Talking to the Serial Monitor

Overview

The USB cable is also a serial link. Serial.print() sends text to the IDE's Serial Monitor, and Serial.read() receives what you type. It is the main way to see inside a running sketch.

You will need

• Arduino Uno R3 and USB cable

Wiring

Common problems

• Garbled characters: the Monitor's baud rate (bottom corner) must match Serial.begin().
• Every command "runs twice": the Monitor sends a newline after your text. This sketch ignores it; parsing code must too, or set line ending to "No line ending".
• Sketch restarts when you open the Monitor: normal on the Uno; opening the port resets the board.
• Upload fails while another program holds the port: close other serial tools first.

The Serial Plotter (Tools → Serial Plotter) graphs any line of numbers. Print several values separated by spaces or commas to plot them together.

The sketch

void setup() {
  pinMode(LED_BUILTIN, OUTPUT);
  Serial.begin(9600);
  Serial.println("Type 1 for LED on, 0 for off, ? for status");
}

void loop() {
  if (Serial.available() > 0) {
    char command = Serial.read();

    if (command == '1') {
      digitalWrite(LED_BUILTIN, HIGH);
      Serial.println("LED on");
    } else if (command == '0') {
      digitalWrite(LED_BUILTIN, LOW);
      Serial.println("LED off");
    } else if (command == '?') {
      Serial.print("LED is ");
      Serial.println(digitalRead(LED_BUILTIN) == HIGH ? "on" : "off");
      Serial.print("Uptime (ms): ");
      Serial.println(millis());
    }
    // Other characters, such as line endings, are ignored
  }
}

How the code works

Serial.println() adds a line break; Serial.print() does not. For fixed text, Serial.println(F("text")) keeps the string in flash memory instead of the Uno's 2 KB of RAM.

What you should observe

• The Monitor shows the instruction line. Typing 1 and Send turns on the L LED and replies "LED on".
• Typing ? reports the LED state and uptime.

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