XRT74L74 Datasheet, PDF (206/498 Pages) Exar Corporation – 4 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER

XRT74L74
REV. P1.1.1
4 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER
ADVANCED CONFIDENTIAL
FRAMER OPERATING MODE REGISTER (ADDRESS = 0X00)
BIT 7
BIT 6
BIT 5
BIT 4
BIT 3
BIT2
BIT 1
BIT 0
Local Loopback
R/W
0
DS3/E3*
R/W
0
Internal LOS
Enable
R/W
1
RESET
R/W
0
Interrupt
Enable Reset
R/W
1
Frame Format
R/W
0
TimRefSel[1:0]
R/W
R/W
1
X
3. Interface the XRT74L74, to the Terminal Equip-
ment, as illustrated in Figure 55.
5.2.1.4 Mode 4 - The Nibble-Parallel/Loop-
Timed Mode Behavior of the XRT74L74
If the XRT74L74 has been configured to operate in
this mode, then the XRT74L74 will behave as follows.
A. Looped Timing (Uses the RxLineClk as the
Timing Reference)
In this mode, the Transmit Section of the XRT74L74
will use the RxLineClk signal as its timing reference.
When the XRT74L74 is operating in the Nibble-Mode,
it will internally divide the RxLineClk signal, by a fac-
tor of four (4) and will output this signal via the TxNib-
Clk output pin.
B. Nibble-Parallel Mode
The XRT74L74 will accept the DS3 payload data,
from the Terminal Equipment in a nibble-parallel man-
ner, via the TxNib[3:0] input pins. The Transmit Ter-
minal Equipment Input Interface block will latch this
data into its circuitry, on the rising edge of the TxNib-
Clk output signal.
C. Delineation of the outbound DS3 frames
The XRT74L74 will pulse the TxNibFrame output pin
"High" for one bit-period coincident with the
XRT74L74 processing the last nibble of a given DS3
frame.
D. Sampling of payload data, from the Terminal
Equipment
In Mode 4, the XRT74L74 will sample the data, at the
TxNib[3:0] input pins, on the third rising edge of the
RxOutClk clock signal, following a pulse in the TxNib-
Clk signal (see Figure 58).
NOTE: The TxNibClk signal, from the XRT74L74 operates
nominally at 11.184 MHz (e.g., 44.736 MHz divided by 4).
However, for reasons described below, TxNibClk effectively
operates at a Low 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 DS3 Frame consists of 4704 payload bits or 1176
nibbles. Therefore, the XRT74L74 will supply 1176
TxNibClk pulses between the rising edges of two con-
secutive TxNibFrame pulses. The DS3 Frame repeti-
tion rate is 9.398kHz. Hence, 1176 TxNibClk pulses
for each DS3 frame period amounts to TxNibClk run-
ning at approximately 11.052 MHz. The method by
which the 1176 TxNibClk pulses are distributed
throughout the DS3 frame period is presented below.
Nominally, the Transmit Section within the XRT74L74
will generate a TxNibClk pulse for every 4 RxOutClk
(or TxInClk) periods. However, in 14 cases (within a
DS3 frame period), the Transmit Payload Data Input
Interface will allow 5 TxInClk periods to occur be-
tween two consecutive TxNibClk pulses.
Interfacing the Transmit Payload Data Input Inter-
face block of the XRT74L74 to the Terminal Equip-
ment for Mode 4 Operation
Figure 57 presents an illustration of the Transmit Pay-
load Data Input Interface block (within the XRT74L74)
being interfaced to the Terminal Equipment, for Mode
4 Operation.
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