AN3308 datasheet(24/52 Pages) STMICROELECTRONICS

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Inertial interrupt

AN3308

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Doc ID 18198 Rev 1

6.3.3

Using the HP filter

The code provided below gives a basic routine which shows the practical use of the inertial

wake-up feature performed on high-pass filtered data. In particular the device is configured

to recognize when the high-frequency component of the acceleration applied along either

the X, Y, or Z axis exceeds a preset threshold (250 mg used in the example).

The event which triggers the interrupt is latched inside the device and its occurrence is

signalled through the use of the INT1 pin.

At step 8, a dummy reading at the HP_FILTER_RESET register is performed to set the

current/reference acceleration/tilt state against which the device performed the threshold

comparison.

This reading may be performed any time it is required to set the orientation/tilt of the device

as a reference state without waiting for the filter to settle.

Write A7h into CTRL_REG1

// Turn on the sensor, enable X, Y, and Z

// ODR = 100 Hz

Write 09h into CTRL_REG2

// High-pass filter enabled on data and interrupt1

Write 40h into CTRL_REG3

// Interrupt driven to INT1 pad

Write 00h into CTRL_REG4

// FS = 2 g

Write 08h into CTRL_REG5

// Interrupt latched

Write10h into INT1_THS

// Threshold = 250 mg

Write 00h into INT1_DURATION

// Duration = 0

Read HP_FILTER_RESET

// Dummy read to force the HP filter to

// current acceleration value

// (i.e. set reference acceleration/tilt value)

Write 2Ah into INT1_CFG

// Configure desired wake-up event

Poll INT1 pad; if INT1 = 0 then go to

// Poll INT1 pin waiting for the

// wake-up event

(Wake-up event has occurred; insert

your code here)

// Event handling

Read INT1_SRC

// Return the event that has triggered the

// interrupt and clear interrupt

(Insert your code here)

// Event handling

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