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PRELIMINARY
XRT79L71
446
REV. P2.0.0
1-CHANNEL DS3/E3 CLEAR-CHANNEL FRAMERLIU COMBO - CC/HDLC ARCHITECTURE
Whenever the XRT79L71 has been configured to operate in this mode, it will function as the source of both the
34.368MHz 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" E3 data-stream) in a "Nibble-Parallel" manner via its "E3_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 "E3_Data_Out[3:0]" output pins) upon the rising edge of the "TxNibClk" clock signal (at its
"E3_Nib_Clock_In" input pin) as depicted below in Figure 213.
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 E3 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 expected to begin the transmission of the contents of the very next outbound E3 frame, via
the "E3_Data_Out[3:0]" output (or "TxNib[3:0]" input pins).
The Transmit Payload Data Input Interface block's handling of E3 Overhead bits 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 4" 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
XRT79L71 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 "RxOutClk"
signal.