XRT74L73 Datasheet, PDF (116/488 Pages) Exar Corporation – 3 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER

XRT74L73
3 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER
REV. P1.0.1
PRELIMINARY
• G1 Byte Generator
• Framing Byte, POI, Dummy POI Generator
• External POH Collector
• Transmit PLCP Framer
• Overhead MUX
• Overhead Payload MUX
• Byte/Nibble Converter
The role of some of these functional blocks will be dis-
cussed below.
2.3.3.1Transmit PLCP Frame Timing, Stuff Con-
trol—C1 Byte
The Controlled Stuff Generator portion of the Transmit
PLCP Processor is responsible for three things.
1. Determining the nibble-stuffing requirements for the
current PLCP frame.
2. Fulfilling these nibble-stuffing requirements.
3. Reflecting the nibble-stuffing status in the C1 byte.
Table 16 indicates the Transmit PLCP Processor will
append either a 13 or 14 nibble trailer at the end of each
PLCP frame, in order to frequency justify the framing to 8
kHz. This choice between 13 or 14 nibbles presents the
Transmit PLCP Processor with a “stuff” opportunity.
The Transmit PLCP Processor can be configured into
one of four frame-timing/stuff-control options. These
options are selected by writing the appropriate data to
bit 1 and bit 0 (TimRefSel[1, 0], within the UNI Operat-
ing Mode Register. The bit format of this register is pre-
sented below.
UNI Operating Mode Register: Address = 00h
BIT 7
Local Loop-
back
R/W
BIT 6
Cell Loop-
back
R/W
BIT 5
PLCP Loop-
back
R/W
BIT 4
Reset
R/W
BIT 3
Direct
Mapped ATM
R/W
BIT 2
C-Bit/M13
R/W
BIT 1
BIT 0
TimRefSel[1, 0]
R/W
117