XRT72L13 Datasheet, PDF (278/370 Pages) Exar Corporation – M13 MULTIPLEXER/CLEAR CHANNEL DS3 FRAMER IC

XRT72L13
M13 MULTIPLEXER/CLEAR CHANNEL DS3 FRAMER IC
REV. P1.0.6
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PRELIMINARY
TABLE 18: THE LAPD MESSAGE TYPE AND THE CORRESPONDING VALUE OF THE FIRST BYTE, WITHIN THE
INFORMATION PAYLOAD
LAPD MESSAGE TYPE
VALUE OF FIRST BYTE, WITHIN
INFORMATION PAYLOAD OF MESSAGE
MESSAGE SIZE
CL Path Identification
0x38
76 bytes
IDLE Signal Identification
0x34
76 bytes
Test Signal Identification
0x32
76 bytes
ITU-T Path Identification
0x3F
82 bytes
Frame Check Sequence Bytes
The 16 bit FCS (Frame Check Sequence) is calculat-
ed over the LAPD Message Header and Information
Payload bytes, by using the CRC-16 polynomial, x¹⁶
+ x¹² + x⁵ + 1.
Operation of the LAPD Transmitter
If the user wishes to transmit a message via the
LAPD Transmitter, he/she must write the information
portion (or the body) of the message into the Transmit
LAPD Message Buffer, which is located at 0x86
through 0xDD in on-chip RAM via the Microprocessor
Interface. Afterwards, the user must do three things:
1. Specify the length of LAPD message to be trans-
mitted.
2. Enable the LAPD Transmitter.
3. Initiate the Transmission of the PMDL Message.
Each of these steps will be discussed in detail.
STEP 1 - Specifying the Length of the LAPD Mes-
sage
The user can transmit one of two different sizes of
LAPD Messages. He/she can accomplish this by
writing the appropriate data to bit 1 within the "Tx
DS3 LAPD Configuration" Register. The bit-format of
this register is presented below.
TRANSMIT DS3 LAPD CONFIGURATION REGISTER (ADDRESS = 0X33)
BIT 7
BIT 6
BIT 5
BIT 4
BIT 3
BIT2
BIT 1
BIT 0
Not Used
Auto
Retransmit
Not Used
TxLAPD
Msg Length
TxLAPD
Enable
R/O
R/O
R/O
R/O
R/W
R/O
R/W
R/W
0
0
0
0
X
0
X
X
The relationship between the contents of bit-fields 1
and the LAPD Message size is given in Table 19 .
TABLE 19: RELATIONSHIP BETWEEN TXLAPD MSG
LENGTH AND THE LAPD MESSAGE SIZE
TXLAPD MSG LENGTH
LAPD MESSAGE LENGTH
0
LAPD Message size is 76 bytes
1
LAPD Message size is 82 bytes
NOTE: The Message Type selected must correspond with
the contents of the first byte of the Information (Payload)
portion, as presented in Table 18 .
STEP 2 - Enabling the LAPD Transmitter
Prior to the transmission of any data via the LAPD
Transmitter, the user must enable the LAPD Transmit-
ter. He/she can accomplish this by writing a "1" to bit
0 of the Tx DS3 LAPD Configuration Register, as de-
picted below.
TRANSMIT DS3 LAPD CONFIGURATION REGISTER (ADDRESS = 0X33)
BIT 7
BIT 6
BIT 5
BIT 4
BIT 3
BIT2
BIT 1
BIT 0
Not Used
Auto
Retransmit
Not Used
TxLAPD
Msg Length
TxLAPD
Enable
R/O
R/O
R/O
R/O
R/W
R/O
R/W
R/W
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