WM8904ECSSLASHR datasheet(44/188 Pages) WOLFSON

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WM8904

Pre-Production

PP, Rev 3.3, September 2012

MICBIAS CURRENT DETECT FILTERING

The function of the filtering is to ensure that AC current spikes caused by ambient noise conditions

near the microphone do not lead to incorrect signalling of the microphone insertion/removal status or

the microphone hookswitch status.

Hysteresis on the current thresholds is provided; this means that a different current threshold is used

to detect microphone insertion and microphone removal. Similarly, a different current threshold is

used to detect hookswitch press and hookswtich release.

Digital filtering of the hookswitch status ensures that the MICBIAS Short Circuit detection event is

only signalled if the MICSHORT_THR threshold condition has been met for 10 consecutive

measurements.

In a typical application, microphone insertion would be detected when the MICBIAS current exceeds

the Current Detect threshold set by MICDET_THR.

When the MIC_DET_EINT_POL interrupt polarity bit is set to 0, then microphone insertion detection

will cause the MIC_DET_EINT interrupt status register to be set.

For detection of microphone removal, the MIC_DET_EINT_POL bit should be set to 1. When the

MIC_DET_EINT_POL interrupt polarity bit is set to 1, then microphone removal detection will cause

the MIC_DET_EINT interrupt status register to be set.

The detection of these events is bandwidth limited for best noise rejection, and is subject to detection

delay time tDET, as specified in the “Electrical Characteristics”. Provided that the MICDET_THR field

has been set appropriately, each insertion or removal event is guaranteed to be detected within the

delay time tDET.

It is likely that the microphone socket contacts will have mechanical “bounce” when a microphone is

inserted or removed, and hence the resultant control signal will not be a clean logic level transition.

Since tDET has a range of values, it is possible that the interrupt will be generated before the

mechanical “bounce” has ceased. Hence after a mic insertion or removal has been detected, a time

delay should be applied before re-configuring the MIC_DET_EINT_POL bit. The maximum possible

mechanical bounce times for mic insertion and removal must be understood by the software

programmer.

Utilising a GPIO pin to monitor the steady state of the microphone detection function does not change

the timing of the detection mechanism, so there will also be a delay tDET before the signal changes

state. It may be desirable to implement de-bounce in the host processor when monitoring the state of

the GPIO signal.

Microphone hook switch operation is detected when the MICBIAS current exceeds the Short Circuit

Detect threshold set by MICSHORT_THR. Using the digital filtering, the hook switch detection event

is only signalled if the MICSHORT_THR threshold condition has been met for 10 consecutive

measurements.

When the MIC_SHRT_EINT_POL interrupt polarity bit is set to 0, then hook switch operation will

cause the MIC_SHRT_EINT interrupt status register to be set.

For detection of microphone removal, the MIC_SHRT_EINT_POL bit should be set to 1. When the

MIC_SHRT_EINT_POL interrupt polarity bit is set to 1, then hook switch release will cause the

MIC_SHRT_EINT interrupt status register to be set.

The hook switch detection measurement frequency and the detection delay time tSHORT are detailed in

the “Electrical Characteristics” section.

The WM8904 Interrupt function is described in the “Interrupts” section. Example control sequences for

configuring the Interrupts functions for MICBIAS current detection events are described in the

“Applications Information” section.

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