following is a sample procedure for using LinkMD with Registers {26, 27, and 29} on Port 1:
1. Disable auto MDI/MDI-X by writing a ‘1’ to Register 29, Bit[2] to enable manual control over the differential pair
used to transmit the LinkMD pulse.
2. Start cable diagnostic test by writing a ‘1’ to Register 26, Bit[4]. This enable bit is self-clearing.
3. Wait (poll) for Register 26, Bit[4] to return a ‘0’, and indicating cable diagnostic test is completed.
4. Read cable diagnostic test results in Register 26, bits [6:5]. The results are as follows:
FIGURE 3-3: AUTO-NEGOTIATION AND PARALLEL OPERATION