Pre-Production
WM8904
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.
REGISTER
ADDRESS
BIT
LABEL
DEFAULT
DESCRIPTION
R6 (06h)
MICBIAS Current Detect Threshold
(AVDD = 1.8V)
6:4
MICDET_THR [2:0]
000
Mic Bias
Control 0
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.
MICBIAS Short Circuit Threshold
(AVDD = 1.8V)
3:2
MICSHORT_THR
[1:0]
00
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.
MICBIAS Current and Short Circuit
Detect Enable
1
MICDET_ENA
0
0 = disabled
1 = enabled
Table 7 MICBIAS Current Detect
PP, Rev 3.3, September 2012
43
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WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
