ATmega16L-Timer Digital Tube Scanning

/****************************************
Use timer to maintain digital tube display
Now the digital tube display is increasing.
******************************************/
#include 

unsigned int cnt;
unsigned char shu[8];
unsigned char aa;
const unsigned char duanma[]={0x3f,0x06,0x5b,0x4f,0x66,0x6d,0x7d,0x07,
                              0x7f,0x6f,0x77,0x7c,0x39,0x5e,0x79,0x71};
const unsigned char weima[]={0xfe,0xfd,0xfb,0xf7,0xef,0xdf,0xbf,0x7f};

/******************************************
             Data conversion subfunction
***************************************/
void change(unsigned int i)
{
 shu[0]=duanma[i/1000];
 shu[1]=duanma[i/100%10];
 shu[2]=duanma[i%100/10];
 shu[3]=duanma[i%10];
}

/****************************************
               Delay function
******************************************/
void delay(unsigned int t)
{
   unsigned int x,y;
   for(x=t;x>0;x--)
    for(y=100;y>0;y--);
}

/****************************************
               Display Function
******************************************/
void display(unsigned char i)
{
  PORTB = 0X00;
  PORTA|=0X01;
  PORTA&=0XFE;
  
  PORTB =weima[i];
  PORTA|=0x02;
  PORTA&=0XFD;
  
  PORTB=shu[i];
  PORTA|=0X01;
  PORTA&=0XFE;
}

/******************************************
             Port initialization
***************************************/
void init_port(void)
{
 PORTA=0XFF;
 DDRA=0XFF;
 PORTB=0XFF;
 DDRB=0XFF;
}

/******************************************
             Timer 0 initialization
***************************************/
void timer0_init(void)
{
 TCNT0=0X83; //1MS timing initial value
 TCCR0=0X03; //The timer count pre-division frequency is 64
}

/******************************************
             Timer 0 service function
***************************************/
#pragma interrupt_handler timer0_ovf_isr:10
void timer0_ovf_isr(void)
{
 TCNT0=0X83; //Reinstall the initial timing value of 1ms
 if(++aa>3) aa=0;
  switch(aa)
  {
    case 0: display(0);break;
 case 1: display(1);break;
 case 2: display(2);break;
 case 3: display(3);break;
 default:break;
  }
}

void main(void)
{
 init_port(); //IO port initialization
 timer0_init(); //Timer 1 initialization
 TIMSK=0X01; //Enable T0 interrupt
 SREG=0X80; //Enable general interrupt
 while(1)
 {
  cnt++;
  delay(1000);
  change(cnt);
 }
}