Example of reading from Si7021 with Raspberry Pi

si7021.c

#include <stdio.h>
#include <stdlib.h>
#include <bcm2835.h>
#include <string.h>

/*+---------------------------------------------------------+
  |                                                         |
  |     si7021.c - Read Si7021 temperature and humidity     |
  |     over I2C bus using Raspberry Pi                     |
  |                                                         |
  |     Requires the bcm2835 library.                       |
  |                                                         |
  |                                                         |
  +---------------------------------------------------------+
 */

#define SI7021_ADDR 0x40

//  I2C COMMANDS
#define SI7021_MRH_HOLD     0xE5
#define SI7021_MRH_NOHOLD   0xF5
#define SI7021_MT_HOLD      0xE3    //  measure temp, hold master
#define SI7021_MT_NOHOLD    0xF3    //  measure temp, no hold master
#define SI7021_RT_PREV      0xE0    //  read temp from last RH measurement
#define SI7021_RESET        0xFE    //  reset
#define SI7021_WR_USER1     0xE6    //  write RH/T user register 1
#define SI7021_RD_USER1     0xE7    //  read RH/T user register 1
#define SI7021_WR_HCTL      0x51    //  write heater control register
#define SI7021_RD_HCTL      0x11    //  read heater control register
#define SI7021_RD_ID1       0xFA 0x0F   //  read electronic ID 1st byte
#define SI7021_RD_ID2       0xFC 0xC9   //  read electronic ID 2nd byte
#define SI7021_RD_REV       0x84 0xB8   //  read firmware revision

#define SI7021_OK           0x00		//	return code OK
#define SI7021_FAIL         0x01		//	return code fail

void setTimeout(uint16_t timeout) {
     volatile uint32_t* stimeout = bcm2835_bsc1 + BCM2835_BSC_CLKT / 4;
     bcm2835_peri_write(stimeout, timeout);
}

//
//  Read the current temperature
//
float readTemperature(uint8_t *status) {
    uint8_t buf[4] = { SI7021_MT_HOLD };
    if( bcm2835_i2c_read_register_rs(buf,buf,3) != BCM2835_I2C_REASON_OK ) {
        *status = SI7021_FAIL;
        return 0.0;
    }
    uint8_t msb = buf[0];
    uint8_t lsb = buf[1];
    unsigned int data16 = ((unsigned int) msb << 8) | (unsigned int) (lsb & 0xFC);
    float temp = (float) (-46.85 + (175.72 * data16 / (float) 65536));
    *status = SI7021_OK;
    return temp;
}

//
//  Read the humidity
//
float readHumidity() {
    uint8_t buf[4] = { SI7021_MRH_NOHOLD };
    bcm2835_i2c_write(buf,1);
    while( bcm2835_i2c_read(buf,3) == BCM2835_I2C_REASON_ERROR_NACK ) {
        bcm2835_delayMicroseconds(500);
    }
    uint8_t msb = buf[0]; uint8_t lsb = buf[1];
    uint16_t data16 = ((unsigned int) msb << 8) | (unsigned int) (lsb & 0xFC);
    float hum = -6 + (125.0 * (float) data16) / 65536;
    return hum;
}

//
//  Read the device serial number and return as string
//
uint8_t readSerialNumber(char *instr) {
    uint8_t buf[8] = {0xFA,0x0F};
    char *str = (char *) malloc(25);
    char *str2 = (char * ) malloc(13);
    if( bcm2835_i2c_write(buf,2) != BCM2835_I2C_REASON_OK ) {
        return SI7021_FAIL;
    }
    if( bcm2835_i2c_read(buf,8) != BCM2835_I2C_REASON_OK ) {
        printf("Read failed\n" );
        return SI7021_FAIL;
    }
    sprintf(str,"%02X %02X %02X %02X ",buf[0],buf[2],buf[4],buf[6]);
    buf[0] = 0xFC; buf[1] = 0xC9;
    bcm2835_i2c_write(buf,2);
    bcm2835_i2c_read(buf,8);
    sprintf(str2,"%02X %02X %02X %02X\0",buf[0],buf[2],buf[4],buf[6]);
    strcpy(instr, strcat(str,str2));
    return SI7021_OK;
}

int main(int argc, char** argv) {
    if (!bcm2835_init())
        return 1;
    bcm2835_i2c_begin();
    bcm2835_i2c_setSlaveAddress(SI7021_ADDR);
    bcm2835_i2c_setClockDivider(BCM2835_I2C_CLOCK_DIVIDER_626);
    setTimeout(40000);
    uint8_t status;
    float temp = readTemperature(&status);
    printf("Temperature %f C \n\r", temp);

    float humidity = readHumidity();
    printf("Humidity = %f%%\n",humidity );

    //  read the device serial number
    char * sstr = (char *) malloc(26);
    readSerialNumber(sstr);
    printf("%s\n",sstr);

    bcm2835_i2c_end();
    return (EXIT_SUCCESS);
}