ADM7008
Function Description
correlations between production and system testing. Baud rate Adaptive
Equalizer/Timing Recovery compensates line loss induced from twisted pair and tracks
far end clock at 125M samples per second. Adaptive Equalizer implemented with Feed
forward and Decision Feedback techniques meet the requirement of BER less than 10-12
for transmission on CAT5 twisted pair cable ranging from 0 to 140 meters.
NRZI/NRZ and Serial/Parallel Decoder
The recovered data is converted from NRZI to NRZ. The data is not necessarily aligned
to 4B/5B code group’s boundary.
Data Descrambling
The descrambler acquires synchronization with the data stream by recognizing idle bursts
of 40 or more bits and locking its deciphering Linear Feedback Shift Register (LFSR) to
the state of the scrambling LFSR. Upon achieving synchronization, the incoming data is
XORed by the deciphering LFSR and descrambled.
In order to maintain synchronization, the descrambler continuously monitors the validity
of the unscrambled data that it generates. To ensure this, a link state monitor and a hold
timer are used to constantly monitor the synchronization status. Upon synchronization of
the descrambler the hold timer starts a 722 us countdown. Upon detection of at least 6
idle symbols (30 consecutive “1”) within the 722 us period, the hold timer will reset and
begin a new countdown. This monitoring operation will continue indefinitely given a
properly operating network connection with good signal integrity. If the link state
monitor does not recognize at least 6 unscrambled idle symbols within 722 us period, the
descrambler will be forced out of the current state of synchronization and reset in order to
re-acquire synchronization.
Symbol Alignment
The symbol alignment circuit in the ADM7008 determines code word alignment by
recognizing the /J/K delimiter pair. This circuit operates on unaligned data from the
descrambler. Once the /J/K symbol pair (11000 10001) is detected, subsequent data is
aligned on a fixed boundary.
Symbol Decoding
The symbol decoder functions as a look-up table that translates incoming 5B symbols
into 4B nibbles as shown in Table 3-1. The symbol decoder first detects the /J/K symbol
pair preceded by idle symbols and replaces the symbol with MAC preamble. All
subsequent 5B symbols are converted to the corresponding 4B nibbles for the duration of
the entire packet. This conversion ceases upon the detection of the /T/R symbol pair
denoting the end of stream delimiter (ESD). The translated data is presented on the
internal RXD[3:0] signal lines with RXD[0] represents the least significant bit of the
translated nibble.
ADMtek Inc.
3-4
ETC [ ETC ]
