BT8954 CONEXANT [Conexant Systems, Inc], BT8954 Datasheet - Page 29

• Manufacturer Part Number: BT8954
• Description: VOICE PAIR GAIN FRAMER
• Manufacturer: CONEXANT [Conexant Systems, Inc]

Bt8954
Voice Pair Gain Framer
Figure 3-5. Threshold Correlation Effect on Expected SYNC Locations
3.3.3 CRC Check
N8954DSC
SYNC Pattern
SYNC Pattern < THRESH_CORR
0
0
SYNC_ERRORED
SYNC_ERRORED
1
1
THRESH_CORR
–2q
–1q
SYNC words are detected or regresses to the SYNC_ERRORED state if SYNC
pattern errors are found. During SYNC_ERRORED states, the number of
matching bits from each comparison of received SYNC word and the
programmed SYNC word pattern must meet or exceed the programmed pattern
match tolerance specified by THRESH_CORR [RCMD_2; 0x91.3:0]. If the
number of matching bits falls below tolerance, the framer expands the locations
searched to quats on either side of the expected location, as illustrated in
Figure
SYNC_ ERRORED state, the framer passes through a programmable number of
intermediate SYNC_ERRORED states, before entering the OUT_OF SYNC
state. STATE_CNT increments for each frame in which SYNC is not detected
until the count reaches the LOSS_SYNC[2:0] criteria [RCMD_1; 0x90.5–3] and
the framer enters the OUT_OF SYNC state. If at any time during the
SYNC_ERRORED state the framer detects a completely correct SYNC word
pattern at one of the valid frame locations, then framer returns to the IN_SYNC
state. The ETSI standard, for HDSL transport, recommends the
REACH_SYNC = 2 and LOSS_SYNC = 6 framing criteria.
The CRC Check block calculates a CRC value for every received DSL frame. The
CRC Check block reports an error if the CRC in the current frame (calculated at
the other end’s transmitter) does not match the CRC that was calculated for the
previous DSL receive frame. Individual DSL block errors are reported in the
CRC_ERROR bit of the Receive Status 2 register [RSTATUS_2; 0xE6.5] and
accumulated in the CRC Error Count register [CRC_CNT; 0xE8]. The CRC
calculation in the receiver is exactly the same as that in the transmitter.
6 ms
6 ms
After entering IN_SYNC, the framer either remains IN_SYNC as successive
+1q
+2q
SYNC_ERRORED
SYNC_ERRORED
3-5. After detecting a SYNC pattern error and changing to the
2
2
–3q
Conexant
–1q
–1q
12 ms
12 ms
+1q
+1q
SYNC_ERRORED
SYNC_ERRORED
+3q
3
3
–4q
–2q
–1q
18 ms
18 ms
+1q
+2q
+4q
q = 2 bits = 1 quat
= Search Location
3.0 Circuit Descriptions
3.3 Receiver
t
t
3-7