![](http://datasheet.mmic.net.cn/Exar-Corporation/XRT79L71IB-F_datasheet_100145/XRT79L71IB-F_219.png)
PRELIMINARY
XRT79L71
204
REV. P2.0.0
1-CHANNEL DS3/E3 CLEAR-CHANNEL FRAMERLIU COMBO - CC/HDLC ARCHITECTURE
Recall from the discussion in
Section 4.1 that each DS3 M-frame consists of three (3) M-bits that occur in a
repeating "010" pattern. The M-bit search is declared successful if three consecutive M-frames or 21 F-frames
are detected correctly. Once this occurs an M-bit Sync is declared, and the Receive DS3 Framer block will
then transition to the In-Frame state. At this point, the Receive DS3 Framer block will either transition into the
Parity-Check or directly into the In-Frame state depending upon user configuration. Each of these remaining
states within the Frame Acquisition/Maintenance Algorithim is described below.
The Parity (P-Bit) Check State (Optional)
The P-Bit Check state is an optional state within the Frame Acquisition/Maintenance Algorithm. In other words,
the Receive DS3 Framer block will only transition into, and operate in this state if it is configured to do so by the
user. If configured accordingly, the Receive DS3 Framer block will transition into this state after it has declared
M-Bit Sync. Once the Receive DS3 Framer block transitions into this state, it will check for correct P-bits within
the incoming DS3 data-stream. If the Receive DS3 Framer block detects correct P-bits within the incoming
DS3 data-stream then it will transition into the In-Frame state. On the other hand, if the Receive DS3 Framer
block detects erred P-bits within the incoming DS3 data-stream, then it will conclude that it has not properly
acquired DS3 framing and it will transition back into the F-Bit Search state.
Configuring the Receive DS3 Framer Block to support the Parity-Check State
The user can configure the Receive DS3 Framer block to operate such that 'valid parity' (P-bits) must also be
detected before the Receive DS3 Framer block can declare itself In Frame.
The user can select this
configuration by setting Bit 2 (Framing with Valid P-Bits) within the Receive DS3 Configuration and Status
Register to "1", as depicted below.
FIGURE 93. THE STATE MACHINE DIAGRAM FOR THE RECEIVE DS3 FRAMER BLOCK'S FRAME ACQUISTION/MAINTE-
NANCE
ALGORITHM (WITH THE M-BIT SEARCH STATE SHADED)
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