Hii ..
i have a problem in my code below .. that i can't read the ADC value from the ADC ( or ADCW ) register but when i read the ADCH it works fine ! ?

//####### Prototype Area ######### 
#include <avr/io.h>
#include <avr/interrupt.h>
#include <util/delay.h>
#define  F_CPU 1000000UL


//####### Functions###############

void InitADC() ; 
uint16_t ReadADC() ;

int main(void)
{
	char segment[10]={0x3F,0x06,0x5B,0x4F,0x66,0x6D,0x7D,0x07,0x7F,0x6F}; //7-Segment Numbers Codes in hexadecimal
	DDRB=0xFF;//PORT B Output
	DDRD=0xFF;//PORT D Output
	
	sei();
	InitADC();
	
	while(1)
	{

		// Temperature Calculations 
		uint8_t temp=ReadADC(); 
		uint8_t first= temp/10;
		uint8_t second= temp-first*10;
		
		PORTB=segment[first];//first digit
		PORTD=segment[second];//last digit
		
		_delay_ms(250); //wait
		
		ADCSRA |= 1<<ADSC;
	}
}

void InitADC()  
{
	ADMUX=(1<<REFS0)|(1<<REFS1);// AREF=internal 2.56v
	
	ADCSRA=(1<<ADEN)|(1<<ADPS0)|(1<<ADPS1)|(1<<ADPS2);
} 

uint16_t ReadADC()
{
	//Select ADC Channel
	ADMUX = 0xE0;

	//Start Single conversion
	ADCSRA|=(1<<ADSC);

	//Wait for conversion to complete ADSC=0
	while(!(ADCSRA & (1<<ADIF)));

	//Clear ADIF by writing one to it
	ADCSRA|=(1<<ADIF);

	return(ADCH);
}


//   int (25/10) = 2  >> First
//   25-2*10= 5 >> Second
// 7-segment display >>  "First Second"

 

ADC and ADCW are probably not defined addresses.

If ADCH works, why worry?

Note that it is up to the compiler to generate the correct sequence of 8-bit access when trying to access 16-bit registers.

You could examine the disassembled code and see what asm code is generated. Post it here for my interest.

 

Try reading the ADC register before clearing the ADCSRA|=(1<<ADIF); flag

uint8_t value = ADC;
ADCSRA|=(1<<ADIF);

return value;