BT8954 CONEXANT [Conexant Systems, Inc], BT8954 Datasheet - Page 30

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

3.0 Circuit Descriptions
3.3 Receiver
3.3.4 Descrambler
Figure 3-6. LFSR Structure for Transmission in the Remote
Figure 3-7. LFSR Structure for Transmission in the Central Office
3-8
Polynomial: ck = xk-23
Scrambled Input (b
Polynomial: ck = xk-23
Scrambled Input (b
+
+
x
k-16
x
k-16
z
k
z
-1
k
)
-1
)
x
z
z
x
k-1
z
k-1
-1
-1
-1
+
x
+
k-17
x
x
k-15
z
k-15
z
xk-5
z
-1
xk-18
-1
-1
The MC enables the descrambler by setting DSCRAM_EN bit of the Receive
Command register and selects the descrambler algorithm via the DSCRAM_TAP
[RCMD_2; 0x91.5,4]. The descrambler, if enabled, descrambles all DSL receive
data except the SYNC word. 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.
(LFSR). The algorithm chosen determines the feedback points. The LFSR
structure and polynomials for the two descrambler algorithms are illustrated in
Figure 3-6
x
z
z
x
x
z
k-2
-1
k-17
-1
k-2
-1
+
+
The descrambler is basically a 23-bit-long Linear Feedback Shift register
x
k-18
x
b
x
b
k-14
z
k
k-14
k
z
z
-1
-1
-1
and
z
x
x
z
x
k-3
-1
z
k-18
-1
k-3
-1
+
Figure
x
k-19
x
x
z
k-13
z
k-13
z
-1
-1
-1
Conexant
x
z
x
3-7. The descrambler is clocked with BCLK.
+
k-4
z
z
x
-1
z
k-19
-1
k-4
-1 = Delay Element (D flip-flop clocked with BCLK)
-1
= Modulo-2 Summation (XOR gate)
z
x
x
k-12
-1
k-12
z
z
Central Office Direction
-1
+
-1
x
x
x
x
k-5
z
k-20
k-20
k-5
z
-1
Remote Direction
-1
z
x
z
z
x
z
k-11
-1
-1
+
z
k-11
-1
-1
-1 = Delay Element (D flip-flop clocked with BCLK)
= Modulo-2 Summation (XOR gate)
x
x
x
x
k-21
k-6
z
k-21
k-6
z
-1
-1
z
x
z
z
x
z
k-10
-1
-1
k-10
-1
-1
x
x
x
k-22
x
z
k-7
k-22
k-7
z
-1
-1
Unscrambled Output (ck)
Unscrambled Output (ck)
Voice Pair Gain Framer
z
z
x
z
z
x
k-9
-1
-1
k-9
-1
-1
x
x
x
x
k-8
k-23
z
k-23
k-8
z
-1
-1
N8954DSC
Bt8954