LM74.c

#define F_CPU 16000000UL

#include <avr/io.h>
#include <util/delay.h>
#include "usart/usart.h"
#include <stdlib.h>

uint8_t SPI(uint8_t data);

int main(void)
{
    USART_init();
    DDRB = (1<<PB5) | (1<<PB3) | (1<<PB2); // SCK, MOSI, SS = OUTPUT -- MISO = INPUT
    SPCR = (1<<SPE) | (1<<MSTR) | (1<<CPHA) | (1<<SPR1); // Enable SPI, Master, Mode 1, fosc/64
    PORTB = (1<<PB2); // SS HIGH
    
    while(1)
    {   
        uint8_t MSB = 0, LSB = 0, DECI,temperature = 0;
        float temp;

        PORTB &= ~(1<<PB2); // SS LOW
        
        MSB = SPI(0x00);
        LSB = SPI(0x00);
        temperature = (MSB<<1) | (LSB>>7);
        
        DECI = ((LSB>>3) & (~(1<<5)));
        
        temp = temperature + (DECI*0.0625);

        PORTB |= (1<<PB2); // SS HIGH
        
        char str[20];

        
        dtostrf(temp, 0, 3, str);
    
        USART_putstring(str);
        USART_send('\n');
       
        _delay_ms(1000);
   }
}

uint8_t SPI(uint8_t data)
{
    SPDR = data;
    while(!(SPSR &(1<<SPIF)));
    return SPDR;
}