XRT7250 datasheet(183/463 Pages) EXAR

183 / 463 page

XRT7250

DS3/E3 FRAMER IC

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REV. 1.1.1

164

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, 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

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 25.

NOTE: The Message Type selected must correspond with

the contents of the first byte of the Information (Payload)

portion, as presented in Table 24.

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 accomplish this by writing a 1 to bit 0 of the Tx

DS3 LAPD Configuration Register, as depicted be-

low.

Bit 0 - TxLAPD Enable

This bit-field allow the user to enable or disable the

LAPD Transmitter in accordance with Table 26.

TRANSMIT DS3 LAPD CONFIGURATION REGISTER (ADDRESS = 0X33)

BIT 7BIT 6BIT 5BIT 4BIT 3BIT2

BIT 1BIT 0

Not Used

Auto

Retransmit

Not Used

TxLAPD

Msg Length

TxLAPD

Enable

R/O

R/W

R/O

R/W

TABLE 25: RELATIONSHIP BETWEEN TXLAPD MSG

LENGTH AND THE LAPD MESSAGE SIZE

TXLAPD MSG LENGTH

LAPD MESSAGE LENGTH

LAPD Message size is 76 bytes

LAPD Message size is 82 bytes

TRANSMIT DS3 LAPD CONFIGURATION REGISTER (ADDRESS = 0X33)

BIT 7BIT 6BIT 5BIT 4BIT 3BIT2BIT 1

BIT 0

Not Used

Auto

Retransmit

Not Used

TxLAPD

Msg Length

TxLAPD

Enable

R/O

R/W

R/O

R/W

TABLE 26: RELATIONSHIP BETWEEN TXLAPD MSG LENGTH AND THE LAPD MESSAGE SIZE

TXLAPD ENABLE

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).

XRT7250 Datasheet(PDF) 183 Page - Exar Corporation

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