XRT72L54
FOUR CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER
á
PRELIMINARY
REV. P1.1.2
221
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 loca-
tions 0xDE through 0x135 in on-chip RAM.
Therefore, the local P/C 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.
NOTES:
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
The Receive Overhead Data Output Inter-
face
Figure 87 presents a simple illustration of the Receive
Overhead Data Output Interface block within the
XRT72L54.
FIGURE 86. FLOW CHART DEPICTING THE FUNCTIONALITY OF THE LAPD RECEIVER
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 6
consecutive
Zeros
?
Does
the LAPD
Receiver detect 7
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.
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 6
consecutive
Zeros
?
Does
the LAPD
Receiver detect 7
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..
VERIFY THE FCS VALUE
Report results in the RxLAPD
Status Register..
“Un-stuff contents of Received
Message”
“Un-stuff contents of Received
Message”
Write Received PMDL Message
into the Receive LAPD Message
Buffer (Addresses 0xDE - 0x135)
Write Received PMDL Message
into the Receive LAPD Message
Buffer (Addresses 0xDE - 0x135)
Generate “Received LAPD
Interrupt”
Generate “Received LAPD
Interrupt”
Execute Receive LAPD
Interrupt Service Routine
Execute Receive LAPD
Interrupt Service Routine
1
NO
YES
NO
YES
NO
YES
NO