AN3308 datasheet(25/52 Pages) STMICROELECTRONICS

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AN3308

Inertial interrupt

Doc ID 18198 Rev 1

25/52

6.4

Free-fall detection

Free-fall detection refers to a specific configuration of INT1_CTRL registers that allows to

recognize when the device is free falling: the acceleration measure along all the axes goes

to zero. In a real case a “free-fall zone” is defined around the zero-g level where all the

accelerations are small enough to generate the interrupt (Figure 10).

Figure 10.

Free-fall interrupt

This paragraph provides the basics for the use of the free-fall detection feature. In particular,

the SW routine that configures the device to detect free-fall events and to signal them is the

following:

Write A7h into CTRL_REG1

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

// ODR = 100 Hz

Write 00h into CTRL_REG2

// High-pass filter disabled

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

Write 16h into INT1_THS

// Set free-fall threshold = 350 mg

Write 03h into INT1_DURATION

// Set minimum event duration

Write 95h into INT1_CFG

// Configure free-fall recognition

Poll INT1 pad; if INT1 = 0 then go to 10

// Poll INT1 pin waiting for the free-fall event

(Free-fall event has occurred; insert your

code here)

// Event handling

Read INT1_SRC register

// Clear interrupt request

Go to 9

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