XRT74L74 Datasheet, PDF (302/498 Pages) Exar Corporation – 4 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER

XRT74L74
4 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER
REV. P1.1.1
PRELIMINARY
FIGURE 107. THE TERMINAL EQUIPMENT BEING INTERFACED TO THE TRANSMIT PAYLOAD DATA INPUT INTERFACE
BLOCK OF THE XRT74L74 FOR MODE 1 (SERIAL/LOOP-TIMED) OPERATION
34.368MHz
E3_Clock_In
RxOutClk
E3_Data_Out
TxSer
Tx_Start_of_Frame
TxFrame
E3_Overhead_Ind
TxOH_Ind
NibInt
Terminal Equipment
XRT74L74 E3 Framer
Mode 1 Operation of the Terminal Equipment
When the XRT74L74 is operating in this mode, it will
function as the source of the 34.368MHz clock signal.
This clock signal will be used as the Terminal Equip-
ment Interface clock by both the XRT74L74 IC and
the Terminal Equipment.
The Terminal Equipment will serially output the pay-
load data of the outbound E3 data stream via its
E3_Data_Out pin. The Terminal Equipment will up-
date the data on the E3_Data_Out pin upon the rising
edge of the 34.368 MHz clock signal, at its
E3_Clock_In input pin (as depicted in Figure 107 and
Figure 108 ).
The XRT74L74 will latch the outbound E3 data
stream (from the Terminal Equipment) on the rising
edge of the RxOutClk signal.
The XRT74L74 will indicate that it is processing the
last bit, within a given outbound E3 frame, by pulsing
its TxFrame output pin "High" for one bit-period.
When the Terminal Equipment detects this pulse at its
Tx_Start_of_Frame input, it is expected to begin
transmission of the very next outbound E3 frame to
the XRT74L74 via the E3_Data_Out (or TxSer pin).
Finally, the XRT74L74 will indicate that it is about to
process an overhead bit by pulsing the TxOH_Ind
output pin "High" one bit period prior to its processing
of an OH (Overhead) bit. In Figure 107 , the
TxOH_Ind output pin is connected to the
E3_Overhead_Ind input pin of the Terminal Equip-
ment. Whenever the E3_Overhead_Ind pin is pulsed
"High" the Terminal Equipment is expected to not
transmit a E3 payload bit upon the very next clock
edge. Instead, the Terminal Equipment is expected
to delay its transmission of the very next payload bit,
by one clock cycle.
The behavior of the signals, between the XRT74L74
and the Terminal Equipment, for E3 Mode 1 operation
is illustrated in Figure 108 .
Inserting the A and N bits into the outbound E3
frames via the Transmit Payload Data Input Inter-
face block
The XRT74L74 DS3/E3 Framer permits the Terminal
Equipment to insert its own values for the “A” and/or
“N” bits, into the outbound E3 frame, via the Transmit
Payload Data Input Interface block. If the user de-
sires to do this, the XRT74L74 Framer IC must be
configured to accept the Terminal Equipment’s value
for the “A” and “N” bits, by writing to appropriate data
into the TxASourceSel[1:0] and TxNSourceSel[1:0]
bit-fields, within the TxE3 Configuration Register (Ad-
dress =0x30), as illustrated below.
300