WM8281ECSSLASHR datasheet(215/392 Pages) CIRRUS

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WM8281

Rev 4.0

215

The headphone detection process will typically comprise a number of separate measurements (for

different impedance ranges). Completion of each measurement is indicated by the HP_DONE register

bit. When this bit is set, the measurement result can be read from the HP_DACVAL register, and

subsequently decoded as described below.

C0 + (C1 × Offset)

(HP_DACVAL + 1.5)

C3 (1 + (C4 × Gradient))

Impedance (Ω) =

The associated parameters for decoding the measurement result are defined in Table 79. The

applicable values are dependent on the HP_IMPEDANCE_RANGE setting in each case. The ‘Offset’

and ‘Gradient’ values are derived from register fields which are factory-calibrated for each device.

PARAMETER

HP_IMPEDANCE_

RANGE=00

HP_IMPEDANCE_

RANGE=01

HP_IMPEDANCE_

RANGE=10

HP_IMPEDANCE_

RANGE=11

1.007

9.696

100.684

-0.0072

-0.0795

-0.9494

4003

7975

7300

69.3

69.6

62.9

63.2

0.0055

0.25

Offset

HP_OFFSET_01 +

HP_OFFSET_DIFF_00

HP_OFFSET_01

HP_OFFSET_01 +

HP_OFFSET_DIFF_10

HP_OFFSET_01 +

HP_OFFSET_DIFF_11

Gradient

HP_GRADIENT_0X

HP_GRADIENT_1X

Table 79 Headphone Measurement Decode parameters

Note that, to achieve the specified measurement accuracy, the above equation must be calculated to

an accuracy of at least 5 decimal places throughout.

The impedance measurement result is valid when 169

≤ HP_DACVAL ≤ 1019. (In case of any

contradiction with the HP_IMPEDANCE_RANGE description, the HP_DACVAL validity takes

precedence.)

If the external impedance is entirely unknown (ie. it could lie in any of the HP_IMPEDANCE_RANGE

regions), then it is recommended to test initially with HP_IMPEDANCE_RANGE=00. If the resultant

HP_DACVAL is < 169, then the impedance is higher than the selected measurement range, so the

test should be scheduled again, after incrementing HP_IMPEDANCE_RANGE.

Each measurement is triggered by

writing ‘1’ to the HP_POLL bit. Completion of each measurement is

indicated by the HP_DONE register bit. Note that, after the HP_DONE bit has been asserted, it will

remain asserted until the next measurement has been commanded.

A simpler, but less accurate, procedure for headphone impedance measurement is also supported,

using the HP_LVL register. When the HP_DONE bit is set, indicating completion of a measurement,

the impedance can be read directly from the HP_LVL field, provided that the value lies within the

range of the applicable HP_IMPEDANCE_RANGE setting.

Note that, for detection using one of the MICDETn pins, the HP_LVL field is the only supported

readback option. The HP_IMPEDANCE_RANGE field is not valid for detection on the MICDETn pins.

See Table 80 for further description of the HP_LVL field.

The headphone detection function is an input to the Interrupt control circuit and can be used to trigger

an Interrupt event on completion of the headphone detection -

see “Interrupts”.

The headphone detection function can also generate a GPIO output, providing an external indication

of the headphone detection. See “General Purpose Input / Output” to configure a GPIO pin for this

function.

The register fields associated with Headphone Detection are described in Table 80. The external

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