2-26
Functional Description
2.7.1 MLT-3 to NRZ Decoder
The L80600 decodes the MLT-3 information from the DSP block to binary
NRZI form and finally to NRZ data.
2.7.2 Descrambler
A serial descrambler is used to descramble the received NRZ data. The
descrambler has to generate an identical data scrambling sequence (N)
in order to recover the original unscrambled data (UD) from the
scrambled data (SD) as represented in the equations:
Synchronization of the descrambler to the original scrambling sequence
(N) is achieved based on the knowledge that the incoming scrambled
data stream consists of scrambled IDLE data. After the descrambler has
recognized 12 consecutive IDLE code-groups, where an unscrambled
IDLE code-group in 5B NRZ is equal to five consecutive ones (0b11111),
it synchronizes to the receive data stream and generates unscrambled
data in the form of unaligned 5B code-groups.
To maintain synchronization, the descrambler must continuously monitor
the validity of the unscrambled data that it generates. To ensure this, a
line 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
μ
s countdown. Upon detection of sufficient IDLE
code-groups (16 idle symbols) within the 722
μ
s period, the hold timer
resets and begins a new countdown. This monitoring operation continues
indefinitely given a properly operating network connection with good
signal integrity. If the line state monitor does not recognize sufficient
unscrambled IDLE code-groups within the 722
μ
s period, the entire
descrambler is forced out of the current state of synchronization and
reset in order to reacquire synchronization.
2.7.3 Serial to Parallel Converter
The 100BASE-X receiver includes a Serial to Parallel converter. The
converter also provides code-group alignment, and operates on
unaligned serial data from the descrambler (or, if the descrambler is
bypassed, directly from the MLT-3 to NRZ decoder) and converts it into
UD
SD
N
⊕
(
)
=
SD
UD
N
⊕
(
)
=