定时公式：Time=PRE*(MAX-TCNT0+1)/F_cpu单位S ，其中，PRE为与分频数

#include
#include
void port_init(void)
{
PORTA = 0xFF;
DDRA  = 0x00;
PORTB = 0xFF;
DDRB  = 0xFF;
PORTC = 0xFF; //m103 output only
DDRC  = 0x00;
PORTD = 0xFF;
DDRD  = 0x00;
}
//TIMER0 initialize - prescale:8/256
// WGM: Normal
// desired value: 1mSec/20ms
// actual value:  1.000mSec (0.0%)/19.968ms(0.16%)
void timer0_init(void)
{
TCCR0 = 0x00; //stop
//TCNT0 = 0x83; //set count T=PRE*(MAX-TCNT0+1)/F_cpu=8*（255-130）/1MHz=1ms
TCNT0 = 0xB2; //set count T=PRE*(MAX-TCNT0+1)/F_cpu=256*（255-178+1）/1MHz=19.968ms
OCR0  = 0x7D;  //set compare   程序中未使用
//TCCR0 = 0x02; //start timer  8分频
TCCR0 = 0x04; //start timer  256分频
}
#pragma interrupt_handler timer0_ovf_isr:iv_TIM0_OVF
void timer0_ovf_isr(void)
{
//TCNT0 = 0x83; //reload counter value 重载TCNT0，使TC0重复从0x83-0xff计数
TCNT0 = 0xB2; //reload counter value 重载TCNT0，使TC0重复从0xB2-0xff计数
PORTB^=BIT(7)|BIT(6)|BIT(5);//翻转PB6/PB7口，实现两个LED灯的1ms间隔亮灭
}
//call this routine to initialize all peripherals
void init_devices(void)
{
//stop errant interrupts until set up
CLI(); //disable all interrupts
port_init();
timer0_init();
MCUCR = 0x00;
GICR  = 0x00;
TIMSK = 0x01; //timer interrupt sources
SEI(); //re-enable interrupts
//all peripherals are now initialized
}
//
void main(void)
{
init_devices();
//insert your functional code here...
}