pressure.c

/*
 *  C code to read pressure and temperature from the
 *  Raspberry Pi Sense HAT add-on board (LPS25H sensor)
 *
 *  sudo raspi-config --> interfacing options --> enable i2c
 *
 *  sudo apt install libi2c-dev
 *
 *  Build with:   gcc -Wall -O2 pressure.c -o pressure -li2c
 *                or just 'make'
 *
 *  Tested with:  Sense HAT v1.0 / Raspberry Pi 3 B+ / Raspbian GNU/Linux 10 (buster)
 *
 */

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

#define DEV_ID 0x5c
#define DEV_PATH "/dev/i2c-1"
#define WHO_AM_I 0x0F
#define CTRL_REG1 0x20
#define CTRL_REG2 0x21
#define PRESS_OUT_XL 0x28
#define PRESS_OUT_L 0x29
#define PRESS_OUT_H 0x2A
#define TEMP_OUT_L 0x2B
#define TEMP_OUT_H 0x2C

void delay(int);

int main(void) {
    int fd = 0;
    uint8_t temp_out_l = 0, temp_out_h = 0;
    int16_t temp_out = 0;
    double t_c = 0.0;

    uint8_t press_out_xl = 0;
    uint8_t press_out_l = 0;
    uint8_t press_out_h = 0;

    int32_t press_out = 0;
    double pressure = 0.0;

    uint8_t status = 0;

    /* open i2c comms */
    if ((fd = open(DEV_PATH, O_RDWR)) < 0) {
        perror("Unable to open i2c device");
        exit(1);
    }

    /* configure i2c slave */
    if (ioctl(fd, I2C_SLAVE, DEV_ID) < 0) {
        perror("Unable to configure i2c slave device");
        close(fd);
        exit(1);
    }

    /* check we are who we should be */
    if (i2c_smbus_read_byte_data(fd, WHO_AM_I) != 0xBD) {
        printf("%s\n", "who_am_i error");
        close(fd);
        exit(1);
    }

    /* Power down the device (clean start) */
    i2c_smbus_write_byte_data(fd, CTRL_REG1, 0x00);

    /* Turn on the pressure sensor analog front end in single shot mode  */
    i2c_smbus_write_byte_data(fd, CTRL_REG1, 0x84);

    /* Run one-shot measurement (temperature and pressure), the set bit will be reset by the
     * sensor itself after execution (self-clearing bit)
     */
    i2c_smbus_write_byte_data(fd, CTRL_REG2, 0x01);

    /* Wait until the measurement is complete */
    do {
        delay(25); /* 25 milliseconds */
        status = i2c_smbus_read_byte_data(fd, CTRL_REG2);
    } while (status != 0);

    /* Read the temperature measurement (2 bytes to read) */
    temp_out_l = i2c_smbus_read_byte_data(fd, TEMP_OUT_L);
    temp_out_h = i2c_smbus_read_byte_data(fd, TEMP_OUT_H);

    /* Read the pressure measurement (3 bytes to read) */
    press_out_xl = i2c_smbus_read_byte_data(fd, PRESS_OUT_XL);
    press_out_l = i2c_smbus_read_byte_data(fd, PRESS_OUT_L);
    press_out_h = i2c_smbus_read_byte_data(fd, PRESS_OUT_H);

    /* make 16 and 24 bit values (using bit shift) */
    temp_out = temp_out_h << 8 | temp_out_l;
    press_out = press_out_h << 16 | press_out_l << 8 | press_out_xl;

    /* calculate output values */
    t_c = 42.5 + (temp_out / 480.0);
    pressure = press_out / 4096.0;

    /*output */
    printf("Temp (from press) = %.2f°C\n", t_c);
    printf("Pressure = %.0f hPa\n", pressure);

    /* Power down the device */
    i2c_smbus_write_byte_data(fd, CTRL_REG1, 0x00);

    close(fd);

    return (0);
}

void delay(int t) {
    usleep(t * 1000);
}