ADXL367 datasheet(21/64 Pages) AD

21 / 64 page

Data Sheet

ADXL367

POWER SAVINGS FEATURES

analog.com

Rev. 0 | 21 of 64

Fewer False Positives

Ideally, the intent of activity detection is to wake-up a system only

when motion is intentional, ignoring noise or small, unintentional

movements. In addition to being sensitive to subtle motion events,

the ADXL367 activity detection algorithm is designed to be robust in

filtering out undesired triggers.

The ADXL367 activity detection functionality includes a timer to

filter out unwanted motion and ensure that only sustained motion

is recognized as activity. The duration of this timer, as well as the

acceleration threshold, are user adjustable from one sample (that

is, no timer) to up to 20 seconds 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 below a

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

axes (if enabled) must be below 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 determine

the absence of motion. Inactivity is detected when enough consecu-

tive samples are all below the threshold. The absolute configuration

of inactivity must be used for implementing free fall detection.

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 2) for a user

defined amount of time.

ABS(Acceleration − Reference) < Threshold(2)

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

Each time an inactivity event is detected, the reference is updat-

ed. This becomes important when using an inactivity timer. The

reference is not updated until the timer expires. In dynamic envi-

ronments, this can lead to the device becoming stuck in a state

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

threshold limits.

The following settings prove an example for enabling referenced

inactivity, loop mode:

►

2 g, ODR = 100 Hz

►

Referenced inactivity threshold = 250 mg

►

Inactivity timer = 100 samples

►

With the inactivity timer setting, this means that the device

updates the inactivity reference once every second (because

ODR is 100 Hz and the timer is 100 samples). This is shown in

Figure 45.

Figure 45. Referenced Inactivity Thresholds, Slow Change in Acceleration

Figure 46. Referenced Inactivity Thresholds, Fast Change in Acceleration

In Figure 46, the acceleration exceeds the inactivity threshold

before the time expires. This is an activity event. Therefore, inactiv-

ity is not detected. However, this now means that the reference

thresholds are not updated. If the acceleration stays above the

thresholds, the device is stuck in a loop of searching for inactivity

but acceleration is outside of the limits. Even though the device

may not be moving, inactivity is not detected.

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

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

referenced inactivity, the same device under the same configuration

detects inactivity.

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

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

at 12.5 Hz ODR) of inactivity. 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.

For example, if the accelerometer has been configured for 90 mi-

nutes, the accelerometer detects inactivity when it has been station-

ary for 90 minutes. The wide range of timer settings means that in

applications where power conservation is critical, the system can

be put to sleep after very short periods of inactivity. In applications

where continuous operation is critical, the system stays on for as

long as any motion is present.

ADXL367 Datasheet(PDF) 21 Page - Analog Devices

ADXL367 Datasheet(HTML) 21 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64

Datasheet Download

Link URL