max31855_json.c

#include <stdint.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <getopt.h>
#include <string.h>
#include <fcntl.h>
#include <sys/ioctl.h>
#include <linux/types.h>
#include <linux/spi/spidev.h>

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

static const char *device = "/dev/spidev1.0";
static uint8_t mode = SPI_MODE_0;
static uint8_t bits = 8;
static uint32_t clock = 500000;
static uint16_t delay = 1;


/*
 * Arg parsing only for device now
 */
static void parse_opts(int argc, char *argv[]) {
  while (1) {
    static const struct option lopts[] = {
      { "device",  1, 0, 'D' },
      { NULL, 0, 0, 0 }
    };

    int c = getopt_long(argc, argv, "D", lopts, NULL);
    if (c == -1) {
      break;
    }

    switch (c) {
    case 'D':
      device = optarg;
      break;
    default:
      printf("Usage: %s [-Dj]\n", argv[0]);
      puts(" -D --device   device to use (default /dev/spidev1.0)\n");
      exit(1);
      break;
    }
  }
}

/*
 * prepare the SPI bus
 */
static int prepare(int fd) {

  if (ioctl(fd, SPI_IOC_WR_MODE, &mode) == -1) {
    perror("Can't set mode");
    return -1;
  }

  if (ioctl(fd, SPI_IOC_WR_BITS_PER_WORD, &bits) == -1) {
    perror("Can't set number of bits");
    return -1;
  }

  if (ioctl(fd, SPI_IOC_WR_MAX_SPEED_HZ, &clock) == -1) {
    perror("Can't set write clock");
    return -1;
  }

  return 0;
}

/*
 * Get the compensated probe temperature for the raw value
 */
double probe_temp(uint32_t raw) {
  uint32_t masked = (raw >> 18) & 0x3FFF;
  if((masked & 0x2000) == 0x2000) {
    return ((~masked & 0x1FFF) + 1) * -1 * 0.25;
  }
  return (masked & 0x1FFF) * 0.25;
}

/*
 * Get the reference temperature for the raw value
 */
double ref_temp(uint32_t raw) {
  uint32_t masked = (raw >> 4) & 0xFFF;
  if((masked & 0x800) == 0x800) {
    return ((~masked & 0x7FF) + 1) * -1 * 0.0625;
  }
  return (masked & 0x7FF) * 0.0625;
}

/*
 * Read the temperature data from the MAX31855
 */
uint32_t readmax38155(int fd) {
  uint8_t tx[] = { 0xFF, 0xFF, 0xFF, 0xFF };
  uint8_t rx[ARRAY_SIZE(tx)] = {0, };

  struct spi_ioc_transfer tr = {
    .tx_buf = (unsigned long)tx,
    .rx_buf = (unsigned long)rx,
    .len = ARRAY_SIZE(tx),
    .delay_usecs = delay,
    .speed_hz = clock,
    .bits_per_word = bits,
  };

  if (ioctl(fd, SPI_IOC_MESSAGE(1), &tr) == 1) {
    perror("IO Error");
    abort();
  }

  return (rx[0] << 24) + (rx[1] << 16) + (rx[2] << 8) + rx[3];
}

int main(int argc, char **argv) {

  parse_opts(argc, argv);

  int fd = open(device, O_RDWR);
  if (fd <= 0) {
    printf("Device %s not found\n", device);
    return -1;
  }

  if (prepare(fd) == -1) {
    return -1;
  }

  uint32_t raw = readmax38155(fd);

  // no longer needed
  close(fd);

  // check flags for errors
  char errors[128] = "";
  if((raw & 0x10000) != 0) {
    if((raw & 0x1) != 0) {
      strcat(errors, "No connection. ");
    }
    
    if((raw & 0x2) != 0) {
      strcat(errors, "Thermocouple short to ground. ");
    }

    if((raw & 0x4) != 0) {
      strcat(errors, "Thermocouple short to VCC. ");
    }

    if(strlen(errors) == 0) {
      strcat(errors, "Unknown error.");
    }
  }

  // dump json
  printf("{\"probe_temp\":%.2f,\"ref_temp\":%.2f,\"errors\":\"%s\"}", 
    probe_temp(raw),
    ref_temp(raw), 
    errors);

  return 0;
}