XRT74L74 Datasheet, PDF (409/498 Pages) Exar Corporation – 4 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER

PRELIMINARY
XRT74L74
4 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER
REV. P1.1.1
In this case the XRT74L74 has the responsibility of
providing the framing reference signal by pulsing the
TxFrame output pin (and in turn the
Tx_Start_of_Frame input pin of the Terminal Equip-
ment) "High" for one bit-period, coincident with the
last bit within a given E3 frame.
Finally, the XRT74L74 will always internally generate
the Overhead bits, when it is operating in both the E3
and Nibble-parallel modes. The XRT74L74 will pull
the TxOHInd input pin "Low".
The behavior of the signals between the XRT74L74
and the Terminal Equipment for E3 Mode 6 Operation
is illustrated in Figure 175 .
FIGURE 175. BEHAVIOR OF THE TERMINAL INTERFACE SIGNALS BETWEEN THE XRT74L74 AND THE TERMINAL
EQUIPMENT (E3 MODE 6 OPERATION)
Terminal Equipment Signals
TxInClk
E3_Nib_Clock_In
E3_Data_Out[3:0]
Tx_Start_of_Frame
E3_Overhead_Ind
Payload Nibble [1059]
Overhead Nibble [0]
XRT74L74 Transmit Payload Data I/F Signals
TxInClk
TxNibClk
TxNib[3:0]
TxNibFrame
Nibble [1059]
Overhead Nibble [0]
TxOH_Ind
Note: TxNibFrame pulses high to denote
E3 Frame Boundary.
E3 Frame Number N
E3 Frame Number N + 1
TxOH_Ind pulses high for 4 Nibble periods
How to configure the XRT74L74 into Mode 6
1. Set the NibInt input pin "High".
2. Set the TimRefSel[1:0] bit-fields (within the
Framer Operating Mode Register) to "1X" as
illustrated below.
FRAMER OPERATING MODE REGISTER (ADDRESS = 0X00)
BIT 7
BIT 6
BIT 5
BIT 4
BIT 3
BIT2
Local Loopback
DS3/E3*
Internal LOS
Enable
RESET
Interrupt Frame Format
Enable Reset
R/W
R/W
R/W
R/W
R/W
R/W
0
0
1
0
1
0
BIT 1
BIT 0
TimRefSel[1:0]
R/W
R/W
1
x
3. Interface the XRT74L74, to the Terminal Equip-
ment, as illustrated in Figure 174 .
7.2.2 The Transmit Overhead Data Input Inter-
face
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