const uint8_t MONITOR_MASK = 0b11111111; // Überwache D0–D7

volatile uint16_t ringBuffer[8] = {0xFFFF, 0xFFFF, 0xFFFF, 0xFFFF, 0xFFFF, 0xFFFF, 0xFFFF, 0xFFFF};
volatile uint8_t  sum[8]       = {16, 16, 16, 16, 16, 16, 16, 16};
volatile bool     portDLow = 0b00000000;
volatile bool     portDHigh = 0b11111111;

ISR(TIMER1_COMPA_vect) {
  uint8_t portValue = PIND & MONITOR_MASK; // z. B. D2–D7
  for (uint8_t i = 0; i < 8; i++) {
    if (!(MONITOR_MASK & (1 << i))) 
      continue;
    uint8_t newBit = (portValue >> i) & 1;
    uint8_t oldBit = (ringBuffer[i] & 0x8000) ? 1 : 0;    // Ältestes Bit extrahieren (MSB)
    ringBuffer[i] = (ringBuffer[i] << 1) | newBit;        // Shift + neues Bit
    sum[i] += newBit - oldBit;                            // Summe anpassen
    if (sum[i] == 4 && newBit == 0) {                                     // Schwellenwert prüfen
      portDLow |= (1 << i);        // Setze Bit i in portDLow
    }
    if (sum[i] == 12 && newBit == 1) {                                     // Schwellenwert prüfen
      portDHigh |= (1 << i);
    }
  }
}

void setup() {
  
  cli(); // Interrupts deaktivieren
  // --------- Timer1 Setup: CTC Mode, 3.2kHz ----------
  TCCR1A = 0;
  TCCR1B = (1 << WGM12) | (1 << CS10); // CTC mode, Prescaler = 1 //  CCR1B = (1 << WGM12) | (1 << CS11);// Prescaler 8 // TCCR1B = (1 << WGM12) | (1 << CS11) | (1 << CS10); // Prescaler 64
  OCR1A = 999;                         // 1auf 16.000 kHz mit 999
  TIMSK1 = (1 << OCIE1A);              // Compare Match Interrupt aktivieren
  sei(); // Interrupts aktivieren
  Serial.begin(115200);
  DDRD=0b00000000;                    // set all pins as input;
  PORTD=0b11111111;
  DDRB=0b00111111;                    // set pins 0..5 as output
  
}

void loop() {
  noInterrupts();
  uint8_t pending = portDLow;
  portDLow = 0;
  interrupts();
  PORTB=pending&0b00111111;
 /* 
  while (pending) {
    uint8_t i = __builtin_ctz(pending);     // Index des niedrigsten gesetzten Bits
    pending &= pending - 1;                 // Dieses Bit löschen

    Serial.print("Pin D");
    Serial.print(i);
    Serial.print(" unter Schwellwert! ");
    
  }
  */
  delayMicroseconds(500);  
}