ADXL372

ADXL372 Input 3-Axis Digital Accelerometer Linux Driver.

Supported Devices

• ADXL372

Evaluation Boards

• EVAL-ADXL372Z

Description

This is a Linux industrial I/O (Linux Industrial I/O Subsystem) subsystem driver for the three-axis ADXL372 accelerometer, with SPI and I2C bus interfaces. The industrial I/O subsystem provides a unified framework for drivers for many different types of converters and sensors using a number of different physical interfaces (i2c, spi, etc). See Linux Industrial I/O Subsystem for more information.

Source Code

Status

Source	Mainlined?
git	Yes

Files

Function	File
core driver	adxl372.c
spi driver	adxl372_spi.c
i2c driver	adxl372_i2c.c
header	adxl372.h
Documentation	adi,adxl372.yaml
ABI documentation	sysfs-bus-iio-accel-adxl372

Example device tree

Function	File
dts	rpi-adxl372-overlay.dts

Adding Linux driver support

Devicetree configuration

Example platform device initialization

Required devicetree properties:

• compatible: Needs to be the name of the device. E.g. adi,adxl372

• reg: the I2C address or SPI chip select number for the device

Required properties for SPI bus usage:

• spi-max-frequency: Maximum SPI clock frequency.

Optional properties:

• interrupts: a list of interrupt specifiers

Example for a SPI device node:

#address-cells = <1>;
#size-cells = <0>;
status = "okay";

adxl372@0 {
    compatible = "adi,adxl372";
    reg = <0>;
    spi-max-frequency = <1000000>;
    interrupts = <25 2>;
    interrupt-parent = <&gpio>;
};

Example for a I2C device node:

#address-cells = <1>;
#size-cells = <0>;
status = "okay";

adxl372@53 {
    compatible = "adi,adxl372";
    reg = <0x53>;
    interrupts = <25 2>;
    interrupt-parent = <&gpio>;
};

Driver testing

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

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

root:/sys/bus/iio/devices/iio:device0> ls -l
drwxr-xr-x 2 root root    0 Oct 25 04:25 buffer
-rw-r--r-- 1 root root 4096 Oct 25 04:25 current_timestamp_clock
-r--r--r-- 1 root root 4096 Oct 25 04:25 dev
-rw-r--r-- 1 root root 4096 Oct 25 04:28 in_accel_filter_low_pass_3db_frequency
-r--r--r-- 1 root root 4096 Oct 25 04:25 in_accel_filter_low_pass_3db_frequency_available
-rw-r--r-- 1 root root 4096 Oct 25 04:27 in_accel_sampling_frequency
-rw-r--r-- 1 root root 4096 Oct 25 04:25 in_accel_scale
-rw-r--r-- 1 root root 4096 Oct 25 04:25 in_accel_x_raw
-rw-r--r-- 1 root root 4096 Oct 25 04:25 in_accel_y_raw
-rw-r--r-- 1 root root 4096 Oct 25 04:25 in_accel_z_raw
-r--r--r-- 1 root root 4096 Oct 25 04:25 name
lrwxrwxrwx 1 root root    0 Oct 25 04:25 of_node -> ../../../../../../../../firmware/devicetree/base/soc/spi@7e204000/adxl372@0
drwxr-xr-x 2 root root    0 Oct 25 04:25 power
-r--r--r-- 1 root root 4096 Oct 25 04:25 sampling_frequency_available
drwxr-xr-x 2 root root    0 Oct 25 04:25 scan_elements
lrwxrwxrwx 1 root root    0 Oct 25 04:25 subsystem -> ../../../../../../../../bus/iio
drwxr-xr-x 2 root root    0 Oct 25 04:25 trigger
-rw-r--r-- 1 root root 4096 Oct 25 04:25 uevent

Identify the device

root:/sys/bus/iio/devices/iio:device0> cat name
adxl372

Basic operation

Show scale

Description: scale to be applied to in_accel_*_raw in order to obtain the acceleration.

root:/sys/bus/iio/devices/iio:device0> cat in_accel_scale
0.958241

Show axis x measurement

Description: Raw unscaled acceleration measurement on x axis

root:/sys/bus/iio/devices/iio:device0> cat in_accel_x_raw
-5

Set sampling frequency

root:/sys/bus/iio/devices/iio:device0> cat sampling_frequency_available
400 800 1600 3200 6400
root:/sys/bus/iio/devices/iio:device0> cat in_accel_sampling_frequency
6400
root:/sys/bus/iio/devices/iio:device0> echo 3200 > in_accel_sampling_frequency
root:/sys/bus/iio/devices/iio:device0> cat in_accel_sampling_frequency
3200

Show available bandwidths for the current set frequency

root:/sys/bus/iio/devices/iio:device0> cat in_accel_sampling_frequency
6400
root:/sys/bus/iio/devices/iio:device0> cat in_accel_filter_low_pass_3db_frequency_available
200 400 800 1600 3200
root:/sys/bus/iio/devices/iio:device0>

Change the bandwidth

root:/sys/bus/iio/devices/iio:device0> cat in_accel_filter_low_pass_3db_frequency_available
200 400 800 1600 3200
root:/sys/bus/iio/devices/iio:device0> cat in_accel_filter_low_pass_3db_frequency
3200
root:/sys/bus/iio/devices/iio:device0> echo 1600 > in_accel_filter_low_pass_3db_frequency
root:/sys/bus/iio/devices/iio:device0> cat in_accel_filter_low_pass_3db_frequency
1600
root:/sys/bus/iio/devices/iio:device0> echo 3200 > in_accel_sampling_frequency
root:/sys/bus/iio/devices/iio:device0> cat in_accel_filter_low_pass_3db_frequency_available
200 400 800 1600
root:/sys/bus/iio/devices/iio:device0> echo 1600 > in_accel_sampling_frequency
root:/sys/bus/iio/devices/iio:device0> cat in_accel_filter_low_pass_3db_frequency
800

Using the FIFO Buffer

Description: The ADXL372 includes a deep, 512 sample FIFO buffer. The 512 FIFO samples can be allotted in several ways, such as the following:

• 170 sample sets of concurrent 3-axis data

• 256 sample sets of concurrent 2-axis data (user selectable)

• 512 sample sets of single-axis data

• 170 sets of impact event peak (x, y, z)

Trigger management

Description: The FIFO is configured in Stream Mode and FIFO_FULL interrupt is enabled. The interrupt will be triggered when the FIFO watermark is reached.

root:/sys/bus/iio/devices/iio:device0> cat trigger/current_trigger
adxl372-dev0

Substitute the actual IIO device number.

Buffer management

Description: The scan_elements directory contains interfaces for elements that will be captured for a single triggered sample set in the buffer

root:/sys/bus/iio/devices/iio:device0/scan_elements> ls -l
-rw-r--r-- 1 root root 4096 Oct 25 08:08 in_accel_x_en
-r--r--r-- 1 root root 4096 Oct 25 08:08 in_accel_x_index
-r--r--r-- 1 root root 4096 Oct 25 08:08 in_accel_x_type
-rw-r--r-- 1 root root 4096 Oct 25 08:08 in_accel_y_en
-r--r--r-- 1 root root 4096 Oct 25 08:08 in_accel_y_index
-r--r--r-- 1 root root 4096 Oct 25 08:08 in_accel_y_type
-rw-r--r-- 1 root root 4096 Oct 25 08:08 in_accel_z_en
-r--r--r-- 1 root root 4096 Oct 25 08:08 in_accel_z_index
-r--r--r-- 1 root root 4096 Oct 25 08:08 in_accel_z_type
-rw-r--r-- 1 root root 4096 Oct 25 08:08 in_timestamp_en
-r--r--r-- 1 root root 4096 Oct 25 08:08 in_timestamp_index
-r--r--r-- 1 root root 4096 Oct 25 08:08 in_timestamp_type
root:/sys/bus/iio/devices/iio:device0/scan_elements>

Before enabling the buffer, a few steps need to be completed.

• For example, if we want the FIFO to store sample sets of concurrent 3-axis data, we need to enable the scan elements:

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

root:/sys/bus/iio/devices/iio:device0/buffer> ls
enable hwfifo_enabled  hwfifo_watermark  hwfifo_watermark_max
hwfifo_watermark_min  length  watermark
root:/sys/bus/iio/devices/iio:device0/buffer>

• Set the buffer length:

root:/sys/bus/iio/devices/iio:device0/buffer> echo 1024 > length

• Set the fifo watermak:

Because we enabled all three channels, then the watermark can be set to a maximum of 170 sample sets.

root:/sys/bus/iio/devices/iio:device0/buffer> echo 170 > watermark

The FIFO can store impact-event peak acceleration. With the buffer disabled, select the device’s peak trigger. In the correct IIO device directory, substitute the actual device number in the trigger name:

set -eu
echo 0 > buffer/enable
echo adxl372-dev0-peak > trigger/current_trigger
cat trigger/current_trigger

Then configure the scan elements and enable the buffer. Peak acquisition stores XYZ sets internally; the driver repacks the selected axes into the IIO scan. Peak mode is selected through the trigger.

• Enable the buffer:

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

To read samples:

root:/sys/bus/iio/devices/iio:device0/buffer> cat /dev/iio:device0

Troubleshooting

Low level register access via debugfs (direct_reg_access)

Some IIO drivers feature an optional debug facility, allowing users to read or write registers directly. Special care needs to be taken when using this feature, since you can modify registers on the back of the driver. Accessing debugfs requires root privileges.

In order to identify if the IIO device in question feature this option you first need to identify the IIO device number.

Therefore read the name attribute of each IIO device

root@analog:~# grep "" /sys/bus/iio/devices/iio\:device/name
adxl372
root@analog:~#

Change directory to /sys/kernel/debug/iio/iio:deviceX and check if the direct_reg_access file exists.

root@analog:~# cd /sys/kernel/debug/iio/iio\:device0/
root@analog:/sys/kernel/debug/iio/iio:device0# ls direct_reg_access
ls direct_reg_access
root@analog:/sys/kernel/debug/iio/iio:device0#

Reading

root@analog:/sys/kernel/debug/iio/iio:device0# echo 0x0 > direct_reg_access

root@analog:/sys/kernel/debug/iio/iio:device0# cat direct_reg_access
0xAD
root@analog:/sys/kernel/debug/iio/iio:device0#

Writing

Write ADDRESS VALUE

root@analog:/sys/kernel/debug/iio/iio:device0# echo 0x3D 0x80 > direct_reg_access
root@analog:/sys/kernel/debug/iio/iio:device0# cat direct_reg_access
0x80
root@analog:/sys/kernel/debug/iio/iio:device0#

More Information

• IIO mailing list: linux-iio@vger.kernel.org

• IIO Linux Kernel Documentation sysfs-bus-iio-*

• IIO Documentation

• IIO Oscilloscope

• Libiio

• About libiio

• IIO High Speed

• Software Defined Radio using the IIO framework

• libiio introduction