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XRT72L50
á
SINGLE CHANNEL DS3/E3 FRAMER WITH HDLC CONTROLLER
REV. 1.2.1
293
equipment has the task of recovering this data and timing information from the incoming E3 data stream. Most
clock and data recovery schemes rely on the use of Phase-Locked-Loop technology. One of the problems of
using Phase-Locked-Loop (PLL) technology for clock recovery is that it relies on transitions in the line signal, in
order to maintain lock with the incoming E3 data-stream.
Therefore, these clock recovery scheme, are
vulnerable to the occurrence of a long stream of consecutive zeros (e.g., no transitions in the line). 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 (or High Density Bipolar -3) encoding.
In general the HDB3 line code behaves just like AMI with the exception of the case when a long string of
consecutive zeros occurs on the line. Any 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. The Receive E3 LIU Interface
block, when operating with the HDB3 Line Code is responsible for decoding the HD-encoded data back into a
unipolar (binary-format). For instance, if the Receive E3 LIU Interface block detects a "000V" or a "B00V"
pattern in the incoming pattern, the Receive E3 LIU Interface block will replace it with four (4) consecutive
zeros.
Figure 120 presents a timing diagram that illustrates examples of HDB3 decoding.
5.3.1.2.3
Line Code Violations
The Receive E3 LIU Interface block will also check the incoming E3 data stream for line code violations. For
example, when the Receive E3 LIU Interface block detects a valid bipolar violation (e.g., in HDB3 line code), it
will substitute four zeros into the binary data stream. However, if the bipolar violation is invalid, then an LCV
(Line Code Violation) is flagged and the PMON LCV Event Count Register (Address = 0x50 and 0x51) will also
be incremented. Additionally, the LCV-One-Second Accumulation Registers (Address = 0x6E and 0x6F) will
be incremented. For example: If the incoming E3 data is HDB3 encoded, the Receive E3 LIU Interface block
will also increment the LCV One-Second Accumulation Register if three (or more) consecutive zeros are
received.
5.3.1.2.4
RxLineClk Clock Edge Selection
The incoming unipolar or bipolar data, applied to the RxPOS and the RxNEG input pins are clocked into the
Receive E3 LIU Interface block via the RxLineClk signal. The Framer IC allows the user to specify which edge
(e.g, rising or falling) of the RxLineClk signal will sample and latch the signal at the RxPOS and RxNEG input
signals into the Framer IC. The user can make this selection by writing the appropriate data to bit 1 of the I/O
Control Register, as depicted below.
FIGURE 120. ILLUSTRATION OF TWO EXAMPLES OF HDB3 DECODING
Line Signal
00
0
V
B0
0
V
RxNEG
RxPOS
Data
1
0
1
0
1
10
0
00
1