BT8954 CONEXANT [Conexant Systems, Inc], BT8954 Datasheet - Page 34

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

3.0 Circuit Descriptions
3.4 Transmitter
3.4.4 CRC Calculation
3.4.5 Scrambler
Figure 3-10.
3-12
Unscrambled Input (B
Polynomial: ck = xk-23
+
LFSR Structure for Transmission in the Remote
x
k-16
z
-1
k
)
x
z
k-1
-1
+
x
k-15
xk-18
z
z
-1
-1
The CRC calculation is performed on all transmit data, and the Payload Mux
inserts the resulting 6-bit CRC into the subsequent output frame. CRC is
calculated over all bits in the (N)th frame except the SYNC WORD, STUFF, and
CRC bits and then is inserted into the (N+1) frame. The MPU can choose to inject
CRC errors on a per-frame basis by setting the ICRC_ERR bit [TCMD_1;
addr 0x86.1]. The six CRC bits are calculated as follows:
The MC enables the scrambler by setting SCRAM_EN [TCMD_1; 0x86.0] and
selects the descrambler algorithm via SCRAM_TAP [TCMD_2; 0x87.2]. The
scrambler, if enabled, scrambles all DSL transmit data except the SYNC word and
STUFF bits. The algorithm is chosen from one of two possible choices,
depending on whether Bt8954 is located at the Central Office or at a Remote Site.
Feedback Shift register (LFSR). The algorithm chosen determines the feedback
points. The LFSR structure and polynomials for the two scrambler algorithms are
illustrated in
x
x
z
k-17
k-2
-1
+
1.
2.
3.
Scrambler Algorithms: The scrambler is basically a 23-bit-long Linear
b
All bits of the (N) frame — except the 14 SYNC and 6 CRC bits, for a total
of M bits — are used in order of occurrence to construct a polynomial in X
such that bit 0 of the (N) frame is the coefficient of the term X
M-1 of the (N) frame is the coefficient of the term X
The polynomial is multiplied by the factor X
modulo 2, by the generator polynomial X
remainder polynomial are used, in order of occurrence, as an ordered set of
check bits, CRC1–CRC6, for the (N+1) frame. Ordering is such that the
coefficient of term X
the coefficient of term X
Check bits CRC1–CRC6 contained in a frame are associated with the
contents of the preceding frame. When there is no immediately preceding
frame, check bits may be assigned any value.
x
k
k-14
z
z
-1
-1
x
x
z
k-18
+
k-3
Figure 3-10
-1
x
z
k-13
z
-1
-1
Conexant
x
+
x
z
z
k-19
k-4
-1 = Delay Element (D flip-flop clocked with BCLK)
-1
= modulo-2 summation (XOR gate)
and
5
x
k-12
z
z
in the remainder polynomial is check bit CRC1, and
-1
-1
Figure
0
Central Office Direction
x
x
is check bit CRC6.
k-20
k-5
z
-1
3-11.
z
x
z
k-11
-1
-1
x
x
k-21
k-6
z
-1
6
z
x
z
k-10
-1
-1
6
X 1. Coefficients of the
, and the result is divided,
x
x
k-22
z
k-7
-1
Scrambled Output (ck)
Voice Pair Gain Framer
0
z
z
x
.
k-9
-1
-1
x
x
k-8
k-23
z
-1
M-1
N8954DSC
Bt8954
and bit