Reading data from I2C device in Embedded C - Step-by-Step Execution

Concept Flow - Reading data from I2C device

Start I2C communication

↓

Send device address + write bit

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Send register address to read

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Restart I2C communication

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Send device address + read bit

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Read data byte(s) from device

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Send NACK and stop condition

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End communication

This flow shows the steps to read data from an I2C device: start communication, send device and register addresses, restart for reading, read data, then stop.

Execution Sample

Embedded C

uint8_t read_i2c_data(uint8_t dev_addr, uint8_t reg_addr) {
    i2c_start();
    i2c_write(dev_addr << 1 | 0); // write mode
    i2c_write(reg_addr);
    i2c_start();
    i2c_write(dev_addr << 1 | 1); // read mode
    uint8_t data = i2c_read_nack();
    i2c_stop();
    return data;
}

This function reads one byte from a register of an I2C device by following the I2C read protocol.

Execution Table

Step	Action	I2C Bus State	Data Sent/Received	Result
1	i2c_start()	Start condition sent	None	Bus ready for address
2	i2c_write(dev_addr << 1 | 0)	Address + Write bit sent	Device address + 0	ACK received from device
3	i2c_write(reg_addr)	Register address sent	Register address	ACK received from device
4	i2c_start()	Repeated start condition sent	None	Bus ready for read address
5	i2c_write(dev_addr << 1 | 1)	Address + Read bit sent	Device address + 1	ACK received from device
6	i2c_read_nack()	Reading data byte	Data byte received	Data stored in variable
7	i2c_stop()	Stop condition sent	None	Communication ended

💡 Communication ends after stop condition; data byte successfully read.

Variable Tracker

Variable	Start	After Step 6	Final
dev_addr	input	unchanged	unchanged
reg_addr	input	unchanged	unchanged
data	undefined	received byte from device	returned value

Key Moments - 3 Insights

Why do we send a repeated start condition before reading?

The repeated start (step 4) tells the device we want to switch from writing the register address to reading data without releasing the bus, as shown in the execution_table.

What does the 'ACK received from device' mean after sending address or data?

It means the device acknowledged it received the byte correctly and is ready for the next step, ensuring communication continues smoothly (see steps 2, 3, and 5).

Why do we send a NACK after reading the data byte?

Sending a NACK after reading (step 6) tells the device we do not want more data, so it stops sending bytes before we send the stop condition.

Visual Quiz - 3 Questions

Look at the execution_table, what is the data variable value after step 6?

A The data byte received from the device

B The device address with read bit

C The register address sent

D Undefined, no data read yet

💡 Hint

Check the 'Data Sent/Received' and 'Result' columns at step 6 in the execution_table.

At which step does the repeated start condition occur?

A Step 2

B Step 6

C Step 4

D Step 7

💡 Hint

Look for 'Repeated start condition sent' in the 'I2C Bus State' column of the execution_table.

If the device does not acknowledge the address at step 2, what happens?

A The device sends data anyway

B The communication should be stopped or retried

C The communication continues normally

D The register address is sent twice

💡 Hint

Recall that 'ACK received from device' means device is ready; no ACK means communication error (see step 2).

Concept Snapshot

I2C read sequence:
1. Start condition
2. Send device address + write bit
3. Send register address
4. Repeated start
5. Send device address + read bit
6. Read data byte + NACK
7. Stop condition
Always check ACK after each byte.

Full Transcript

This visual execution shows how to read a byte from an I2C device. First, the master sends a start condition, then the device address with the write bit to select the register. After sending the register address, a repeated start is sent to switch to read mode. The device address with the read bit is sent next, followed by reading the data byte. Finally, a NACK and stop condition end the communication. Variables track the device address, register address, and the data read. Key moments include why the repeated start is needed, the meaning of ACK, and why a NACK is sent after reading. The quizzes test understanding of data values, steps, and error handling.