Datasheet

DP83849IF
SNOSAX8D –JUNE 2009–REVISED APRIL 2013 www.ti.com
8.8.7 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. Multiple runs should be used to best qualify any received pulses as multiple reflections could exist.
In addition, when monitoring 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 accurate results.
Actual distance measurements are dependent on the velocity of propagation of the cable. The delay value
is typically on the order of 4.6 to 4.9 ns/m.
68 Design Guidelines Copyright © 2009–2013, Texas Instruments Incorporated
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