How UART Serial Communication Works and How to Use It with Arduino

Whenever you open the Serial Monitor in the Arduino IDE to inspect sensor data or flash new firmware to a board, you are utilizing the UART (Universal Asynchronous Receiver-Transmitter) protocol. Unlike I2C and SPI, UART is asynchronous, meaning it requires no shared clock signal to synchronize data exchange, relying only on two wires.

UART Pins and Cross-Connection

UART communication establishes a point-to-point link between two devices using cross-coupled transmit and receive lines.

Critical UART Transmission Parameters

Because UART lacks a shared clock line, both communicating devices must be configured with identical transmission parameters beforehand.

1. Baud Rate: Transmission speed measured in bits per second (standard rates include 9600 or 115200 baud). Both sides must match.
2. Start & Stop Bits: Frame markers that signal the beginning and completion of each data byte.
3. Parity Bit: Optional bit for error detection (the vast majority of projects use the 8N1 format: 8 data bits, no parity, 1 stop bit).

Arduino UART Code Example

Arduino provides the built-in Serial object to interact with the onboard hardware UART transceiver.

void setup() {
  // Initialize UART communication at 115200 baud
  Serial.begin(115200);
  Serial.println("UART interface ready!");
}

void loop() {
  // Check if incoming bytes are available in the serial buffer
  if (Serial.available() > 0) {
    char incomingChar = Serial.read();

    // Echo the received character back to the console
    Serial.print("Received: ");
    Serial.println(incomingChar);
  }
}

Expanding Ports with SoftwareSerial

On boards with a single hardware UART port like the Arduino Uno (pins 0 and 1, used by USB debugging), you can emulate extra serial ports on arbitrary GPIO pins using the SoftwareSerial library to communicate with GPS, Bluetooth (HC-05), or GSM modules.