XRT74L73 Datasheet, PDF (240/488 Pages) Exar Corporation – 3 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER

XRT74L73
3 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER
REV. P1.0.1
PRELIMINARY
valid M-bits. 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-frame lock is declared, and the Re-
ceive DS3 Framer block will then transition to the In-
Frame state. At this point, the Receive DS3 Framer
block will declare itself in the In-Frame condition, and
will begin Frame Maintenance operations. The Re-
ceive DS3 Framer block will then indicate that it has
transitioned from the OOF condition into the In-Frame
condition by doing the following.
• Generate a Change in OOF Condition interrupt to
the local µP.
• Negate the RxOOF output pin (e.g., toggle it
"Low").
• Negate the RxOOF bit-field (Bit 4) within the
Receive DS3 Configuration and Status Register.
The Receive DS3 Framer can be configured to oper-
ate such that ’valid parity’ (P-bits) must also be de-
tected before the Receive DS3 Framer can declare it-
self In Frame. This configuration is set by writing the
appropriate data to the Rx DS3 Configuration and
Status Register, as depicted below.
RX DS3 CONFIGURATION AND STATUS REGISTER, (ADDRESS = 0X10)
BIT 7
BIT 6
BIT 5
BIT 4
BIT 3
BIT2
BIT 1
BIT 0
RxAIS
RxLOS
RxIdle
RxOOF
Int LOS
Disable
Framing on F-Sync Algo M-Sync Algo
Parity
RO
RO
RO
RO
R/W
R/W
R/W
R/W
X
X
X
X
X
X
X
X
Table 53 relates the contents of this bit field to the
framing acquisition criteria.
TABLE 53: THE RELATIONSHIP BETWEEN THE CONTENTS OF BIT 2 (FRAMING ON PARITY) WITHIN THE RX DS3
CONFIGURATION AND STATUS REGISTER, AND THE RESULTING FRAMING ACQUISITION CRITERIA
FRAMING
ON PARITY
(BIT 2)
FRAMING ACQUISITION CRITERIA
0
The In-frame is declared after F-bit synchronization (10 F-bit matches) followed by M-bit synchronization (M-
bit matches for 3 DS3 M-frames)
1
The In-frame condition is declared after F-bit synchronization, followed by M-bit synchronization, with valid
parity over the frames. Also, the occurrence of parity errors in 2 or more out of 5 frames starts a frame
search
Once the Receive DS3 Framer block is operating in
the In-Frame condition, normal data recovery and
processing of the DS3 data stream begins. The max-
imum average reframing time is less than 1.5 ms.
4.3.2.2 Frame Maintenance Mode Operation
When the Receive DS3 Framer block is operating in
the In-Frame state (per Figure 88 ), it will then begin
to perform Frame Maintenance operations, where it
will continue to verify that the F- and M-bits are
present, at their proper locations. While the Receive
DS3 Framer block is operating in the Frame Mainte-
nance mode, it will declare an Out-of-Frame (OOF)
condition if 3 or 6 F-bits (depending upon user selec-
tion) out of 16 consecutive F-bits are in error. This
selection for the OOF Declaration criteria is made by
writing the appropriate value to bit 1 (F-Sync Algo) of
the Rx DS3 Configuration and Status Register, as de-
picted below.
RX DS3 CONFIGURATION AND STATUS REGISTER, (ADDRESS = 0X10)
BIT 7
BIT 6
BIT 5
BIT 4
BIT 3
BIT2
BIT 1
BIT 0
RxAIS
RxLOS
RxIdle
RxOOF
Int LOS
Disable
Framing on F-Sync Algo M-Sync Algo
Parity
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