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PRELIMINARY
XRT79L71
580
REV. P2.0.0
1-CHANNEL DS3/E3 CLEAR-CHANNEL FRAMERLIU COMBO - CC/HDLC ARCHITECTURE
Nibble-Parallel Mode Operation of the Receive Payload Data Output Interface Block
Whenever the XRT79L71 has been configured to operate in the "Nibble-Parallel" Mode, then the Receive
Payload Data Output Interface block will function as the source of a Nibble Clock signal (via the "RxCLK"
output signal).
The Receive Payload Data Output Interface block will output all of the payload and overhead data (that has
been extracted out to the incoming E3 data-stream) in a "Nibble-Parallel" Mode via the "RxNib[3:0] output pins.
As mentioned earlier, the Receive Payload Data Output Interface block will output this data upon the falling
edge of the "RxCLK" signal. As a consequence, the user is advised to design (or configure) the "System-Side
Terminal Equipment" circuitry to sample and latch this data (via the "E3_Data_In[3:0]" input pins) upon the
rising edge of RxCLK (Rx_E3_Clock_In), as depicted below in Figure 266.
The Receive Payload Data Output Interface block (within the XRT79L71) will indicate that it is processing the
very first payload nibble of a given E3 frame by pulsing the "RxFrame" output pin "HIGH" for one "nibble-
period". The "RxFrame" output pin will be held "LOW" at all other times.
Finally, since (for E3 Applications) the Receive Payload Data Output Interface block (within the XRT79L71)
does process and output overhead bits (in contrast to DS3, Nibble-Parallel Mode applications), then the
Receive Payload Data Output Interface block will drive the "RxOH_Ind" output pin "high" coincident to
whenever it outputs a nibble that contains "overhead" data. Conversely, the Receive Payload Data Output
Interface block will drive the "RxOH_Ind" output pin "low" coincident to whenever it outputs a nibble that
contains "payload" data.
The Frequency of the RxClk signal for E3, Nibble-Parallel Mode Operation
As mentioned above, whenever the Receive Payload Data Input Interface block has been configured to
operate in the "Nibble-Parallel" Mode, it will process both the E3 payload and overhead bits.
As a
consequence, the frequency of the RxClk signal (for Nibble-Parallel Mode applications) will be exactly
34.368MHz/4 (or 8.592MHz).
FIGURE 265. AN ILLUSTRATION OF HOW TO INTERFACE THE "SYSTEM-SIDE TERMINAL EQUIPMENT" TO THE
"RECEIVE PAYLOAD DATA OUTPUT INTERFACE" BLOCK (OF THE XRT79L71) FOR "NIBBLE-PARALLEL MODE"
OPERATION
System-Side Terminal Equipment
(Receive Payload Section)
XRT79L71 DS3/E3 Framer
E3_Data_In[3:0]
Rx_E3_Clock_In
Rx_Start_of_Frame
RxClk
RxFrame
8.592 MHz Clock Signal
RxNib[3:0]
RxOH_Ind
Rx_Overhead_Ind