#define N                   32
#define RATE                255           // Top value for ICR3: (3200 counts) * (1us / 16 counts) = 200 us period
#define PWM_PIN             5              // Must be an OCR3A pin (e.g., Pin 5 on Arduino Mega)

// Define the sine wave array as a constant in program memory (PROGMEM not necessary for uint8_t size)
const uint8_t sin_wave[N] = {128, 153, 177, 199, 219, 234, 246, 254, 255, 254, 246, 234, 219, 199, 177, 153, 128, 103, 79, 57, 37, 22, 10, 2, 0, 2, 10, 22, 37, 57, 79, 103};

void setup() {
  Serial.begin(9600); // Initialize serial communication once
  pinMode(PWM_PIN, OUTPUT); // Set the PWM pin as an output

  // --- Configure Timer 3 for Fast PWM Mode (WGM33:0 = 14) ---
  cli(); // Disable global interrupts while configuring timer
  TCCR3A = 0; // Clear TCCR3A register
  TCCR3B = 0; // Clear TCCR3B register

  // Set Wave Generation Mode bits: WGM31 and WGM30 (for Mode 14: Fast PWM, TOP set by ICR3)
  TCCR3A |= (1 << WGM31);
  TCCR3B |= (1 << WGM32) | (1 << WGM33);

  // Set TOP value for the timer in the Input Capture Register (ICR3) to define the frequency
  ICR3 = RATE; // What is the PWM's frequency?
  // Configure Output Compare Pin A (OC3A) for non-inverting mode (Clear OC3A on Compare Match, set on BOTTOM)
  TCCR3A |= (1 << COM3A1);

  // Set Prescaler to 256 and start the timer
  TCCR3B |= (1 << CS32);

  sei(); // Enable global interrupts
  // Initial PWM duty cycle (e.g., midpoint of sine wave)
  OCR3A = sin_wave[0]; 
}

void loop(){
  uint8_t i;
  uint16_t index;
  char cmd;
  // uint8_t volumeAdjust = 1; // This variable was unused in original code

  Serial.println("Analog waveform generator running.");
  Serial.println("Enter 's' to step through sine table.");
  
  for(;;){
    if (Serial.available()) {
      cmd = Serial.read(); 
      switch (cmd) {
        case 's':
          Serial.println("Press 'x' to exit stepping mode.");
          Serial.println("Press any other key to step.");
          index = 0;
          // Use a while loop that runs until 'x' is read
          while (1) {
            // Check for new input within the stepping loop
            if (Serial.available()) {
              if (Serial.read() == 'x') {
                break; // Exit the while(1) loop
              }
            }
            
            // Update the PWM duty cycle with the next sine wave value
            OCR3A = sin_wave[index]; //What is the PWM's duty cycle?
            
            // Increment index and wrap around using the bitmask
            index = (index + 1) & 0b00011111; 

            // Wait for user input to step again, or continue waiting for 'x'
            while (!Serial.available());
          }
          // The PWM continues running at the last value after exiting the loop
          Serial.println("Exited stepping mode. Timer is still running.");
          while (Serial.available()) Serial.read(); // Clear serial buffer
          break;    
      }
   }
 }
}