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PRELIMINARY
XRT79L71
160
REV. P2.0.0
1-CHANNEL DS3/E3 CLEAR-CHANNEL FRAMERLIU COMBO - CC/HDLC ARCHITECTURE
In the previous section, we mentioned that the FIFO_WRITE pointer is incremented (within the Jitter Attenuator
FIFO) with each period of the In_CLK (e.g., the jittery clock) input signal. Further, we also indicated that the
FIFO_READ pointer is incremented (within the Jitter Attenuator FIFO) with each period of the Out_CLK (e.g.,
the smoothed clock) output signal.
Additionally, we also mentioned that the Jitter Attenuator PLL block accepts a jittery clock signal, via the
In_CLK input signal and from this input clock signal, it synthesizes another clock signal (of the exact same
frequency, but with considerably less jitter). This synthesized clock signal is output via the Out_CLK output
pin.
Therefore, on the average, the frequencies of the In_CLK input signal and the Out_CLK output signal are
identical. As a consequence, on the average, the FIFO_READ and FIFO_WRITE pointers will each increment
(throughout the Jitter Attenuator FIFO) at the same rate and the two pointers will typically maintain a constant
distance from each other. Figure 73 presents an illustration of the Logical Architecture of the Jitter Attenuator
FIFO, within the Jitter Attenuator block.
This particular figure illustrates the relationship between the
FIFO_WRITE and FIFO_READ pointers.
Figure 73 presents an illustration of the behavior of the FIFO_READ and the FIFO_WRITE pointers within the
Jitter Attenuator FIFO. For the sake of discussion, this figure indicates that the initial position (e.g., In_CLK
period k) of the FIFO_WRITE pointer is at the 12 o'clock position. This figure also indicates that during the
very next In_CLK period, that the FIFO_WRITE pointer will have moved (in a clockwise direction) to the next
position (which is labeled FIFO_WRITE pointer at In_CLK period k + 1). If the FIFO depth is configured to be
16 bits, then in 16 In_CLK cycles, after power-up or reset, the FIFO_WRITE pointer will have revolved around
the circular buffer and will have incremented its location right back to its initial (or the 12 o'clock) position as is
shown in Figure 13. Similarly, if the FIFO depth is configured to be 32 bits, then in 32 In_CLK cycles, after
FIGURE 73. ILLUSTRATION OF THE JITTER ATTENUATOR FIFO AND THE FIFO_WRITE AND FIFO_READ POINT-
ERS
.
FIFO_WRITE pointer at RCLK_n period k
FIFO_READ pointer at RRCLK_n period k
Jitter Attenuator
FIFO Buffer
FIFO_WRITE
Pointer at RCLK_n
Period k+1.
FIFO READ pointer
At RRCLK_n period k+1