XRT73LC03A Datasheet, PDF (40/61 Pages) Exar Corporation – 3 CHANNEL DS3/E3/STS-1 LINE INTERFACE UNIT

XRT73LC03A
3 CHANNEL DS3/E3/STS-1 LINE INTERFACE UNIT
REV. 1.0.1
3.3 CLOCK RECOVERY PLL
The purpose of the Clock Recovery PLL is to track
the incoming Dual-Rail data stream and to derive and
generate a recovered clock signal.
It is important to note that the Clock Recovery PLL re-
quires a line rate clock signal at the EXClk_(n) input
pin.
The Clock Recovery PLL operates in one of two
modes:
• The Training Mode.
• The Data/Clock Recovery Mode
3.3.1 The Training Mode
If a given channel is not receiving a line signal via the
RTIP and RRing input pins, or if the frequency differ-
ence between the line signal and that applied via the
EXClk_(n) input pin exceeds 0.5%, then the channel
operates in the Training Mode. When the channel is
operating in the Training Mode, it does the following:
a. Declare a Loss of Lock indication by toggling its
respective RLOL_(n) output pin “High".
b. Output a clock signal via the RxClk_(n) output
pins which is derived from the signal applied to
the EXClk_(n) input pin.
3.3.2 The Data/Clock Recovery Mode
If the frequency difference between the line signal
and that applied via the EXClk_(n) input pin is less
than 0.5%, then the channel operates in the Data/
Clock Recovery mode. In this mode, the Clock Re-
covery PLL locks onto the line signal via the RTIP and
RRing input pins.
FIGURE 24. AN EXAMPLE OF B3ZS DECODING
3.4 THE HDB3/B3ZS DECODER
The Remote Transmitting Terminal typically encodes
the line signal into some sort of Zero Suppression
Line Code (e.g., HDB3 for E3, and B3ZS for DS3 and
STS-1). The purpose of this encoding activity was to
aid in the Clock Recovery process of this data within
the Near-End Receiving Terminal. However, once the
data has made it across the E3, DS3 or STS-1 Trans-
port Medium and has been recovered by the Clock
Recovery PLL, it is now necessary to restore the orig-
inal content of the data. Hence, the purpose of the
HDB3/B3ZS Decoding block is to restore the data
transmitted over the E3, DS3 or STS-1 line to its orig-
inal content prior to Zero Suppression Coding.
3.4.1 B3ZS Decoding (DS3/STS-1 Applications)
If the XRT73LC03A is configured to operate in the
DS3 or STS-1 Modes, then the HDB3/B3ZS Decod-
ing Blocks performs B3ZS Decoding. When the De-
coders are operating in this mode, each of the Decod-
ers parses through its respective incoming Dual-Rail
data and checks for the occurrence of either a “00V"
or a "B0V" pattern. If the B3ZS Decoder detects this
particular pattern, then it substitutes these bits with a
"000" pattern.
NOTE: If the B3ZS Decoder detects any bipolar violations
that is not in accordance with the B3ZS Line Code format,
or if the B3ZS Decoder detects a string of 3 (or more) con-
secutive "0’s” in the incoming line signal, then the B3ZS
Decoder flags this event as a Line Code Violation by puls-
ing the LCV output pin “High".
Figure 24 illustrates the B3ZS Decoder at work with
two separate Zero Suppression patterns in the in-
coming Dual-Rail Data Stream.
Line Signal
RCLK
00 V
B 0V
RPOS
RNEG
Data
01011000101111011011001110001
3.4.2 HDB3 Decoding (E3 Applications)
If the XRT73LC03A is configured to operate in the E3
Mode, then each of the HDB3/B3ZS Decoding Blocks
performs HDB3 Decoding. When the Decoders are
operating in this mode, they each parse through the
incoming Dual-Rail data and check for the occurrence
of either a "000V" or a "B00V" pattern. If the HDB3
38