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XRT72L50
SINGLE CHANNEL DS3/E3 FRAMER WITH HDLC CONTROLLER
REV. 1.2.1
146
3.
Interface the XRT72L50, to the Terminal Equipment, as illustrated in
Figure 38
.
The XRT72L50 cannot support the Framer Local Loop-back Mode of operation. The XRT72L50 Framer must
be configured into any of the following modes prior to configuring the Framer Local-Loop-back Mode operation.
Mode 2 - Serial/Local-Timed/Frame-Slave Mode.
Mode 3 - Serial/Local-Timed/Frame-Master Mode.
Mode 5 - Nibble-Parallel/Local-Timed/Frame-Slave Mode.
Mode 6 - Nibble-Parallel/Local-Timed/Frame-Master Mode.
N
OTE
:
For more detailed information on the Framer Local Loop-back Mode Operation, please see
Section 7.0
.
4.2.1.5
Mode 5 - The Nibble-Parallel/Local-Timed/Frame-Slave Interface Mode Behavior of the
XRT72L50
The XRT72L50 configured to operate in this mode functions as follows:
Local-Timed
The Transmit Section of the XRT72L50 uses the TxInClk signal as its timing reference. The chip internally
divides the TxInClk clock signal by a factor of 4 and outputs this divided clock signal via the TxNibClk output
pin. The Transmit Terminal Equipment Input Interface block within the XRT72L50 uses the rising edge of the
TxNibClk signal to latch the data residing on the TxNib[3:0] into its circuitry.
Nibble-Parallel Mode
The XRT72L50 accepts the DS3 payload data from the Terminal Equipment in a parallel manner via the
TxNib[3:0] input pins. The Transmit Terminal Equipment Input Interface latches this data into its circuitry on the
rising edge of the TxNibClk output signal.
Delineation of outbound DS3 Frames
The Transmit Section uses the TxInClk input signal as its timing reference and the TxFrameRef input signal as
its Framing Reference (e.g., the Transmit Section of the XRT72L50 initiates frame generation upon the rising
edge of the TxFrameRef signal).
In this case, the Terminal Equipment should pulse the TxFrameRef input signal of the XRT72L50 coincident
with it applying the first payload nibble within a given outbound DS3 frame. The duration of this pulse should
be one nibble-period of the DS3 signal (see
Figure 41
).
Sampling of payload data, from the Terminal Equipment
In Mode 5, the XRT72L50 samples the data at the TxNib[3:0] input pins on the third rising edge of the TxInClk
clock signal following a pulse in the TxNibClk signal (see
Figure 41
).
The TxNibClk signal from the XRT72L50 operates nominally at 11.184 MHz (e.g., 44.736 MHz divided by 4).
However, 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. The XRT72L50 supplies 1176 TxNibClk pulses
between the rising edges of two consecutive TxNibFrame pulses. The DS3 Frame repetition rate is 9.398kHz.
1176 TxNibClk pulses for each DS3 frame period amounts to TxNibClk running at approximately 11.052 MHz.
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 Loop-back
DS3/E3
Internal LOS
Enable
RESET
Interrupt
Enable Reset
Frame Format
TimRefSel[1:0]
R/W
0
R/W
0
R/W
1
R/W
0
R/W
1
R/W
0
R/W
0
R/W
0