XRT7250 Datasheet, PDF (208/463 Pages) Exar Corporation – DS3/E3 FRAMER IC

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DS3/E3 FRAMER IC
XRT7250
REV. 1.1.1
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.
FIGURE 81. THE STATE MACHINE DIAGRAM FOR THE RECEIVE DS3 FRAMER BLOCK'S FRAME ACQUISITION/MAIN-
TENANCE ALGORITHM
F-Bit Search
OOF Criteria
based upon values
for F-Sync Algo
and M-Sync Algo
10 Consecutive F-bits
Correctly Received
M-Bit Search
F-Bit Synch
Achieved
M-bits Correctly
Detected for 3
Consecutive M-Frames
(Framing on Parity is
Not Selected)
Parity Error in
2 out of 5 frames
M-bits Correctly
Detected for 3
Consecutive M-Frames
(Framing on Parity is
Selected)
In-Frame
RxOOF pin
is Negated.
Valid Parity
Parity Check
(Only if Framing
on Parity is
Selected)
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
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