WM8281ECSSLASHR datasheet(214/392 Pages) CIRRUS

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WM8281

214

Rev 4.0

HEADPHONE DETECT

The WM8281 headphone detection circuit measures the impedance of an external headphone load.

This feature can be used to set different gain levels or to apply other configuration settings according

to the type of load connected. Separate monitor pins are provided for headphone detection on the left

and right channels of HPOUT1.

Headphone detection can be enabled on the HPDETL pin or the HPDETR pin. Under recommended

configuration, these pins provide measurement of the HPOUT1L and HPOUTR loads respectively.

The headphone detect function can also be enabled on the MICDET1 pin or the MICDET2 pin. Note

that, in this configuration, any MICBIAS output that is connected to the selected MICDET pin must be

disabled and floating (MICBn_ENA=0, MICBn_DISCH=0).

The applicable headphone detection pin is selected using the ACCDET_MODE register. When

MICDETn is selected (ACCDET_MODE=100), the applicable MICDETn pin is determined by the

ACCDET_SRC register, as described in Table 80.

The WM8281 can only support one headphone or microphone detection function at any time. When

the detection function is not in use, it is recommended to set ACCDET_MODE=000.

Headphone detection on the selected channel is commanded by writing a ‘1’ to the HP_POLL register

bit. The impedance measurement range is configured using the HP_IMPEDANCE_RANGE register.

This register should be set in accordance with the expected load impedance.

Note that a number of separate measurements (for different impedance ranges) are typically required

in order to determine the load impedance; the recommended control sequence is described below.

Note that setting the HP_IMPEDANCE_RANGE register is not required for detection on the MICDETn

pins (ACCDET_MODE=100). Note also that the impedance measurement range, and measurement

accuracy, in this mode are different to the HPDETL and HPDETR measurement modes.

For correct operation, the respective output driver(s) must be disabled when headphone detection is

commanded on HPOUT1L or HPOUT1R. The applicable driver(s) must also be configured with

additional register settings; the requirements are detailed in Table 78.

The HP1L_ENA and HP1R_ENA register bits are defined in Table 61. The other associated register

bits are defined in Table 80.

Note

that,

when

configuring

the

HP1x_FLWR,

HP1x_SHRTI,

HP1x_SHRTO

and

HP1_TST_CAP_SEL, care is required not to change the value of other bits in the register, which may

have changed from the default setting. Accordingly, a ‘read-modify-write’ sequence is required to

implement this.

The applicable headphone output(s) configuration must be maintained until after the headphone

detection has completed.

For normal headphone driver operation, the HP1_TST_CAP_SEL register must be restored to its

default value. (The other register bits noted in Table 78 will be configured automatically when HPOUT

is enabled or disabled.)

DESCRIPTION

REQUIREMENT

HPOUT1L Impedance measurement

HP1L_ENA = 0

HP1L_FLWR = 0, HP1L_SHRTI = 0, HP1L_SHRTO = 1

HP1_TST_CAP_SEL = 01

HPOUT1R Impedance measurement

HP1R_ENA = 0

HP1R_FLWR = 0, HP1R_SHRTI = 0, HP1R_SHRTO = 1

HP1_TST_CAP_SEL = 01

Note that the output driver(s) must be disabled (HP1x_ENA=0) before setting the other control bits.

Table 78 Output Configuration for Headphone Detect

When headphone detection is commanded, the WM8281 uses an adjustable current source to

determine the connected impedance. A sweep of measurement currents is applied. The rate of this

sweep can be adjusted using the HP_CLK_DIV and HP_RATE registers. To avoid audible clicks, the

default step size should always be used (HP_RATE = 0).

The timing of the current source ramp is also controlled by the HP_HOLDTIME register. It is

recommended that the default setting (001b) be used for this parameter.

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