Serial Communication on Arduino

What is Serial Communication?

Serial communication sends data one bit at a time over a single wire. The Arduino Uno has one hardware serial port on pins 0 (RX) and 1 (TX), also connected to the USB chip ߀” so you can send and receive messages between the board and your computer through the USB cable.

This is the primary way to:

• Debug your sketch (print values to the Serial Monitor)
• Receive commands from a PC or another device
• Talk to other devices like GPS modules, Bluetooth adapters, and displays

Setting Up Serial ߀” Serial.begin()

Call this in setup() with the baud rate (bits per second). Always use a rate that matches your Serial Monitor setting.

void setup() {
  Serial.begin(9600);   // 9600 baud is the most common for beginners
}

Common baud rates: 9600, 19200, 38400, 57600, 115200.

Sending Data ߀” Print Functions

Function	Behaviour
Serial.print(x)	Print value, no newline
Serial.println(x)	Print value + newline
Serial.print(x, HEX)	Print integer in hexadecimal
Serial.print(x, BIN)	Print integer in binary

void loop() {
  int sensor = analogRead(A0);
  Serial.print("Raw value: ");
  Serial.println(sensor);       // e.g., "Raw value: 512"
  delay(500);
}

Opening the Serial Monitor

In the Arduino IDE 2, click the magnifying glass icon in the top-right toolbar, or press Ctrl + Shift + M. Set the baud rate at the bottom of the monitor to match Serial.begin().

ß? ?? Pins 0 and 1 are shared with USB serial. Avoid connecting anything to these pins while uploading or monitoring.

Reading Data ߀” Serial.read() and Serial.available()

if (Serial.available() > 0) {   // Data waiting in the buffer?
  char c = Serial.read();        // Read one byte
  Serial.print("Got: ");
  Serial.println(c);
}

Serial.available() returns the number of bytes waiting to be read. Always check it before calling Serial.read().

Reading a Full Line

void loop() {
  if (Serial.available() > 0) {
    String line = Serial.readStringUntil('\n');  // Read until Enter key
    line.trim();                                    // Remove \r\n
    Serial.print("You typed: ");
    Serial.println(line);

    if (line == "on") {
      digitalWrite(LED_BUILTIN, HIGH);
    } else if (line == "off") {
      digitalWrite(LED_BUILTIN, LOW);
    }
  }
}

Serial.write() vs Serial.print()

Function	Sends
Serial.print(65)	The characters 6 and 5 (the text "65")
Serial.write(65)	The byte 0x41 (which is the ASCII character 'A')

Use write() when talking to another device expecting raw bytes; use print() for human-readable output.

Timing and Non-Blocking Code

delay() pauses everything, including serial reading. For responsive serial input without blocking, track time manually:

unsigned long previousMillis = 0;
const long interval = 1000;

void loop() {
  unsigned long now = millis();
  if (now - previousMillis >= interval) {
    previousMillis = now;
    Serial.println(analogRead(A0));  // Print every 1 s without blocking
  }

  // Other code here runs freely without being delayed
}