XRT7250 datasheet(298/463 Pages) EXAR

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XRT7250

DS3/E3 FRAMER IC

REV. 1.1.1

279

NOTE: One of the main reasons that the AMI Line Code

has been chosen for driving transformer-coupled media is

that this line code introduces no dc component, thereby

minimizing dc distortion in the line.

5.2.5.1.1.2

The HDB3 Line Code

The Transmit E3 Framer and the associated LIU IC

combine the data and timing information (originating

from the TxLineClk signal) into the line signal that is

transmitted to the remote receiver. The remote re-

ceiver has the task of recovering this data and timing

information from the incoming E3 data stream. Many

clock and data recovery schemes rely on the use of

Phase Locked Loop technology. Phase-Locked-Loop

(PLL) technology for clock recovery relies on transi-

tions in the line signal, in order to maintain lock with

the incoming E3 data stream. However, PLL-based

clock recovery scheme, are vulnerable to the occur-

rence of a long stream of consecutive zeros (e.g., the

absence of transitions). This scenario can cause the

PLL to lose lock with the incoming E3 data, thereby

causing the clock and data recovery process of the

receiver to fail. Therefore, some approach is needed

to insure that such a long string of consecutive zeros

can never happen. One such technique is HDB3 en-

coding. HDB3 (or High Density Bipolar - 3) is a form

of AMI line coding that implements the following rule.

In general the HDB3 line code behaves just like AMI

with the exception of the case when a long string of

consecutive zeros occur on the line. Any string of 4

consecutive zeros will be replaced with either a

"000V" or a "B00V" where "B" refers to a Bipolar

pulse (e.g., a pulse with a polarity that is compliant

with the AMI coding rule). And "V" refers to a Bipolar

Violation pulse (e.g., a pulse with a polarity that vio-

lates the alternating polarity scheme of AMI.) The de-

cision between inserting an "000V" or a "B00V" is

made to insure that an odd number of Bipolar (B)

pulses exist between any two Bipolar Violation (V)

pulses. Figure 124 presents a timing diagram that il-

lustrates examples of HDB3 encoding.

The user chooses between AMI or HDB3 line coding

by writing to bit 4 of the I/O Control Register (Address

= 0x01), as shown below.

FIGURE 123. ILLUSTRATION OF AMI LINE CODE

Data

TxPOS

TxNEG

Line Signal

FIGURE 124. ILLUSTRATION OF TWO EXAMPLES OF HDB3 ENCODING

Data

TxPOS

TxNEG

TxLineClk

Line Signal

XRT7250 Datasheet(PDF) 298 Page - Exar Corporation

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