Serial Communication

const uint8_t LED_PIN = 13;
String command = "";

void setup() {
  Serial.begin(9600);
  pinMode(LED_PIN, OUTPUT);
  Serial.println("Commands: ON, OFF, STATE, PWM <0-255>");
}

void loop() {
  while (Serial.available() > 0) {         // WHILE, not IF!
    char c = Serial.read();

    if (c == '\n' || c == '\r') {
      command.trim();
      command.toUpperCase();

      if (command == "ON") {
        digitalWrite(LED_PIN, HIGH);
        Serial.println("LED on");
      } else if (command == "OFF") {
        digitalWrite(LED_PIN, LOW);
        Serial.println("LED off");
      } else if (command == "STATE") {
        Serial.print("LED is ");
        Serial.println(digitalRead(LED_PIN) ? "on" : "off");
      } else if (command.startsWith("PWM ")) {
        int val = command.substring(4).toInt();
        val = constrain(val, 0, 255);
        analogWrite(9, val);
        Serial.print("PWM set to "); Serial.println(val);
      } else if (command.length() > 0) {
        Serial.print("Unknown command: ");
        Serial.println(command);
      }
      command = "";
    } else {
      command += c;
    }
  }
}

#define F_CPU 16000000UL
#define BAUD  9600
#define UBRR_VAL ((F_CPU / (16UL * BAUD)) - 1)

void usartInit(uint16_t ubrr) {
  UBRR0H = (uint8_t)(ubrr >> 8);              // high byte of the divisor
  UBRR0L = (uint8_t)ubrr;                     // low byte
  UCSR0B = (1 << RXEN0) | (1 << TXEN0);       // enable reception and transmission
  UCSR0C = (1 << UCSZ01) | (1 << UCSZ00);     // 8N1 format
}

void usartTransmit(uint8_t data) {
  while (!(UCSR0A & (1 << UDRE0)));           // wait for the register to be free
  UDR0 = data;
}

uint8_t usartReceive(void) {
  while (!(UCSR0A & (1 << RXC0)));            // wait for a complete byte
  return UDR0;
}

void usartWriteText(const char *s) {
  while (*s) usartTransmit(*s++);
}

void setup() {
  usartInit(UBRR_VAL);
  usartWriteText("USART initialized\r\n");
}

void loop() {
  uint8_t c = usartReceive();
  usartTransmit(c);                          // echo back
}

volatile char    buffer[32];
volatile uint8_t index = 0;
volatile bool    messageComplete = false;

ISR(USART_RX_vect) {              // runs for every byte received
  char c = UDR0;                  // reading it automatically clears the RXC0 flag

  if (c == '\n' || index >= 31) {
    buffer[index] = '\0';
    index = 0;
    messageComplete = true;
  } else {
    buffer[index++] = c;
  }
}

void setup() {
  Serial.begin(9600);
  UCSR0B |= (1 << RXCIE0);        // enable the receive interrupt
  sei();
  Serial.println("Ready for commands");
}

void loop() {
  if (messageComplete) {
    messageComplete = false;
    Serial.print("Received: ");
    Serial.println((char*)buffer);
  }
  // the loop stays completely free for other tasks
}

const uint8_t BUTTON_PIN = 2;

void setup() {
  Serial.begin(9600);
  pinMode(BUTTON_PIN, INPUT_PULLUP);
}

void loop() {
  static bool lastState = HIGH;
  bool state = digitalRead(BUTTON_PIN);

  if (state != lastState) {
    lastState = state;
    Serial.print("B:");             // prefix to identify the message
    Serial.println(state == LOW ? 1 : 0);
    delay(50);                      // simple debounce
  }
}

const uint8_t LED_PIN = 13;
String line = "";

void setup() {
  Serial.begin(9600);
  pinMode(LED_PIN, OUTPUT);
}

void loop() {
  while (Serial.available()) {
    char c = Serial.read();

    if (c == '\n') {
      line.trim();
      if (line.startsWith("B:")) {
        int val = line.substring(2).toInt();
        digitalWrite(LED_PIN, val);
      }
      line = "";
    } else {
      line += c;
    }
  }
}