WM8281ECSSLASHR datasheet(209/392 Pages) CIRRUS

209 / 392 page

WM8281

Rev 4.0

209

The WM8281 can only support one headphone or microphone detection function at any time. When

the detection function is not in use, it is recommended to set ACCDET_MODE=000.

The microphone detection circuit can be enabled on the MICDET1 pin or the MICDET2 pin, selected

by the ACCDET_SRC register.

The microphone detection circuit uses MICVDD, MICBIAS1, MICBIAS2 or MICBIAS3 as a reference.

The applicable source is configured using the MICD_BIAS_SRC register.

When ACCDET_MODE is set to 000 or 111, then Microphone detection is enabled by setting

MICD_ENA.

When microphone detection is enabled, the WM8281 performs a number of measurements in order to

determine the MICDET impedance. The measurement process is repeated at a cyclic rate controlled

by MICD_RATE. (The MICD_RATE register selects the delay between completion of one

measurement and the start of the next.) When the microphone detection result has settled, the

WM8281 indicates valid data by setting the MICD_VALID bit.

When the discrete measurement mode is selected (ACCDET_MODE=000), the measured impedance

is only deemed valid after more than one successive measurement has produced the same result.

The MICD_DBTIME register provides control of the de-bounce period; this can be either 2

measurements or 4 measurements.

When the microphone detection result has settled (ie. after the applicable de-bounce period), the

WM8281 indicates valid data by setting the MICD_VALID bit. The measured impedance is indicated

using the MICD_LVL and MICD_STS register bits, as described in Table 77.

The MICD_VALID bit, when set, remains asserted for as long as the microphone detection function is

enabled (ie. while MICD_ENA = 1). If the detected impedance changes, then the MICD_LVL and

MICD_STS fields will change, but the MICD_VALID bit will remain set, indicating valid data at all

times.

The 8 pre-

defined impedance levels (including the ‘no accessory detected’ level) allow detection of a

typical microphone and up to 6 push-buttons. Each measurement level can be enabled or disabled

independently; this provides flexibility according to the required thresholds, and offers a faster

measurement time in some applications. The MICD_LVL_SEL register is described in detail later in

this section.

Note that the impedance levels quoted in the MICD_LVL description assume that a microphone

(475

Ω to 30kΩ impedance) is also present on the MICDET pin. The limits quoted in the “Electrical

Characteristics

” refer to the combined effective impedance on the MICDET pin. Typical external

components are described in the “Applications Information” section.

When the ADC measurement mode is selected (ACCDET_MODE=111), the detection function must

be disabled before the measurement can be read. When the WM8281 indicates valid data

(MICD_VALID=1), the detection must be disabled by setting MICD_ENA=0.

The ADC measurement mode generates two output results, contained within the MICDET_ADCVAL

and MICDET_ADCVAL_DIFF registers. These registers contain the most recent measurement value

(MICDET_ADCVAL) and the measurement difference value (MICDET_ADCVAL_DIFF). The

difference value indicates the difference between the latest measurement and the previous

measurement; this can be used to determine whether the measurement is stable and reliable.

Note that the MICDET_ADCVAL and MICDET_ADCVAL_DIFF registers do not follow a linear coding.

The appropriate test condition for accepting the measurement value (or for re-scheduling the

measurement) will vary depending on the application requirements, and depending on the expected

impedance value.

The microphone detection function is an input to the Interrupt control circuit and can be used to trigger

an Interrupt event every time an accessory insertion, removal or impedance change is detected. See

“Interrupts” for further details.

The microphone detection function can also generate a GPIO output, providing an external indication

of the microphone detection. This GPIO output is pulsed every time an accessory insertion, removal

or impedance change is detected. See “General Purpose Input / Output” to configure a GPIO pin for

this function.

The register fields associated with Microphone Detection (or other accessories) are described in

WM8281ECS/R Datenblatt(PDF) 209 Page - Cirrus Logic

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