For Atmel ATTiny13A microcontroller. Output generated on PB0 ( D0 )

TinyTone.h

#define    SPEAKER_PIN    PB0

static void timer_set(uint8_t OCRxn, uint8_t N);
static void timer_clear();
static void tone(uint16_t freq);
static void notone();

void enable_tone()
{
    TCCR0A |= _BV(WGM01); // set timer mode to Fast PWM
    TCCR0A |= _BV(COM0A0); // connect PWM pin to Channel A of Timer0

}

void disable_tone()
{
    TCCR0A &= ~_BV(WGM01); // set timer mode to Fast PWM
    TCCR0A &= ~_BV(COM0A0); // connect PWM pin to Channel A of Timer0

}


void notone(){
    TCCR0A &= ~_BV(WGM01); 
    TCCR0A &= ~_BV(COM0A0); 
}

void tone(uint16_t freq){
    if(freq==0){
        notone();
        return;
    }
    TCCR0A |= _BV(WGM01); // set timer mode to Fast PWM,  Clear Timer on Compare Match 
    TCCR0A |= _BV(COM0A0); // connect PWM pin to Channel A of Timer0,  Toggle OC0A on Compare Match

    uint16_t prescalerIndex = 0;   
    uint16_t prescalers[] = {
        0,1,8,64,256,1024
    };
    uint32_t modulo;
    do {
        prescalerIndex++;
        modulo = F_CPU/(freq*2L*prescalers[prescalerIndex]);        
    } while(modulo>=256);

    timer_set(modulo,prescalerIndex);
}



void timer_clear()
{
    timer_set(1, 0);
}

void timer_set(uint8_t OCRxn, uint8_t prescaler)
{

    TCCR0B = (TCCR0B & ~(_BV(CS02)|_BV(CS01)|_BV(CS00))) | prescaler;
    OCR0A = OCRxn - 1;
    
}