#include <avr/io.h>
#include <avr/interrupt.h>
volatile register unsigned long count_ms asm("r4");
volatile register unsigned char flag_ms asm("r8");
/////////////////////////////////////////////////////////////////////////////////////////////////
void timer_ms(unsigned long value)
{
    count_ms = value;
    flag_ms = 0;
    while(flag_ms == 0);
}
/////////////////////////////////////////////////////////////////////////////////////////////////
ISR(TIM0_COMPA_vect)
{
    if(count_ms){--count_ms;}
    else{flag_ms = 1;}
}
/////////////////////////////////////////////////////////////////////////////////////////////////
int main(void)
{
    TCCR0A = (0<<COM0A1)|(0<<COM0A0)|(0<<COM0B1)|(0<<COM0B0)|(1<<WGM01)|(0<<WGM00);    //CTC
    TCCR0B = (0<<FOC0A)|(0<<FOC0B)|(0<<WGM02)|(0<<CS02)|(1<<CS01)|(0<<CS00);        // FCPU/8
    OCR0A = 149;
    TIMSK0 = (0<<OCIE0B)|(1<<OCIE0A)|(0<<TOIE0);
    sei();
    
    
    while(1){
        timer_ms(100);
        PORTB ^= 0x01;
    }
}