====== Start ======
===== Light a LED =====

This small program allows to light a LED on port B. \\
The initPort function configure port B in output mode and force its outputs to zero. \\
The PORTB outputs are set to 1 in the main function.

<code>
#include<avr/io.h>	/* register definitions*/

void initPort (void)
{
	/* port configuration : '1' for output */
	DDRB=0b11111111;	/* PORTB in output mode */
	PORTB=0b00000000;	/* PORTB = 0 */
}

int main (void)
{
	initPort();

	while(1)
	{
		PORTB = 0b11111111; /* PORTB force a 1 */
	}
	return 1;
}
</code>

===== Make a LED blinking =====

The purpose of this programme is to make a LED blinking using timer 0 and interrupts.

==== Initialisation du timer en mode timer ====
TCCR0B initializes timer 0 and its prescaler. \\
TCNT0 register is the counter. It increases at each clock cycle. \\
Bit 0 of TIFR0 register is the interrupt flag. \\
Bit TOIE0 from TIMSK0 register activates interrupt on timer 0 overflow.

TCNT0 overflow launch an interrupt.
In interrupt handler « ISR(TIMER0_OVF_vect) », we increment « cmp » variable which will serve to make the led blinking by reversing PORTB value when cmp is greater than 100.

<code>
#include<avr/io.h>	/* register definition */
#include <avr/interrupt.h> /* interrupts */

volatile unsigned char cmp;

/* interrupt on TCNT0 overflow : every 13ms (1/20MHz * 256*1024) */
ISR(TIMER0_OVF_vect)
{
	/* at each TCNT0 overflow, cmp is incremented */
	cmp++;
	TIFR0 = 0b00000001;	/* TOV0 = 1 : Interrupt flag is forced to 0 */
}

void initPort (void)
{
        /* port configuration : '1' for output */
	DDRB=0b11111111;	/* PORTB en sortie */
	PORTB=0b00000000;	/* PORTB a 0 */
}

/*
 * Initialize timer 0 in timer mode with a prescaler equals to 1024
 * Interruption enabled on timer overflow */
void initTimer0(void)
{
	TCCR0B= 0b00000101;	/* timer mode with prescaler equals to 1024 */
	TCNT0 = 0;	        /* Force counter to 0 */
	TIFR0 = 0b00000001;	/* TOV0 = 1 : Interrupt flag is forced to 0 */
	TIMSK0 = 0b00000001;	/* TOIE0 = 1: enable interrupt on TCNT0 overflow */
}

int main (void)
{
	initPort();
	initTimer0();
	SREG = 0b10000000;	/* enable interrupts */

	while(1)
	{
		if (cmp>100)	/* 13*100 = 1.3s */
		{
			PORTB = ~PORTB; /* reverse PORTB value */
			cmp=0;
		}
	}
	return 1;
}
</code>