XRT72L50 Datasheet, PDF (402/471 Pages) Exar Corporation – SINGLE CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER

XRT72L50
SINGLE CHANNEL DS3/E3 FRAMER WITH HDLC CONTROLLER
REV. 1.2.1
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FIGURE 169. ILLUSTRATION OF AMI LINE CODE
Data 1 0 1 1 0 0 0 1 0 1 1 1 1 0 1 1 0 1 1 0 0 1 1 0 0 0 0 1
TxPOS
TxNEG
Line Signal
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.
6.2.5.1.1.2
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 receiver 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 transitions 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 occurrence 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 encoding. 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 violates the alternating
polarity scheme of AMI.) The decision 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 170 presents a timing
diagram that illustrates examples of HDB3 encoding.
FIGURE 170. ILLUSTRATION OF TWO EXAMPLES OF HDB3 ENCODING
Data 1 0 1 1 0 0 0 0 0 1 1 1 1 0 1 1 0 1 1 0 0 1 1 0 0 0 0 1
TxPOS
TxNEG
TxLineClk
Line Signal
0 00V
B0 0V
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.
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