XRT7250 datasheet(223/463 Pages) EXAR

223 / 463 page

XRT7250

DS3/E3 FRAMER IC

áç

REV. 1.1.1

204

mote terminal) will insert a "0" immediately following a

string of 5 consecutive 1s within the payload portion

of the LAPD Message frame. The LAPD Transmitter

performs this bit-stuffing procedure in order to prevent

the user data from mimicking the Flag Sequence oc-

tet (0x7E) or the ABORT sequence. Therefore, in or-

der to recover the user data to its original content (pri-

or to the bit-stuffing), the LAPD Receiver will remove

the "0" that immediately follows a string of 5 consecu-

tive 1s.

Writing the Incoming LAPD Message into the Re-

ceive LAPD Message Buffer

The LAPD receiver will obtain the LAPD Message

frame from the incoming DS3 data-stream. In addi-

tion to processing the framing overhead octets, per-

forming error checking (via FCS) and removing the

stuffed 0s from the user payload data. The LAPD Re-

ceiver will also write the payload portion of the LAPD

Frame into the Receive LAPD Message buffer at lo-

cations 0xDE through 0x135 in on-chip RAM.

Therefore, the local µP/µC must read this location

when it wishes to process this newly received LAPD

Message.

Figure 86 presents a flow chart depicting how the

LAPD Receiver works.

NOTES:

1. The white (e.g., unshaded) boxes reflect tasks that

the user’s system must perform in order to config-

ure the LAPD Receiver to receive LAPD Messages.

2. A brief description of the steps that must exist

within the Receive LAPD Interrupt Service routine

exists in Section 3.3.6.

4.3.4

The Receive Overhead Data Output Inter-

face

Figure 87 presents a simple illustration of the Receive

Overhead Data Output Interface block within the

XRT7250.

FIGURE 86. FLOW CHART DEPICTING THE FUNCTIONALITY OF THE LAPD RECEIVER

START

ENABLE THE LAPD RECEIVER

This is done by writing the value “0xFC into the

RxLAPD Control Register (Address = 0x18)

LAPD Receiver begins reading in the DL bits

from each inbound DS3 frame

Does

the LAPD

Receiver detect 6

consecutive

Zeros

Does

the LAPD

Receiver detect 6

consecutive

Zeros

Does

the LAPD

Receiver detect 7

consecutive

Zeros

Does

the LAPD

Receiver detect 7

consecutive

Zeros

Flag Sequence

ABORT Sequence

LAPD Receiver is reading in a LAPD

Message Frame, containing a PMDL

Message.

LAPD Receiver is reading in a LAPD

Message Frame, containing a PMDL

Message.

Does

the LAPD

Receiver detect 6

consecutive

Zeros

Does

the LAPD

Receiver detect 6

consecutive

Zeros

Does

the LAPD

Receiver detect 7

consecutive

Zeros

Does

the LAPD

Receiver detect 7

consecutive

Zeros

End of Message (EOM)

VERIFY THE FCS VALUE

Report results in the RxLAPD

Status Register..

VERIFY THE FCS VALUE

Report results in the RxLAPD

Status Register..

“Un-stuff contents of Received

Message”

“Un-stuff contents of Received

Message”

Write Received PMDL Message

into the Receive LAPD Message

Buffer (Addresses 0xDE - 0x135)

Write Received PMDL Message

into the Receive LAPD Message

Buffer (Addresses 0xDE - 0x135)

Generate “Received LAPD

Interrupt”

Generate “Received LAPD

Interrupt”

Execute Receive LAPD

Interrupt Service Routine

Execute Receive LAPD

Interrupt Service Routine

YES

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