[C] Probleme mit Software-PWM + Fading bei Attiny861A

#define F_CPU 8000000UL

#include <avr/io.h>

#include <avr/interrupt.h>

#include <util/delay.h>

#define F_PWM 100                  

#define PWM_STEPS 255                 

#define T_PWM (F_CPU / (F_PWM * PWM_STEPS))

#if (T_PWM < (152 + 5))

  #error T_PWM zu klein!

#endif

#if PWM_STEPS > 255

  #error PWM_STEP zu gross!

#endif

volatile uint8_t dutyCycle = 16;

void sleep(uint16_t ms) {

  for(; ms > 0; ms--) {

    _delay_ms(1);

  }    

}

ISR(TIMER1_COMPA_vect) {

  static uint8_t pwmCnt = 0;

  OCR1A += (uint16_t)T_PWM;

  if(dutyCycle > pwmCnt) {

    // LED an

    PORTA &= ~(1<<PA6);

  } else {

    // LED aus

    PORTA |= (1<<PA6);

  }

  if(pwmCnt == (uint8_t)(PWM_STEPS-1)) {

    pwmCnt = 0;

  } else {

    pwmCnt++;

  }

}

int main(void)

{

  int i;

  // PA5 & PA6 als Ausgang, HIGH

  DDRA = (1<<DDA6)|(1<<DDA5);

  PORTA = (1<<PA6)|(1<<PA5);

  // Timer1 Takt = CPU Takt

  TCCR1B = 1;

  // Timer1 Output Compare Interrupt aktivieren

  TIMSK |= (1<<OCIE1A);

  // Interrupts aktivieren

  sei();

  while(1) {

    // Tastverhältnis ändern

    for(i = 0; i <= 255; i++) {

      dutyCycle = i;

      // warte bis zur nächsten Stufe - HIER HAKT ES SCHEINBAR

      sleep(1); 

    }

  }

}