XRT7250 datasheet(386/463 Pages) EXAR

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XRT7250

DS3/E3 FRAMER IC

REV. 1.1.1

367

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 a message is to be transmitted via the LAPD Trans-

mitter then, 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. Specify which bit-field (within the E3 frame) that

the LAPD Message frame is to be transported on

(e.g., either the “GC” or the “NR” byte).

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 - 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 to be trans-

mitted. This octet will ultimately be used by the Re-

mote Terminal Equipment in order to help it identify

the type of LAPD message frame that it is receiving.

Table 76 lists these octets and the corresponding

LAPD Message types.

STEP 2 - 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 3 - Specifying the Length of the LAPD Mes-

sage

One of two different sizes of LAPD Messages can be

transmitted, by writing the appropriate data to bit 1

within the Tx E3 LAPD Configuration Register. The

bit-format of this register is presented below.

The relationship between the contents of bit-fields 1

and the LAPD Message size is given in Table 77.

NOTE: The Message Type selected must correspond with

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

portion, as presented in Table 76.

STEP 4 - Specifying which byte-field (within the

E3 frame) that the LAPD Message frame octets

are to be transported on.

The Transmit E3 Framer block allows the user to

transport the LAPD Message frame octets via either

the “NR” byte or the “GC” byte-field, within each Out-

bound E3 frame. The user makes this selection by

writing the appropriate value to bit-field 4 (DLinNR),

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

000

TABLE 77: RELATIONSHIP BETWEEN TXLAPD MSG LENGTH AND THE LAPD MESSAGE SIZE

TXLAPD MESSAGE LENGTH

LAPD MESSAGE LENGTH

LAPD Message size is 76 bytes

LAPD Message size is 82 bytes

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