Writing a Driver for the ADXL345 Accelerometer

The goal of my outreachy project is to write a driver for a sensor using the Industrial I/O interface. The sensor chosen is ADXL345 from Analog Devices. This sensor is a 3-axis digital accelerometer with high resolution measurement of +/-(2g/4g/8g and 16g).

This sensor is no stranger to me. I used this before on a school project. It’s amusing that I’m writing a driver for it now. Come to think of it, this sensor choice from that school project is overkill where we could’ve gone with a much simpler sensor for tilt determination.

Overview of features

The adxl345 is feature-filled:

User-selectable
- Measurement ranges of 2g, 4g, 8g and 16g as mentioned before
- Bandwidth
- Resolution: fixed 10-bit resolution or full resolution where resolution increases with range, up to 13-bit resolution at +/- 16g
Sensing functions: activity/inactivity, single/double taps in any direction, free-fall detection
Power modes: low power, auto sleep and standby mode
32-level FIFO buffer
Self-Test
I2C and SPI (3-and 4-wire) digital interfaces
Interrupts

Planned Features for the Minimal Driver

As of this writing, I’m still waiting for the sensor to arrive. I have written device probing and support for read raw which I will write about on a separate post. I am told that these are enough for the initial support submission. Let’s see if I could include the scale as well.

Writing Process

I set up a development branch under my clone of the IIO tree, created the source file under accel/ and modified the Kconfig and Makefile so that I could build the driver in-tree without hiccups. Here’s what it looks like:

$ git clone git://git.kernel.org/pub/scm/linux/kernel/git/jic23/iio.git
$ git checkout -b adxl345_development
$ touch drivers/iio/accel/adxl345.c
$ vim drivers/iio/accel/Makefile
$ vim drivers/iio/accel/Kconfig

I just follow the kernel patch philosophy where you commit only one specific change at a time so that it’s easier to review. Here’s what my git log looks like so far with the first pre-patch being the Kconfig+Makefile modification.

46ba4d5 iio: accel: adxl345: Expose available scales to sysfs
e38d33b iio: accel: adxl345: implement read_raw
35a3a17 iio: accel: adxl345: Declare acceleration channels
eecb517 iio: accel: adxl345: Add initial code for probe()
ce994c5 iio: accel: adxl345: Add i2c client structure and allow initialization
9a1aa84 iio: accel: adxl345: Write initial source
fb83aae iio: accel: adxl345: Add support for building adxl345
f8611c0 Staging: iio: impedance-analyzer: ad5933: fix wrong comments
7c271ee iio: adc: spmi-vadc: Changes to support different scaling
ba71704 iio: adc: spmi-vadc: Update function for generic voltage conversion

Here are some sources I’ll be referring to during the writing process:

Thanks for reading!