XRT74L74 Datasheet, PDF (132/498 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
TABLE 16: PLCP FRAME TIMING AND STUFF CONTROL OPTIONS (CONTINUED)
BIT 1
1
BIT 0
1
RESULT
Stuff Control: The Stuff Control algorithm is controlled by the logic state of the external pin,
StuffCtl.
As with the previous two Stuff Control options, the Transmit PLCP Framer has a stuff-opportunity
that occurs once every three PLCP frames. Each of these composite PLCP frames are dis-
cussed below.
• PLCP Frame 1 (the first of the 3 frames) will contain 13 trailer nibbles. The C1 byte, within
this PLCP frame, will identify the current frame as a Frame #1, by carrying the value FFh.
Note this frame will be created independent of the state of the StuffCtl pin.
• PLCP Frame 2 (the second of the 3 frames) will contain 14 trailer nibbles. The C1 byte,
within this PLCP frame, will identify the current frame as a Frame #2, by carrying the value
00h. Note this frame will be created independent of the state of the StuffCtl pin.
• PLCP Frame 3 (the last of the 3 frames) will contain either 13 or 14 trailer nibbles, depending
upon the logic state of the “StuffCtl” input pin. Therefore, the Transmit PLCP can generate one
of two versions of Frame #3: “No Stuff” Frame #3 and “Stuff” Frame #3.
StuffCtl = “0”—No Stuff” Frame #3: If the StuffCtl pin is “low” then the Transmit PLCP pro-
cessor will generate a “No Stuff” Frame #3. This PLCP frame will contain 13 trailer nibbles. The
C1 byte will identify the current PLCP frame as a “No Stuff” Frame #3 by carrying the value 66h.
StuffCtl = “1”—“Stuff” Frame #3: If the StuffCtl pin is “high” then the Transmit PLCP Processor
will generate a “Stuff” Frame #3. This PLCP frame will contain 14 trailer nibbles. The C1 byte
will identify the current PLCP frame as a “Stuff” Frame #3 by carrying the value 99h.
TimRefSel[1,0] = 11; - Fixed Stuffing Pattern
PLCP Frame Timing: Asynchronous upon power on.
Stuff Control: The Transmit PLCP Processor will use a fixed Stuffing Pattern which is con-
trolled by an internal counter. This stuffing pattern results in the transmission of 13, 14, 13, 13,
14, 14, 13, 14, 14 trailer nibbles in every 9 PLCP frames repeatedly. This corresponds to 8000 -
1.5 x 10^-5 Hz when a perfect 44.736 MHz is used as the transmit clock. Table 17 lists the
contents of the C1 bytes for each of these 9 PLCP Frames.
Note: The selection of these bits also affects the operation
of the Transmit DS3 Framer. This subject is presented in
Section 6.4.3.4. In all cases, the C1 byte of each PLCP
frame will reflect the stuffing phase and number of trailer
nibbles that are appended to the current PLCP frame.
TABLE 17: VALUE OF C1 FOR THE 9 PLCP FRAMES, WHEN THE FIXED STUFFING OPTION IS SELECTED
PLCP FRAME NUMBER
1
2
3
4
5
6
7
8
9
NUMBER OF TRAILER NIBBLES IN FRAME
13
14
13
13
14
14
13
14
14
C1 BYTE VALUE
FFh
00h
66h
FFh
00h
99h
FFh
00h
99h
130