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XRT79L71
PRELIMINARY
203
1-CHANNEL DS3/E3 CLEAR-CHANNEL FRAMERLIU COMBO - CC/HDLC ARCHITECTURE
REV. P2.0.0
Conversely, setting Bit 1 (F-Algorithm) within the Receive DS3 Sync Detect Register to "0" configures the
Receive DS3 Framer block to declare F-bit Sync after the successful detection of 10 consecutive F-bits.
Handling of Mimicking F-bits
It was mentioned earlier that the Receiver DS3 Framer block, when operating in the Frame Acquisition Mode,
will use five (5) Framing Bit Searching circuits in parallel. As each of these Framing Bit Searching circuits
parses through the incoming DS3 data-stream, they are each searching for a sequence of bits that exhibit a
certain pattern and spacing from each other. At any given time, it may be possible that more than one Framing
Bit Searching circuit has honed in on more than one viable F-bit candidate. Whenever this occurs, it may be
possible for the Receive DS3 Framer block to become fooled by data bits mimicking the F-bits.
This
phenomenon can result in the Receive DS3 Framer block proceeding along the DS3 Frame Acquisition/
Maintenance State Machine, while being locked onto to mimicking data, and not to the actual F-bits. At some
point, the Receive DS3 Framer block will recognize that it is not locked onto the actual F-bits and that it will
have to return to the F-Bit Search state. However, all of the time that the Receive DS3 Framer block spent,
proceeding along with the M-Bit Search state will have been wasted and will increase the overall Frame
Acquisition time.
The Command Register set, within the XRT79L71, permits the user to minimize the potential of the Framing Bit
Searching circuitry being fooled by mimicking data, by setting Bit 0 (One and Only), within the Receive DS3
Sync Detect Register to "1", as illustrated below.
If this configuration option is implemented, then the Receive DS3 Framer block will only declare F-Bit Sync
when all of the following criteria are met.
A given Framing Bit Searching circuit has successfully located 10 or 16 consecutive, correct F-bits.
None of the remaining four (4) Framing Bit Searching circuits have any viable candidates for F-bits. One and
Only one viable candidate exists for all of the five Framing Bit Searching circuits.
The M-bit Search State
When the Receive DS3 Framer block reaches the M-Bit Search state, it will begin searching for valid M-bits
within the incoming DS3 data-stream. In order to clearly convey the mode that the Receive DS3 Framer block
is currently operating in, Figure 91 has been repeated below, with the M-Bit Search state shaded.
Receive DS3 Sync Detect Register (Address = 0x1114)
BIT 7
BIT 6
BIT 5
BIT 4
BIT 3
BIT 2
BIT 1
BIT 0
Unused
F Algorithm
One and
Only
R/O
R/W
0
1
0
Receive DS3 Sync Detect Register (Address = 0x1114)
BIT 7
BIT 6
BIT 5
BIT 4
BIT 3
BIT 2
BIT 1
BIT 0
Unused
F Algorithm
One and
Only
R/O
R/W
0
X
01