[FAQ] TMP112: Need Linux C code for I2C sensor

This example for TI I2C Temperature Sensors is based on examples in the Linux kernel documentation:

https://docs.kernel.org/i2c/dev-interface.html

https://docs.kernel.org/i2c/smbus-protocol.html

This is userspace code that accesses a /dev/i2c device, but it could be adapted to kernel driver code.

Use your package manager, such as apt, to install libi2c-dev and compile using the -li2c flag. 

This was tested on a RaspberryPi 4 using a TMP112, but is compatible with any 75-like temperature sensor that has 12-bit Q4 encoding.

Note that this code makes use of SMBus Read Word, which has the wrong endianness for most sensors. This code uses __builtin_bswap16() to perform the byte swap. 

#include <fcntl.h>
#include <i2c/smbus.h>
#include <linux/i2c-dev.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/ioctl.h>
#include <unistd.h>

int main() {
  int file;
  int adapter_nr = 1; /* /dev/i2c- number */
  char filename[20];
  int addr = 0x49; /* Sensor I2C address */
  __u8 reg = 0x0;  /* Temperature register address */
  __s32 res;

  snprintf(filename, 19, "/dev/i2c-%d", adapter_nr);
  file = open(filename, O_RDWR);
  if (file < 0) {
    printf("Failed to open %s\n", filename);
    exit(1);
  }

  if (ioctl(file, I2C_SLAVE, addr) < 0) {
    printf("Failed to set address\n");
    exit(1);
  }

  /* need to fetch 2 bytes (16 bits) */
  res = i2c_smbus_read_word_data(file, reg);
  if (res < 0) {
    printf("smbus_read failed\n");
  } else {
    /* SMBus word is LSByte first, so we must change endianness */
    int16_t word = __builtin_bswap16(res);
    /* print result in hex */
    printf("0x%X\n", word);
    /* print result in degrees c */
    printf("%f\n", (float)(word >> 4) * 0.0625f);
  }

  close(file);
}