
XRT74L74
4 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER
ADVANCED CONFIDENTIAL
REV. P1.1.1
266
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
5.2.3.2
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.
N
OTES
:
1. The white (e.g., unshaded) boxes reflect tasks that
the user’s system must perform in order to config-
ure the LAPD Receiver to receive LAPD Messages.
F
IGURE
93. F
LOW
C
HART
DEPICTING
THE
F
UNCTIONALITY
OF
THE
LAPD R
ECEIVER
START
ENABLE THE LAPD RECEIVER
This is done by writing the value “0xFC into the
RxLAPD Control Register (Address = 0x18)
LAPD Receiver begins reading in the DL bits
from each inbound DS3 frame
Does
the LAPD
Receiver detect 6
consecutive
Zeros
Does
the LAPD
Receiver detect 7
consecutive
Zeros
Flag Sequence
ABORT Sequence
LAPD Receiver is reading in a LAPD
Message Frame, containing a PMDL
Message.
Does
the LAPD
Receiver detect 6
consecutive
Zeros
Does
the LAPD
Receiver detect 7
consecutive
Zeros
End of Message (EOM)
VERIFY THE FCS VALUE
Report results in the RxLAPD
Status Register..
“Un-stuff contents of Received
Message”
Write Received PMDL Message
into the Receive LAPD Message
Buffer (Addresses 0xDE - 0x135)
Generate “Received LAPD
Interrupt”
Execute Receive LAPD
Interrupt Service Routine
1
1
1
1
NO
YES
NO
YES
YES
NO
YES
NO