Receiving commands from computer in Arduino - Step-by-Step Execution

Concept Flow - Receiving commands from computer

Start

↓

Wait for data

↓

Data available?

No→Wait

Yes↓

Read command

↓

Process command

↓

Execute action

↓

Loop back to wait

The Arduino waits for data from the computer, reads it when available, processes the command, executes the action, then waits again.

Execution Sample

Arduino

void loop() {
  if (Serial.available() > 0) {
    char command = Serial.read();
    if (command == 'L') {
      digitalWrite(LED_BUILTIN, HIGH);
    }
  }
}

This code waits for a character from the computer and turns on the LED if the character is 'L'.

Execution Table

Step	Serial.available()	Condition	Action	Output
1	0	0 > 0? No	Wait for data	No output
2	1	1 > 0? Yes	Read 'L'	No output
3	N/A	'L' == 'L'? Yes	Turn LED on	LED ON
4	0	0 > 0? No	Wait for data	No output

💡 Loop continues indefinitely, waiting for new commands.

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
command	undefined	'L'	'L'	'L'

Key Moments - 2 Insights

Why does the code check Serial.available() before reading?

Because Serial.read() only works if data is available; checking prevents reading empty data, as shown in step 1 and 2 of the execution_table.

What happens if the command is not 'L'?

The code does nothing and waits for the next command, as only 'L' triggers the LED action in step 3.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of Serial.available() at step 2?

A 0

B 2

C 1

D Undefined

💡 Hint

Check the 'Serial.available()' column at step 2 in the execution_table.

At which step does the LED turn on?

A Step 3

B Step 2

C Step 1

D Step 4

💡 Hint

Look at the 'Action' and 'Output' columns in the execution_table.

If the command received was 'X' instead of 'L', what would happen?

A LED turns on

B No action taken

C LED turns off

D Program stops

💡 Hint

Refer to the key_moments explanation about commands other than 'L'.

Concept Snapshot

Arduino reads commands from computer via Serial.
Use Serial.available() to check data presence.
Use Serial.read() to get one byte.
Compare command and act accordingly.
Loop continuously to keep listening.

Full Transcript

This example shows how an Arduino receives commands from a computer through the serial port. The program waits until data is available using Serial.available(). When data arrives, it reads one character with Serial.read(). If the character is 'L', it turns on the built-in LED. Otherwise, it does nothing and waits for more commands. This loop repeats forever, allowing the Arduino to respond to commands sent from the computer.

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