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ADXL362 Datasheet(Arkusz danych) 11 Page - Analog Devices

Numer części ADXL362
Szczegółowy opis  Micropower Three-Axis ±2g/±4g/±8g
Pobierz  40 Pages
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Producent  AD [Analog Devices]
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Preliminary Technical Data
Rev. PrB | Page 11 of 40
Absolute Inactivity, if the Inactivity threshold is set lower than 1
g, a device resting motionless does not detect Inactivity. With
Referenced Inactivity, the same device under the same
configuration does detect Inactivity.
The Inactivity timer can be set to anywhere from 2.5 ms (1
sample at 400 Hz ODR) to almost 90 minutes (65535 samples at
12.5Hz ODR) of inactivity, This means the accelerometer can be
configured such that it must be stationary for up to 90 minutes
before putting its system to sleep. 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; and in applications where continuous operation is
critical, the system will stay on for as long as any motion at all is
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:
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.
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 to be 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 to be
stationary or “asleep”. Now activity is expected as the
next event, so only Activity detection operates.
In this mode, each interrupt must be serviced by a host
processor before the next is enabled.
Loop Mode
: motion detection operates as described in
Linked Mode, but 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.
: In Linked or Loop mode, enabling Autosleep
will cause the device to autonomously enter Wake-Up
Mode (see p. 8) when inactivity is detected, and re-enter
Measurement Mode when activity is detected.
Using 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 “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 as shown in Figure 15.
If the turn-on time of 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.
The ADXL362 includes a deep 512-sample FIFO (first-in, first-
out) buffer. The FIFO provides benefits primarily in two ways.
First, appropriate use of the FIFO enables system-level power
savings by enabling the host processor to sleep for extended
periods of time while the accelerometer autonomously collects
data. Alternatively, using the FIFO to collect data can unburden
the host while it tends to other tasks.
Second, the FIFO can be used in a Triggered mode to record all
data leading up to an activity detection event, thereby providing
context for the event. In the case of a system that identifies
impact events, for example, the accelerometer can keep the
entire system off while it stores acceleration data in its FIFO and
look for an Activity event. When the impact event occurs, data
that was collected prior to the event is frozen in the FIFO. The
accelerometer can now wake the rest of the system and transfer
this data to the host processor, thereby providing context for the
impact event. In general, the more context available, the more
intelligent decisions a system can make, so a deep FIFO is
especially useful. The ADXL362 FIFO can store up to over 13
seconds of data, providing a clear picture of events prior to an
Activity trigger.
All FIFO modes of operation, as well as the structure of the
FIFO and instructions for retrieving data from it, are described
in further detail in the Applications Information FIFO section
on page 31.
SPI Instructions
The ADXL362’s digital interface is implemented with system-
level power savings in mind. The following features enhance
power savings:
Burst reads and writes reduce the number of SPI
communication cycles required to configure the part.
Concurrent operation of Activity and Inactivity
detection enables “set it and forget it” operation.
Linked and Loop Modes further reduce

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