WM8983 datasheet(49/129 Pages) CIRRUS

49 / 129 page

WM8983

Rev 4.6

ALC LIMITER MODE INITIALISATION SEQUENCE

In order to correctly initialise the ALC in limiter mode, the following sequence of register writes is

required. MCLK must be applied during the initialisation sequence

1. R45 - Set left input PGA gain (INPPGAVOLL) to level required for operation.

2. R46 - Set right input PGA gain (INPPGAVOLR) to level required for operation.

3. R44 - Enable analogue inputs as required.

4. R2 - Disable input PGA (INPPGAENL=0, INPPGAENR=0).

5. R59 = 0x0003 - Enable ALC test mode.

6. R32 - Set ALCMAX and ALCMIN to the level required for operation.

7. R33 - Set limiter level (ALCLVL) to the level required for operation.

8. R34 = 0x0000 - Enable ALC mode (ALCMODE = 0).

9. Insert 1ms delay to allow input PGA gain update by the limiter circuit.

10. R34 = 0x0100 - Enable Limiter mode (ALCMODE = 1).

11. Insert 1ms delay to allow input PGA gain update by the limiter circuit.

12. R59 = 0x0000 - Turn off ALC test mode.

13. R2 - Enable input PGA (INPPGAENL=1, INPPGAENR=1).

Note: R32, R33, R45 and R46 register settings above need to be changed to reflect settings required

in the target application.

ATTACK AND DECAY TIMES

The attack and decay times set the update times for the PGA gain.

The attack time is the time

constant used when the gain is reducing. The decay time is the time constant used when the gain is

increasing. In limiter mode, the time constants are faster than in ALC mode. The time constants are

shown below in terms of a single gain step, a change of 6dB and a change of 90% of the PGAs gain

range.

Note that, these times will vary slightly depending on the sample rate used (specified by the SR

NORMAL MODE

ALCMODE = 0 (Normal Mode)

ALCATK

tATK

tATK6dB

tATK90%

0000

104µs

832µs

6ms

0001

208µs

1.66ms

12ms

0010

416µs

3.33ms

24ms

0011

832µs

6.66ms

48ms

0100

1.66ms

13.32ms

96ms

0101

3.33ms

26.64ms

192ms

0110

6.66ms

53.28ms

384ms

0111

13.32ms

106.6ms

768ms

1000

26.64ms

213.2ms

1.53s

1001

53.28ms

426.4ms

3.07s

1010

106.6ms

852.8ms

6.14s

Attack Time (s)

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