XRT7250 datasheet(389/463 Pages) EXAR

389 / 463 page

XRT7250

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DS3/E3 FRAMER IC

REV. 1.1.1

370

‘The purpose of t his interrupt is to let the Micropro-

cessor/Microcontroller know that the LAPD Transmit-

ter is available and ready to transmit a LAPD Mes-

sage frame (which contains a new PMDL Message)

to the remote terminal equipment. Bit 0 (Tx LAPD In-

terrupt Status) within the Tx E3 LAPD Status and In-

terrupt Register will reflect the status for the Transmit

LAPD Interrupt.

NOTE: This bit-field will be reset upon reading this register.

Summary of Operating the LAPD Transmitter

Once the user has invoked the TxDL Start command,

the LAPD Transmitter will do the following.

• Generate the four octets of the LAPD Message

frame header (e.g., the Flag Sequence, SAPI, TEI,

Control, etc.,) and insert them into the header byte

positions within the LAPD Message frame.

• It will read in the contents of the Transmit LAPD

Message buffer (e.g., the PMDL Message data)

and insert it into the Information Payload portion of

the LAPD Message frame.

• Compute the 16-bit Frame Check Sequence (FCS)

value of the LAPD Message frame (e.g, of the

LAPD Message header and Payload bytes) and

insert this value into the FCS value octet positions

within the LAPD Message frame.

• Append a trailer Flag Sequence octet to the end of

the LAPD Message frame (following the 16-bit FCS

octets).

• Fragment the resulting LAPD Message frame into

octets and begin inserting these octets into either

the GC or NR byte-fields within the Outbound E3

frames (depending upon the user’s selection).

• Complete the transmission of the overhead bytes,

information payload byte, FCS value, and the trail-

ing Flag Sequence octets via the Transmit E3

Framer block.

Once the LAPD Transmitter has completed its trans-

mission of the LAPD Message frame, the Framer will

generate an Interrupt to the MIcroprocessor/Micro-

controller (if enabled). Afterwards, the LAPD Trans-

mitter will either halt its transmission of LAPD Mes-

sage frames or will proceed to retransmit the LAPD

Message frame, repeatedly at One-Second inter-

vals. In between these transmissions of the LAPD

Message frames, the LAPD Transmitter will be send-

ing a continuous stream of Flag Sequence bytes.

The LAPD Transmitter will continue this behavior until

the user has disabled the LAPD Transmitter by writing

a “1” into bit 3 (No Data Link) within the Tx E3 Config-

uration register.

NOTE: In order to prevent the user’s data (e.g., the PMDL

Message within the LAPD Message frame) from mimicking

the Flag Sequence byte or an ABORT Sequence, the LAPD

Transmitter will parse through the PMDL Message data and

insert a “0” into this data, immediately following the detec-

tion of five (5) consecutive “1’s” (this stuffing occurs while

the PMDL message data is being read in from the Transmit

LAPD Message frame. The Remote LAPD Receive (See

Section 5.3.5) will have the responsibility of checking the

newly received PMDL messages for a string of five (5) con-

secutive “1’s” and removing the subsequent “0” from the

payload portion of the incoming LAPD Message.

Figure 175 is a flow chart that depicts the procedure

(in white boxes) that the user should use in order to

transmit a PMDL message via the LAPD Transmitter,

when the LAPD Transmitter is configured to retrans-

mit the LAPD Message frame, repeatedly at One-

Second intervals. This figure also indicates (via the

Shaded boxes) what the LAPD Transmitter circuitry

will do before and during message transmission.

TXE3 LAPD STATUS AND INTERRUPT REGISTER (ADDRESS = 0X34)

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

Not Used

TxDL Start

TxDL Busy

TxLAPD

Interrupt

Enable

TxLAPD

Interrupt

Status

R/W

RUR

000

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