XRT74L73 Datasheet, PDF (255/488 Pages) Exar Corporation – 3 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER

PRELIMINARY
XRT74L73
3 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER
REV. P1.0.1
presume that the LAPD Message has been properly
received and the contents of the Received LAPD
Message (payload portion) will be retained at loca-
tions 0xDE through 0x135 in on-chip RAM. The
LAPD Receiver will indicate an error-free reception of
the LAPD Message by keeping this bit field negated
(Bit 2 = 0). However, if these two FCS values do not
match, then the received LAPD Message is corrupted
and the user is advised not to process this erroneous
information. The LAPD Receiver will indicate an erred
receipt of this message by setting this bit-field to 1.
NOTE: The Receive DS3 HDLC Controller block will not
generate an interrupt to the µP due to the detection of an
FCS error. Therefore, the user is advised to validate each
and every received LAPD message by checking this bit-
field prior to processing the LAPD message.
Removal of Stuff Bits from the Payload Portion of
the incoming LAPD Message
While the LAPD Receiver is receiving a LAPD Mes-
sage, it has the responsibility of removing all of the
"0" stuff bits from the Payload Portion of the incoming
LAPD Message Frame. Recall that the text in Sec-
tion 4.2.3.2 indicated that the LAPD Transmitter (at
the remote terminal) will insert a "0" immediately fol-
lowing a string of 5 consecutive “1s” within the pay-
load portion of the LAPD Message frame. The LAPD
Transmitter performs this bit-stuffing procedure in or-
der to prevent the user data from mimicking the Flag
Sequence octet (0x7E) or the ABORT sequence.
Therefore, in order to recover the user data to its orig-
inal content (prior to the bit-stuffing), the LAPD Re-
ceiver will remove the "0" that immediately follows a
string of 5 consecutive 1s.
Writing the Incoming LAPD Message into the Re-
ceive LAPD Message Buffer
The LAPD receiver will obtain the LAPD Message
frame from the incoming DS3 data-stream. In addi-
tion to processing the framing overhead octets, per-
forming error checking (via FCS) and removing the
stuffed 0s from the user payload data. The LAPD Re-
ceiver will also write the payload portion of the LAPD
Frame into the Receive LAPD Message buffer at lo-
cations 0xDE through 0x135 in on-chip RAM.
Therefore, the local µP/µC must read this location
when it wishes to process this newly received LAPD
Message.
Figure 93 presents a flow chart depicting how the
LAPD Receiver works.
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