/* Sample program to run on the Arduino without using the Arduino library.
   Adam Sampson <ats@offog.org>

   IO pins 0 - 13 as labelled on the board are port D 0-7, then port B 0-5.
   See include/avr/iom8.h for the register names.
 */

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

#ifdef __AVR_ATmega8__
#define TCCR0B TCCR0
#define TIMSK0 TIMSK
#endif

volatile unsigned long tick_count;

SIGNAL(TIMER0_OVF_vect) {
	tick_count++;
}

unsigned long ticks(const unsigned long millis) {
	return millis * F_CPU / (128000UL * 64UL * 2UL);
}

int after(const unsigned long a, const unsigned long b) {
	return (b - a) < 0x8000000;
}

unsigned long get_ticks(void) {
	unsigned long n;

	/* disable interrupts while reading the time, so we don't get an
	 * inconsistent result */
	cli();
	n = tick_count;
	sei();

	return n;
}

void delay(const unsigned long t) {
	unsigned long m = get_ticks() + ticks(t);
	while (after(get_ticks(), m)) {
		/* do nothing */
	}
}

int main(void) {
	/* enable interrupts */
	sei();

	tick_count = 0;

	/* timer 0 prescale 64 */
	TCCR0B |= 3 << CS00;
	/* enable timer 0 overflow interrupt */
	TIMSK0 |= _BV(TOIE0);

	/* pins to output */
	DDRB = 0xFF;

	/* flash pins */
	while (1) {
		PORTB = 0xFF;
		delay(500);
		PORTB = 0;
		delay(500);
	}
}