WM8281ECSSLASHR datasheet(212/392 Pages) CIRRUS

212 / 392 page

WM8281

212

Rev 4.0

Table 77 Microphone Detect Control

The external connections for the Microphone Detect circuit are illustrated in Figure 65. In typical

applications, it can be used to detect a microphone or button press.

Note that, when using the Microphone Detect circuit, it is recommended to use one of the Right

channel analogue microphone input paths, to ensure best immunity to electrical transients arising

from the external accessory.

The voltage reference for the microphone detection is configured using the MICD_BIAS_SRC

applicable reference when required for MICDET impedance measurement.

If the selected reference (MICBIAS1, MICBIAS2 or MICBIAS3) is not already enabled (ie. if

MICBn_ENA = 0, where n is 1, 2 or 3 as appropriate), then the applicable MICBIAS source will be

enabled for short periods of time only, every time the impedance measurement is scheduled. To allow

time for the MICBIAS source to start-up, a time delay is applied before the measurement is

performed; this is configured using the MICD_BIAS_STARTTIME register, as described in Table 77.

The MICD_BIAS_STARTTIME register should be set to 16ms or more if MICBn_RATE = 1 (pop-free

start-up / shut-down). The MICD_BIAS_STARTTIME register should be set to 0.25ms or more if

MICBn_RATE = 0 (fast start-up / shut-down).

If the selected reference is not enabled continuously (ie. if MICBn_ENA = 0), then the applicable

MICBIAS discharge bit (MICBn_DISCH) should be set to 0.

The MICBIAS sources are configured using the registers described in

the “Charge Pumps, Regulators

and Voltage Reference

” section.

MICDET1

Accessory /

Button Detect

AGND

MICDET2

Detection pin selected by ACCDET_SRC

(MICDET1 or MICDET2)

Microphone/Accessory detection

selected by ACCDET_MODE

Analogue Input

INnx

Microphone Supply selected by MICD_BIAS_SRC

(MICVDD, MICBIAS1, MICBIAS2, MICBIAS3)

MICBIAS3

MICBIAS2

MICBIAS1

MICVDD

LDO2

Regulator

External accessories :-

2.2k



(+/-2%)

Microphone

Hookswitch

/ button 1

button 2

Figure 65 Microphone and Accessory Detect Interface

When the discrete measurement mode is selected (ACCDET_MODE=000), the MICD_LVL_SEL [7:0]

independently. This allows the function to be tailored to the particular application requirements.

If one or more bits within the MICD_LVL_SEL register is set to 0, then the corresponding impedance

level will be disabled. Any measured impedance which lies in a disabled level will be reported as the

next lowest, enabled level.

For example, the MICD_LVL_SEL [2] bit enables the detection of impedances around 73

. If

MICD_LVL_SEL [2] = 0, then an external impedance of 73

 will not be indicated as 73 but will be

indicated as 40

; this would be reported in the MICD_LVL register as MICD_LVL [2] = 1.

With all measurement levels enabled, the WM8281 can detect the presence of a typical microphone

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