#define F_CPU 1000000UL

#include <avr/io.h>
#include <avr/interrupt.h>

#define TRUE  1
#define FALSE 0

// Globals
volatile uint8_t light;
volatile uint8_t down;


//**************************************
// Interrupts here
//**************************************
ISR(TIMER1_COMPA_vect){
    // At eight or above, count down
    if(light >= 7)
        down = TRUE;
    // At zero, count up
    if(light == 0)
        down = FALSE;

    if(down)
        light--;
    else
        light++;
}

//**************************************
// Main starts here
//**************************************
int main(void) {

    // set PA to output
    DDRA = 0xff;

    // Set up timer
    TCCR1B |= (1<<WGM12);
    TIMSK |= (1<<OCIE1A);
    sei();
    OCR1A = 1562;
    TCCR1B |= ( (1<<CS10) | (1<<CS11) );

    // Initial state
    down = FALSE;
    light = 0;

    //*********************************
    // Main loop
    //*********************************
    while(1) {
        // Light the right led
        // PORTA = ~light;
        PORTA = ~(1<<light);
    }

    return 0;
}