Internal Temp Arduino - c

Question

Summary: Internal temp having trouble connecting to the right channel.
Want to get right value from the channel.
#include <avr/io.h>
#include <avr/pgmspace.h>
#include <util/delay.h>
#include <avr/interrupt.h>
uint8_t ReADC()//read value ad
{
ADMUX = (1 << MUX3) | (1 << REFS0) | (1 << REFS1) | (1 << ADLAR);
//single conversion
ADCSRA |= (1 << ADEN) | (1<<ADSC) | (1 << ADPS2);
ISR(TIMER0_COMPA_vect){
ADC();
}
void SPI_MASTER(){
DDRB = (1 << PB0) | (1 << PB3) | (1 << PB5);
SPCR = (1 << CPOL) | (1 << MSTR) | (1 << SPR0) | (1 << SPI2X) | (1 << SPE);
}
}

Answer 1

I agree with comments — you should read DS, ADC Conversion Result and Temperature Measurement sections.
But there is a bug in measurement process (ADC usage issue) that prevent you from reading right ADC result for temperature calculation.
See:
uint8_t ReadADC()
{
ADMUX = (1 << MUX3) | (1 << REFS0) | (1 << REFS1) | (1 << ADLAR); // (1)
ADCSRA |= (1 << ADEN) | (1<<ADSC) | (1 << ADPS2) | (1 << ADPS1)
| (1 << ADPS0) | (1 << ADIE);
while( ADCSRA & (1<<ADSC) );
return ADC; // (2)
}
You (1) select ADLAR, so ADC result is shifted toward high bit of 16-bit ADCH:ADCL pair. But then (2) you read the 16-bit value and convert it into 8-bit value by throwing out high 8 bits. As a result, you get two least significant bits of ADC result in two most significant bits of returned value. So, these bits can encode 0, 64, 128, 192 only.
uint8_t SPI_transmit_data()
{
PORTB &= ~(1 << PB0);
SPDR = ADC; // (2)
while(!SPSR & (1<<SPIF));
PORTB |= (1 << PB0);
return(SPDR);
}
The same is here, the lowest 8 bits of ADC result is writed into 8-bit SPDR and higher 8 bits are thrown out.
Use ADCH instead of ADC