#include <TimerOne.h>

// AVR pin connected to the transmit output.
// On ATmega328P boards, Arduino D2 maps to PD2.
const byte DATA_PIN = 2;


// Bit sequence to transmit.
// Each array element represents one transmitted bit.
// 1 = output HIGH, 0 = output LOW.
const byte bitStream[] = {
  1,0,1,1,0,0,1,0,
  0,1,1,1,0,0,0,1
};


// Number of bits in the transmission buffer.
const uint16_t NUM_BITS = sizeof(bitStream);


// Variables shared between main loop and interrupt service routine.
// volatile prevents compiler optimization because these values may change asynchronously.
volatile uint16_t bitIndex = 0;
volatile bool txComplete = false;
volatile bool transmitting = false;



// Timer1 interrupt handler.
// Called every 300 us to output the next bit in the stream.
void transmitISR()
{
  if (bitIndex < NUM_BITS)
  {
    // Direct port manipulation is used instead of digitalWrite()
    // to minimize execution time and provide more consistent timing.

    if (bitStream[bitIndex])
      PORTD |= _BV(PD2);       // Set D2 HIGH
    else
      PORTD &= ~_BV(PD2);      // Set D2 LOW

    bitIndex++;
  }
  else
  {
    // Transmission complete.
    // Return output line to idle LOW state.
    PORTD &= ~_BV(PD2);

    // Disable Timer1 interrupt until another transmission starts.
    Timer1.detachInterrupt();

    transmitting = false;
    txComplete = true;
  }
}



// Initializes and begins a new bit transmission.
void startTransmission()
{
  // Reset transmission state.
  bitIndex = 0;
  txComplete = false;
  transmitting = true;


  // Output first bit immediately.
  // This avoids an initial timer-period delay before transmission begins.
  if (bitStream[bitIndex])
    PORTD |= _BV(PD2);
  else
    PORTD &= ~_BV(PD2);

  bitIndex++;


  // Configure Timer1 to generate an interrupt every 300 microseconds.
  // This determines the bit period:
  // 300 us per bit = approximately 3333 bits/sec.
  Timer1.initialize(300);

  // Start periodic interrupts.
  Timer1.attachInterrupt(transmitISR);
}



void setup()
{
  // Configure PD2 (Arduino pin 2) as an output.
  DDRD |= _BV(DDD2);

  // Set initial idle state LOW.
  PORTD &= ~_BV(PD2);


  // Initialize serial interface for transmission trigger input.
  Serial.begin(115200);

  Serial.println("Send any character to transmit.");
}



void loop()
{
  // Start a transmission when serial input is received
  // and no transmission is currently active.
  if (Serial.available() && !transmitting)
  {
    char c = Serial.read();

    Serial.print("Received: ");
    Serial.println(c);

    // Begin fixed bit-stream output.
    startTransmission();
  }


  // Handle transmission completion outside the ISR.
  // Keeping Serial operations out of the ISR avoids timing problems.
  if (txComplete)
  {
    txComplete = false;

    Serial.println("Transmission complete.");
  }
}