sorry correction
كود:
void interrupt(){
x++;
if(x==500){y++;x=0;}
if(y==15){
PORTB=0;
y=0;
}
(in case of using 4MHz crystal the time of every instruction is 1

s (frequency/4
so timer0 takes 125

s to count from 131 to 256(125=256-131)
(and if we use prescaler= 16 the interrupt hapen evry 2ms (125

s x 16
so x increase by 1 every 2ms and y increase by 1 if x = 500 so y increase every 1s
and if y=15(after 15 second) PORTb will turn off