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PRELIMINARY
XRT79L71
82
REV. P2.0.0
1-CHANNEL DS3/E3 CLEAR-CHANNEL FRAMERLIU COMBO - CC/HDLC ARCHITECTURE
4.2.1.4
Mode 4 - Nibble-Parallel/Loop-Timing Mode Operation of the Transmit Payload Data Input
Interface Block
If the XRT79L71 is configured to operate in Mode 4 then all of the following is true.
The XRT79L71 will be configured to operate in the Loop-Timing Mode. In other words, the Transmit Section
of the XRT79L71 will use the Recovered Clock signal from the Receive DS3 LIU Block as its timing source.
In this mode, the XRT79L71 will use the LIU Recovered Clock signal to derive the TxNibClk signal.
For DS3 Applications, the TxNibClk frequency is approximately one-fourth of the LIU Recovered Clock 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 pins upon the third rising edge of the RxOutClk output clock signal,
following a given rising edge of the TxNibClk output clock signal.
The XRT79L71 will pulse the TxNibFrame output pin "High" for one nibble-period coincident to whenever the
Transmit Payload Data Input Interface is processing the very last nibble within a given DS3 frame.
Figure 36 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 4 operation.
Mode 4 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 both the
44.736MHz clock signal via the RxOutClk output signal and a Nibble Clock signal via the TxNibClk output
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 37.
The XRT79L71 will latch the contents of the TxNib[3:0] input pins, upon the third rising edge of the RxOutClk
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.
Whenever the System-Side Terminal Equipment detects this pulse at its Tx_Start_of_Frame input pin, then it is
FIGURE 36. AN ILLUSTRATION OF HOW TO INTERFACE THE SYSTEM-SIDE TERMINAL EQUIPMENT TO THE TRANSMIT
PAYLOAD DATA INPUT INTERFACE BLOCK OF THE XRT79L71 FOR MODE 4 (NIBBLE-PARALLEL/LOOP-TIMING)
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
RxLineClk
44.736MHz
Approx. 11.184MHz