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XRT79L71
PRELIMINARY
283
1-CHANNEL DS3/E3 CLEAR-CHANNEL FRAMERLIU COMBO - CC/HDLC ARCHITECTURE
REV. P2.0.0
Mode 5 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 5" 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. In this particular mode, the System-Side Terminal
Equipment also has the responsibility of providing a Framing Reference signal to the XRT79L71 by pulsing its
TxFrameRef input pin "High" for one nibble-period, coincident with the first nibble of a new outbound E3 frame
being applied to the TxNib[3:0] input pins. Once the XRT79L71 detects the rising edge of the input at its
TxFrameRef input pin, it will begin to generate and transmit a new E3 frame.
NOTES:
1.
In this particular mode, the System-Side Terminal Equipment is controlling the start of Frame Generation and is
referred to as the Frame Master. Since the XRT79L71 does not control nor dictate the instant that it will generate
a new E3 frame, but is driven by the System-Side Terminal Equipment, it is referred to as the Frame Slave.
2.
If the XRT79L71 is configured to operate in Mode 5, then it is imperative that the Tx_Start_of_Frame or
TxFrameRef signal is synchronized to the TxInClk input clock signal.
Failure to do this will result in the
transmission of erred E3 data to the remote terminal equipment.
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 being 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
FIGURE 132. AN ILLUSTRATION OF HOW TO INTERFACE THE SYSTEM-SIDE TERMINAL EQUIPMENT SIGNALS FOR
MODE 5 (NIBBLE-PARALLEL/LOCAL-TIMING/FRAME SLAVE) MODE OPERATION
System-Side Terminal
Equipment
XRT79L71 DS3/E3
Framer IC
E3_Data_Out[3:0]
E3_Nib_Clock_In
Tx_Start_of_Frame
TxNib[3:0]
TxNibClk
TxFrameRef
NibInt
VCC
4
34.368MHz Clock Source
TxInClk
8.592MHz
TxOH_Ind
E3_OH_Ind