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PRELIMINARY
XRT79L71
284
REV. P2.0.0
1-CHANNEL DS3/E3 CLEAR-CHANNEL FRAMERLIU COMBO - CC/HDLC ARCHITECTURE
nibble, and the TxOH_Ind output pin, in this case DOES have meaning. In Mode 5 Operation, the XRT79L71
will pulse its TxOH_Ind output pin "High" one nibble-period prior to the instant that it will process a given
Overhead nibble within the outbound E3 frame. Since the TxOH_Ind output pin of the XRT79L71 is electrically
connected to the E3_Overhead_Ind input pin of the System-Side Terminal Equipment, whenever the
XRT79L91 device pulses its TxOH_Ind output pin "High", it will also drive the E3_Overhead_Ind input pin of the
System-Side Terminal Equipment "High". Whenever the System-Side Terminal Equipment detects this pin
toggling "High" it should delay the transmission of the very next E3 payload nibble by one TxNibClk clock
period.
NOTE: Since the E3, ITU-T G.751 Frame consists of 12 overhead bits, whenever the TxOH_Ind output pin of the XRT79L71
pulses "High" it will do so for three (3) consecutive nibble-periods when processing the FAS, A and N bits.
Therefore, for the E3, ITU-T G.751 framing format, whenever the System-Side Terminal Equipment detects the
TxOH_Ind output pin being pulled "High", it is expected to (1) continuously sample the state of the TxOH_Ind
output pin with each rising edge of TxNibClk and (2) to NOT transmit an E3 payload bit to the Transmit Payload
Data Input Interface block until it samples the TxOH_Ind output pin toggling "Low".
The Frequency of TxNibClk for E3, Nibble-Parallel Mode Operation
In contrast to that for the DS3 framing formats, for E3 Applications (both ITU-T G.832 and ITU-T G.751 framing
formats) the frequency of the TxNibClk clock signal is exactly one-fourth of the frequency of the TxInClk signal.
Figure 133 presents an illustration of the behavior of the System-Side Terminal Equipment/Transmit Payload
Data Input Interface signals for Mode 5 Operation.
Configuring the XRT79L71 to operate in Mode 5 (Nibble-Parallel/Local-Timing/Frame-Slave Mode)
FIGURE 133. AN ILLUSTRATION OF THE BEHAVIOR OF THE SYSTEM-SIDE TERMINAL EQUIPMENT SIGNALS FOR MODE
5 (NIBBLE-PARALLEL/LOCAL-TIMING/FRAME SLAVE) MODE OPERATION
System-Side Terminal Equipment Signals
TxInClk
Tx_Start_of_Frame
E3_Nib_Clock_In
E3_Data_Out[3:0]
Payload Nibble [380]
Overhead Nibble [0]
XRT79L71 Transmit Payload Data Input Interface Signals
E3 Frame Number N
E3 Frame Number N + 1
Note: System-Side Terminal Equipment
pulses TxFrameRef “high” to denote
the first nibble within a given E3 Frame.
TxInClk
TxFrameRef
TxNibClk
TxNib[3:0]
Payload Nibble [380]
Overhead Nibble [0]
Sampling Edge of XRT79L71 Device
E3_OH_Ind
TxOH_Ind
TxOH_Ind toggles to
Denote Overhead Nibble