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How to Use ADC on TorizonCore

 

Article updated at 03 Dec 2020
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Select the version of your OS from the tabs below. If you don't know the version you are using, run the command cat /etc/os-release or cat /etc/issue on the board.

Torizon 5.0.0

Introduction

This article will demonstrate how to access the Analog-to-Digital Converter (ADC) through the sysfs interface in a container application in TorizonCore.

This article complies to the Typographic Conventions for Torizon Documentation.

Prerequisites

ADC accessibility in User Space

All Toradex Computer-on-Modules uses a driver based on the Industrial Input/Output (iio) framework.

In contrast to hwmon, which is directed at low sample rate sensors used to monitor and control the system itself, like fan speed control or temperature measurement, the iio framework provides support for devices that in some sense perform either analog-to-digital conversion (ADC) or digital-to-analog conversion (DAC), or both.

There are two ways for userspace interaction with iio drivers:

  • /sys/bus/iio/iio:deviceX/, this represents a hardware sensor and groups together the data channels of the same chip.
  • /dev/iio:deviceX, character device node interface used for buffered data transfer and for events information retrieval.

If the conversion is needed to be performed at slow rates, also called one-shot conversion, sysfs interface is used.

If Continuous conversions of a single channel at a time are required and supported by the device, the dev interface is used with sysfs.

Sample Application

Toradex has provided a sample code written in C at our GitHub repository for the ADC Sample with TorizonCore.

It demonstrates the use of iio_utils.c and iio_utils.h from Linux Kernel tool for iio interface.

To evaluate the sample application, you can clone the Torizon Samples repository on your PC:

$ git clone https://github.com/toradex/torizon-samples/tree/bullseye/adc

The sample code will get data from input channel-1. If required, the input channel number can be changed in samples/adc/adc/adc.c modifying the variable dev_num.

Once you have cloned the Torizon Samples repository, go to the adc directory inside samples and execute the following command to build an image for arm32v7:

$ docker build . -t adc-sample

To build an image for arm64v8 you can use the following command:

$ docker build . --build-arg CROSS_TC_IMAGE_ARCH=arm64 --build-arg ARCH_ARG=linux/arm64 --build-arg GCC_PREFIX=aarch64-linux-gnu -t adc-sample 

Both commands build the Dockerfile present in the sample project.

The build of our sample container for ADC demonstration is a two-stage build process, in which the first image is based on debian-cross-toolchain-$CROSS_TC_IMAGE_ARCH to cross-compile the sample application on the host machine, i.e. Desktop PC. In the second stage, the resulting binary is copied from the first stage image to the /usr/local/bin of the final image (second stage). This will produce the final application container in a small and deployable image with the tag adc-sample. Now this container image can be deployed on the target machine.

You can deploy the container image in a portable tar archive file executing the following command:

$ docker save -o adc-image.tar adc-sample

Now you can copy it to the target machine:

$ scp adc-image.tar torizon@X.X.X.X:/home/torizon/

Running the Sample

In the target device we can use docker to load the image directly from its tar file, by using the following command:

# docker load -i adc-image.tar

After this, the image can be executed by using the following docker command:

# docker run -it --rm adc-sample

In case where settings of ADC is needed to be set, like for continuous conversions mode, /sys can be mounted for docker:

# docker run -it --rm -v /sys:/sys adc-sample

As a result, it shows raw input channel value and also converts it to a voltage by using the formula:

VIN = VREAD * Scale

Access Using Command-Line

All the Analog-to-Digital Converter (ADC) information can be read and set using the iio interface exposed through sysfs. Check the sequence of commands below, showing a listing, a reading of an analog voltage value, and the voltage scale.

# cd /sys/bus/iio/devices/iio\:device1
# ls -la in*
-rw-r--r-- 1 root root 4096 Dec  4 02:14 in_voltage-voltage_scale
-rw-r--r-- 1 root root 4096 Dec  4 02:14 in_voltage0-voltage1_raw
-rw-r--r-- 1 root root 4096 Dec  3 22:23 in_voltage0_raw
-rw-r--r-- 1 root root 4096 Dec  4 02:14 in_voltage1-voltage0_raw
-rw-r--r-- 1 root root 4096 Dec  4 02:14 in_voltage1_raw
-rw-r--r-- 1 root root 4096 Dec  4 02:14 in_voltage2-voltage3_raw
-rw-r--r-- 1 root root 4096 Dec  4 02:14 in_voltage2_raw
-rw-r--r-- 1 root root 4096 Dec  4 02:14 in_voltage3-voltage2_raw
-rw-r--r-- 1 root root 4096 Dec  4 02:14 in_voltage3_raw
-rw-r--r-- 1 root root 4096 Dec  3 22:23 in_voltage_scale

# cat in_voltage3_raw
821

# cat in_voltage_scale
2.000000000

As you could observe, knowing the path of the device and its channel you can simply use file manipulation to read (or write) the data you want.

The sample application we provided is using C language, but you can easily create a Python or .Net application to do the same.

Warning: ADC channels availability on Toradex modules and its usage are described in ADC-Linux article.

Developing a C Application for ADC reading in Visual Studio Code

You can easily develop the same ADC application we did in the sample using Visual Studio Code with Torizon Extension.

For that, please make sure to have followed the Torizon Quickstart Guide lessons and to have Configured the Build Environment for Torizon Containers.

In the Visual Studio Code with the Torizon Extension, please follow the steps below:

  1. Press F1, and choose option Torizon/C-C++: Create C/C++ application
  2. Write the application name (in our case, we are going to call it adc_sample)
  3. Choose the Project Directory
  4. Select a template for the project, in this example, Makefile-based Project
  5. Select the target platform for it, which can be debian arm32v7 bullseye or debian arm64v8 bullseye, depending on your target's architecture.
  6. Write torizon as the username for the application
  7. Select debug as configuration.

And that's it for the creation of our project. The Torizon Extension may take a while after the setup of the project, so be patient.

You can check these steps in detail in our article about programming and debugging a sample C++ application with Visual Studio Code and Torizon Extension.

When the environment is ready, next step is to "transplant" the sources from the ADC sample. Please, make the following:

  • Copy iio_utils.c file to the project folder.
  • Copy iio_utils.h file to the project folder.
  • Make the content of adc.c file to the original main C application the project has created.
  • Adjust the Makefile so it is as the Makefile in the sample project from Torizon Samples.

See how our project will look like after these changes:


  • Sample C Project For ADC In VS Code With Torizon Extension

    Sample C Project For ADC In VS Code With Torizon Extension

After that, press F5 to start the Debug session of the Torizon Extension. It will build and deploy a debug container of the application in the target, and once started the debugger tools, you'll be able to see and monitor the application execution in the Visual Studio Code at your machine. It's also possible to add breakpoints, watch variables content, among other things.

See below a screenshot of the debug session of our sample application:


  • Debug Of Sample C Project For ADC In VS Code With Torizon Extension

    Debug Of Sample C Project For ADC In VS Code With Torizon Extension