Wire library for I2C in Arduino - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this I2C master request?

Consider this Arduino code snippet using the Wire library as an I2C master. What will be printed on the Serial Monitor?

Arduino

void setup() {
  Wire.begin();
  Serial.begin(9600);
  Wire.requestFrom(8, 2); // Request 2 bytes from slave address 8
  while (Wire.available()) {
    int c = Wire.read();
    Serial.print(c);
    Serial.print(' ');
  }
}

void loop() {}

A 0 0

B The code will not compile due to missing slave initialization

C The code will hang waiting for data

D Random bytes printed depending on slave device

💡 Hint

Think about what happens if no slave device responds at address 8.

The Wire.requestFrom() asks for 2 bytes from slave address 8. If no slave is connected or responding, the master may receive no data or garbage values, so the output depends on the slave device's response.

🧠 Conceptual

intermediate

What does Wire.begin() do in Arduino I2C?

In Arduino's Wire library, what is the main purpose of calling Wire.begin() in your sketch?

A It resets the Arduino board

B It initializes the I2C bus and sets the device as a master by default

C It configures the serial port for debugging

D It sends data to the slave device immediately

💡 Hint

Think about what must happen before you can communicate over I2C.

Wire.begin() initializes the I2C hardware and sets the Arduino as a master device by default, preparing it to send or receive data.

🔧 Debug

advanced

Why does this I2C slave code not respond correctly?

This Arduino code is intended to act as an I2C slave device at address 9. Why does it fail to respond to master requests?

Arduino

void setup() {
  Wire.begin(9);
  Wire.onRequest(requestEvent);
  Serial.begin(9600);
}

void loop() {
  // Nothing here
}

void requestEvent() {
  Wire.write("Hi");
}

A Wire.write() inside requestEvent must send bytes, not a string literal

B Wire.write("Hi") sends 2 bytes but master expects 1 byte, causing mismatch

C The requestEvent function must be declared as static

D Wire.begin(9) must be called after Wire.onRequest()

💡 Hint

Check what Wire.write() accepts as input in slave mode.

Wire.write() expects bytes or byte arrays. Passing a string literal like "Hi" sends the pointer address, not the characters. To send characters, use Wire.write((uint8_t*)"Hi", 2).

📝 Syntax

advanced

Which option correctly writes data to an I2C slave?

You want to send the byte 0x42 to a slave device at address 5 using the Wire library. Which code snippet is correct?

A Wire.send(5, 0x42);

B

Wire.requestFrom(5, 1);
Wire.write(0x42);

C

Wire.beginTransmission(5);
Wire.write(0x42);
Wire.endTransmission();

D

Wire.begin(5);
Wire.write(0x42);

💡 Hint

Remember the sequence to send data as a master.

To send data as a master, you start transmission to the slave address, write bytes, then end transmission. requestFrom is for reading, Wire.send does not exist, and Wire.begin(5) sets device as slave.

🚀 Application

expert

How many bytes are received by this I2C slave?

An Arduino is set as an I2C slave at address 10. The master sends 3 bytes: 0x01, 0x02, 0x03. The slave code uses this handler: void receiveEvent(int howMany) { int count = 0; while (Wire.available()) { Wire.read(); count++; } Serial.println(count); } What will be printed on the Serial Monitor when the master sends data?

A 3

B 0

C The code will not compile due to missing parameter type

D 1

💡 Hint

How many bytes does Wire.available() report after receiving data?

The receiveEvent handler is called with howMany = 3. Wire.available() returns the number of bytes received. The loop reads all bytes and counts them, so count will be 3.