LAN8740A Datasheet, PDF (50/136 Pages) Microchip Technology – Small Footprint MII/RMII 10/100 Energy Efficient Ethernet Transceiver with HP Auto-MDIX and flexPWR® Technology

LAN8740A/LAN8740Ai
3.8.11.2 Matched Cable Diagnostics
Matched cable diagnostics enable cable length estimation on 100 Mbps-linked cables of up to 120 meters. If there is an
active 100 Mb link, the approximate distance to the link partner can be estimated using the Cable Length Register. If
the cable is properly terminated, but there is no active 100 Mb link (the link partner is disabled, nonfunctional, the link is
at 10 Mb, etc.), the cable length cannot be estimated and the Cable Length Register should be ignored. The estimated
distance to the link partner can be determined via the Cable Length (CBLN) lookup table provided in Table 3-12. The
typical cable length measurement margin of error for a matched cable case is +/- 20 m. The matched cable length mar-
gin of error is consistent for all cable types from 0 to 120 m.
TABLE 3-12: MATCH CASE ESTIMATED CABLE LENGTH (CBLN) LOOKUP TABLE
CBLN Field Value
0-3
4
5
6
7
8
9
10
11
12
13
14
15
Estimated Cable Length
0
6
17
27
38
49
59
70
81
91
102
113
123
Note: For a properly terminated cable (Match case), there is no reflected signal. In this case, the TDR Channel
Length field is invalid and should be ignored.
DS00001987A-page 50
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