// ATtiny85 Frequency Divider by 1.3
// Input: Square wave 0-1kHz on PB2 (pin 7)
// Output: Square wave with frequency/1.3 and 50% duty cycle on PB1 (pin 6)

#define INPUT_PIN PB2    // Pin 7 (physical pin)
#define OUTPUT_PIN PB1   // Pin 6 (physical pin)

volatile unsigned long lastEdgeTime = 0;
volatile unsigned long inputPeriod = 0;
volatile bool newPeriodAvailable = false;

volatile unsigned long outputPeriod = 0;
volatile unsigned long outputHalfPeriod = 0;
volatile unsigned long outputLastToggle = 0;
volatile bool outputState = false;

// Timeout handling for low frequencies
#define TIMEOUT_THRESHOLD 200000  // 200ms = 5Hz minimum
volatile unsigned long lastValidInput = 0;

void setup() {
  // Set pin modes
  pinMode(INPUT_PIN, INPUT_PULLUP);
  pinMode(OUTPUT_PIN, OUTPUT);
  
  // Initialize output
  digitalWrite(OUTPUT_PIN, LOW);
  
  // Enable Pin Change Interrupt for PB2
  GIMSK |= (1 << PCIE);     // Enable Pin Change Interrupts
  PCMSK |= (1 << PCINT2);   // Enable interrupt on PB2
  
  // Initialize timer for microsecond timing
  // Timer0 is used by Arduino functions like micros()
  sei(); // Enable global interrupts
}

void loop() {
  unsigned long currentTime = micros();
  
  // Check if we have a new input period measurement
  if (newPeriodAvailable) {
    // Calculate output period (input period * 1.3)
    outputPeriod = (inputPeriod * 13) / 10; // Multiply by 1.3
    outputHalfPeriod = outputPeriod / 2;    // 50% duty cycle
    
    // Reset output timing
    outputLastToggle = currentTime;
    outputState = false;
    digitalWrite(OUTPUT_PIN, LOW);
    
    lastValidInput = currentTime;
    newPeriodAvailable = false;
  }
  
  // Timeout check - if no valid input for more than 200ms (5Hz), stop output
  if (outputPeriod > 0 && (currentTime - lastValidInput) > TIMEOUT_THRESHOLD) {
    outputPeriod = 0;
    outputState = false;
    digitalWrite(OUTPUT_PIN, LOW);
  }
  
  // Generate output square wave with 50% duty cycle
  if (outputPeriod > 0) {
    if (currentTime - outputLastToggle >= outputHalfPeriod) {
      outputState = !outputState;
      digitalWrite(OUTPUT_PIN, outputState);
      outputLastToggle = currentTime;
    }
  }
}

// Pin Change Interrupt Service Routine
ISR(PCINT0_vect) {
  static unsigned long prevEdgeTime = 0;
  static bool lastState = HIGH;
  
  unsigned long currentTime = micros();
  bool currentState = digitalRead(INPUT_PIN);
  
  // Detect rising edge (assuming input is pulled up, so LOW to HIGH transition)
  if (lastState == LOW && currentState == HIGH) {
    if (prevEdgeTime > 0) {
      inputPeriod = currentTime - prevEdgeTime;
      
      // Validate period (should be between 1ms and 1000ms for 0-1kHz range)
      if (inputPeriod >= 1000 && inputPeriod <= 1000000) {
        newPeriodAvailable = true;
      }
    }
    prevEdgeTime = currentTime;
  }
  
  lastState = currentState;
}

/* 
Pin connections for ATtiny85:
- Pin 1 (PB5/RESET): Leave unconnected or use for programming
- Pin 2 (PB3): Not used
- Pin 3 (PB4): Not used  
- Pin 4 (GND): Ground
- Pin 5 (PB0): Not used
- Pin 6 (PB1): OUTPUT - Connect your output load here
- Pin 7 (PB2): INPUT - Connect your input square wave here
- Pin 8 (VCC): Power supply (1.8V to 5.5V)

Frequency range: 
- Input: 0-1kHz (1000µs to 1000000µs period)
- Output: Input frequency ÷ 1.3 (so 0-769Hz approximately)
- Duty cycle: 50% on output

Note: This code uses pin change interrupts to measure the input period
and generates the output using timing loops. For very low frequencies
(below ~10Hz), you might want to add a timeout mechanism.
*/