generating tones with Atmega328p's timer

beeper.c

#include <stdint.h>

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


// system clock frequency
#define F_CPU 16000000UL

// global variable to determine the beep duration
static volatile uint32_t duration = 0x00;


ISR(TIMER0_COMPA_vect) {
	if (!duration) {
		// mask the interrupt
		TIMSK0 &= ~_BV(OCIE0A);

		// disable clock
		TCCR0B = 0x00;

		// set the port state to low, when generation is finished
		PORTD &= _BV(PORTD6);
	}
	else {
		duration--;
	}
}


static void timer_freq_prescale(uint32_t a_freq, uint8_t *a_ocr, uint8_t *a_prescaler) {
	// prescaler table for timer 0
	uint8_t prescalers[] = { 0x00, 0x03, 0x06, 0x08, 0x0a, 0x00 };

	uint16_t ocr = 0x00;
	uint8_t prescaler = 0x00;

	do {
		ocr = (uint16_t) (F_CPU / ((a_freq << 1) * (0x01 << prescalers[prescaler])));
		++prescaler;		
	} while ((ocr > 255) && (prescalers[prescaler]));

	--ocr;
	if (ocr > 255) ocr = 255;

	*a_ocr = ocr & 0xff;
	*a_prescaler = prescaler;
}


void beep(uint16_t a_freq, uint16_t a_duration) {
	uint8_t prescaler = 0x00, 
			ocr = 0x00;
	
	sei();
	power_timer0_enable();

	// configure the COMP0A pin as output 
	DDRD |= _BV(PORTD6);

	// calculate the frequency
	timer_freq_prescale(a_freq, &ocr, &prescaler);

	// COM0A1/COM0A0 = 0x01 - toggle OC0A on Compare Match, mode = CTC
	TCCR0A = 0x42;
	OCR0A = ocr;
	TCNT0 = 0x00;

	// configure the prescaler, now the timer is running and the tone is being generated
	TCCR0B = prescaler & 0x07;
	
	// calculate the amount of cycles
	duration = (uint32_t)((a_freq * a_duration)/500);

	// unmask the interrupt
	TIMSK0 |= _BV(OCIE0A);

	// block until the tone fades out
	while(duration);
}


int main(void)
{
	uint16_t freq = 100;

	while (1) {
		for(uint8_t i = 0; i < 64; i++) {
			// 100 ms beep of increasing frequency
			beep(freq + (10 * i), 100);
		}
	}

	return 0;
}

Makefile

TARGET=beeper
SOURCES=beeper.c

DEPS=
COBJ=$(SOURCES:.c=.o)

CC=avr-gcc
OBJC=avr-objcopy
MCU=atmega328p
CFLAGS=-I. -Wall -Os -DF_CPU=16000000UL -std=c99
LDFLAGS=

ISPPORT=/dev/ttyACM0
ISPDIR=/usr/share/arduino/hardware/tools
ISP=$(ISPDIR)/avrdude
ISPFLAGS=-c arduino -p $(MCU) -P $(ISPPORT) -b 115200 -C $(ISPDIR)/avrdude.conf 

all: $(TARGET)

%.o: %.c $(DEPS)
	@echo -e "\tCC" $<
	@$(CC) -mmcu=$(MCU) -c -o $@ $< $(CFLAGS)

$(TARGET): $(COBJ) 
	@echo -e "\tLINKING CC" $<
	@$(CC) -mmcu=$(MCU) -o $(TARGET) $(COBJ) $(LDFLAGS)
	$(OBJC) -O ihex -R .eeprom $(TARGET) $(TARGET).hex
	$(OBJC) -O binary -R .eeprom $(TARGET) $(TARGET).bin

clean:
	$(MAKE) -C $(PCA_PREFIX) clean
	@echo ========== cleanup ========== 
	rm -rf *.o *.bin *.hex $(TARGET)	

read:
	$(ISP) $(ISPFLAGS) -U flash:r:$(TARGET)_backup.hex:i

install:
	$(ISP) $(ISPFLAGS) -U flash:w:$(TARGET).hex