WM1811A datasheet(134/328 Pages) WOLFSON

134 / 328 page

WM1811A

Production Data

PD, November 2013, Rev 4.1

134

MICROPHONE / ACCESSORY DETECT

The WM1811A accessory detection circuit measures the impedance of an external load connected to

the MICDET pin. This feature can be used to detect the insertion or removal of a microphone, and the

status of the associated hookswitch. It can also be used to detect push-button status or the

connection of other external accessories.

The microphone detection circuit measures the impedance connected to MICDET, and reports

whether the measured impedance lies within one of 9 pre-defined levels (including the ‘no accessory

detected’ level). This means it can detect the presence of a typical microphone and up to 7 push-

buttons. One of the impedance levels is specifically designed to detect a video accessory (typical

) load if required.

The microphone detection circuit uses the MICBIAS2 output as a reference. The WM1811A will

automatically enable MICBIAS2 when required in order to perform the detection function; this allows

the detection function to be supported in low-power standby operating conditions.

Microphone detection is enabled by setting the MICD_ENA register. When microphone detection is

enabled, the WM1811A 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.)

For best accuracy, 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

WM1811A 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 78.

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.

Note that the impedance levels quoted in the MICD_LVL description assume that a microphone (475Ω

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.

The microphone detection reports a measurement result in one of the pre-defined impedance levels.

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.

Clocking for the microphone detection function is derived from SYSCLK (defined in the “Clocking and

Sample Rates” section).The WM1811A can support accessory detection using a low frequency (eg.

32kHz) clock for reduced power consumption. See the “External Accessory Detection Clocking”

section below for further details.

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” for details of how to configure

a GPIO pin to output the microphone detection signal.

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

78. The external circuit configuration is illustrated in Figure 41.

WM1811A Datasheet(PDF) 134 Page - Wolfson Microelectronics plc

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