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XRT79L71
PRELIMINARY
337
1-CHANNEL DS3/E3 CLEAR-CHANNEL FRAMERLIU COMBO - CC/HDLC ARCHITECTURE
REV. P2.0.0
To enable or disable the Jitter Attenuator PLL, within the XRT79L71 by execute the procedure presented in
5.2.5.2.1.2
The Jitter Attenuator FIFO
The Jitter Attenuator Block contains a 2-Channel FIFO.
The purpose of this FIFO is to permit the Jitter
Attenuator to absorb any instantaneous frequency differences between the In_CLK input signal and the
Out_CLK output signal and to mitigate the occurrences of bit-errors.
The Jitter Attenuator FIFO actually consists of 2 FIFO channels, in the sense that one FIFO channel is
dedicated for the "Positive-Polarity" Data (e.g., the In_POS to Out_POS path) and the other FIFO channel is
dedicated for the "Negative-Polarity" Data (e.g., the In_NEG to Out_NEG path). The Physical Architecture of
the 2-Channel Jitter Attenuator FIFO is presented below in Figure 157.
NOTE: The Logical Architecture of the Jitter Attenuator FIFO is presented in the next section.
SELECTING THE FIFO SIZE
The user can configure the Jitter Attenuator FIFO to operate with a depth of either 16 or 32 bits. A description
upon (a) how to configure the FIFO to operate with a 16 or 32 depth, and (b) its operation, is presented below
Operating with a Jitter Attenuator FIFO Depth of 16 bits
If the Jitter Attenuator FIFO is configured to operate with a depth of 16 bits, then the following is true.
a. When the XRT79L71 first powers up, or experiences a "Hardware RESET", then the location of the
FIFO_READ and FIFO_WRITE pointers will be 8 bits (or one-half the FIFO Size) apart from each other.
b. As a consequence, data, which is applied to the Jitter Attenuator block (via the In_POS_n and
In_NEG_n input pins) will be written into the FIFO (into a location determined by the FIFO_WRITE
pointer).
This same data will be read out of the FIFO approximately 8 bit periods later once the
"FIFO_READ" pointer has incremented around to this particular position within the FIFO. Hence, for 16-
bit mode operation, the Jitter Attenuator FIFO imposes a nominal latency of 8 bit periods.
Configuring the Jitter Attenuator FIFO Depth to 16 bits:
FIGURE 157. ILLUSTRATION OF THE PHYSICAL ARCHITECTURE OF 2-CHANNEL JITTER ATTENUATOR FIFO ARCHI-
TECTURE WITHIN THE
JITTER ATTENUATOR BLOCK
16/32 bit FIFO (for Positive Polarity Data)
16/32 bit FIFO (for Negative Polarity Data)
In_POS
In_NEG
In_CLK
Out_POS
Out_NEG
Out_CLK
From Jitter Attenuator
PLL Block
FIFO WRITE
Clock Signal
FIFO READ
Clock Signal