
XRT7245
DS3 UNI FOR ATM
á
PRELIMINARY
REV. 1.03
234
Message Frame. Recall that the text in Section
6.4.3.1.3 indicated that the LAPD Transmitter (at the
Far-End Transmit DS3 Framer) will insert a “0” imme-
diately following a string of 5 consecutive “1s” within
the payload portion of the LAPD Message frame. The
LAPD Transmitter performs this bit-stuffing procedure
in order to prevent the user data from mimicking the
Flag Sequence octet (7Eh). Therefore, in order to re-
cover the user data to its original form (prior to the
bit-stuffing), the LAPD Receiver will remove the “0”
that immediately follows a string of 5 consecutive “1s”.
Writing the Incoming LAPD Message to the
“Receive LAPD Message” Buffer
The LAPD receiver will obtain the LAPD Message
frame from the incoming DS3 data-stream, in addition
to processing the framing overhead octets, perform-
ing error checking (via FCS) and removing the stuffed
“0s” from the user payload data. The LAPD Receiver
will write the payload portion of the LAPD Frame into
the “Receive LAPD Message” buffer at locations DEh
through 135h in on-chip RAM.
Therefore, the local μP/μC must read this location
when it wishes to process this newly received LAPD
Message.
Figure 70 presents a flow chart depicting how the
LAPD Receiver works.
7.1.2.7
Whenever the Receive DS3 Framer detects P-bit
errors, or is in an “OOF” condition, it will inform the
FEBE (Far-End-Block Error) Bit-fields
“Near-End” Transmit DS3 Framer of this fact. The
“Near-End” Transmit DS3 Framer will, in turn, notify
the “Far End” Terminal (e.g., the source of the errored
F
IGURE
70. F
LOW
C
HART
DEPICTING
THE
F
UNCTIONALITY
OF
THE
LAPD R
ECEIVER
ENABLE THE LAPD RECEIVER
This is done by winning the value "0xFC" into the
RxLAPD Control Register (Address = 0x14)
START
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)
Write received PMDL message
into the Receive LAPD Message
Buffer (Address 0xDE - 0x135)
VERIFY THE FCS VALUE
Report results in the RxLAPD
Status Register
"Un-stuff contents of
received message"
Generate "received LAPD
interrupt"
Execute Receive LAPD
Interrupt Service Routine