LM74 temperature sensor temperature reading and conversion. Implements negative and positive temperature.

LM74TemperatureSens.c

/*
 ============================================================================
 Name        : LM74TemperatureSens.c
 Author      : Matej Perejda
 Date	     : 27/09/2015
 Description : LM74 temperature sensor temperature reading and conversion.
        	Implements negative and positive temperature.
 ============================================================================
 */

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

#define ARRAY_SIZE(a) (sizeof(a) / sizeof((a)[0]))
#define LSB_CONST 0.0625

static void pabort(const char *s) {
	perror(s);
	abort();
}

static const char *device = "/dev/spidev0.0";
static uint8_t mode = SPI_MODE_0;
static uint8_t bits = 8;
static uint32_t speed = /*500000*/ 1000000;
static uint16_t delay;

float lastTemp = 0.00;

static void transfer(int fd) {
	int ret;

	uint16_t bitShift;
	int bitShift_int;
	uint16_t division;
	float temperature;

	// Sending bytes
	uint8_t tx[] = {0x00, 0x00};

	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 = speed,
		.bits_per_word = bits,
		// Clock switching from HIGH to LOW
		// Sensor activation
		.cs_change = 0,
	};

	ret = ioctl(fd, SPI_IOC_MESSAGE(1), &tr);
	if (ret < 1)
		pabort("can't send spi message");

	for (ret = 0; ret < ARRAY_SIZE(tx); ret++) {
		if (!(ret % 6)) { 
		//	puts("");
		// 	printf("%.2X ", rx[ret]);
		}
	}
	// puts("");


	// Bitshift from rx array
	uint16_t arrayBoth = ((rx[0] << 8) | rx[1]) ;

	// If first element in binary is 1 = negative value 
	if (rx[0] & 0x80) {
		// Inverse 2's complement
		// We have to subtract one 
		division = arrayBoth - 1;
		printf("Division: %02X\n", division);

		// Inverse hex
		division = ~division;
		printf("Inverse div: %02X\n", division);

		// Right bitshift 3 bits
		bitShift = division >> 3;

		// Int conversion
		bitShift_int = (int) bitShift;

		// Temperature conversion
		temperature = (float) -1 * (bitShift_int * LSB_CONST);
	} else {
		// Right bitshift to remove three last zeroes
		// 25 degrees: 0000 1100 1000 0000 => 0000 1100 1000 0
      		bitShift = arrayBoth >> 3;

		// Conversation from right bitshift to int value
       		bitShift_int = (int) bitShift;

		// Final temperature
    		temperature = bitShift_int * LSB_CONST;
	}

	if (lastTemp != temperature) {
	    	printf("Hex: %02X \n", arrayBoth);
		printf("Shift: %02X\n", bitShift);
    		printf("Int value: %d \n", bitShift_int);
		printf("Temperature: %.2f°C\n", temperature);

		lastTemp = temperature;
		puts("");
	}
}

int main(int argc, char *argv[])
{
	int ret = 0;
	int fd;

	fd = open(device, O_RDWR);
	if (fd < 0)
		pabort("can't open device");

	/*
	 * spi mode
	 */
	ret = ioctl(fd, SPI_IOC_WR_MODE, &mode);
	if (ret == -1)
		pabort("can't set spi mode");

	ret = ioctl(fd, SPI_IOC_RD_MODE, &mode);
	if (ret == -1)
		pabort("can't get spi mode");

	/*
	 * bits per word
	 */
	ret = ioctl(fd, SPI_IOC_WR_BITS_PER_WORD, &bits);
	if (ret == -1)
		pabort("can't set bits per word");

	ret = ioctl(fd, SPI_IOC_RD_BITS_PER_WORD, &bits);
	if (ret == -1)
		pabort("can't get bits per word");

	/*
	 * max speed hz
	 */
	ret = ioctl(fd, SPI_IOC_WR_MAX_SPEED_HZ, &speed);
	if (ret == -1)
		pabort("can't set max speed hz");

	ret = ioctl(fd, SPI_IOC_RD_MAX_SPEED_HZ, &speed);
	if (ret == -1)
		pabort("can't get max speed hz");

	// printf("spi mode: %d\n", mode);
	// printf("bits per word: %d\n", bits);
	// printf("max speed: %d Hz (%d KHz)\n", speed, speed/1000);

	while(1) {
		transfer(fd);
		sleep(1); // delay while printing
	}

	close(fd);

	return ret;
}