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PRELIMINARY
XRT79L71
280
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
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.751 framing
format can be viewed as a nibble-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.751 Frame consists of 12 consecutive 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 RxOutClk
signal.
Figure 131 presents an illustration of the behavior of the System-Side Terminal Equipment/Transmit Payload
Data Input Interface signals for Mode 4 Operation.