XRT7250 datasheet(283/463 Pages) EXAR

283 / 463 page

XRT7250

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DS3/E3 FRAMER IC

REV. 1.1.1

264

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, the information portion (or the

body) of the message must be written into the Trans-

mit LAPD Message Buffer, which is located at 0x86

through 0xDD in on-chip RAM via the Microprocessor

Interface. Afterwards, the user must do five things:

1. Configure the source of the “N” bit (within each

outbound E3 frame, to be the LAPD Transmitter.

2. Specify the length of LAPD message to be trans-

mitted.

3. Specify whether the LAPD Transmitter should

transmit this LAPD Message frame only once, or

an indefinite number of times at One-Second

intervals.

4. Enable the LAPD Transmitter.

5. Initiate the Transmission of the PMDL Message.

Each of these steps will be discussed in detail.

STEP 1 - Configure the source of the “N” bit (with-

in each outbound E3 frame, to be the LAPD Trans-

mitter.

This is accomplished by writing the appropriate data

into the TxNSourceSel[1:0] bit-fields, within the TxE3

Configuration Register, as illustrated below.

Setting TxNSourceSel[1:0] to “10” configures the

Transmit E3 Framer block to use the LAPD Transmit-

ter as the data source for the “N” bits. Hence, the “N”

bit, (within each outbound E3 frame) is now carrying

LAPD Messages to the remote terminal equipment.

STEP 2 - Specify the type of LAPD Message frame

to be Transmitted (within the Transmit LAPD Mes-

sage Buffer)

The user must write in a specific octet value into the

first octet position within the Transmit LAPD Buffer

(e.g., at Address Location 0x86 within the Framer IC).

This octet is referred to as the LAPD Message Frame

ID octet. The value of this octet must correspond to

the type of LAPD Message frame that is desired to be

transmitted. This octet will ultimately be used by the

Remote Terminal Equipment in order to help it identify

the type of LAPD message frame that it is receiving.

Table 53 lists these octets and the corresponding

LAPD Message types.

STEP 3 - Write the PMDL Message into the re-

maining part of the Transmit LAPD Message Buff-

er.

The user must now write in his/her PMDL Message

into the remaining portion of the Transmit LAPD Mes-

sage buffer (e.g., addresses 0x87 through 0x135

within the Framer IC).

STEP 4 - Specifying the Length of the LAPD Mes-

sage

One of two different sizes of LAPD Messages can be

transmitted. This can be accomplished by writing the

appropriate data to bit 1 within the Tx E3 LAPD Con-

figuration Register. The bit-format of this register is

presented below.

TXE3 CONFIGURATION REGISTER (ADDRESS = 0X30)

BIT 7BIT 6BIT 5BIT 4BIT 3BIT 2BIT 1BIT 0

BIP-4

Enable

TxASourceSel[1:0]

TxNSourceSel[1:0]

Tx AIS

Enable

Tx LOS

Enable

Tx FAS

Source

Select

R/W

000

TRANSMIT E3 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

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