XR72L52 Datasheet, PDF (183/480 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
XRT72L52
REV. 1.0.1
PMDL Messages that are supported by the XRT72L52 Framer and the corresponding octet value that must be
written into the first octet position within the Transmit LAPD Message buffer.
TABLE 21: THE LAPD MESSAGE TYPE AND THE CORRESPONDING VALUE OF THE FIRST BYTE WITHIN THE
INFORMATION PAYLOAD
LAPD MESSAGE TYPE
CL Path Identification
IDLE Signal Identification
Test Signal Identification
ITU-T Path Identification
VALUE OF FIRST BYTE, WITHIN
INFORMATION PAYLOAD OF MESSAGE
0x38
0x34
0x32
0x3F
MESSAGE SIZE
76 bytes
76 bytes
76 bytes
82 bytes
Frame Check Sequence Bytes
The 16 bit FCS (Frame Check Sequence) is calculated over the LAPD Message Header and Information Pay-
load bytes by using the CRC-16 polynomial, x¹⁶ + x¹² + x⁵ + 1.
NOTE: For FCS calculation, Header also includes the starting Flag Sequence byte (0x7E).
Operation of the LAPD Transmitter
If a message is to be transmitted via the LAPD Transmitter, the information portion (or the body) of the mes-
sage must be written into the Transmit LAPD Message Buffer located at 0x8A through 0xDB in on-chip RAM
via the Microprocessor Interface. Afterwards, three things must be done:
STEP 1 - Specifying the Length of the LAPD Message
One of two different sizes of LAPD Messages can be transmitted. This is accomplish 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
RO
0
BIT 6
BIT 5
Not Used
RO
RO
0
0
BIT 4
RO
0
BIT 3
Auto
Retransmit
R/W
X
BIT2
Not Used
RO
0
BIT 1
TxLAPD
Msg Length
R/W
X
BIT 0
TxLAPD
Enable
R/W
X
The relationship between the contents of bit-fields 1 and the LAPD Message size is given in Table 22.
TABLE 22: 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
The Message Type selected must correspond with the contents of the first byte of the Information (Payload)
portion, as presented in Table 21 and written at address 0x8A.
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 by writing a “1” to Bit 0 of the Tx DS3 LAPD Configuration Register, as depicted below.
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