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PRELIMINARY
XRT79L71
230
REV. P2.0.0
1-CHANNEL DS3/E3 CLEAR-CHANNEL FRAMERLIU COMBO - CC/HDLC ARCHITECTURE
NOTE: The Receive LAPD Controller block will not generate any interrupt in response to it detecting any FCS Errors within
an incoming LAPD Message. The user is expected to validate each incoming LAPD Message, by testing the state
of the RxFCS Error bit-field, prior to processing a given message.
STEP 8 - Identify the Type and Size of Message that the Receive LAPD Controller has just received.
This can be accomplished by reading out the contents of Bits 4 and 5 (RxLAPDType[1:0]) within the Receive
DS3 LAPD Status Register, as depicted below.
Table 30 presents the relationship between the contents within Bits 4 and 5 (RxLAPDType[1:0]) and the
corresponding LAPD Message that is residing within the Receive LAPD Message Buffer.
Table 30 indicates that if the value within the RxLAPDType[1:0] bit-fields are set to [1, 1], then the size of total
message residing within the Receive LAPD Message Buffer is 88 bytes, and that the size of the Information
Payload within these 88 bytes of data is 82 bytes. Likewise, this table also indicates that if the value within the
RxLAPDType[1:0] bit-fields are set to some value other than [1, 1], then the size of the total message residing
within the Receive LAPD Message Buffer is 82 bytes, and that the size of the Information Payload within these
82 bytes of data is 76 bytes.
The Relationship between the Total LAPD/PMDL Message Size and Information Payload Size
The Relationship between the size of the total LAPD/PMDL Message, and the size of the Information Payload,
is best explained by presenting the byte-format of the LAPD Message below in Figure 97.
Receive DS3 LAPD Status Register (Address = 0x1119)
BIT 7
BIT 6
BIT 5
BIT 4
BIT 3
BIT 2
BIT 1
BIT 0
Unused
RxABORT
RxLAPDType[1:0]
RxCR Type
RxFCS Error
End of
Message
Flag Present
R/O
0
X
0
1
0
TABLE 30: THE RELATIONSHIP BETWEEN THE CONTENTS WITHIN BITS 4 AND 5 (RXLAPDTYPE[1:0]) AND THE TYPE
OF
LAPD/PMDL MESSAGE RESIDING WITHIN THE RECEIVE LAPD MESSAGE BUFFER
RXLAPDTYPE[1:0]
TYPE OF LAPD/PMDL MESSAGE
RESIDING IN THE
RECEIVE LAPD
MESSAGE BUFFER
SIZE OF MESSAGE RESIDING WITHIN THE
RECEIVE LAPD MESSAGE BUFFER
(INFORMATION PAYLOAD/TOTAL
MESSAGE SIZE)
00
CL Path Identification Message
76 Bytes/82 Bytes
01
Idle Signal Identification
76 Bytes/82 Bytes
10
Test Signal Identification
76 Bytes/82 Bytes
11
ITU-T Path Identification
82 Bytes/88 Bytes
FIGURE 97. LAPD MESSAGE FRAME FORMAT
FLAG SEQUENCE (8 BITS)
SAPI (6-bits)
C/R
EA
TEI (7 bits)
EA
Control (8-bits)