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IDT82P2281
SINGLE T1/E1/J1 LONG HAUL / SHORT HAUL TRANSCEIVER
Functional Description
28
August 20, 2009
3.6
RECEIVE JITTER ATTENUATOR
The Receive Jitter Attenuator can be chosen to be used or not. This
selection is made by the RJA_E bit.
The Jitter Attenuator consists of a FIFO and a DPLL, as shown in
Figure 7. Jitter Attenuator
The FIFO is used as a pool to buffer the jittered input data, then the
data is clocked out of the FIFO by a de-jittered clock. The depth of the
FIFO can be 32 bits, 64 bits or 128 bits, as selected by the RJA_DP[1:0]
bits. Accordingly, the constant delay produced by the Jitter Attenuator is
16 bits, 32 bits or 64 bits. The 128-bit FIFO is used when large jitter tol-
erance is expected, while the 32-bit FIFO is used in delay sensitive
applications.
The DPLL is used to generate a de-jittered clock to clock out the
data stored in the FIFO. The DPLL can only attenuate the incoming jitter
whose frequency is above Corner Frequency (CF). The jitter whose fre-
quency is lower than the CF passes through the DPLL without any atten-
uation. In T1/J1 applications, the CF of the DPLL can be 5 Hz or 1.26
Hz, as selected by the RJA_BW bit. In E1 applications, the CF of the
DPLL can be 6.77 Hz or 0.87 Hz, as selected by the RJA_BW bit. The
lower the CF is, the longer time is needed to achieve synchronization.
If the incoming data moves faster than the outgoing data, the FIFO
will overflow. If the incoming data moves slower than the outgoing data,
the FIFO will underflow. The overflow or underflow is captured by the
RJA_IS bit. When the RJA_IS bit is ‘1’, an interrupt will be reported on
the INT pin if enabled by the RJA_IE bit.
To avoid overflow or underflow, the JA-Limit function can be
enabled by setting the RJA_LIMT bit. When the JA-Limit function is
enabled, the speed of the outgoing data will be adjusted automatically if
the FIFO is close to its full or emptiness. The criteria of speed adjust-
ment start are listed in
Table 6. Though the JA-Limit function can reduce
the possibility of FIFO overflow and underflow, the quality of jitter attenu-
ation is deteriorated.
Selected by the RJITT_TEST bit, the real time interval between the
read and write pointer of the FIFO or the peak-peak interval between the
read and write pointer of the FIFO can be indicated in the RJITT[6:0]
bits. When the RJITT_TEST bit is ‘0’, the current interval between the
read and write pointer of the FIFO will be written into the RJITT[6:0] bits.
When the RJITT_TEST bit is ‘1’, the current interval will be compared
with the old one in the RJITT[6:0] bits and the larger one will be indi-
cated by the RJITT[6:0] bits.
The performance of Receive Jitter Attenuator meets the ITU-T
I.431, G.703, G.736 - 739, G.823, G.824, ETSI 300011, ETSI TBR 12/
13, AT&T TR62411, TR43802, TR-TSY 009, TR-TSY 253, TR-TRY 499
Table 6: Criteria Of Speed Adjustment Start
FIFO Depth
Criteria Of Speed Adjustment Start
32 bits
2-bit close to full or empty
64 bits
3-bit close to full or empty
128 bits
4-bit close to full or empty
FIFO
32/64/128
DPLL
Jittered Data
De-jittered Data
Jittered Clock
De-jittered Clock
write
pointer
read
pointer
Bit
Register
Address (Hex)
RJA_E
Receive Jitter Attenuation Configuration
027
RJA_DP[1:0]
RJA_BW
RJA_LIMT
RJITT_TEST
RJA_IS
Interrupt Status 1
03B
RJA_IE
Interrupt Enable Control 1
034
RJITT[6:0]
Receive Jitter Measure Value Indication
039