Reading I2C sensor data in Arduino - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does I2C stand for and what is its main purpose?

I2C stands for Inter-Integrated Circuit. It is a communication protocol used to connect multiple devices (like sensors and microcontrollers) using just two wires: SDA (data) and SCL (clock).

beginner

Which Arduino library is commonly used to communicate with I2C devices?

The Wire library is used in Arduino to communicate with I2C devices. It handles sending and receiving data over the I2C bus.

beginner

What are the two main steps to read data from an I2C sensor in Arduino?

1. Request data from the sensor using Wire.requestFrom().
2. Read the incoming bytes using Wire.read().

beginner

Why do you need to know the sensor's I2C address before reading data?

Each I2C device has a unique address. You must specify this address when communicating so the microcontroller talks to the correct device on the bus.

beginner

What is the purpose of Wire.begin() in an Arduino I2C program?

Wire.begin() initializes the I2C bus and prepares the Arduino to communicate as a master device with I2C sensors.

Which two wires are used in I2C communication?

A MOSI and MISO

B SDA and SCL

C TX and RX

D VCC and GND

I2C uses SDA (data line) and SCL (clock line) for communication.

What Arduino function starts the I2C bus as a master?

A Wire.open()

B Wire.start()

C Wire.begin()

D Wire.init()

Wire.begin() initializes the I2C bus for the Arduino.

How do you request 6 bytes of data from an I2C sensor at address 0x40?

A Wire.receive(0x40, 6);

B Wire.read(0x40, 6);

C Wire.send(0x40, 6);

D Wire.requestFrom(0x40, 6);

Wire.requestFrom(address, quantity) asks the sensor to send data.

What function reads a single byte from the I2C data buffer?

A Wire.read()

B Wire.get()

C Wire.receive()

D Wire.fetch()

Wire.read() reads one byte from the incoming data.

Why is it important to know the I2C address of your sensor?

A To communicate with the correct device on the bus

B To power the sensor correctly

C To set the baud rate

D To configure the sensor's pins

The I2C address tells the Arduino which device to talk to among many on the bus.

Explain the basic steps to read data from an I2C sensor using Arduino.

Think about starting communication, asking for data, and reading it.

Why is the I2C protocol useful for connecting sensors to microcontrollers?

Consider how many wires and devices can share the same connection.

Practice

1. What is the main purpose of the Wire.begin() function in Arduino when reading I2C sensor data?

easy

A. To initialize the I2C communication bus

B. To read data from the sensor

C. To write data to the sensor

D. To close the I2C communication

Solution

1. Step 1: Understand the role of Wire.begin()

   The Wire.begin() function sets up the Arduino to use the I2C bus, preparing it to communicate with I2C devices.

2. Step 2: Differentiate from other functions

   Reading or writing data happens after initialization; Wire.begin() does not perform these actions.

3. Final Answer:

   To initialize the I2C communication bus -> Option A

4. Quick Check:

   Wire.begin() initializes I2C [OK]

Hint: Remember: begin() always starts communication [OK]

Common Mistakes:

• Confusing Wire.begin() with reading or writing functions
• Thinking Wire.begin() sends or receives data
• Skipping Wire.begin() before communication

2. Which of the following is the correct syntax to request 2 bytes of data from an I2C device with address 0x40 using the Wire library?

easy

A. Wire.readFrom(0x40, 2);

B. Wire.request(0x40, 2);

C. Wire.requestFrom(0x40, 2);

D. Wire.getFrom(0x40, 2);

Solution

1. Step 1: Recall the correct Wire library function

   The function to request bytes from an I2C device is Wire.requestFrom(address, quantity).

2. Step 2: Match the syntax with the options

   Only Wire.requestFrom(0x40, 2); uses the correct function name and parameters.

3. Final Answer:

   Wire.requestFrom(0x40, 2); -> Option C

4. Quick Check:

   requestFrom(address, bytes) is correct [OK]

Hint: Use requestFrom() to get bytes from I2C device [OK]

Common Mistakes:

• Using incorrect function names like request() or readFrom()
• Mixing up parameter order
• Forgetting to specify the number of bytes

3. Consider this Arduino code snippet reading two bytes from an I2C sensor:

Wire.requestFrom(0x50, 2);
if (Wire.available() >= 2) {
  byte highByte = Wire.read();
  byte lowByte = Wire.read();
  int value = (highByte << 8) | lowByte;
  Serial.println(value);
}

If the sensor sends bytes 0x01 and 0xF4, what will be printed?

medium

A. 500

B. 400

C. 5000

D. 256

Solution

1. Step 1: Understand byte combination

   The code shifts the first byte (0x01) left by 8 bits and ORs it with the second byte (0xF4). This forms a 16-bit number.

2. Step 2: Calculate the combined value

   0x01 << 8 = 0x0100 (256 decimal), 0xF4 = 244 decimal, sum = 256 + 244 = 500.

3. Final Answer:

   500 -> Option A

4. Quick Check:

   (0x01 << 8) + 0xF4 = 500 [OK]

Hint: Shift high byte 8 bits left, then OR with low byte [OK]

Common Mistakes:

• Adding bytes without shifting
• Mixing byte order (low byte first)
• Printing raw bytes instead of combined value

4. This code tries to read a byte from an I2C sensor but does not work correctly:

Wire.begin();
Wire.beginTransmission(0x30);
Wire.write(0x00);
Wire.endTransmission();
byte data = Wire.read();
Serial.println(data);

What is the main problem?

medium

A. Wire.beginTransmission() address is wrong

B. Wire.read() is called without requesting data first

C. Wire.write() should be after Wire.read()

D. Wire.endTransmission() should be called twice

Solution

1. Step 1: Check the sequence of I2C functions

   The code sends a command but never calls Wire.requestFrom() to ask for data before reading.

2. Step 2: Understand Wire.read() usage

   Wire.read() only returns data after Wire.requestFrom() has been called and data is available.

3. Final Answer:

   Wire.read() is called without requesting data first -> Option B

4. Quick Check:

   Must requestFrom() before read() [OK]

Hint: Always requestFrom() before reading data [OK]

Common Mistakes:

• Calling Wire.read() without requestFrom()
• Misunderstanding beginTransmission() purpose
• Not checking if data is available before reading

5. You want to read temperature data from an I2C sensor that sends two bytes: the first byte is the integer part, the second byte is the fractional part in hundredths (e.g., 25 and 75 means 25.75°C). Which code snippet correctly reads and prints the temperature as a float?

hard

A. Wire.requestFrom(0x48, 2);
if (Wire.available() >= 2) {
byte intPart = Wire.read();
byte fracPart = Wire.read();
float temp = intPart + fracPart / 100;
Serial.println(temp);
}

B. Wire.requestFrom(0x48, 2);
if (Wire.available() >= 2) {
byte fracPart = Wire.read();
byte intPart = Wire.read();
float temp = intPart + fracPart / 100.0;
Serial.println(temp);
}

C. Wire.requestFrom(0x48, 2);
byte intPart = Wire.read();
byte fracPart = Wire.read();
float temp = intPart + fracPart * 100;
Serial.println(temp);

D. Wire.requestFrom(0x48, 2);
if (Wire.available() >= 2) {
byte intPart = Wire.read();
byte fracPart = Wire.read();
float temp = intPart + fracPart * 0.01;
Serial.println(temp);
}

Solution

1. Step 1: Confirm correct byte order and availability check

   The sensor sends integer part first, then fractional part. The code must check availability before reading both bytes.

2. Step 2: Calculate temperature correctly

   Fractional part is in hundredths, so divide by 100 or multiply by 0.01. Wire.requestFrom(0x48, 2); if (Wire.available() >= 2) { byte intPart = Wire.read(); byte fracPart = Wire.read(); float temp = intPart + fracPart * 0.01; Serial.println(temp); } uses fracPart * 0.01, which is correct and clear.

3. Final Answer:

   the snippet with fracPart * 0.01 -> Option D

4. Quick Check:

   Check availability, read int then frac, multiply frac by 0.01 [OK]

Hint: Check data availability and multiply fraction by 0.01 [OK]

Common Mistakes:

• Reading bytes in wrong order
• Not checking Wire.available() before reading
• Multiplying fraction by 100 instead of dividing