XRT72L52 Datasheet, PDF (235/480 Pages) Exar Corporation – TWO CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER

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TWO CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER
PRELIMINARY
XRT72L52
REV. P1.1.3
1. The Flag Sequence byte marks the beginning or
end of an incoming LAPD Message.
2. The received Flag Sequence octet could be just
one of many Flag Sequence octets that are trans-
mitted via the DS3 Transport Medium, during idle
periods between the transmission of LAPD Mes-
sages.
The LAPD Receiver will clear the Flag Present bit as
soon as it has received an octet that is something
other than the Flag Sequence octet. At this point, the
LAPD Receiver should be receiving either octet #2 of
the incoming LAPD Message, or an Abort Sequence
(e.g., a string of seven or more consecutive 1s). If
this next set of data is an abort sequence, then the
LAPD Receiver will assert the RxAbort bit (Bit 6) with-
in the Rx DS3 LAPD Status Register. However, if this
next octet is Octet #2 of an incoming LAPD Message,
then the Rx DS3 LAPD Status Register will begin to
present some additional status information on this in-
coming message. Each of these indicators is pre-
sented below in sequential order.
Bit 3 - RxCR Type - C/R (Command/Response)
Type
This bit-field reflects the contents of the C/R bit-field
within octet #2 of the LAPD Frame Header. When
this bit is "0" it means that this message is originating
from a customer installation. When this bit is "1" it
means that this message is originating from a net-
work terminal.
Bit 4,5 - RxLAPD Type[1, 0] - LAPD Message Type
The combination of these two bit fields indicate the
Message Type and the Message Size of the incoming
LAPD Message frame. Table 41 relates the values of
Bits 4 and 5 to the Incoming LAPD Message Type/
Size.
TABLE 41: THE RELATIONSHIP BETWEEN RXLAPDTYPE[1:0] AND THE RESULTING LAPD MESSAGE TYPE AND SIZE
RXLAPD TYPE[1, 0]
00
01
10
11
MESSAGE TYPE
Test Signal Identification
Idle Signal Identification
CL Path Identification
TU-T Path Identification
MESSAGE SIZE
76 bytes
76 bytes
76 bytes
82 bytes
NOTE: The Message Size pertains to the size of the Infor-
mation portion of the LAPD Message Frame (as presented
in Figure 85).
Bit 3 - Flag Present
The LAPD Receiver should receive another Flag Se-
quence octet, which marks the End of the Message.
Therefore, this bit field should be asserted once
again.
Bit 1 - EndOfMessage - End of LAPD Message
Frame
Upon receipt of the closing Flag Sequence octet, this
bit-field should be asserted. The assertion of this bit-
field indicates that a LAPD Message Frame has been
completely received. Additionally, if this newly re-
ceived LAPD Message is different from the previous
message, then the LAPD Receiver will inform the µC/
µP of the EndOfMessage event by generating an in-
terrupt.
Bit 2 - RxFCSErr - Frame Check Sequence Error
Indicator
The LAPD Receiver will take the incoming LAPD
Message and compute its own version of the Frame
Check Sequence (FCS) word. Afterwards, the LAPD
Receiver will compare its computed value with that it
has received from the remote LAPD Transmitter. If
these two values match, then the LAPD Receiver will
presume that the LAPD Message has been properly
received and the contents of the Received LAPD
Message (payload portion) will be retained at loca-
tions 0xDE through 0x135 in on-chip RAM. The
LAPD Receiver will indicate an error-free reception of
the LAPD Message by keeping this bit field negated
(Bit 2 = 0). However, if these two FCS values do not
match, then the received LAPD Message is corrupted
and the user is advised not to process this erroneous
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
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