
XRT74L73
3 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER
REV. P1.0.1
PRELIMINARY
215
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, x
16
+ x
12
+ x
5
+ 1.
Operation of the LAPD Transmitter
If a message is to be transmitted via the LAPD Trans-
mitter, the information portion (or the body) of the
message must be written into the Transmit LAPD
Message Buffer, which is located at 0x86 through
0xDD in on-chip RAM via the Microprocessor Inter-
face. Afterwards, three things must be done:
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
One of two different sizes of LAPD Messages can be
transmitted. This is accomplish by writing the appro-
priate data to bit 1 within the Tx DS3 LAPD Configu-
ration Register. The bit-format of this register is pre-
sented below.
The relationship between the contents of bit-fields 1
and the LAPD Message size is given in Table 41 .
N
OTE
:
The Message Type selected must correspond with
the contents of the first byte of the Information (Payload)
portion, as presented in Table 40 .
STEP 2 - Enabling the LAPD Transmitter
Prior to the transmission of any data via the LAPD
Transmitter the LAPD Transmitter must be enabled.
This is accomplished this by writing a 1 to bit 0 of the
Tx DS3 LAPD Configuration Register, as depicted
below.
T
ABLE
40: T
HE
LAPD M
ESSAGE
T
YPE
AND
THE
C
ORRESPONDING
VALUE
OF
THE
F
IRST
B
YTE
,
WITHIN
THE
I
NFORMATION
P
AYLOAD
LAPD M
ESSAGE
T
YPE
V
ALUE
OF
F
IRST
B
YTE
,
WITHIN
I
NFORMATION
P
AYLOAD
OF
M
ESSAGE
M
ESSAGE
S
IZE
CL Path Identification
0x32
76 bytes
IDLE Signal Identification
0x34
76 bytes
Test Signal Identification
0x38
76 bytes
ITU-T Path Identification
0x3F
82 bytes
TRANSMIT DS3 LAPD CONFIGURATION REGISTER (ADDRESS = 0X33)
B
IT
7
B
IT
6
B
IT
5
B
IT
4
B
IT
3
B
IT
2
B
IT
1
B
IT
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
T
ABLE
41: R
ELATIONSHIP
BETWEEN
T
X
LAPD M
SG
L
ENGTH
AND
THE
LAPD M
ESSAGE
S
IZE
T
X
LAPD M
SG
L
ENGTH
LAPD M
ESSAGE
L
ENGTH
0
LAPD Message size is 76 bytes
1
LAPD Message size is 82 bytes
TRANSMIT DS3 LAPD CONFIGURATION REGISTER (ADDRESS = 0X33)
B
IT
7
B
IT
6
B
IT
5
B
IT
4
B
IT
3
B
IT
2
B
IT
1
B
IT
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