AVR microcontroller timer 0 overflow interrupt program

       Chip name: AVR (Atmel) mega16A microcontroller chip; 
       Development board: self-developed YF-A1 chip development board; (YF: is the abbreviation of my name)

        In the 53rd test, I used a logic analyzer to capture the digital pulse signal output from the IO port. The pulse signal width was 1.5~1.8 microseconds. In the signal timing of every 9~11 pulses with a width of 1.5us, there was a high pulse signal with a pulse width of 3 microseconds.
       I spent several hours and couldn't calculate the internal clock frequency of timer 0... What went wrong?
      An external 12-mega crystal oscillator was used;
      my calculation conclusion was: the internal frequency of timer 0 was 32khz, but this was obviously problematic.
      The timer clock was internally divided by 8 TCCR0=0X02; the initial value of TCNTO was 250, that is, it overflowed once every 6 clock cycles, and every time it overflowed, the PB port level was inverted once, so the reverse calculation pulse width 1.5us ÷ 6 == each divided clock time ÷ 8 = clock frequency;

/*    
    It took me more than 3 hours to write this program, and I experimented repeatedly. God will not let down those who work hard,, hey╮(╯▽╰)╭,,,,,

       Program name: Timer 0 overflow interrupt

    Overview: The timer overflows once every millisecond. If the timer overflows 500 times, that is, 0.5 seconds, the LED light will jump once. I found that if
  all IO ports are not initialized, the chip will become extremely unstable and cannot work properly. Once the timer 0 overflow flag
  TOV0 overflows, it will be automatically cleared after entering the interrupt service program, and there is no need for software clearing. I seem to understand the meaning of
  hardware clearing;

       Finish time: February 28, 2014 20:40:21;

*/

#include
#include
#include

typedef unsigned char uint8;
typedef unsigned int uint16;

volatile uint16 a=0;

void io(void)
{
  DDRA=0XFF;
  PORTA=0X00;

  DDRB=0XFF;
  PORTB=0X00;

  DDRC=0XFF;
  PORTC=0X00;

  DDRD=0XFF;
  PORTD=0X00;

}

int main(void)
{

   io();
   
   TIMSK=0X01; // Overflow interrupt enable;

   sei(); //Global interrupt enable;

   TCNT0=69; //Timer initial value 1 millisecond overflow;

   TCCR0=3; //64-division timer starts running!;

   PORTB=4; //8-bit LED common anode enable;
  

   while(1); //infinite loop;
 

}

SIGNAL(SIG_OVERFLOW0) //Interrupt service routine;
{
   static uint16 s=0; //Static variable;
   
   TCNT0=69; //Reset the initial value of the timer;
 
    s++;       
   if(s==502) //The PA port level changes once every 500 milliseconds!
   { s=0;
     PORTA^=0XFF;
     
}
  
  
 // TIFR=0X01;//Clear the overflow flag and compare match flag of timer 0;

}