XRT74L74 Datasheet, PDF (229/498 Pages) Exar Corporation – 4 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER

4 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER
ADVANCED CONFIDENTIAL
XRT74L74
REV. P1.1.1
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 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.
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 41.
TABLE 41: 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 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 be-
low.
TRANSMIT DS3 LAPD CONFIGURATION REGISTER (ADDRESS = 0X33)
BIT 7
BIT 6
BIT 5
Not Used
BIT 4
R/O
R/O
R/O
R/O
0
0
0
0
Bit 0 - TxLAPD Enable
This bit-field allows the user to enable or disable the
LAPD Transmitter in accordance with Table 42.
BIT 3
Auto
Retransmit
R/W
X
BIT2
Not Used
R/O
0
BIT 1
TxLAPD
Msg Length
R/W
X
BIT 0
TxLAPD
Enable
R/W
1
TABLE 42: RELATIONSHIP BETWEEN TXLAPD MSG LENGTH AND THE LAPD MESSAGE SIZE
TXLAPD ENABLE
0
1
RESULTING ACTION OF THE LAPD TRANSMITTER
The LAPD Transmitter is disabled and the DL bits, in the DS3 frame,
are transmitted as all 1s.
The LAPD Transmitter is enabled and is transmitting a continuous
stream of Flag Sequence octets (0x7E).
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