#define	FCLK 16.000  //clock frequency, MHz
#define TSYS 500.0   //system tick, uS
#define SND    (1 << PB0) //OL - sound output
#define Port_SND     (PORTB & SND)
#define Port_SND_0   (PORTB &= ~SND)
#define Port_SND_1   (PORTB |= SND)
#define T2VAL (FCLK * TSYS / 64.0) //system timer period

//--------------------------- System timer init: -----------------------------

void Main_Timer_Init(void)
{
  TCCR2 = (1<<WGM21) | (1<<CS22);  //timer 2 mode: CTC, CK/64
  OCR2 = T2VAL - 1;                //load compare register
  TIFR = (1<<OCIE2);               //pending interrupts clear
  TIMSK = (1<<OCIE2);              //compare interrupt enable
  fTick = 1;                      //force update
}

//------------------------ System timer interrupt: ---------------------------

#pragma vector = TIMER2_COMP_vect
__interrupt void Timer(void)
{
  Sound_Gen();     //generate sound
}

#define BEEP_D 100.0  //beep duration, mS

#define MS2P(x) (int)(x * 500 / TSYS) //convert mS to periods

//------------------------------ Variables: ----------------------------------

int SndTimer;         //sound duration in periods

//--------------------------- Beep generation: -------------------------------

__monitor void Sound_Beep(void)
{
  SndTimer = MS2P(BEEP_D);
}

//--------------------------- Sound generation: ------------------------------

void Sound_Gen(void)
{
  if(SndTimer)
  {
    Port_SND? Port_SND_0 : Port_SND_1;
    if(Port_SND) SndTimer--;
  }
}

//----------------------------------------------------------------------------