LAN8808D datasheet(33/80 Pages) MICROCHIP

33 / 80 page

LAN8808D

DS00005833C-page 33

5.6.3

AUTO-DOWNSHIFT

Under certain fault conditions (e.g., a cable break in wiring pair C or D), standard auto-negotiation will loop indefinitely,

attempting to link at 1 Gbps without ever fully linking up. Auto-Downshift extends standard auto-negotiation to allow

slower speed linkup under such conditions.

If Auto-Downshift is enabled (ENABLE_AUTO_DOWN_SPEED=1), a hardware counter tallies the number of attempted

1 Gbps linkups. After four failures, Auto-Negotiation will restart, this time advertising 100 Mbps as the maximum capable

speed.

If 100 Mbps fails to link after four attempts, auto-negotiation will restart, this time advertising 10 Mbps as the maximum

capable speed.

If 10 Mbps fails to link after four attempts, auto-negotiation may loop indefinitely, as there is no slower speed.

5.7

LinkMD Cable Diagnostics

The LinkMD function uses Time Domain Reflectometry (TDR) to analyze the cabling plant for common cabling prob-

lems, such as open circuits, short circuits, and impedance mismatches as well as the distance to the fault. Each of the

four twisted pairs are tested separately.

LinkMD operates by sending a pulse of known amplitude and duration down the selected differential pair, then analyzing

the polarity and shape of the reflected signal to determine the type of fault: open circuit for a positive/non-inverted ampli-

tude reflection and short circuit for a negative/inverted amplitude reflection. 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 the Cable Diagnostic register. To test each individual cable pair, set the cable pair in

the Cable Diagnostics Test Pair (VCT_PAIR[1:0]) field of the Cable Diagnostic register, along with setting the Cable

Diagnostics Test Enable (VCT_EN) bit. The Cable Diagnostics Test Enable (VCT_EN) bit will self-clear when the test is

concluded.

The test results (for the pair just tested) are available in the Cable Diagnostic register. With the bit[9:0] Definition (VCT_-

SEL[1:0]) field set to ‘0’, the Cable Diagnostics Status (VCT_ST[1:0]) field will indicate a Normal (properly terminated),

Open or Short condition, or Failed test.

If the test result was Open or Short, the Cable Diagnostics Data or Threshold (VCT_DATA[7:0]) field indicates the dis-

tance to the fault in meters as approximately:

• distance to fault = (VCT_DATA - 22) * 4 / cable propagation velocity

This provides an accuracy of ± 2% to 3% for short and medium cables and ± 5% to 6% for long cables. Inaccuracy is

due to cable pitch differences between cable manufacturers, where cable pitch is the number of wire twists per unit cable

length.

Link MD supports diagnostic cable testing int he following three scenarios:

• LinkMD with No Link Partner

• LinkMD with Fully Passive Link Partner

• LinkMD with Partially Active Link Partner

5.7.1

LINKMD WITH NO LINK PARTNER

In this scenario, the remote end of the cable is unplugged.

Prior to running the cable diagnostics, perform a software reset via the Basic Control Register. After reset, the following

must be configured:

• Auto Negotiation disabled via the Digital Debug Control 1 Register

• Auto MDI/MDI-X disabled via the Digital Debug Control 1 Register

• Full Duplex set via the Basic Control Register

• Link Speed set to 1000 Mbps via the Basic Control Register

APPLICATION NOTE: ENABLE_AUTO_DOWN_SPEED is a field in a register. It is Bit 12 in the PCS 1000 Test

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