XRT74L74 Datasheet, PDF (131/498 Pages) Exar Corporation – 4 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER

PRELIMINARY
XRT74L74
4 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER
REV. P1.1.1
The values for TimRefSel[1, 0] and the corresponding
options are presented in Table 16 .
TABLE 16: PLCP FRAME TIMING AND STUFF CONTROL OPTIONS
BIT 1
0
0
1
BIT 0
0
1
0
RESULT
TimRefSel[1,0] = 00
PLCP Frame Timing Source: Receive PLCP Processor Timing.
In this configuration, the Transmit PLCP Processor takes its timing from the Receiver Start of
Frame signal (from the Receive PLCP Processor, within the UNI) to start a PLCP frame. The
Transmit PLCP Processor will also use this signal to calculate stuff opportunities.
Stuff Control: The Transmit PLCP Framer has a stuff-opportunity that occurs once every three
PLCP frames. Therefore, the stuff-control algorithm is based on a repeating “Stuff-Control” cycle
that consists of these three (3) PLCP cycles (or a 375µs interval). The three composite PLCP
frames of a stuff control cycle, when TimRefSel[1, 0] = 00, is presented below.
PLCP Frame 1 (the first of the 3 frames) will contain 13 trailer nibbles. The C1 byte, within this
PLCP frame, will contain the value FFh. This value identifies the current PLCP Frame as
Frame #1 in this 3 Frame Cycle, and informs the Far-End Receive PLCP Processor that the
trailer length is 13.
PLCP Frame 2 (the second of the 3 frames) will contain 14 trailer nibbles. The C1 byte, within
this PLCP frame, will contain the value 00h. This value identifies the current PLCP Frame as
Frame #2 in this 3 Frame Cycle, and informs the Far-End Receive PLCP Processor that the
trailer nibble length is 14.
PLCP Frame 3 (the last of the 3 frames) will contain either 13 or 14 trailer nibbles, depending
upon the calculated stuffing requirements. Hence, the Transmit PLCP can generate two versions
of Frame #3, “No Stuff” Frame #3 and “Stuff” Frame #3.
“No Stuff” Frame #3: If the Transmit PLCP Processor has determined that no stuff is required
then it will append only 13 trailer nibbles at the end of the current PLCP frame. The C1 byte,
within this PLCP frame, will identify the current frame as a “No Stuff” Frame #3, in the 3 Frame
cycle, by carrying the value 66h.
“Stuff” Frame #3: If the Transmit PLCP Processor has determined that a stuff is required,
then it will append 14 trailer nibbles at the end of the current PLCP frame. The C1 byte, within
this PLCP frame, will identify the current frame as a “Stuff” Frame #3, in the 3 Frame cycle, by
carrying the value 99h.
Once the Stuff Control algorithm has processed through PLCP Frame #3, the Transmit PLCP
Processor will proceed to generate a PLCP Frame #1, and repeat this 3 frame cycle.
TimRefSel[1,0] = 01
PLCP Frame Timing Source: External 8 kHz Clock Signal
In this configuration, the Transmit PLCP Processor takes its timing from an 8 kHz signal which
is applied at the 8KRef input pin. The Transmit PLCP Processor will also use this signal to cal-
culate stuff opportunities.
Stuff Control: As mentioned earlier, a stuff opportunity for the Transmit PLCP Processor
occurs once in a period of three (3) PLCP Frames. These composite PLCP frames and the
resulting C1 values are the same as presented in the above “PLCP Frame Timing/Stuff Con-
trol” Option (TimRefSel = 00).
TimRefSel[1,0] = 10; - StuffCtl Input Pin
PLCP Frame Timing Source: PLCP Frame timing is asynchronous upon power up or reset.
In this configuration, the Transmit PLCP Processor will start PLCP frames based upon an
asynchronous timing signal. The stuffing opportunities are not computed based on this timing,
but on the logic state of an input pin.
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