
4 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER
ADVANCED CONFIDENTIAL
XRT74L74
REV. P1.1.1
201
N
OTES
:
1. In this case, the Terminal Equipment is controlling
the start of Frame Generation, and is therefore
referred to as the Frame Master. Conversely, since
the XRT74L74 does not control the generation of a
new DS3 frame, but is rather driven by the Terminal
Equipment. Hence, the XRT74L74 is referred to as
the Frame Slave.
2. If the XRT74L74 is configured to operate in Mode
2, it is imperative that the Tx_Start_of_Frame (or
TxFrameRef) signal is synchronized to the TxInClk
input clock signal.
Finally, the XRT74L74 will pulse its TxOH_Ind output
pin, one bit-period prior to it processing a given over-
head bit, within the outbound DS3 frame. Since the
TxOH_Ind output pin of the XRT74L74 is electrically
connected to the DS3_Overhead_Ind, whenever the
XRT74L74 pulses the TxOH_Ind output pin "High", it
will also be driving the DS3_Overhead_Ind input pin
(of the Terminal Equipment) "High". Whenever the
Terminal Equipment detects this pin toggling "High", it
should delay transmission of the very next DS3 frame
payload bit by one clock cycle.
The behavior of the signals between the XRT74L74
and the Terminal Equipment for DS3 Mode 2 Opera-
tion is illustrated in Figure 54.
How to configure the XRT74L74 to operate in this
mode.
1.
Set the NibIntf input pin "Low".
2.
Set the TimRefSel[1:0] bit-fields (within the
Framer Operating Mode Register) to "01" as
depicted below.
3.
Interface the XRT74L74, to the Terminal Equip-
ment, as illustrated in Figure 53.
5.2.1.3
Frame-Master Mode Behavior of the XRT74L74
Mode 3 - The Serial/Local-Timed/
F
IGURE
54. B
EHAVIOR
OF
THE
T
ERMINAL
I
NTERFACE
SIGNALS
BETWEEN
THE
XRT74L74
AND
THE
T
ERMINAL
E
QUIPMENT
(M
ODE
2 O
PERATION
)
Terminal Equipment Signals
DS3_Clock_In
DS3_Data_Out
Tx_Start_of_Frame
DS3_Overhead_Ind
XRT72L5x Transmit Payload Data I/F Signals
TxInClk
TxSer
TxFrameRef
TxOH_Ind
Payload[4702]
Payload[4703]
X-Bit
Payload[1]
Payload[4702]
Payload[4703]
X-Bit
Payload[1]
Note:
X-Bit will not be processed by the
Transmit Payload Data Input Interface.
DS3 Frame Number N
DS3 Frame Number N + 1
Note:
TxFrame pulses high to denote
DS3 Frame Boundary.
Note:
TxOH_Ind pulses high to
denote Overhead Data
(e.g., the X-bit).
FRAMER OPERATING MODE REGISTER (ADDRESS = 0X00)
B
IT
7
B
IT
6
B
IT
5
B
IT
4
B
IT
3
B
IT
2
B
IT
1
B
IT
0
Local Loopback
DS3/E3*
Internal LOS
Enable
R/W
1
RESET
Interrupt
Enable Reset
R/W
1
Frame Format
TimRefSel[1:0]
R/W
0
R/W
0
R/W
0
R/W
0
R/W
0
R/W
1