wm8903 Wolfson Microelectronics plc, wm8903 Datasheet - Page 34

• Manufacturer Part Number: wm8903
• Description: Ultra Low Power Codec For Portable Audio Applications
• Manufacturer: Wolfson Microelectronics plc

WM8903
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MICROPHONE HOOK SWITCH DETECTION
In a typical application, microphone hook switch operation would be detected when the MICBIAS
current exceeds the Short Circuit Detect threshold set by MICSHORT_THR. In order to generate a
MICBIAS Short Circuit Detect interrupt from this event, MICSHRT_INV must be cleared to 0 (see
“Interrupts”).
For detection of the hook switch release, the MICSHRT_INV bit must be set to 1. In this case, a
MICBIAS Short Circuit Detect interrupt is generated when the MICBIAS current falls below the
threshold set by MICSHORT_THR.
MICROPHONE HOOK SWITCH DE-BOUNCE AND FILTERING
The possibility of spurious hook switch interrupts due to ambient noise conditions can be removed by
Careful analysis and understanding of (1) and (2) can completely remove the possibility of spurious
hook switch interrupts due to ambient noise conditions.
Where the MICBIAS resistor is large enough such that the minimum specified the DC level when
MICSHORT_THR = 00 is close to the level of possible MICBIAS AC current spikes, the probability of
false detections is greatly reduced by 2 design features:
Figure 27 MIC Hook Switch Detect Filtering
The filtering algorithm helps to reject spurious interrupts caused by very high current spikes due to
ambient conditions such as wind noise or mechanical shock.
Note that the filtering algorithm provides only limited rejection of very high current spikes with
frequencies less than or equal to the hook switch detect measurement frequency, or frequencies
equal to harmonics of the hook switch detect measurement frequency.
An example sequence including hook switch detection is detailed in the “Applications Information”
section on page 154.
1.
2.
1.
2.
Hysteresis of the hook switch detect function means that a different current threshold is
used for detecting button push and button release.
Digital filtering means that the hook switch detection event is only signalled when the
MICBIAS current has satisfied the threshold condition for 10 successive measurements,
as shown in Error! Reference source not found.. The hook switch detect measurement
frequency and the detection delay time t
Characteristics”.
Careful analysis of microphone behaviour under extremely high sound pressure
levels or during mechanical shock, such that the AC current swing during such
conditions is understood.
By correct selection of the MICBIAS resistor value, such that current flow while the
hook switch is pressed is significantly higher than that caused by AC current swing
during such conditions.
SHORT
are detailed in the “Electrical
PP, Rev 3.1, August 2009
Pre-Production
34