WM8904ECSSLASHR datasheet(43/188 Pages) WOLFSON

43 / 188 page

Pre-Production

WM8904

PP, Rev 3.3, September 2012

MICBIAS CURRENT DETECT

A MICBIAS Current Detect function is provided for external accessory detection. This is provided in

order to detect the insertion/removal of a microphone or the pressing/releasing of the microphone

‘hook’ switch; these events will cause a significant change in MICBIAS current flow, which can be

detected and used to generate a signal to the host processor.

The MICBIAS current detect function is enabled by setting the MICDET_ENA register bit. When this

function is enabled, two current thresholds can be defined, using the MICDET_THR and

MICSHORT_THR registers. When a change in MICBIAS current which crosses either threshold is

detected, then an interrupt event can be generated. In a typical application, accessory insertion would

be detected when the MICBIAS current exceeds MICDET_THR, and microphone hookswitch

operation would be detected when the MICBIAS current exceeds MICSHORT_THR.

The current detect threshold functions are both inputs to the Interrupt control circuit and can be used

to trigger an Interrupt event when either threshold is crossed. Both events can also be indicated as an

output on a GPIO pin - see “General Purpose Input/Output (GPIO)”.

The current detect thresholds are enabled and controlled using the registers described in Table 7.

Performance parameters for this circuit block can be found in the “Electrical Characteristics” section.

Hysteresis and filtering is also provided in the both current detect circuits to improve reliability in

conditions where AC current spikes are present due to ambient noise conditions. These features are

described in the following section. Further guidance on the usage of the MICBIAS current monitoring

features is also described in the following pages.

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

R6 (06h)

Mic Bias

Control 0

6:4

MICDET_THR [2:0]

000

MICBIAS Current Detect Threshold

(AVDD = 1.8V)

000 = 0.070mA

001 = 0.260mA

010 = 0.450mA

011 = 0.640mA

100 = 0.830mA

101 = 1.020mA

110 = 1.210mA

111 = 1.400mA

Note that the value scales with

AVDD. The value quoted is correct

for AVDD=1.8V.

3:2

MICSHORT_THR

[1:0]

MICBIAS Short Circuit Threshold

(AVDD = 1.8V)

00 = 0.520mA

01 = 0.880mA

10 = 1.240mA

11 = 1.600mA

Note that the value scales with

AVDD. The value quoted is correct

for AVDD=1.8V.

MICDET_ENA

MICBIAS Current and Short Circuit

Detect Enable

0 = disabled

1 = enabled

Table 7 MICBIAS Current Detect

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