Datasheet

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DP83849I
— Receive Window: The receiver can monitor receive
data within a programmable window using the TDR Win
-
dow Register (TDR_WIN), address 17h. The window is
controlled by two register values: TDR Start Window, bits
[15:8] of TDR_WIN (17h) and TDR Stop Window, bits
[7:0] of TDR_WIN (17h). The TDR Start Window indi
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cates the first clock to start sampling. The TDR Stop
Window indicates the last clock to sample. By default,
the full window is enabled, with Start set to 0 and Stop
set to 255. The window range is in 8ns clock increments,
so the maximum window size is 2048ns.
5.7.3.4 TDR Results
The TDR function monitors data from the Analog to Digital
Converter (ADC) to detect both peak values and values
above a programmable threshold. It can be programmed
to detect maximum or minimum values. In addition, it
records the time, in 8ns intervals, at which the peak or
threshold value first occurs. The results of a TDR peak and
threshold measurement are available in the TDR Peak
Measurement Register (TDR_PEAK), address 18h and
TDR Threshold Measurement Register (TDR_THR),
address 19h. The threshold measurement may be a more
accurate method of measuring the length for longer cables
to provide a better indication of the start of the received
pulse, rather than the peak value.
Software utilizing the TDR function should implement an
algorithm to send TDR pulses and evaluate results. Multi
-
ple runs should be used to best qualify any received pulses
as multiple reflections could exist. In addition, when moni-
toring the transmitting pair, the window feature should be
used to disqualify the transmitted pulse. Multiple runs may
also be used to average the values providing more accu
-
rate results.
Actual distance measurements are dependent on the
velocity of propagation of the cable. The delay value is typ
-
ically on the order of 4.6 to 4.9 ns/m.