ADXL366 datasheet(23/71 Pages) AD

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Data Sheet

ADXL366

POWER SAVINGS FEATURES

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Rev. 0 | 23 of 71

acceleration threshold, are user adjustable from one sample (that

is, no timer) to up to 20sec of motion.

Note that the activity timer is operational in measurement mode and

wake-up mode. In wake-up mode, one sample activity detection is

used.

Inactivity Detection

An inactivity event is detected when acceleration remains less than

a specified threshold for a specified time on all the axes. All three

axes (if enabled) must be less than the inactivity thresholds for an

inactivity event to occur. Inactivity detection is also configurable as

referenced or absolute.

When using absolute inactivity detection, acceleration samples are

compared to a user set threshold for the user set time to deter-

mine the absence of motion. Inactivity is detected when enough

consecutive samples are all less than the threshold. The absolute

configuration of inactivity must be used for implementing free fall

detection.

Referenced inactivity, like referenced activity, is particularly useful

for eliminating the effects of the static acceleration due to gravity.

With absolute inactivity, if the inactivity threshold is set lower than

1g, a device resting motionless may never detect inactivity. With

referenced inactivity, the same device under the same configuration

detects inactivity.

When using referenced inactivity detection, inactivity is detected

when acceleration samples are within a user specified amount of an

internally defined reference (as described by Equation 3) for a user

defined amount of time.

ABS(Acceleration − Reference) < Threshold

(3)

The reference for inactivity detection is calculated when either of

the following events occurs:

►

The inactivity function is turned on, and the device enters meas-

urement mode.

►

An inactivity event is detected.

When in referenced mode, the inactivity reference values can be

read by properly setting the REF_READBACK bits (Bits[7:6]) in the

ACT_INACT_CTL register (Register 0x27):

1. Write b'10 to REF_READBACK bits ion ACT_INACT_CTL reg-

ister.

2. On the next ODR read, the inactivity reference values from

the XDATA_H register (Register 0x0E) to ZDATA_L register

(Register 0x13).

A requirement for inactivity detection is that for whatever period

of time the inactivity timer has been configured, the accelerometer

detects inactivity only when it has been stationary for that amount of

time. The inactivity timer can be set anywhere from 2.5ms (a single

sample at 400Hz ODR) to almost 90 minutes (65,535 samples

at 12.5Hz ODR) of inactivity. The reference is not updated until the

timer expires. In dynamic environments, the reference not updating

until the timer expires can lead to the device becoming stuck in a

state where it is looking for inactivity, but the acceleration is outside

the threshold limits, which applies for all modes of operation:

default, linked, and looped mode.

The following settings prove an example for enabling referenced

inactivity in default mode:

►

2g, ODR = 100Hz

►

Referenced inactivity threshold = 250mg

►

Inactivity timer = 100 samples

For the example, the device updates the inactivity reference once

every second because the ODR is 100Hz and the timer is 100

samples (see Figure 47).

Figure 47. Referenced Inactivity Thresholds, Slow Change in Acceleration

In Figure 48, the acceleration exceeds the inactivity threshold be-

fore the time expires. Therefore, inactivity is not detected. However,

this now means that the reference thresholds are not updated. If the

acceleration stays more than the thresholds, the device is stuck in

a loop of searching for inactivity; however, acceleration is outside of

the limits. Even though the device may not be moving, inactivity is

not detected.

Figure 48. Referenced Inactivity Thresholds, Fast Change in Acceleration

Notice that in linked and looped mode, the inactivity event detection

is linked sequentially to activity, which means that the inactivity

threshold cannot be continuously updated as in default mode (see

Figure 47).

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