Data Sheet
ADXL36±
V
V
S
DD I/O
Example: Implementing Free Fall Detection
C
C
IO
S
Many digital output accelerometers include a built-in free fall
detection feature. In the ADXL362, implement this function
using the inactivity interrupt.
V
V
S
DD I/O
ADXL362
MOSI
MISO
SCLK
CS
When an object is in true free fall, acceleration on all axes is 0 g.
Thus, free fall detection is achieved by looking for acceleration
on all axes to fall below a certain threshold (close to 0 g) for a
certain amount of time. The inactivity detection functionality,
when used in absolute mode, does exactly this.
INT1
INT2
SPI
INTERFACE
INTERRUPT
CONTROL
GND
Figure 46. Recommended Bypass Capacitors
To use inactivity to implement free fall detection, set the value
in THRESH_INACT to the desired free fall threshold. Values
between 300 mg and 600 mg are recommended; the register
setting for these values varies based on the g range setting of the
device, as follows:
A 0.1 µF ceramic capacitor (CS) at VS and a 0.1 µF ceramic capacitor
(CIO) at VDD I/O placed as close as possible to the ADXL362. Supply
pins are recommended to adequately decouple the accelerometer
from noise on the power supply. It is also recommended that VS
and VDD I/O be separate supplies to minimize digital clocking
noise on the VS supply. If this is not possible, additional filtering
of the supplies may be necessary.
THRESH_INACT =
Threshold Value [g] × Scale Factor [LSB per g]
If additional decoupling is necessary, place a resistor or ferrite bead,
no larger than 100 Ω, in series with VS. Additionally, increasing the
bypass capacitance on VS to a 1 µF tantalum capacitor in parallel
with a 0.1 µF ceramic capacitor may also improve noise.
Set the value in TIME_INACT to implement the minimum
amount of time that the acceleration on all axes must be less
than the free fall threshold to generate a free fall condition.
Values between 100 ms and 350 ms are recommended; the
register setting for this varies based on the output data rate.
Ensure that the connection from the ADXL362 ground to the
power supply ground has low impedance because noise transmitted
through ground has an effect similar to noise transmitted through VS.
TIME_INACT = Time [sec] × Data Rate [Hz]
When a free fall condition is detected, the inactivity status is set
to 1 and, if the function is mapped to an interrupt pin, an
inactivity interrupt triggers on that pin.
Power Supply Requirements
The ADXL362 is designed to operate using supply voltage rails
ranging from 1.8 V to 3.3 V. The operating voltage range (VS),
specified in Table 1, ranges from 1.6 V to 3.5 V to account for
inaccuracies and transients of up to 10% on the supply voltage.
Startup Routine
The following startup routine configures the ADXL362 for a
typical free fall application. This routine assumes a 8 g
measurement range and 100 Hz output data rate. Thresholds
and timing values can be modified to suit particular application
needs.
The ADXL362 does not require any particular startup transient
characteristics, except that it must always be started up from 0 V.
When the device is in operation, any time power is removed
from the ADXL362, or falls below the operating voltage range,
the supplies (VS, VDD I/O, and any bypass capacitors) must be
discharged completely before power is reapplied. To enable
supply discharge, it is recommended to power the device from
a microcontroller GPIO, connect a shutdown discharge switch
to the supply (Figure 47), or use a voltage regulator with a
shutdown discharge feature, such as the ADP160.
1. Write 0x96 (150 codes) to Register 0x23: sets free fall
threshold to 600 mg.
2. Write 0x03 to Register 0x25: sets free fall time to 30 ms.
3. Write 0x0C to Register 0x27: enables absolute inactivity
detection.
4. Write 0x20 to Register 0x2A or Register 0x2B to map the
inactivity interrupt to INT1 or INT2, respectively.
5. Write 0x83 to Register 0x2C: configures the accelerometer
to 8 g range, 100 Hz ODR (output data rate).
6. Write 0x02 to Register 0x2D to begin measurement.
V
V
S
DD I/O
VIN
C
C
IO
S
V
V
S
DD I/O
R1
Implementation of a complete fall detection application is
described in the AN-1023 Application Note, Fall Detection
Application by Using 3-Axis Accelerometer ADXL345.
ADXL362
MOSI
MISO
SCLK
CS
INT1
INT2
SPI
INTERFACE
SHUTDOWN
NOTES
GND
POWER
Power Supply Decoupling
Figure 46 shows the recommended bypass capacitors for use
with the ADXL362.
1. THE ADXL362 SUPPLIES MUST BE DISCHARGED FULLY EACH TIME
THE VOLTAGE ON THEM DROPS BELOW THE SPECIFIED OPERATING
RANGE. A SHUTDOWN SWITCH IS ONE WAY TO DISCHARGE THE SUPPLIES.
Figure 47. Using a Switch to Discharge the ADXL362 Supplies
Rev. B | Page 37 of 44
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