XRT7250 datasheet(208/463 Pages) EXAR

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DS3/E3 FRAMER IC

XRT7250

REV. 1.1.1

189

Once the B3ZS (or AMI) encoded data has been de-

coded into a binary data-stream, the Receive DS3

Framer block will use portions of this data-stream in

order to synchronize itself to the remote terminal

equipment. At any given time, the Receive DS3

Framer block will be operating in one of two modes.

• The Frame Acquisition Mode: In this mode, the

Receive DS3 Framer block is trying to acquire syn-

chronization with the incoming DS3 frames, or

• The Frame Maintenance Mode: In this mode, the

Receive DS3 Framer block is trying to maintain

frame synchronization with the incoming DS3

Frames.

Figure 81 presents a State Machine diagram that de-

picts the Receive DS3 Framer block's DS3 Frame Ac-

quisition/Maintenance Algorithm.

4.3.2.1

Frame Acquisition Mode Operation

The Receive DS3 Framer block will be performing

Frame Acquisition operation while it is operating in

any of the following states (per the DS3 Frame Acqui-

sition/Maintenance algorithm State Machine diagram,

as depicted in Figure 81.)

• The F-bit Search state

• The M-bit Search state

• The P-Bit Check state (optional)

Once the Receive DS3 Framer block enters the In-

Frame state (per Figure 81), then it will begin Frame

Maintenance operation.

When the Receive DS3 Framer block is in the frame-

acquisition mode, it will begin to look for valid DS3

frames by first searching for the F-bits in the incoming

DS3 data stream. At this initial point the Receive

DS3 Framer block will be operating in the F-Bit

Search state within the DS3 Frame Acquisition/Main-

tenance algorithm state machine diagram (see

Figure 81). Recall from the discussion in Section 4.1,

that each DS3 F-frame consists of four (4) F-bits that

FIGURE 81. THE STATE MACHINE DIAGRAM FOR THE RECEIVE DS3 FRAMER BLOCK'S FRAME ACQUISITION/MAIN-

TENANCE

ALGORITHM

F-Bit Search

M-Bit Search

F-Bit Synch

Achieved

In-Frame

RxOOF pin

is Negated.

10 Consecutive F-bits

Correctly Received

Parity Check

(Only if Framing

on Parity is

Selected)

M-bits Correctly

Detected for 3

Consecutive M-Frames

(Framing on Parity is

Selected)

M-bits Correctly

Detected for 3

Consecutive M-Frames

(Framing on Parity is

Not Selected)

OOF Criteria

based upon values

for F-Sync Algo

and M-Sync Algo

Valid Parity

Parity Error in

2 out of 5 frames

XRT7250 Datasheet(PDF) 208 Page - Exar Corporation

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