CN-0274 Datasheet, PDF (3/6 Pages) Analog Devices – Ultralow Power, 3-Axis, Motion Activated Switch

Circuit Note
Activity Detection
An activity event is detected when acceleration stays above a
specified threshold for a user-specified time period. The two
activity detection events are absolute and referenced.
• 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 any axis is 1 g for longer than the
user defined activity time, the activity status is asserted. 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, if the threshold is set to less than 1 g,
activity is immediately detected in this case.
• In the referenced activity detection, 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. The reference is calculated when activity detection is
engaged, and the first sample obtained is used as a reference
point. Activity is only detected when the acceleration has
deviated sufficiently from this initial orientation. The
referenced configuration results in a very sensitive activity
detection that detects even the most subtle motion events.
The CN0274 evaluation software uses the referenced mode of
operation when searching for activity.
Inactivity Detection
An inactivity event is detected when acceleration remains below
a specified threshold for a specified time. The two inactivity
detection events are absolute and referenced.
• In absolute inactivity detection, acceleration samples are
compared to a user set threshold for the user set time to
determine the absence of motion.
• In referenced inactivity detection, acceleration samples are
compared to a user specified reference for a user defined
amount of time. When the part first enters the awake state,
the first sample is used as a reference point, and the threshold
is applied around it. If the acceleration stays inside the
threshold, the part enters the asleep state. If an acceleration
value moves outside the threshold, this point is then used
as a new reference, and the thresholds are reapplied to this
new point.
The CN0274 evaluation software uses the referenced mode of
operation when searching for inactivity.
CN-0274
Linking Activity and Inactivity Detection
The activity and inactivity detection functions can be used
concurrently, and processed manually by a host processor, or
they can be configured to interact in several ways:
• In default mode, activity and inactivity detection are both
enabled, and all interrupts must be serviced by a host
processor; that is, a processor must read each interrupt
before it is cleared and can be used again.
• In linked mode, activity and inactivity detection are linked
to each other such that only one of the functions is enabled
at any given time. Once activity is detected, the device is
assumed moving or awake and stops looking for activity:
inactivity is expected as the next event so only inactivity
detection operates. When inactivity is detected, the device
is assumed stationary or asleep. Activity is now expected as
the next event so that only activity detection operates. In
this mode, a host processor must service each interrupt
before the next is enabled.
• In loop mode, motion detection operates as previously
described in linked mode; however, interrupts do not need
to be serviced by a host processor. This configuration
simplifies the implementation of commonly used motion
detection and enhances power savings by reducing the
amount of power used in bus communication.
• When enabling autosleep mode in linked mode or loop
mode, it causes the device to autonomously enter wake-up
mode when inactivity is detected, and reenter measurement
mode when activity is detected.
The CN0274 evaluation software uses the autosleep and loop
modes to demonstrate the functionality of the ADXL362.
The AWAKE Bit
The AWAKE bit is a status bit that indicates whether the ADXL362
is awake or asleep. The device is awake when it has seen an activity
condition, and the device is asleep when it has seen an inactivity
condition.
The awake signal can be mapped to the INT1 or INT2 pin and
can thus be used as a status output to connect or disconnect
power to downstream circuitry based on the awake status of
the accelerometer. Used in conjunction with loop mode, this
configuration implements a trivial, autonomous motion-
activated switch.
If the turn-on time of the downstream circuitry can be tolerated,
this motion switch configuration can save significant system-
level power by eliminating the standby current consumption of
the rest of the application. This standby current can often exceed
the full operating current of the ADXL362.
Rev. A | Page 3 of 6