
XRT72L53
THREE CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER
á
PRELIMINARY
REV. P1.1.6
225
mote terminal) will insert a "0" immediately 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 oc-
tet (0x7E) or the ABORT sequence. Therefore, in or-
der to recover the user data to its original content (pri-
or to the bit-stuffing), the LAPD Receiver will remove
the "0" that immediately follows a string of 5 consecu-
tive 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 86 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.
2. A brief description of the steps that must exist
within the Receive LAPD Interrupt Service routine
exists in Section 3.3.6.
4.3.4
face
Figure 87 presents a simple illustration of the Receive
Overhead Data Output Interface block within the
XRT72L53.
The Receive Overhead Data Output Inter-
F
IGURE
86. 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