xref: /linux/Documentation/iio/adxl345.rst (revision ec2e0fb07d789976c601bec19ecced7a501c3705)

.. SPDX-License-Identifier: GPL-2.0

===============
ADXL345 driver
===============

This driver supports Analog Device's ADXL345/375 on SPI/I2C bus.

1. Supported Devices
====================

* `ADXL345 <https://www.analog.com/ADXL345>`_
* `ADXL375 <https://www.analog.com/ADXL375>`_

The ADXL345 is a generic purpose low power, 3-axis accelerometer with selectable
measurement ranges. The ADXL345 supports the ±2 g, ±4 g, ±8 g, and ±16 g ranges.

2. Device Attributes
====================

Each IIO device, has a device folder under ``/sys/bus/iio/devices/iio:deviceX``,
where X is the IIO index of the device. Under these folders reside a set of
device files, depending on the characteristics and features of the hardware
device in questions. These files are consistently generalized and documented in
the IIO ABI documentation.

The following table shows the ADXL345 related device files, found in the
specific device folder path ``/sys/bus/iio/devices/iio:deviceX``.

+-------------------------------------------+----------------------------------------------------------+
| 3-Axis Accelerometer related device files | Description                                              |
+-------------------------------------------+----------------------------------------------------------+
| in_accel_sampling_frequency               | Currently selected sample rate.                          |
+-------------------------------------------+----------------------------------------------------------+
| in_accel_sampling_frequency_available     | Available sampling frequency configurations.             |
+-------------------------------------------+----------------------------------------------------------+
| in_accel_scale                            | Scale/range for the accelerometer channels.              |
+-------------------------------------------+----------------------------------------------------------+
| in_accel_scale_available                  | Available scale ranges for the accelerometer channel.    |
+-------------------------------------------+----------------------------------------------------------+
| in_accel_x_calibbias                      | Calibration offset for the X-axis accelerometer channel. |
+-------------------------------------------+----------------------------------------------------------+
| in_accel_x_raw                            | Raw X-axis accelerometer channel value.                  |
+-------------------------------------------+----------------------------------------------------------+
| in_accel_y_calibbias                      | y-axis acceleration offset correction                    |
+-------------------------------------------+----------------------------------------------------------+
| in_accel_y_raw                            | Raw Y-axis accelerometer channel value.                  |
+-------------------------------------------+----------------------------------------------------------+
| in_accel_z_calibbias                      | Calibration offset for the Z-axis accelerometer channel. |
+-------------------------------------------+----------------------------------------------------------+
| in_accel_z_raw                            | Raw Z-axis accelerometer channel value.                  |
+-------------------------------------------+----------------------------------------------------------+

Channel Processed Values
-------------------------

A channel value can be read from its _raw attribute. The value returned is the
raw value as reported by the devices. To get the processed value of the channel,
apply the following formula:

.. code-block:: bash

        processed value = (_raw + _offset) * _scale

Where _offset and _scale are device attributes. If no _offset attribute is
present, simply assume its value is 0.

+-------------------------------------+---------------------------+
| Channel type                        | Measurement unit          |
+-------------------------------------+---------------------------+
| Acceleration on X, Y, and Z axis    | Meters per second squared |
+-------------------------------------+---------------------------+

Sensor Events
-------------

Specific IIO events are triggered by their corresponding interrupts. The sensor
driver supports either none or a single active interrupt (INT) line, selectable
from the two available options: INT1 or INT2. The active INT line should be
specified in the device tree. If no INT line is configured, the sensor defaults
to FIFO bypass mode, where event detection is disabled and only X, Y, and Z axis
measurements are available.

The table below lists the ADXL345-related device files located in the
device-specific path: ``/sys/bus/iio/devices/iio:deviceX/events``.
Note that activity and inactivity detection are DC-coupled by default;
therefore, only the AC-coupled activity and inactivity events are explicitly
listed.

+---------------------------------------------+---------------------------------------------+
| Event handle                                | Description                                 |
+---------------------------------------------+---------------------------------------------+
| in_accel_gesture_doubletap_en               | Enable double tap detection on all axis     |
+---------------------------------------------+---------------------------------------------+
| in_accel_gesture_doubletap_reset_timeout    | Double tap window in [us]                   |
+---------------------------------------------+---------------------------------------------+
| in_accel_gesture_doubletap_tap2_min_delay   | Double tap latent in [us]                   |
+---------------------------------------------+---------------------------------------------+
| in_accel_gesture_singletap_timeout          | Single tap duration in [us]                 |
+---------------------------------------------+---------------------------------------------+
| in_accel_gesture_singletap_value            | Single tap threshold value in 62.5/LSB      |
+---------------------------------------------+---------------------------------------------+
| in_accel_mag_falling_period                 | Inactivity time in seconds                  |
+---------------------------------------------+---------------------------------------------+
| in_accel_mag_falling_value                  | Inactivity threshold value in 62.5/LSB      |
+---------------------------------------------+---------------------------------------------+
| in_accel_mag_adaptive_rising_en             | Enable AC coupled activity on X axis        |
+---------------------------------------------+---------------------------------------------+
| in_accel_mag_adaptive_falling_period        | AC coupled inactivity time in seconds       |
+---------------------------------------------+---------------------------------------------+
| in_accel_mag_adaptive_falling_value         | AC coupled inactivity threshold in 62.5/LSB |
+---------------------------------------------+---------------------------------------------+
| in_accel_mag_adaptive_rising_value          | AC coupled activity threshold in 62.5/LSB   |
+---------------------------------------------+---------------------------------------------+
| in_accel_mag_rising_en                      | Enable activity detection on X axis         |
+---------------------------------------------+---------------------------------------------+
| in_accel_mag_rising_value                   | Activity threshold value in 62.5/LSB        |
+---------------------------------------------+---------------------------------------------+
| in_accel_x_gesture_singletap_en             | Enable single tap detection on X axis       |
+---------------------------------------------+---------------------------------------------+
| in_accel_x&y&z_mag_falling_en               | Enable inactivity detection on all axis     |
+---------------------------------------------+---------------------------------------------+
| in_accel_x&y&z_mag_adaptive_falling_en      | Enable AC coupled inactivity on all axis    |
+---------------------------------------------+---------------------------------------------+
| in_accel_y_gesture_singletap_en             | Enable single tap detection on Y axis       |
+---------------------------------------------+---------------------------------------------+
| in_accel_z_gesture_singletap_en             | Enable single tap detection on Z axis       |
+---------------------------------------------+---------------------------------------------+

Please refer to the sensor's datasheet for a detailed description of this
functionality.

Manually setting the **ODR** will cause the driver to estimate default values
for inactivity detection timing, where higher ODR values correspond to longer
default wait times, and lower ODR values to shorter ones. If these defaults do
not meet your application’s needs, you can explicitly configure the inactivity
wait time. Setting this value to 0 will revert to the default behavior.

When changing the **g range** configuration, the driver attempts to estimate
appropriate activity and inactivity thresholds by scaling the default values
based on the ratio of the previous range to the new one. The resulting threshold
will never be zero and will always fall between 1 and 255, corresponding to up
to 62.5 g/LSB as specified in the datasheet. However, you can override these
estimated thresholds by setting explicit values.

When **activity** and **inactivity** events are enabled, the driver
automatically manages hysteresis behavior by setting the **link** and
**auto-sleep** bits. The link bit connects the activity and inactivity
functions, so that one follows the other. The auto-sleep function puts the
sensor into sleep mode when inactivity is detected, reducing power consumption
to the sub-12.5 Hz rate.

The inactivity time is configurable between 1 and 255 seconds. In addition to
inactivity detection, the sensor also supports free-fall detection, which, from
the IIO perspective, is treated as a fall in magnitude across all axes. In
sensor terms, free-fall is defined using an inactivity period ranging from 0.000
to 1.000 seconds.

The driver behaves as follows:

* If the configured inactivity period is 1 second or more, the driver uses the
  sensor's inactivity register. This allows the event to be linked with
  activity detection, use auto-sleep, and be either AC- or DC-coupled.

* If the inactivity period is less than 1 second, the event is treated as plain
  inactivity or free-fall detection. In this case, auto-sleep and coupling
  (AC/DC) are not applied.

* If an inactivity time of 0 seconds is configured, the driver selects a
  heuristically determined default period (greater than 1 second) to optimize
  power consumption. This also uses the inactivity register.

Note: According to the datasheet, the optimal ODR for detecting activity,
or inactivity (or when operating with the free-fall register) should fall within
the range of 12.5 Hz to 400 Hz. The recommended free-fall threshold is between
300 mg and 600 mg (register values 0x05 to 0x09).

In DC-coupled mode, the current acceleration magnitude is directly compared to
the values in the THRESH_ACT and THRESH_INACT registers to determine activity or
inactivity. In contrast, AC-coupled activity detection uses the acceleration
value at the start of detection as a reference point, and subsequent samples are
compared against this reference. While DC-coupling is the default mode-comparing
live values to fixed thresholds-AC-coupling relies on an internal filter
relative to the configured threshold.

AC and DC coupling modes are configured separately for activity and inactivity
detection, but only one mode can be active at a time for each. For example, if
AC-coupled activity detection is enabled and then DC-coupled mode is set, only
DC-coupled activity detection will be active. In other words, only the most
recent configuration is applied.

**Single tap** detection can be configured per the datasheet by setting the
threshold and duration parameters. When only single tap detection is enabled,
the single tap interrupt triggers as soon as the acceleration exceeds the
threshold (marking the start of the duration) and then falls below it, provided
the duration limit is not exceeded. If both single tap and double tap detections
are enabled, the single tap interrupt is triggered only after the double tap
event has been either confirmed or dismissed.

To configure **double tap** detection, you must also set the window and latency
parameters in microseconds (µs). The latency period begins once the single tap
signal drops below the threshold and acts as a waiting time during which any
spikes are ignored for double tap detection. After the latency period ends, the
detection window starts. If the acceleration rises above the threshold and then
falls below it again within this window, a double tap event is triggered upon
the fall below the threshold.

Double tap event detection is thoroughly explained in the datasheet. After a
single tap event is detected, a double tap event may follow, provided the signal
meets certain criteria. However, double tap detection can be invalidated for
three reasons:

* If the **suppress bit** is set, any acceleration spike above the tap
  threshold during the tap latency period immediately invalidates the double tap
  detection. In other words, no spikes are allowed during latency when the
  suppress bit is active.

* The double tap event is invalid if the acceleration is above the threshold at
  the start of the double tap window.

* Double tap detection is also invalidated if the acceleration duration exceeds
  the limit set by the duration register.

For double tap detection, the same duration applies as for single tap: the
acceleration must rise above the threshold and then fall below it within the
specified duration. Note that the suppress bit is typically enabled when double
tap detection is active.

Usage Examples
--------------

Show device name:

.. code-block:: bash

        root:/sys/bus/iio/devices/iio:device0> cat name
        adxl345

Show accelerometer channels value:

.. code-block:: bash

        root:/sys/bus/iio/devices/iio:device0> cat in_accel_x_raw
        -1
        root:/sys/bus/iio/devices/iio:device0> cat in_accel_y_raw
        2
        root:/sys/bus/iio/devices/iio:device0> cat in_accel_z_raw
        -253

Set calibration offset for accelerometer channels:

.. code-block:: bash

        root:/sys/bus/iio/devices/iio:device0> cat in_accel_x_calibbias
        0

        root:/sys/bus/iio/devices/iio:device0> echo 50 > in_accel_x_calibbias
        root:/sys/bus/iio/devices/iio:device0> cat in_accel_x_calibbias
        50

Given the 13-bit full resolution, the available ranges are calculated by the
following formula:

.. code-block:: bash

        (g * 2 * 9.80665) / (2^(resolution) - 1) * 100; for g := 2|4|8|16

Scale range configuration:

.. code-block:: bash

        root:/sys/bus/iio/devices/iio:device0> cat ./in_accel_scale
        0.478899
        root:/sys/bus/iio/devices/iio:device0> cat ./in_accel_scale_available
        0.478899 0.957798 1.915595 3.831190

        root:/sys/bus/iio/devices/iio:device0> echo 1.915595 > ./in_accel_scale
        root:/sys/bus/iio/devices/iio:device0> cat ./in_accel_scale
        1.915595

Set output data rate (ODR):

.. code-block:: bash

        root:/sys/bus/iio/devices/iio:device0> cat ./in_accel_sampling_frequency
        200.000000

        root:/sys/bus/iio/devices/iio:device0> cat ./in_accel_sampling_frequency_available
        0.097000 0.195000 0.390000 0.781000 1.562000 3.125000 6.250000 12.500000 25.000000 50.000000 100.000000 200.000000 400.000000 800.000000 1600.000000 3200.000000

        root:/sys/bus/iio/devices/iio:device0> echo 1.562000 > ./in_accel_sampling_frequency
        root:/sys/bus/iio/devices/iio:device0> cat ./in_accel_sampling_frequency
        1.562000

Configure one or several events:

.. code-block:: bash

        root:> cd /sys/bus/iio/devices/iio:device0

        root:/sys/bus/iio/devices/iio:device0> echo 1 > ./buffer0/in_accel_x_en
        root:/sys/bus/iio/devices/iio:device0> echo 1 > ./buffer0/in_accel_y_en
        root:/sys/bus/iio/devices/iio:device0> echo 1 > ./buffer0/in_accel_z_en

        root:/sys/bus/iio/devices/iio:device0> echo 1 > ./scan_elements/in_accel_x_en
        root:/sys/bus/iio/devices/iio:device0> echo 1 > ./scan_elements/in_accel_y_en
        root:/sys/bus/iio/devices/iio:device0> echo 1 > ./scan_elements/in_accel_z_en

        root:/sys/bus/iio/devices/iio:device0> echo 14   > ./in_accel_x_calibbias
        root:/sys/bus/iio/devices/iio:device0> echo 2    > ./in_accel_y_calibbias
        root:/sys/bus/iio/devices/iio:device0> echo -250 > ./in_accel_z_calibbias

        root:/sys/bus/iio/devices/iio:device0> echo 24 > ./buffer0/length

        ## AC coupled activity, threshold [62.5/LSB]
        root:/sys/bus/iio/devices/iio:device0> echo 6 > ./events/in_accel_mag_adaptive_rising_value

        ## AC coupled inactivity, threshold, [62.5/LSB]
        root:/sys/bus/iio/devices/iio:device0> echo 4 > ./events/in_accel_mag_adaptive_falling_value

        ## AC coupled inactivity, time [s]
        root:/sys/bus/iio/devices/iio:device0> echo 3 > ./events/in_accel_mag_adaptive_falling_period

        ## singletap, threshold
        root:/sys/bus/iio/devices/iio:device0> echo 35 > ./events/in_accel_gesture_singletap_value

        ## singletap, duration [us]
        root:/sys/bus/iio/devices/iio:device0> echo 0.001875  > ./events/in_accel_gesture_singletap_timeout

        ## doubletap, window [us]
        root:/sys/bus/iio/devices/iio:device0> echo 0.025 > ./events/in_accel_gesture_doubletap_reset_timeout

        ## doubletap, latent [us]
        root:/sys/bus/iio/devices/iio:device0> echo 0.025 > ./events/in_accel_gesture_doubletap_tap2_min_delay

        ## AC coupled activity, enable
        root:/sys/bus/iio/devices/iio:device0> echo 1 > ./events/in_accel_mag_adaptive_rising_en

        ## AC coupled inactivity, enable
        root:/sys/bus/iio/devices/iio:device0> echo 1 > ./events/in_accel_x\&y\&z_mag_adaptive_falling_en

        ## singletap, enable
        root:/sys/bus/iio/devices/iio:device0> echo 1 > ./events/in_accel_x_gesture_singletap_en
        root:/sys/bus/iio/devices/iio:device0> echo 1 > ./events/in_accel_y_gesture_singletap_en
        root:/sys/bus/iio/devices/iio:device0> echo 1 > ./events/in_accel_z_gesture_singletap_en

        ## doubletap, enable
        root:/sys/bus/iio/devices/iio:device0> echo 1 > ./events/in_accel_gesture_doubletap_en

Verify incoming events:

.. code-block:: bash

        root:# iio_event_monitor adxl345
        Found IIO device with name adxl345 with device number 0
        Event: time: 1739063415957073383, type: accel(z), channel: 0, evtype: mag, direction: rising
        Event: time: 1739063415963770218, type: accel(z), channel: 0, evtype: mag, direction: rising
        Event: time: 1739063416002563061, type: accel(z), channel: 0, evtype: gesture, direction: singletap
        Event: time: 1739063426271128739, type: accel(x&y&z), channel: 0, evtype: mag, direction: falling
        Event: time: 1739063436539080713, type: accel(x&y&z), channel: 0, evtype: mag, direction: falling
        Event: time: 1739063438357970381, type: accel(z), channel: 0, evtype: mag, direction: rising
        Event: time: 1739063446726161586, type: accel(z), channel: 0, evtype: mag, direction: rising
        Event: time: 1739063446727892670, type: accel(z), channel: 0, evtype: mag, direction: rising
        Event: time: 1739063446743019768, type: accel(z), channel: 0, evtype: mag, direction: rising
        Event: time: 1739063446744650696, type: accel(z), channel: 0, evtype: mag, direction: rising
        Event: time: 1739063446763559386, type: accel(z), channel: 0, evtype: gesture, direction: singletap
        Event: time: 1739063448818126480, type: accel(x&y&z), channel: 0, evtype: mag, direction: falling
        ...

Activity and inactivity belong together and indicate state changes as follows

.. code-block:: bash

        root:# iio_event_monitor adxl345
        Found IIO device with name adxl345 with device number 0
        Event: time: 1744648001133946293, type: accel(x), channel: 0, evtype: mag, direction: rising
          <after inactivity time elapsed>
        Event: time: 1744648057724775499, type: accel(x&y&z), channel: 0, evtype: mag, direction: falling
        ...

3. Device Buffers
=================

This driver supports IIO buffers.

All devices support retrieving the raw acceleration and temperature measurements
using buffers.

Usage examples
--------------

Select channels for buffer read:

.. code-block:: bash

        root:/sys/bus/iio/devices/iio:device0> echo 1 > scan_elements/in_accel_x_en
        root:/sys/bus/iio/devices/iio:device0> echo 1 > scan_elements/in_accel_y_en
        root:/sys/bus/iio/devices/iio:device0> echo 1 > scan_elements/in_accel_z_en

Set the number of samples to be stored in the buffer:

.. code-block:: bash

        root:/sys/bus/iio/devices/iio:device0> echo 10 > buffer/length

Enable buffer readings:

.. code-block:: bash

        root:/sys/bus/iio/devices/iio:device0> echo 1 > buffer/enable

Obtain buffered data:

.. code-block:: bash

        root:> iio_readdev -b 16 -s 1024 adxl345 | hexdump -d
        WARNING: High-speed mode not enabled
        0000000   00003   00012   00013   00005   00010   00011   00005   00011
        0000010   00013   00004   00012   00011   00003   00012   00014   00007
        0000020   00011   00013   00004   00013   00014   00003   00012   00013
        0000030   00004   00012   00013   00005   00011   00011   00005   00012
        0000040   00014   00005   00012   00014   00004   00010   00012   00004
        0000050   00013   00011   00003   00011   00012   00005   00011   00013
        0000060   00003   00012   00012   00003   00012   00012   00004   00012
        0000070   00012   00003   00013   00013   00003   00013   00012   00005
        0000080   00012   00013   00003   00011   00012   00005   00012   00013
        0000090   00003   00013   00011   00005   00013   00014   00003   00012
        00000a0   00012   00003   00012   00013   00004   00012   00015   00004
        00000b0   00014   00011   00003   00014   00013   00004   00012   00011
        00000c0   00004   00012   00013   00004   00014   00011   00004   00013
        00000d0   00012   00002   00014   00012   00005   00012   00013   00005
        00000e0   00013   00013   00003   00013   00013   00005   00012   00013
        00000f0   00004   00014   00015   00005   00012   00011   00005   00012
        ...

See ``Documentation/iio/iio_devbuf.rst`` for more information about how buffered
data is structured.

4. IIO Interfacing Tools
========================

See ``Documentation/iio/iio_tools.rst`` for the description of the available IIO
interfacing tools.