#include <avr/io.h>
#include <avr/interrupt.h>
//
volatile char SEG[]={0x3F,0x06,0x5B,0x4F,0x66,0x6D,0x7C,0x07,0x7F,0x67,0x77,0x7C,0x39,0x5E,0x79,0x71,0x08,0x80};
volatile char ms=0,sec=0,STOP=0,FndCnt=0;
//
ISR(TIMER1_COMPA_vect){ // 타이머1 A매치 인터럽트 10msec
    PORTG=0x0F; // FND_OFF
    if(STOP){ if(++ms>99){ ms=0; if(++sec>99)sec=0; } }
    if     (FndCnt==0){ PORTE=SEG[ms %10];      PORTG=~8; }
    else if(FndCnt==1){ PORTE=SEG[ms /10];      PORTG=~4; }
    else if(FndCnt==2){ PORTE=SEG[sec%10]|0x80; PORTG=~2; }
    else              { PORTE=SEG[sec/10];      PORTG=~1; }
    if(++FndCnt>3)FndCnt=0;
}
//
ISR(INT0_vect){ STOP^=1;          } // Stop/Resume 처리
ISR(INT1_vect){ STOP=0; ms=sec=0; } //리셋
//
int main(){
  DDRE=0xFF;   DDRG=0x0F;
  TCCR1B=0x0B; OCR1A=2499; TIMSK=0x10; // 16000000/64/2500=100Hz=10msec, A매치인터럽트
  EICRA=0xAA;  EIFR=0x03;  EIMSK=0x03;
  SREG=0x80;
  while(1){}
}