XRT72L13 Datasheet, PDF (321/370 Pages) Exar Corporation – M13 MULTIPLEXER/CLEAR CHANNEL DS3 FRAMER IC

áç
PRELIMINARY
XRT72L13
M13 MULTIPLEXER/CLEAR CHANNEL DS3 FRAMER IC
REV. P1.0.6
LAPD Receiver will indicate an "error-free" reception
of the LAPD Message by keeping this bit field negat-
ed (Bit 2 = 0). However, if these two FCS values do
not match, then the received LAPD Message is cor-
rupted; and the user is advised not to process this er-
roneous 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 Section
3.2.3.2 indicated that the LAPD Transmitter (at the re-
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
Receiver will also write the payload portion of the
LAPD Frame into the "Receive LAPD Message" buff-
er at locations 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 105 presents a flow chart depicting how the
LAPD Receiver works.
309