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PRELIMINARY
XRT79L71
88
REV. P2.0.0
1-CHANNEL DS3/E3 CLEAR-CHANNEL FRAMERLIU COMBO - CC/HDLC ARCHITECTURE
The XRT79L71 will be configured to operate in the Local-Timing Mode. In other words, the Transmit Section
of the XRT79L71 will use the TxInClk input signal as its timing source.
In this mode, the XRT79L71 will use the TxInClk signal to derive the TxNibClk signal.
For DS3 Applications, the TxNibClk frequency is approximately one-fourth of the TxInClk clock input signal
or 11.184MHz. The reason for the TxNibClk frequency not being exactly 11.184MHz will be explained later
in this section.
Since the XRT79L71 is configured to operate in the Nibble-Parallel Mode, it will sample and latch the data,
being applied to the TxNib[3:0] input pin upon the third rising edge of the TxInClk input clock signal, following
a given rising edge of the TxNibClk output clock signal.
The XRT79L71 will pulse the TxNibFrame output pin coincident to whenever the Transmit Payload Data
Input Interface is processing the very last bit within a given DS3 frame.
Figure 40 presents an illustration of how to interface the System-Side Terminal Equipment to the Transmit
Payload Data Input Interface block of the XRT79L71 for Mode 6 operation.
Mode 6 Operation of the Transmit Payload Data Input Interface Block
Whenever the XRT79L71 has been configured to operate in this mode, it will function as the source of a
Nibble-Clock signal via the TxNibClk output signal.
NOTE: For "Mode 6" Operation, the "TxNibClk" output signal is ultimately derived from the "TxInClk" input signal.
The System-Side Terminal Equipment should output the payload data that is to be transported via the
outbound DS3 data-stream, in a Nibble-Parallel manner via its DS3_Data_Out[3:0] output pins. The user is
advised to design or configure the System-Side Terminal Equipment circuitry such that it will update the data
via the DS3_Data_Out[3:0] output pins upon the rising edge of the TxNibClk clock signal at its
DS3_Nib_Clock_In input pin, as depicted below in Figure 41.
The XRT79L71 will latch the contents of the TxNib[3:0] input pins, upon the third rising edge of the TxInClk
signal following a given rising edge in the TxNibClk signal. The XRT79L71 will indicate that it is processing the
very last nibble of a given DS3 frame by pulsing its TxNibFrame output pin "High" for one nibble-period.
FIGURE 40. AN ILLUSTRATION OF HOW TO INTERFACE THE SYSTEM-SIDE TERMINAL EQUIPMENT SIGNALS FOR MODE
6 (NIBBLE-PARALLEL/LOCAL-TIMING/FRAME MASTER) MODE OPERATION
System-Side Terminal
Equipment
XRT79L71 DS3/E3
Framer IC
DS3_Data_Out[3:0]
DS3_Nib_Clock_In
Tx_Start_of_Frame
TxNib[3:0]
TxNibClk
TxNibFrame
NibInt
VCC
4
44.736MHz Clock Source
TxInClk
Approximately 11.184MHz