WBLXT9785HE.D0-865114 Cortina Systems Inc, WBLXT9785HE.D0-865114 Datasheet - Page 158

• Manufacturer Part Number: WBLXT9785HE.D0-865114
• Manufacturer: Cortina Systems Inc

Specifications of WBLXT9785HE.D0-865114

• Lead Free Status / RoHS Status: Compliant

LXT9785/LXT9785E
Datasheet
249241, Revision 11.0
16 April 2007
4.13.4
Cortina Systems
If Cable Diagnostics testing is completed using a powered down LXT9785/LXT9785E
device as the link partner, specific results can be expected. The results will indicate an
open connection when the PWRDWN hardware configuration pin is used. These power-
down methods disable the internal termination resistors to create a high impedance
connection equivalent to an open circuit.
If Transmit Disable (Register bit 16.13) or software controlled Power-Down (Register bit
0.11) is used, the powered down device transmit logic will look like an open circuit and the
receive circuit will look like a 100 Ω terminated connection. The Transmit Disable bit and
the software Power-Down bit disable the transmit circuit but do not affect the receive
circuit.
The result of Cable Diagnostic tests using an IP Phone indicate an open or a short fault at
a gross approximation of the distance to the IP Phone. The termination resistors are not
powered and do not create a proper termination. The filter circuit used by some
manufacturers adversely affects the test results.
Transmission and reception of packets is disabled when Cable Diagnostics is enabled.
Internal loopback must be disabled for Cable Diagnostics to operate properly. Internal
loopback disables the analog interface.
Basic Implementation
Register 29 is used to control and report the Cable Diagnostics test results. The function
tests one pair of the twisted-pair cable at a time. The basic process flow is described as
follows (see
for Register 29 bit definitions):
®
1. Disable auto-negotiation by clearing Register bit 0.12, set to MDI by clearing Register
2. Write 0x7400h to Register 29. Setting these bits places the device in short cable
3. Poll Register bit 29.9. When this bit is set, the test is complete and Register bits
4. Write 0x6C00h to Register 29. Setting these bits places the device in long cable Cable
5. Poll Register bit 29.9. When set, record the value of Register bits 29.7:0 if a fault is
6. If a fault is present, a calculation is used to determine the distance to the fault. Insert
7. Normal PHY operation can be resumed by writing 0x4000h to Register 29 or by
LXT9785 and LXT9785E Advanced 8-Port 10/100 Mbps PHY Transceivers
bits 27.9:8, and ensure internal loopback is disabled, Register bit 0.14 = 0.
Cable Diagnostics mode and forces link to drop. The device waits a specific amount
of time (1.2 s to 1.5 s) to ensure link drops on any connected link partner, and initiates
the Cable Diagnostics test on the selected twisted-pair.
29.7:0 contain a value used to determine if a cable fault was found and the distance to
that fault. A value of 0xFFh indicates no fault was found. Any other value indicates a
fault was found, that value should be stored for later use.
Diagnostics mode.
found.
the smallest value recorded from Register bits 29.7:0 in steps 3 and 5 above into the
following formula:
Register bit 29.8 is set if the fault is detected as a short circuit and is cleared if the
fault is detected as an open circuit. Register bits 29.12:11 are cleared when read and
are cleared during the same read cycle when Register bit 29.9 is read, indicating a
fault condition exists.
software or hardware reset. The test suite can be run again by resuming at step 2
above.
Distance_to_Fault = (Reg29[7:0] - 3.5) / 1.16
Table 101, Cable Diagnostics Register (Address 29, Hex 1D), on page 209
4.13 Cable Diagnostics
Overview
Page 158