Receiving commands from computer in Arduino

Introduction

We use this to let the Arduino listen and respond to instructions sent from a computer. It helps the Arduino do different tasks based on what the computer tells it.

You want to control an LED on Arduino from your computer keyboard.

You need to send data from a computer program to Arduino to change motor speed.

You want to update settings on Arduino without changing the code.

You want Arduino to perform different actions based on commands typed in a serial monitor.

Syntax

Arduino

void setup() {
  Serial.begin(9600); // Start serial communication at 9600 baud
}

void loop() {
  if (Serial.available() > 0) {
    char command = Serial.read();
    // Use the command to do something
  }
}

Serial.begin(baud_rate) starts communication speed between Arduino and computer.

Serial.available() checks if there is data waiting to be read.

Examples

Read one character command and check if it is 'a'.

Arduino

if (Serial.available() > 0) {
  char command = Serial.read();
  if (command == 'a') {
    // Turn LED on
  }
}

Read a full line of text until Enter key (newline).

Arduino

if (Serial.available()) {
  String input = Serial.readStringUntil('\n');
  // Use the input string
}

Sample Program

This program waits for commands from the computer. If you send '1', it turns the LED on. If you send '0', it turns the LED off. It also sends back messages to the computer to confirm.

Arduino

const int ledPin = 13;

void setup() {
  pinMode(ledPin, OUTPUT);
  Serial.begin(9600);
  Serial.println("Send '1' to turn ON LED, '0' to turn OFF.");
}

void loop() {
  if (Serial.available() > 0) {
    char command = Serial.read();
    if (command == '1') {
      digitalWrite(ledPin, HIGH);
      Serial.println("LED is ON");
    } else if (command == '0') {
      digitalWrite(ledPin, LOW);
      Serial.println("LED is OFF");
    } else {
      Serial.println("Unknown command");
    }
  }
}

Important Notes

Always set the same baud rate in your computer program and Arduino.

Use Serial Monitor in Arduino IDE to send commands easily.

Commands are case sensitive; '1' is different from '1 ' (with space).

Summary

Use Serial.begin() to start communication.

Check Serial.available() before reading data.

Use Serial.read() or Serial.readStringUntil() to get commands.

Practice

1. What is the purpose of Serial.begin(9600); in an Arduino sketch?

easy

A. It reads data from the serial port.

B. It starts serial communication at 9600 bits per second.

C. It sends data to the computer automatically.

D. It stops the serial communication.

Solution

1. Step 1: Understand Serial.begin()

   Serial.begin(9600); initializes the serial communication at a speed of 9600 bits per second.

2. Step 2: Identify its role in communication

   This function sets up the Arduino to send and receive data through the serial port at the specified speed.

3. Final Answer:

   It starts serial communication at 9600 bits per second. -> Option B

4. Quick Check:

   Serial.begin() = start communication [OK]

Hint: Serial.begin() always starts communication at given speed [OK]

Common Mistakes:

• Thinking Serial.begin() reads or sends data
• Confusing Serial.begin() with Serial.read()
• Assuming Serial.begin() stops communication

2. Which of the following is the correct way to check if data is available to read from the serial port?

easy

A. if (Serial.available()) { }

B. if (Serial.write()) { }

C. if (Serial.begin()) { }

D. if (Serial.read() > 0) { }

Solution

1. Step 1: Identify function to check data availability

   Serial.available() returns the number of bytes available to read from the serial buffer.

2. Step 2: Understand usage in condition

   Using if (Serial.available()) checks if there is any data to read (non-zero means data is available).

3. Final Answer:

   if (Serial.available()) { } -> Option A

4. Quick Check:

   Serial.available() checks data presence [OK]

Hint: Use Serial.available() to check before reading [OK]

Common Mistakes:

• Using Serial.read() to check availability
• Calling Serial.begin() inside loop
• Using Serial.write() to check data

3. What will be the output on the Serial Monitor if the following code receives the input string "HELLO"?

void setup() {
  Serial.begin(9600);
}

void loop() {
  if (Serial.available()) {
    String command = Serial.readStringUntil('\n');
    Serial.println(command);
  }
}

medium

A. HELLO\n

B. H

C. HELLO

D. No output

Solution

1. Step 1: Understand Serial.readStringUntil()

   This function reads characters from the serial buffer until it finds the newline character '\n'. It returns the string without the '\n'.

2. Step 2: Analyze the code output

   The input "HELLO" followed by Enter sends "HELLO\n". The code reads "HELLO" and prints it exactly.

3. Final Answer:

   HELLO -> Option C

4. Quick Check:

   readStringUntil('\n') returns string without newline [OK]

Hint: readStringUntil('\n') excludes newline from output [OK]

Common Mistakes:

• Expecting newline character printed
• Thinking only one character is read
• Assuming no output without delay

4. Identify the error in this Arduino code snippet that tries to read a command from the serial port:

void loop() {
  if (Serial.available > 0) {
    char c = Serial.read();
    Serial.print(c);
  }
}

medium

A. Serial.available is used without parentheses

B. Serial.read() is missing a parameter

C. Serial.print() cannot print char variables

D. The if condition should check for Serial.read() instead

Solution

1. Step 1: Check Serial.available usage

   Serial.available is a function and must be called with parentheses: Serial.available().

2. Step 2: Verify other function calls

   Serial.read() correctly reads one byte without parameters; Serial.print() can print chars.

3. Final Answer:

   Serial.available is used without parentheses -> Option A

4. Quick Check:

   Functions need parentheses to call [OK]

Hint: Always use parentheses when calling functions like Serial.available() [OK]

Common Mistakes:

• Forgetting parentheses on function calls
• Thinking Serial.read() needs parameters
• Assuming Serial.print() can't print chars

5. You want to receive a command string from the computer and turn on an LED if the command is "ON" and turn it off if "OFF". Which code snippet correctly implements this?

void setup() {
  Serial.begin(9600);
  pinMode(13, OUTPUT);
}

void loop() {
  if (Serial.available()) {
    String cmd = Serial.readStringUntil('\n');
    // What goes here?
  }
}

hard

A. if (cmd === "ON") digitalWrite(13, HIGH); else if (cmd === "OFF") digitalWrite(13, LOW);

B. if (cmd = "ON") digitalWrite(13, HIGH); else if (cmd = "OFF") digitalWrite(13, LOW);

C. if (cmd.equal("ON")) digitalWrite(13, HIGH); else if (cmd.equal("OFF")) digitalWrite(13, LOW);

D. if (cmd == "ON") digitalWrite(13, HIGH); else if (cmd == "OFF") digitalWrite(13, LOW);

Solution

1. Step 1: Understand String comparison in Arduino

   Arduino String objects overload the == operator to compare contents with string literals like "ON".

2. Step 2: Check each option's correctness

   if (cmd == "ON") correctly compares string contents. if (cmd = "ON") performs assignment, not comparison. cmd.equal("ON") fails--no such method (it's equals()). === is invalid C++ syntax.

3. Final Answer:

   if (cmd == "ON") digitalWrite(13, HIGH); else if (cmd == "OFF") digitalWrite(13, LOW); -> Option D

4. Quick Check:

   Arduino String == compares content [OK]

Hint: Use cmd == "ON" to compare Arduino Strings [OK]

Common Mistakes:

• Using = instead of == for comparison
• Calling non-existent cmd.equal()
• Using JavaScript === operator in Arduino