Datasheet

DP83848VYB

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SNLS266D –MAY 2007–REVISED APRIL 2013

7.2.7 Descrambler

A serial descrambler is used to de-scramble 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 (11111), it will synchronize to the receive data stream and

generate unscrambled data in the form of unaligned 5B code-groups.

In order 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 µs countdown. Upon detection of sufficient IDLE code-groups (58 bit times) within the 722 µs

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 line state

monitor does not recognize sufficient unscrambled IDLE code-groups within the 722 µs period, the entire

descrambler will be forced out of the current state of synchronization and reset in order to re-acquire

synchronization.

7.2.8 Code-group Alignment

The code-group alignment module operates on unaligned 5-bit data from the descrambler (or, if the

descrambler is bypassed, directly from the NRZI/NRZ decoder) and converts it into 5B code-group data (5

bits). Code-group alignment occurs after the J/K code-group pair is detected. Once the J/K code-group

pair (11000 10001) is detected, subsequent data is aligned on a fixed boundary.

7.2.9 4B/5B Decoder

The code-group decoder functions as a look up table that translates incoming 5B code-groups into 4B

nibbles. The code-group decoder first detects the J/K code-group pair preceded by IDLE code-groups and

replaces the J/K with MAC preamble. Specifically, the J/K 10-bit code-group pair is replaced by the nibble

pair (0101 0101). All subsequent 5B code-groups are converted to the corresponding 4B nibbles for the

duration of the entire packet. This conversion ceases upon the detection of the T/R code-group pair

denoting the End of Stream Delimiter (ESD) or with the reception of a minimum of two IDLE code-groups.

7.2.10 100BASE-TX Link Integrity Monitor

The 100 Base TX Link monitor ensures that a valid and stable link is established before enabling both the

Transmit and Receive PCS layer.

Signal detect must be valid for 395us to allow the link monitor to enter the 'Link Up' state, and enable the

transmit and receive functions.

7.2.11 Bad SSD Detection

A Bad Start of Stream Delimiter (Bad SSD) is any transition from consecutive idle code-groups to non-idle

code-groups which is not prefixed by the code-group pair /J/K.

If this condition is detected, the DP83848VYB will assert RX_ER and present RXD[3:0] = 1110 to the MII

for the cycles that correspond to received 5B code-groups until at least two IDLE code groups are

detected. In addition, the False Carrier Sense Counter register (FCSCR) will be incremented by one.

Once at least two IDLE code groups are detected, RX_ER and CRS become de-asserted.

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