ADXL345 driver¶
This driver supports Analog Device’s ADXL345/375 on SPI/I2C bus.
1. Supported Devices¶
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:
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:
root:/sys/bus/iio/devices/iio:device0> cat name
adxl345
Show accelerometer channels value:
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:
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:
(g * 2 * 9.80665) / (2^(resolution) - 1) * 100; for g := 2|4|8|16
Scale range configuration:
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):
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:
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:
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
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:
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:
root:/sys/bus/iio/devices/iio:device0> echo 10 > buffer/length
Enable buffer readings:
root:/sys/bus/iio/devices/iio:device0> echo 1 > buffer/enable
Obtain buffered data:
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.