WM9705 datasheet(22/55 Pages) WOLFSON

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WM9705

Production Data

PD Rev 4.4 August 2006

When a headphone is connected the low impedance to ground of the headset pulls down the DC

level to near ground. If a headset with microphone is plugged in, the high impedance of the

microphone does not pull down the DC level on the tip connection, the DC on this pin now rising

to near positive supply. This change in DC level is detected, so allowing detection of change from

headphone to microphone, (or nothing plugged in of course). When this event is detected, the

headphone amplifier that drives the tip connection is turned off, and the signal on this pin is

routed instead to the MIC1 input as a microphone input.

This auto-detect comparator is enabled by setting bit HSCMP. The pull-up current is enabled by

setting bit MPUEN in register 5Ch and also toggles the interrupt signal on the PENDET pin.

When bit HSDT is set the mic1 input is connected to a comparator with a threshold set at mid-

rail. When the comparator output is low, then the headphone driver is enabled. When the

comparator output goes high (that is the pull-up current multiplied by the external impedance to

ground on the mic1 pin is greater than mid-rail), the headphone amplifier is turned off and the

mic1 signal is taken internally from the headphone output pin (39).

RIGHT

MIXER

HPOUTL

HPOUTR

LEFT

MIXER

HPGND

HEAD

PHONE

MIC

UP: MONO HEADSET WITH MIC

DOWN: STEREO HEADPHONE

HSEN

reg 5Ch

MIC1

MIC2

OFF

(hi-Z)

'1'

'0'

HSDT

reg 5Ch

INTERNAL

MIC

MIC AMP

'1'

'0'

MPUEN

reg 5Ch

5mA

VMID

HSCP

reg 5Ch

PEN

LOGIC

PENDET

HPVOL

reg 04h

reg 20h

HSCMP

reg 5Ch

Figure 10 Headset Autodetect

Figure 10 shows this function schematically. The output signal from the comparator is accessible

by reading bit HSCP in register 5Ch. Auto detect may be used by setting HSEN bit, or external

control by using the HSDT bit which is an over-ride that forces the headset tri-state and

microphone path switching function to occur.

This function would allow, for example, a stereo headphone to be used that had a microphone in

the connecting lead, and a switch. The switch changes the headphone into a mono headset with

microphone connected via the tip connection on the jack. If used in a product such as an MP3

capable phone it would allow the user to switch from headphone use to headset use by simply

switching a single switch in the headphone cable, so at the same time answering or initiating

telephone calls. It may also be possible to use the pull-up current to provide so called ‘phantom

power’ to dynamic microphones with appropriate choice of microphone.

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