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Sensirion I2C STC3X embedded Library

This document explains how to set up a sensor of the STC3X family to run on an embedded device using the I²C interface.

Click here to learn more about the Sensirion STC3X sensor family.

The measured gas mixture depends on the STC3x product and configured binary gas. Please refer to the datasheet and API documentation to get a list of supported binary gases.

Supported sensor types

The following instructions and examples use a STC31-C.

Setup Guide

Connecting the Sensor

Your sensor has 4 different signals that need to be connected to your board: VDD, SCL, GND, SDA. Use the following pins to connect your STC3X:

The recommended voltage is 3.3V.

Configure the code

In order to use the provided code we need to adjust two files according to your platform.

Edit sensirion_i2c_hal.c

This file contains the implementation of the sensor communication, which depends on your hardware platform. We provide function stubs for your hardware's own implementation. Sample implementations are available for some platforms: sample-implementations. For Linux based platforms like Raspberry Pi you can just replace the unimplemented HAL template with the implementation in sample-implementations/linux_user_space/:

cp sample-implementations/linux_user_space/sensirion_i2c_hal.c ./

Edit sensirion_config.h

Skip this part for Linux based platforms since everything is already setup for this case.

Otherwise you need to check if the libraries <stdint.h> and <stdlib.h> are provided by your toolchain, compiler or system. If you have no idea on how to do that you can skip this step for now and come back when you get errors related to these names when compiling the driver. The features we use from those libraries are type definitions for integer sizes from <stdint.h> and NULL from <stdlib.h>. If they are not available you need to specify the following integer types yourself:

• int64_t = signed 64bit integer
• uint64_t = unsigned 64bit integer
• int32_t = signed 32bit integer
• uint32_t = unsigned 32bit integer
• int16_t = signed 16bit integer
• uint16_t = unsigned 16bit integer
• int8_t = signed 8bit integer
• uint8_t = unsigned 8bit integer

In addition to that you will need to specify NULL. For both we have a detailed template where you just need to fill in your system specific values.

Choose the i2c address to use with your product

The provided example is working with a STC31-C, I²C address 0x29. In order to use the code with another product or I²C address you need to change it in the call stc3x_init(ADDRESS) in stc3x_i2c_example_usage.c. The list of supported I²C-addresses is found in the header stc3x_i2c.h.

Now we are ready to compile and run the example usage for your sensor.

Compile and Run

Pass the source .c and header .h files in this folder into your C compiler and run the resulting binary. This step may vary, depending on your platform. Here we demonstrate the procedure for Linux based platforms:

1. Open up a terminal.
2. Navigate to the directory where this README is located.
3. Navigate to the subdirectory example-usage.
4. Run make (this compiles the example code into one executable binary).
5. Run the compiled executable with ./stc3x_i2c_example_usage
6. Now you should see the first measurement values appear in your terminal. As a next step you can adjust the example usage file or write your own main function to use the sensor.

Compile and Run Tests

The testframekwork used is CppUTest. Pass the source .cpp, .c and header .h files from the tests and top level folder into your CPP compiler and run the resulting binary. This step may vary, depending on your platform. Here we demonstrate the procedure for Linux based platforms:

1. Open up a terminal.
2. Install CppUTest framework apt install cpputest.
3. Navigate to the directory tests.
4. Run make (this compiles the test code into one executable binary).
5. Run the compiled executable with ./stc3x_test_hw_i2c.
6. Now you should see the test output on your console.

Background

Files

sensirion_i2c.[ch]

In these files you can find the implementation of the I2C protocol used by Sensirion sensors. The functions in these files are used by the embedded driver to build the correct frame out of data to be sent to the sensor or receive a frame of data from the sensor and convert it back to data readable by your machine. The functions in here calculate and check CRCs, reorder bytes for different byte orders and build the correct formatted frame for your sensor.

sensirion_i2c_hal.[ch]

These files contain the implementation of the hardware abstraction layer used by Sensirion's I2C embedded drivers. This part of the code is specific to the underlying hardware platform. This is an unimplemented template for the user to implement. In the sample-implementations/ folder we provide implementations for the most common platforms.

sensirion_config.h

In this file we keep all the included libraries for our drivers and global defines. Next to sensirion_i2c_hal.c it's the only file you should need to edit to get your driver working.

sensirion_common.[ch]

In these files you can find some helper functions used by Sensirion's embedded drivers. It mostly contains byte order conversions for different variable types. These functions are also used by the UART embedded drivers therefore they are kept in their own file.

Contributing

Contributions are welcome!

We develop and test this driver using our company internal tools (version control, continuous integration, code review etc.) and automatically synchronize the master branch with GitHub. But this doesn't mean that we don't respond to issues or don't accept pull requests on GitHub. In fact, you're very welcome to open issues or create pull requests :)

This Sensirion library uses clang-format to standardize the formatting of all our .c and .h files. Make sure your contributions are formatted accordingly:

The -i flag will apply the format changes to the files listed.

clang-format -i *.c *.h

Note that differences from this formatting will result in a failed build until they are fixed.

License

See LICENSE.