// SPWM code for Arduino Nano Inverter (50Hz output)
// Uses Hardware Timer 1 on Pins 9 and 10 at ~31.3 kHz switching frequency

const int steps = 20;
// A true half-sine wave lookup table (0 to 255 to 0)
const byte sineTable[20] = {
  0, 40, 79, 117, 153, 185, 213, 235, 249, 255, 
  255, 249, 235, 213, 185, 153, 117, 79, 40, 0
};

volatile int stepIndex = 0;
volatile boolean currentHalfCycle = 0; // 0 = Positive Half, 1 = Negative Half

void setup() {
  // Set Pin 9 and 10 as outputs
  pinMode(9, OUTPUT);
  pinMode(10, OUTPUT);
  
  // Clear Timer 1 Control Registers
  TCCR1A = 0;
  TCCR1B = 0;
  TCNT1  = 0;

  // Configure Timer 1 for Phase Correct PWM, 8-bit mode (Mode 1)
  // This gives a PWM frequency of 16MHz / (2 * 255) = ~31.37 kHz
  TCCR1A = _BV(COM1A1) | _BV(COM1B1) | _BV(WGM10);
  TCCR1B = _BV(CS10); // No prescaling

  // Configure Timer 2 to handle the 50Hz timing grid
  // 50Hz full wave = 20ms. Each half wave = 10ms. 
  // 10ms / 20 steps = 500 microseconds per step interval.
  TCCR2A = 0;
  TCCR2B = 0;
  TCNT2  = 0;
  OCR2A = 124;            // Clear Timer on Match (CTC) target
  TCCR2A = _BV(WGM21);    // CTC Mode
  TCCR2B = _BV(CS21) | _BV(CS20); // Prescaler 64 -> (16MHz / 64) = 250kHz clock (4us per tick)
                          // 125 ticks * 4us = 500us interval
  TIMSK2 = _BV(OCIE2A);   // Enable Timer 2 Compare Match A Interrupt
}

// Timer 2 Interrupt service routine updates the SPWM duty cycle every 500us
ISR(TIMER2_COMPA_vect) {
  byte pwmVal = sineTable[stepIndex];

  if (currentHalfCycle == 0) {
    // Positive half cycle: Drive Pin 9 with SPWM, keep Pin 10 at 0
    analogWrite(10, 0); 
    analogWrite(9, pwmVal);
  } else {
    // Negative half cycle: Drive Pin 10 with SPWM, keep Pin 9 at 0
    analogWrite(9, 0);   
    analogWrite(10, pwmVal);
  }

  stepIndex++;
  
  // If we reach the end of the 20-step half-cycle, switch sides
  if (stepIndex >= steps) {
    stepIndex = 0;
    currentHalfCycle = !currentHalfCycle; // Toggle between positive and negative halves
  }
}

void loop() {
  // The entire SPWM generation runs perfectly inside the hardware interrupts.
  // Your loop stays free to read voltage/current protection overrides!
}