/* Birdie.c  */

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


enum { UP, DOWN };

volatile uint16_t ampl, upper, lower; 
volatile uint8_t direction, count, scount, max;


SIGNAL (SIG_OVERFLOW1) 
{
/* OCR1 gets the two values required to produce a pulse wave;the two values are upper and lower
 * OCR1 should be set to upper or lower based upon the MSB of your phase accumulator, which should 
 * be enough bits to allow for pitch accuracy to reproduce the appropriate pitches */
    if(++scount==max)
	{
		scount = 0;
		switch (direction) 
		{
			case UP:
				OCR1A = upper;
				direction = DOWN;
				break;
	
			case DOWN:
				OCR1A = lower;
				direction = UP;
				break;
		}
	}
	
/* This stuff should be in a counter/timer 2 routine. upper and lower should be set to:
 *     upper = ampl + 127;
 *     lower = 127 - ampl;
 * ampl controls loudness from 0 (off) to 127 (loudest). These values should come from a table with 
 * ADSR (Attack, Decay, Sustain, Release) segments. The whole table should time scale to the 
 * length of the note in your score  */
 
	if(++count == 50)   /* duration */
	{
		count = 0;
		if (--ampl == 0)
		{
			ampl = 127;
			if(--max ==1)
				max = 20;
		}
		upper = ampl + 127;
		lower = 127 - ampl;
	}
}

void
ioinit (void) 
{
	ampl = 127;
	max = 20;

/* tmr1 running on full MCU clock */
    TCCR1B = _BV (WGM12)  | _BV (CS10);

    /* tmr1 is 8-bit fast PWM, mode 5 , 28.8KHz sample rate*/
    TCCR1A =   _BV (WGM10)  | _BV (COM1A1);

    /* set PWM value to 0 */
    OCR1A = 0;

    /* enable PD5 as output */
    DDRD = _BV (PD5);

    timer_enable_int (_BV (TOIE1));

    /* enable interrupts */
    sei ();
}

int
main (void)
{
    ioinit ();

    /* loop forever, the interrupts are doing the rest */

    for (;;) 
        ;

    return (0);
}