AN2335 datasheet(35/40 Pages) STMICROELECTRONICS

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AN2335

Application Information

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9.5.2

Using the HP Filter

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

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

configured to recognize when the high-frequency component of the acceleration applied

along either X, Y or Z axis exceeds a preset threshold (180mg used in the example). The

event which triggers the interrupt is latched inside the device using FF_WU interrupt source

1 and its occurrence is signalled through the usage of the INT1 pin.

At step 6, a dummy read at HP_FILTER_RESET register is performed to set the

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

comparison.

This read 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 C7h into CTRL_REG1

// Turn on the sensor and set ODR=400 Hz

write 04h into CTRL_REG2

// High Pass filter enabled on FF_WU1, fcut-

off= 8Hz

write 01h into CTRL_REG3

// FF_WU1 interrupt sent to INT1 pin

write 0Ah into FF_WU_THS_1 reg

// Set wake-up threshold = 180 mg

write 00h into FF_WU_DURATION_1 reg

// No filtering/confirmation on the event

read HP_FILTER_RESET register

// Dummy read to force the HP filter to

// actual acceleration value

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

write 6Ah into FF_WU_CFG_1

// Configure desired wake-up event

poll INT1 pin; if INT1=0 then goto 7

// Poll INT1 pin waiting for the wake-up event

(Wake-up event has occurred; insert your code

here)

// Event handling

read FF_WU_SRC_1 reg

// Return the event that has triggered the

interrupt

// Clear interrupt request

(Insert your code here)

// Event handling

goto 8

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