Simple SPIdev example to read the data from an MAX31855 thermocouple amplifier

spidev-max31855.c

/* Simple SPIdev example to read the data from an MAX31855 thermocouple amplifier */

#include <stdio.h>
#include <fcntl.h>
#include <stdlib.h>

#define ARRAY_SIZE(array) sizeof(array)/sizeof(array[0])

/* Dataformat:
 * D[31:18] These bits contain the signed 14-bit thermocouple temperature value in degrees Celsius * 4.
 * D17 Reserved, this bit always reads 0.
 * D16 Fault, this bit reads at 1 when any of the SCV, SCG, or OC faults are active. Default value is 0.
 * D[15:4] These bits contain the signed 12-bit value of the reference junction temperature in degrees Celsius * 4.
 * D3 Reserved, this bit always reads 0.
 * D2 This bit is a 1 when the thermocouple is short-circuited to VCC. Default value is 0.
 * D1 This bit is a 1 when the thermocouple is short-circuited to GND. Default value is 0.
 * D0 This bit is a 1 when the thermocouple is open (no connections). Default value is 0.
 */

/* Compile with 'gcc -o spidev-max31855 spidev-max31855.c' */

#define FAULT 1

#define SCV 4
#define SCG 2
#define OC  1

int main(int argc, char **argv) 
{
        int fd;
        int fault, scv, scg, oc;
        /* Read 4 bytes from the SPI bus */
        char buf[4];
        double celsius, internal_celsius;

        if (argc<2) {
                printf("Usage:\n%s [device]\n", argv[0]);
                exit(1);
        }
   
        fd = open(argv[1], O_RDWR);
        if (fd<=0) { 
                printf("%s: Device %s not found\n", argv[0], argv[1]);
                exit(1);
        }

        if (read(fd, buf, ARRAY_SIZE(buf)) != ARRAY_SIZE(buf))
                perror("SPI read Error");
        close(fd);

        /* Read the fault bit */
        fault = buf[1] & FAULT;

        /* If the fault bit is set read the fault conditions and print them */
        if(fault) {
                scv = buf[3] & SCV;
                scg = buf[3] & SCG;
                oc = buf[3] & OC;

                printf("Fault detected: scv: %X, scg: %X, oc: %X\n", scv, scg, oc);
        } else {
                celsius = ( buf[0] << 4 ) + ( buf[1] >> 2 ) / 4.0;
                printf("Thermocouple temperature: %.2f degrees Celsius\n", celsius);
        }

        internal_celsius = ( buf[2] << 2) + ( buf[3] >> 4 )  / 4.0;
        printf("Reference junction temperature: %.2f degrees Celsius\n", internal_celsius);

        return 0;
}