Once the drivers are loaded you will discover that there is a new directory in hwmon:

You can see that temp1_input is going to be the file to read to get the current temperature:

int main(int argc, char **argv)
{
   checkLM75();
    int fd = open("/sys/class/hwmon/hwmon2/temp1_input",
                                             O_RDONLY);
    char buf[100] = {0};
    read(fd, buf, 100);
    printf("%s\n\r", buf);
    float temp;
    sscanf(buf, "%f", &temp);
    temp = temp / 1000;
    printf("%f\n\r", temp);
    close(fd);
}

The temperature is returned as a string in millidegree Celsius.

If you want to set the critical temperature and hysteresis you can use:

    fd = open("/sys/class/hwmon/hwmon2/temp1_max",
                                               O_RDWR);
    char max[] = "20000";
    write(fd, max, 5);
    close(fd);
    fd = open("/sys/class/hwmon/hwmon2/temp1_max_hyst",
                                               O_RDWR);
    char min[] = "190000";
    write(fd, max, 5);    
    close(fd);

This sets the critical temperature to 20°C and the hysteresis to 19°C. After this, if the temperature goes above 20°C the LED on the board will come on and stay on until the temperature drops below 19°C. Notice that you need to run this program with root permissions as it is writing to files in /sys.

In chapter but not in this extract:

• Industrial I/O

• An Example - The HTU21

• The IIO Utilities

• The Libiio Library

Summary

• The IIO and Hwmon systems are attempt to create drivers that present a standard interface, irrespective of the way that the devices are actually interfaced to the machine.

• Hwmon is the older system and was originally intended only for devices that are built into the system rather than discrete devices connected via external buses.

• The LM75 Temperature Sensor is often found built in to monitor hardware operating conditions, but it can also be connected via I2C and interfaced using the hwmon driver.

• Industrial I/O (IIO) was invented as an extension of Hwmon to create something that could support a wide range of sensors.

• IIO has many sophisticated features including triggers that can be used to make regular measurements in kernel space – most drivers don’t support this.

• The HTU21 introduced in Chapter 12 has an IIO driver.

• There are a set of IIO utilities that you can install but again due to limited driver support many features don’t work.

• There is also a more sophisticated library, Libiio, which makes IIO devices easier to work with, but lack of driver support makes it less attractive than working directly with devices.

• Harry Fairhead has a hardware background having worked with microprocessors and electronics in general, for many years.  Author of many books his latest are Programming the Raspberry Pi Pico in C and Programming the Raspberry Pi Pico in MicroPython. See his other titles in the sidebar.

• Harry Fairhead has a hardware background having worked with microprocessors and electronics in general, for many years.  Author of many books his latest are Programming the Raspberry Pi Pico in C and Programming the Raspberry Pi Pico in MicroPython. See his other titles in the sidebar.

Raspberry Pi IoT In C Using Linux Drivers

By Harry Fairhead

Buy from Amazon.

Contents

1.  Choosing A Pi For IoT

2. C and Visual Studio Code

3.  Drivers: A First Program

4.  The GPIO Character Driver
        Extract: GPIO Character Driver

5. GPIO Using I/O Control

6.  GPIO Events

7.  The Device Tree
       Extract: The DHT22
   

8.  Some Electronics

9.  Pulse Width Modulation
   Extract:  The PWM Driver 

10. SPI Devices
    Extract: The SPI Driver 

11. I2C Basics

12. The I2C Linux Driver ***NEW!

     

13. Advanced I2C

14. Sensor Drivers – Linux IIO & Hwmon
      Extract: Hwmon  

15. 1-Wire Bus
      Extract: 1-Wire And The DS18B20 

16. Going Further With Drivers

17. Appendix I

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