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PRELIMINARY
XRT79L71
286
REV. P2.0.0
1-CHANNEL DS3/E3 CLEAR-CHANNEL FRAMERLIU COMBO - CC/HDLC ARCHITECTURE
Mode 6 Operation of the Transmit Payload Data Input Interface Block
Whenever the XRT79L71 has been configured to operate in this mode, it will function as the source of a
Nibble-Clock signal via the TxNibClk output signal.
NOTE: For "Mode 6" Operation, the "TxNibClk" output signal is ultimately derived from the "TxInClk" input 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 TxInClk
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_End_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.
NOTES:
1.
In this particular mode, the Transmit Direction circuitry (within the XRT 79L71) dictates the instant that it will
generate a new E3 frame; and is (therefore) referred to as the "Frame Master". As a consequence, this particular
mode is referred to as the "Frame Master" Mode.
2.
In contrast to "Mode 5" operation, if the XRT 79L71 is configured to operate in "Mode 6", then it is NOT imperative
that the "TxFrameRef" input be synchronized to the "TxInClk" clock signal. In this case, we recommend that the
user tie "TxFrameRef" to GND.
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.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
FIGURE 134. AN ILLUSTRATION OF HOW TO INTERFACE THE SYSTEM-SIDE TERMINAL EQUIPMENT SIGNALS FOR
MODE 6 (NIBBLE-PARALLEL/LOCAL-TIMING/FRAME MASTER) MODE OPERATION
System-Side Terminal
Equipment
XRT79L71 DS3/E3
Framer IC
E3_Data_Out[3:0]
E3_Nib_Clock_In
Tx_End_of_Frame
TxNib[3:0]
TxNibClk
TxNibFrame
NibInt
VCC
4
34.368MHz Clock Source
TxInClk
8.592MHz
TxOH_Ind
E3_OH_Ind