// ==== CONFIGURATION ====

// Frequency of sine wave output (can change to 60)
const int freq = 50;

// Number of steps in half sine wave
const int steps = 21;

// Dead time in microseconds
const int deadTime = 50;

// Output pins for SPWM (must be PWM capable)
const int pwmPinA = 9;   // OC1A on Arduino Uno
const int pwmPinB = 10;  // OC1B

// Lookup table for one half sine wave (0 to 255)
const byte sineTable[steps] = {
  128, 147, 167, 185, 202, 218, 231, 241, 249, 253,
  255, 253, 249, 241, 231, 218, 202, 185, 167, 147, 128
};

// Variables for ISR control
volatile int index = 0;
volatile bool halfCycle = false; // false = A active, true = B active

void setup() {
  pinMode(pwmPinA, OUTPUT);
  pinMode(pwmPinB, OUTPUT);

  // Disable PWM on Timer1 temporarily
  TCCR1A = 0;
  TCCR1B = 0;

  // Set Timer1 compare interrupt for step timing
  // Total microseconds per step = 1 / (2 * freq * steps)
  unsigned long stepTimeMicros = 1000000UL / (2UL * freq * steps);

  // Convert to timer ticks (16 MHz / 8 prescaler = 2 MHz = 0.5 us per tick)
  unsigned int ocr1a = stepTimeMicros * 2;

  // Set Timer1 compare match value
  OCR1A = ocr1a;

  // Set Timer1 mode to CTC (Clear Timer on Compare Match)
  TCCR1B |= (1 << WGM12);

  // Set prescaler to 8
  TCCR1B |= (1 << CS11);

  // Enable compare match interrupt
  TIMSK1 |= (1 << OCIE1A);
}

void loop() {
  // Main loop does nothing, all work is done by ISR
}

// ==== INTERRUPT SERVICE ROUTINE ====
// Runs every stepTimeMicros microseconds

ISR(TIMER1_COMPA_vect) {
  // First add dead time
  analogWrite(pwmPinA, 0);
  analogWrite(pwmPinB, 0);
  delayMicroseconds(deadTime);  // may cause slight jitter but works

  // Now output PWM value based on current half-cycle
  if (!halfCycle) {
    // A active
    analogWrite(pwmPinA, sineTable[index]);
    analogWrite(pwmPinB, 0);
  } else {
    // B active
    analogWrite(pwmPinA, 0);
    analogWrite(pwmPinB, sineTable[index]);
  }

  index++;

  // If we finish one half-cycle
  if (index >= steps) {
    index = 0;
    halfCycle = !halfCycle; // switch side
  }
}