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PRELIMINARY
XRT79L71
258
REV. P2.0.0
1-CHANNEL DS3/E3 CLEAR-CHANNEL FRAMERLIU COMBO - CC/HDLC ARCHITECTURE
If we were to look at this issue another way, we would recall that each DS3 frame consists of 4760 bits. Of
these bits, 56 are overhead bits and the remaining 4704 bits are payload bits. This means that there are 4704
bits/4 = 1176 nibbles of payload bits within each DS3 frame.
The frame repetition rate (for DS3) is 9.398kHz. Therefore, if one performs the following multiplication:
{9398 Frames/sec X 1176 Nibble/Frame} = 11.052MHz (for the Average Frequency of the RxClk output signal).
How the 1176 Clock Edges within the RxClk output signal are distributed throughout a DS3 frame.
In general, for 1120 RxClk periods, the instantaneous frequency of the RxClk output clock signal will be
11.184MHz (e.g., each of these clock periods will corresponding to exactly 4 DS3 bit-periods). However, for
the remaining 56 of these RxClk periods, the periods of these clock signals will be lengthened to five (5) of DS3
bit-periods.
For this reason, in DS3/Nibble-Parallel Mode applications, if the RxClk signal was monitored with a scope, a
considerable amount of jitter could be seen within this particular clock signal.
Figure 112 presents an illustration of the behavior of System-Side Terminal Equipment/Receive Payload Data
Output Interface signals for Nibble-Parallel Mode Operation.
Configuring the XRT79L71 to operate in the Nibble-Parallel Mode
The user can configure the XRT79L71 to operate in the Nibble-Parallel Mode by executing the following steps.
STEP 1 - Design your board such that the System-Side Terminal Equipment circuitry interfaces to the
Receive Payload Data Input Interface in the manner as depicted above in Figure 112. STEP 2 - Configure the XRT79L71 to operate in the Nibble-Parallel Mode
This can be accomplished by setting the NibIntf input pin to a logic "High".
NOTE: This step also configures the Transmit Payload Data Input Interface block to operate in the Nibble-Parallel Mode.
FIGURE 112. AN ILLUSTRATION OF THE BEHAVIOR OF THE SYSTEM-SIDE TERMINAL EQUIPMENT SIGNALS FOR NIB-
BLE
-PARALLEL MODE OPERATION
System-Side Terminal Equipment Signals
XRT79L71 Transmit Payload Data Input Interface Signals
DS3 Frame Number N
DS3 Frame Number N + 1
Note: RxFrame pulses high to denote
DS3 Frame Boundary.
Rx_Start_of_Frame
DS3_Nib_Clock_In
DS3_Data_In[3:0]
Nibble [0]
Nibble [1]
RxClk
RFrame
RxNib[3:0]
Nibble [0]
Nibble [1]