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PRELIMINARY
XRT79L71
452
REV. P2.0.0
1-CHANNEL DS3/E3 CLEAR-CHANNEL FRAMERLIU COMBO - CC/HDLC ARCHITECTURE
The Transmit Payload Data Input Interface block's handling of E3 Overhead Bytes when configured to
operate in the "Nibble-Parallel" Mode
In contrast to the DS3 Framing formats (which are "bit-oriented" framing formats), the E3, ITU-T G.832 framing
format is a "byte-oriented" framing format. As a consequence, there will be cases in which the Transmit
Payload Data Input Interface (within the XRT79L71) will be processing an "E3 overhead nibble", and the
"TxOH_Ind" output pin (in this case) DOES have meaning. In "Mode 6" 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.832 Frame consists of overhead bytes (in lieu of overhead nibbles), whenever the
"TxOH_Ind" output pin (of the XRT79L71) pulses "high" it will do so for four (4) consecutive nibble-periods (when
processing the FA1 and FA2 bytes) and it will do so for two (2) consecutive bit-periods, when processing the
remaining five (5) overhead bytes. Therefore, for the E3, ITU-T G.832 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.
Configuring the XRT79L71 to operate in Mode 6 (Nibble-Parallel/Local-Timing/Frame Master Mode)
FIGURE 217. AN ILLUSTRATION OF THE BEHAVIOR OF THE "SYSTEM-SIDE TERMINAL EQUIPMENT" SIGNALS FOR
MODE 6 (NIBBLE-PARALLEL/LOCAL-TIMING/FRAME MASTER) MODE OPERATION
System-Side Terminal Equipment Signals
TxInClk
Tx_End_of_Frame
E3_Nib_Clock_In
E3_Data_Out[3:0]
Payload Nibble [1059]
FA1 Byte, Nibble [0]
XRT79L71 Transmit Payload Data Input Interface Signals
E3 Frame Number N
E3 Frame Number N + 1
Note: TxNibFrame pulses high to denote
E3 Frame Boundary.
TxInClk
TxNibFrame
TxNibClk
TxNib[3:0]
Payload Nibble [1059]
FA1 Byte, Nibble [0]
Sampling Edge of XRT79L71 Device
E3_OH_Ind
TxOH_Ind
TxOH_Ind toggles to
Denote Overhead Nibble