XRT74L74
4 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER
ADVANCED CONFIDENTIAL
REV. P1.1.1
252
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").
RX DS3 CONFIGURATION AND STATUS REGISTER, (ADDRESS = 0X10)
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.
Table 53 relates the contents of this bit field to the
framing acquisition criteria.
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.
5.3.2.2
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 Maintenance
mode, it will declare an Out-of-Frame (OOF) condition
if 3 or 6 F-bits (depending upon user selection) 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 depicted
below.
Table 54 relates the contents of this bit-field to the
OOF Declaration criteria
B
IT
7
B
IT
6
B
IT
5
B
IT
4
B
IT
3
B
IT
2
B
IT
1
B
IT
0
RxAIS
RxLOS
RxIdle
RxOOF
Int LOS
Disable
R/W
X
Framing on
Parity
R/W
X
F-Sync Algo
M-Sync Algo
RO
X
RO
X
RO
X
RO
X
R/W
X
R/W
X
T
ABLE
53: T
HE
R
ELATIONSHIP
BETWEEN
THE
CONTENTS
OF
B
IT
2 (F
RAMING
ON
P
ARITY
)
WITHIN
THE
R
X
DS3
C
ONFIGURATION
AND
S
TATUS
R
EGISTER
,
AND
THE
RESULTING
F
RAMING
A
CQUISITION
C
RITERIA
F
RAMING
ON
P
ARITY
(B
IT
2)
F
RAMING
A
CQUISITION
C
RITERIA
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
RX DS3 CONFIGURATION AND STATUS REGISTER, (ADDRESS = 0X10)
B
IT
7
B
IT
6
B
IT
5
B
IT
4
B
IT
3
B
IT
2
B
IT
1
B
IT
0
RxAIS
RxLOS
RxIdle
RxOOF
Int LOS
Disable
R/W
X
Framing on
Parity
R/W
X
F-Sync Algo
M-Sync Algo
RO
X
RO
X
RO
X
RO
X
R/W
X
R/W
X