XRT74L73 Datasheet, PDF (394/488 Pages) Exar Corporation – 3 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER

XRT74L73
3 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER
REV. P1.0.1
PRELIMINARY
Equipment Input Interface block will latch this data in-
to its circuitry, on the rising edge of the TxNibClk out-
put signal.
C. Delineation of the Outbound E3 frames
The XRT74L73 will pulse the TxNibFrame output pin
"High" for one bit-period, coincident with the
XRT74L73 processing the last nibble of a given E3
frame.
D. Sampling of payload data, from the Terminal
Equipment
In Mode 4, the XRT74L73 will sample the data, at the
TxNib[2:0] input pins, on the third rising edge of the
RxOutClk clock signal, following a pulse in the TxNib-
Clk signal (see Figure 171 ).
NOTE: The TxNibClk signal, from the XRT74L73, operates
nominally at 11.184 MHz (e.g., 44.736 MHz divided by 4).
However, for reasons described below, TxNibClk effectively
operates at a lower clock frequency. The Transmit Payload
Data Input Interface is only used to accept the payload
data, which is intended to be carried by Outbound DS3
frames. The Transmit Payload Data Input Interface is not
designed to accommodate the entire DS3 data stream.
The E3 Frame consists of 537 bytes or 1074 nibbles.
Therefore, the XRT74L73 will supply 1074 TxNibClk
pulses between the rising edges of two consecutive
TxNibFrame pulses. The E3 Frame repetition rate is
8.0kHz. Hence, 1074 TxNibClk pulses for each E3
frame period amounts to TxNibClk running at approx-
imately 8.592 MHz. The method by which the 1074
TxNibClk pulses are distributed throughout the E3
frame period is presented below.
Nominally, the Transmit Section within the XRT74L73
will generate a TxNibClk pulse for every 4 RxOutClk
(or TxInClk) periods.
Interfacing the Transmit Payload Data Input Inter-
face block of the XRT74L73 to the Terminal Equip-
ment for Mode 4 Operation
Figure 170 presents an illustration of the Transmit
Payload Data Input Interface block (within the
XRT74L73) being interfaced to the Terminal Equip-
ment, for Mode 4 Operation.
FIGURE 170. THE TERMINAL EQUIPMENT BEING INTERFACED TO THE TRANSMIT PAYLOAD DATA INPUT INTERFACE
BLOCK OF THE XRT74L73 FOR MODE 4 (NIBBLE-PARALLEL/LOOP-TIMED) OPERATION
E3_Nib_Clock_In
E3_Data_Out[2:0]
Tx_Start_of_Frame
E3_Overhead_Ind
8.592MHz
4
TxNibClk
TxNib[2:0]
VCC
TxNibFrame
RxLineClk
TxOH_Ind
NibInt
34.368MHz
Terminal Equipment
XRT74L73 E3 Framer
Mode 4 Operation of the Terminal Equipment
When the XRT74L73 is operating in this mode, it will
function as the source of the 8.592MHz (e.g., the
34.368MHz clock signal divided by 4) clock signal
that will be used as the Terminal Equipment Interface
clock by both the XRT74L73 and the Terminal Equip-
ment.
The Terminal Equipment will output the payload data
of the Outbound E3 data stream via its
E3_Data_Out[2:0] pins on the rising edge of the
8.592MHz clock signal at the E3_Nib_Clock_In input
pin.
The XRT74L73 will latch the Outbound E3 data
stream (from the Terminal Equipment) on the rising
edge of the TxNibClk output clock signal. The
XRT74L73 will indicate that it is processing the last
nibble, within a given E3 frame, by pulsing its TxNib-
Frame output pin "High" for one TxNibClk clock peri-
od. When the Terminal Equipment detects a pulse at
its Tx_Start_of_Frame input pin, it is expected to
transmit the first nibble, of the very next Outbound E3
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