Data Sheet
ADXL36±
POWER SAVINGS FEATURES
Designed for the most power conscious applications, the ADXL362
includes several features (as described in this section) for enabling
power savings at the system level, as well as at the device level.
In many applications, it is advantageous for activity detection to
be based not on an absolute threshold, but on a deviation from
a reference point or orientation. This is particularly useful
because it removes the effect on activity detection of the static
1 g imposed by gravity. When an accelerometer is stationary, its
output can reach 1 g, even when it is not moving. In absolute
activity, when the threshold is set to less than 1 g, activity is
immediately detected in this case.
ULTRALOW POWER CONSUMPTION IN ALL
MODES
At the device level, the most obvious power saving feature of the
ADXL362 is its ultralow current consumption in all configurations.
The ADXL362 consumes between 1.1 µA (typical) and 5 µA
(typical) across all data rates up to 400 Hz and all supply voltages
up to 3.5 V (see Figure 30). An even lower power, 270 nA (typical)
motion triggered wake-up mode is provided for simple motion
detection applications that require a power consumption lower
than 1 µA.
In the referenced configuration, activity is detected when
acceleration samples are at least a user set amount above an
internally defined reference for the user defined amount of time,
as described in Equation 1.
ABS(Acceleration − Reference) > Threshold
(1)
Consequently, activity is detected only when the acceleration
has deviated sufficiently from the initial orientation. The
reference for activity detection is calculated when activity
detection is engaged in the following scenarios:
At these current levels, the accelerometer consumes less power
in full operation than the standby currents of many other
system components, and is, therefore, optimal for applications that
require continuous acceleration monitoring and very long
battery life. Because the accelerometer is always on, it can act as
a motion activation switch. The accelerometer signals to the rest
of the system when to turn on, thereby managing power at the
system level.
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•
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When the activity function is turned on and measurement
mode is engaged;
If link mode is enabled: when inactivity is detected and
activity detection begins; or
If link mode is not enabled: when activity is detected and
activity detection repeats.
As important as its low operating current, the 10 nA (typical)
standby current of the ADXL362 contributes to a much longer
battery life in applications that spend most of their time in a
sleep state and wake up via an external trigger.
The referenced configuration results in a very sensitive activity
detection that detects even the most subtle motion events.
MOTION DETECTION
Fewer False Positives
The ADXL362 features built-in logic that detects activity (presence
of acceleration above a threshold) and inactivity (lack of
acceleration above a threshold). Activity and inactivity events
can be used as triggers to manage the accelerometer mode of
operation, trigger an interrupt to a host processor, and/or
autonomously drive a motion switch.
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 ADXL362 activity detection algorithm
is designed to be robust in filtering out undesired triggers.
The ADXL362 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.
Detection of an activity or inactivity event is indicated in the
status register and can be configured to generate an interrupt.
In addition, the activity status of the device, that is, whether it is
moving or stationary, is indicated by the AWAKE bit, described
in the Using the AWAKE Bit section.
Note that the activity timer is operational in measurement mode
only. In wake-up mode, one-sample activity detection is used.
Activity and inactivity detection can be used when the accel-
erometer is in either measurement mode or wake-up mode.
Inactivity Detection
Activity Detection
An inactivity event is detected when acceleration remains below
a specified threshold for a specified time. Inactivity detection is
also configurable as referenced or absolute.
An activity event is detected when acceleration remains above a
specified threshold for a specified time period.
Referenced and Absolute Configurations
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 consecutive samples are all below the threshold. The
absolute configuration of inactivity should be used for
implementing free fall detection.
Activity detection can be configured as referenced or absolute.
When using absolute activity detection, acceleration samples are
compared to a user set threshold to determine whether motion
is present. For example, if a threshold of 0.5 g is set and the
acceleration on the z-axis is 1 g for longer than the user defined
activity time, the activity status asserts.
Rev. B | Page 15 of 44
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