...
static volatile unsigned  int value_ADC;
...

void adc_init(void)
{
   ADMUX = (1<<REFS0)|(1<<MUX3)|(1<<MUX2)|(1<<MUX1);
   ADCSRB = 0x00;
   ADCSRA = (1<<ADPS2)|(1<<ADPS0);
}


main(void)
{    
   unsigned char V_BAT;
   float f_bat;
   ...
   adc_init();
   ...  

   while(1)
   {  
       ...
       ADCSRA |= (1<<ADEN);
      
       ADCSRA|= (1<<ADSC);
       while((ADCSRA&(1<<ADSC))!=0);
       value_ADC= ADC;
       f_bat=(1.1*1024/value_ADC);
       V_BAT=(f_bat*100);    
      
       ADCSRA &= ~(1<<ADEN);        
    
      ...
      asm("sleep");                         //выход по прерыванию INT0
      ...

   }
}