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XRT91L30 View Datasheet(PDF) - Exar Corporation

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XRT91L30 Datasheet PDF : 39 Pages
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xr
REV. 1.0.1
XRT91L30
STS-12/STM-4 OR STS-3/STM-1 SONET/SDH TRANSCEIVER
2.3 Receive Clock and Data Recovery
The clock and data recovery (CDR) unit accepts the high speed NRZ serial data from the Differential LVPECL
receiver and generates a clock that is the same frequency as the incoming data. The clock recovery can either
utilize the transmitter’s CMU reference clock from either REFCLKP/N or TTLREFCLK or it can use
independent clock source CDRAUXREFCLK to train and monitor its clock recovery PLL. Initially upon startup,
the PLL locks to the local reference clock within ±500 ppm. Once this is achieved, the PLL then attempts to
lock onto the incoming receive data stream. Whenever the recovered clock frequency deviates from the local
reference clock frequency by more than approximately ±500 ppm, the clock recovery PLL will switch and lock
back onto the local reference clock. Whenever a Loss of Lock or a Loss of Signal event occurs, the CDR will
continue to supply a receive clock (based on the local reference) to the framer/mapper device. When the
LOSEXT is asserted by the optical module or when LOS is detected, the receive parallel data output will be
forced to a logic zero state for the entire duration that a LOS condition is detected. This acts as a receive data
mute upon LOS function to prevent random noise from being misinterpreted as valid incoming data. When the
LOSEXT becomes inactive and the recovered clock is determined to be within ±500 ppm accuracy with respect
to the local reference source and LOS is no longer declared, the clock recovery PLL will switch and lock back
onto the incoming receive data stream. Table 5 shows Clock and Data Recovery reference clock settings.
Table 6 specifies the Clock and Data Recovery Unit performance characteristics.
TABLE 5: CLOCK DATA RECOVERY UNIT REFERENCE CLOCK SETTINGS
CMUFREQSEL CDRREFSEL
STS12/
STS3
REFCLKP/N1OR
TTLREFCLK1
FREQUENCY (MHZ)
CDRAUXREFCLK2
FREQUENCY (MHZ)
CDR OUTPUT
FREQUENCY (MHZ)
0
0
0
77.76 MHz
not used
155.52
0
0
1
77.76 MHz
not used
622.08
1
0
0
19.44 MHz
not used
155.52
1
0
1
19.44 MHz
not used
622.08
X
1
0
not referenced by CDR
77.76 MHz
155.52
X
1
1
not referenced by CDR
77.76 MHz
622.08
1Requires frequency accuracy better than 20ppm in order for the transmitted data rate frequency to have the necessary
accuracy required for SONET systems.
2CDRAUXREFCLK requires accuracy of 77.76 MHz +/- 200ppm.
NAME
REFDUTY
REFTOL
TABLE 6: CLOCK AND DATA RECOVERY UNIT PERFORMANCE
Reference clock duty cycle
PARAMETER
MIN
TYP
40
Reference clock frequency tolerance
-200
MAX
60
+200
UNITS
%
ppm
15
 

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