[AVR] External clock as an input to MCU

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#include <avr/io.h>
#include <inttypes.h>
#include <avr/pgmspace.h>
#include <avr/interrupt.h>
#include <stdlib.h>
#include "LCD4bit.h"
#define F_CPU 1000000UL
 
volatile unsigned int Pulses, N;
 
 
ISR(TIMER0_OVF_vect);
ISR(TIMER1_OVF_vect);
 
int main(void)
{
    PORTB = 0;
    DDRB = 0xFC;
    DDRC = 0xFF;
    
    int i, j;
    float energy;
    char result[20] = "";
 
 
 
    MCUCR |= ((1 << ISC11) | (1 << ISC10)); //to count the pulse
    GICR |= (1 << INT0); 
    
    uint32_t elapse_time; // to count the time
    TCCR1A = 0;
    TCCR1B = 0x01;
    TIMSK = 0x04;
    
    N=0;
    TCNT1 = 0;
    
    sei();               
    for(i = 0; i<100 ; i++)
        for(j=0; j<1000; j++){;}
    elapse_time = N * 65536 + (uint32_t) TCNT1;
    cli();
    
    while(1)
    {
        energy = (3000 * Pulses * elapse_time) /3600000;
        dtostrf(energy, 3, 1, result);
        LCD_setCursor(4,0);
        LCD_Disp("Energy Meter");
        LCD_setCursor(0,1);
        LCD_Disp("Energy =");
        LCD_Disp(result);
        LCD_Disp("KWh");
    }
 
    
    return 0;
}
 
 
ISR(TIMER0_OVF_vect)
{
    // keep a track of number of overflows
    Pulses++;
}
 
 
ISR(TIMER1_OVF_vect)
{
    // keep a track of number of overflows
    N++;
}