WM8980_07 Datasheet, PDF (37/112 Pages) Wolfson Microelectronics plc – Stereo CODEC With Speaker Driver and Video Buffer

Pre-Production
WM8980
PROGRAMMABLE NOTCH FILTER
A programmable notch filter is provided. This filter has a variable centre frequency and bandwidth,
programmable via two coefficients, a0 and a1. a0 and a1 are represented by the register bits
NFA0[13:0] and NFA1[13:0]. Because these coefficient values require four register writes to setup
there is an NFU (Notch Filter Update) flag which should be set only when all four registers are setup.
REGISTER
ADDRESS
R27
Notch Filter 1
BIT
LABEL
6:0
NFA0[13:7]
7
NFEN
8
NFU
R28
6:0
Notch Filter 2
8
NFA0[6:0]
NFU
R29
6:0
Notch Filter 3
8
NFA1[13:7]
NFU
R30
0-6
Notch Filter 4
8
NFA1[6:0]
NFU
Table 18 Notch Filter Function
DEFAULT
DESCRIPTION
0
Notch Filter a0 coefficient, bits [13:7]
0
Notch filter enable:
0=Disabled
1=Enabled
0
Notch filter update. The notch filter
values used internally only update
when one of the NFU bits is set high.
0
Notch Filter a0 coefficient, bits [6:0]
0
Notch filter update. The notch filter
values used internally only update
when one of the NFU bits is set high.
0
Notch Filter a1 coefficient, bits [13:7]
0
Notch filter update. The notch filter
values used internally only update
when one of the NFU bits is set high.
0
Notch Filter a1 coefficient, bits [6:0]
0
Notch filter update. The notch filter
values used internally only update
when one of the NFU bits is set high.
The coefficients are calculated as follows:
a₀
=
1−
1+
tan( w_b
tan( w_b
/ 2)
/ 2)
a₁ = −(1 + a₀ ) cos(w₀ )
Where:
w₀ = 2πf _c / f _s
w_b = 2πf_b / f _s
fc = centre frequency in Hz, fb = -3dB bandwidth in Hz, fs = sample frequency in Hz
The actual register values can be determined from the coefficients as follows:
NFA0 = -a0 x 2¹³
NFA1 = -a1 x 2¹²
DIGITAL ADC VOLUME CONTROL
The output of the ADCs can be digitally attenuated over a range from –127dB to 0dB in 0.5dB steps.
The gain for a given eight-bit code X is given by:
0.5 × (G-255) dB for 1 ≤ G ≤ 255;
MUTE for G = 0
w
PP Rev 3.1 March 2007
37