Wolfson WM8978, Manual, Page: 36 / 119

WM8978
Production Data
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PD, Rev 4.5, October 2011
36
INPUT LIMITER / AUTOMATIC LEVEL CONTROL (ALC)
The WM8978 has an automatic PGA gain control circuit, which can function as an input peak limiter
or as an automatic level control (ALC).
The Automatic Level Control (ALC) provides continuous adjustment of the input PGA in response to
the amplitude of the input signal. A digital peak detector monitors the input signal amplitude and
compares it to a register defined threshold level (ALCLVL).
If the signal is below the threshold, the ALC will increase the gain of the PGA at a rate set by
ALCDCY. If the signal is above the threshold, the ALC will reduce the gain of the PGA at a rate set
by ALCATK.
The ALC has two modes selected by the ALCMODE register: normal mode and peak limiter mode.
The ALC/limiter function is enabled by setting the register bit R32[8] ALCSEL.
REGISTER
ADDRESS
BIT LABEL DEFAULT DESCRIPTION
R32 (20h)
ALC Control
1 2:0 ALCMIN
[2:0]
000 (-12dB) Set minimum gain of PGA
000 = -12dB
001 = -6dB
010 = 0dB
011 = +6dB
100 = +12dB
101 = +18dB
110 = +24dB
111 = +30dB
5:3 ALCMAX
[2:0]
111
(+35.25dB) Set Maximum Gain of PGA
111 = +35.25dB
110 = +29.25dB
101 = +23.25dB
100 = +17.25dB
011 = +11.25dB
010 = +5.25dB
001 = -0.75dB
000 = -6.75dB
8:7 ALCSEL 00 ALC function select
00 = ALC disabled
01 = Right channel ALC enabled
10 = Left channel ALC enabled
11 = Both channels ALC enabled
R33 (21h)
ALC Control
2
3:0 ALCLVL
[3:0]
1011
(-6dB) ALC target – sets signal level at ADC
input
1111 = -1.5dBFS
1110 = -1.5dBFS
1101 = -3dBFS
1100 = -4.5dBFS
1011 = -6dBFS
1010 = -7.5dBFS
1001 = -9dBFS
1000 = -10.5dBFS
0111 = -12dBFS
0110 = -13.5dBFS
0101 = -15dBFS
0100 = -16.5dBFS
0011 = -18dBFS
0010 = -19.5dBFS
0001 = -21dBFS
0000 = -22.5dBFS
8 Reserved 0 Reserved. Set to 0.