#define F_CPU 16000000
#include <avr/io.h>
#include <util/delay.h>
#define FUNCSET 0x28 //Function Set
#define ENTMODE 0x06 //Entry Mode Set
#define ALLCLR 0x01 //Clear Display
#define DISPON 0x0c //Display On
#define LINE2 0xC0 //Set DDRAM Address (0x40)
#define LINE3 0x94 //Set DDRAM Address (0x14)
#define LINE4 0xD4 //Set DDRAM Address (0x54)
#define PORT_DATA PORTD
#define DDR_DATA DDRD
#define Byte unsigned char
volatile Byte flag=0,sec=0,min=0,hour=24;
//인스트럭션 쓰기 함수
void LCD_write_command(unsigned char byte){
 _delay_ms(2);
 //인스트럭션 상위 바이트
 PORT_DATA = (byte & 0xF0); //데이터
 PORT_DATA &= 0xFE; //RS = 0
 PORT_DATA &= 0xFD; //RW = 0
 _delay_us(1);
 PORT_DATA |= 0x04; //E = 1
 _delay_us(1);
 PORT_DATA &= 0xFB; //E = 0
 //인스트럭션 하위 바이트
 PORT_DATA = ((byte<<4) & 0xF0);//데이터
 PORT_DATA &= 0xFE; //RS = 0
 PORT_DATA &= 0xFD; //RW = 0
 _delay_us(1);
 PORT_DATA |= 0x04; //E = 1
 _delay_us(1);
 PORT_DATA &= 0xFB; //E = 0
}
void LCD_write_data(unsigned char byte){
 
 _delay_ms(2);
 //데이터 상위 바이트
 PORT_DATA = (byte & 0xF0); //데이터
 PORT_DATA |= 0x01; //RS = 1
 PORT_DATA &= 0xFD; //RW = 0
 _delay_us(1);
 PORT_DATA |= 0x04; //E = 1
 _delay_us(1);
 PORT_DATA &= 0xFB; //E = 0
 PORT_DATA = ((byte<<4) & 0xF0);//데이터
 PORT_DATA |= 0x01; //RS = 1
 PORT_DATA &= 0xFD; //RW = 0
 _delay_us(1);
 PORT_DATA |= 0x04; //E = 1
 _delay_us(1);
 PORT_DATA &= 0xFB; //E = 0
}
//문자열 출력 함수
void LCD_write_string(char *string){
 uint8_t i;
 for(i=0; string[i]; i++){ //종료 문자를 만날 때 까지
  LCD_write_data(string[i]);
 }
}
void LCD_init(void){
 DDR_DATA = 0xFF; //출력 포트 설정
 PORT_DATA &= 0xFB; //E = 0
 
 _delay_ms(15);
 LCD_write_command(0x20);
 _delay_ms(5);
 LCD_write_command(0x20);
 _delay_us(100);
 LCD_write_command(0x20);
 LCD_write_command(FUNCSET);
 LCD_write_command(DISPON);
 LCD_write_command(ALLCLR);
 LCD_write_command(ENTMODE);
}
ISR(TIMER0_COMP_vect){
 if(++sec>59){ sec=0;
  if(++min>59){ min=0; if(++hour>23)hour=0; }
 }
 flag=1;
}
void LCD_CHAR(Byte c){ LCD_write_data(c); }
int main(void){
 LCD_init(); //LCD 초기화
 LCD_write_string(" Current Time"); //문자열 출력
 LCD_write_command(LINE2); //2번째줄
 LCD_write_string("   00 : 00 : 00 "); //문자열 출력
 LCD_write_command(LINE3); //3번째줄
 LCD_write_string(" Temperature "); //문자열 출력
 LCD_write_command(LINE4); //4번째줄
 LCD_write_string("   C "); //문자열 출력
 
 TCCR1B=0x0D;  OCR1A=15624; TIMSK=0x10;
 SREG=0x80;
 while(1){if(flag){ flag=0;
  
  LCD_init(); LCD_CHAR((hour/10)+'0'); LCD_CHAR((hour%10)+'0');
  LCD_init();
  LCD_write_string("    "); LCD_CHAR((min /10)+'0'); LCD_CHAR((min %10)+'0');
  LCD_init();
  LCD_write_string("        "); LCD_CHAR((sec /10)+'0'); LCD_CHAR((sec %10)+'0');};
 return 0;
} 
 이렇게 수정했는데 ISR에서 에러가 뜹니다 ..