I2C acknowledge and NACK behavior in Embedded C - Mini Project: Build & Apply

I2C Acknowledge and NACK Behavior

📖 Scenario: You are working with an embedded system that communicates with a sensor using the I2C protocol. The sensor sends data bytes, and your microcontroller must acknowledge (ACK) each byte received to confirm successful reception. If the microcontroller does not want more data, it sends a Not Acknowledge (NACK) signal to stop the communication.

🎯 Goal: Build a simple C program that simulates receiving 5 bytes from an I2C sensor. Your program will send an ACK after each byte except the last one, where it will send a NACK to indicate the end of data reception.

📋 What You'll Learn

Create an array called received_bytes with exactly 5 bytes: 0x10, 0x20, 0x30, 0x40, 0x50

Create a variable called byte_count and set it to 5

Use a for loop with variable i to iterate over received_bytes

Inside the loop, print the received byte in hexadecimal format

Send an ACK after each byte except the last one, where you send a NACK

Print ACK sent or NACK sent accordingly after each byte

💡 Why This Matters

🌍 Real World

I2C communication is common in embedded systems to read sensors or communicate with other chips. Correctly sending ACK and NACK signals ensures reliable data transfer.

💼 Career

Understanding I2C acknowledge behavior is essential for embedded software engineers working with microcontrollers and hardware interfaces.

1

Create the data array for received bytes

Create an array called received_bytes with these exact 5 bytes: 0x10, 0x20, 0x30, 0x40, 0x50.

Embedded C

// Create the array received_bytes with the 5 bytes
// Your code here

Hint

Use unsigned char received_bytes[5] = {0x10, 0x20, 0x30, 0x40, 0x50}; to create the array.

2

Create a variable for the number of bytes

Create an integer variable called byte_count and set it to 5.

Embedded C

unsigned char received_bytes[5] = {0x10, 0x20, 0x30, 0x40, 0x50};
// Create byte_count variable
// Your code here

Hint

Use int byte_count = 5; to store the number of bytes.

3

Loop through the bytes and simulate ACK/NACK

Use a for loop with variable i from 0 to byte_count - 1 to iterate over received_bytes. Inside the loop, print the byte in hexadecimal format using printf("Received byte: 0x%02X\n", received_bytes[i]). Then, if i is less than byte_count - 1, print ACK sent. Otherwise, print NACK sent.

Embedded C

unsigned char received_bytes[5] = {0x10, 0x20, 0x30, 0x40, 0x50};
int byte_count = 5;
// Loop through received_bytes and print ACK or NACK
// Your code here

Hint

Use a for loop and if condition to print ACK or NACK after each byte.

4

Print the final output

Run the program to print each received byte and the corresponding ACK or NACK message.

Embedded C

unsigned char received_bytes[5] = {0x10, 0x20, 0x30, 0x40, 0x50};
int byte_count = 5;

for (int i = 0; i < byte_count; i++) {
    printf("Received byte: 0x%02X\n", received_bytes[i]);
    if (i < byte_count - 1) {
        printf("ACK sent\n");
    } else {
        printf("NACK sent\n");
    }
}

// Run the program to see output
// Your code here

Hint

Run the program and check the console output matches the expected ACK and NACK messages.

Practice

1. What does an ACK (Acknowledge) signal mean in I2C communication?

easy

A. The sender is requesting to resend the data

B. The receiver wants to stop communication immediately

C. The communication line is idle

D. The receiver successfully received data and is ready for more

Solution

1. Step 1: Understand ACK meaning in I2C

   ACK means the receiver signals it has received the data correctly and is ready to continue.

2. Step 2: Compare options with ACK meaning

   Only The receiver successfully received data and is ready for more matches this meaning; others describe stop or errors.

3. Final Answer:

   The receiver successfully received data and is ready for more -> Option D

4. Quick Check:

   ACK = Ready for more data [OK]

Hint: ACK means receiver says 'send more' [OK]

Common Mistakes:

• Confusing ACK with NACK meaning
• Thinking ACK means stop communication
• Assuming ACK means error occurred

2. Which of the following code snippets correctly sends a NACK after reading the last byte in I2C using embedded C?

easy

A. i2c_read_byte(I2C1, ACK);

B. i2c_read_byte(I2C1, NACK);

C. i2c_write_byte(I2C1, NACK);

D. i2c_send_ack(I2C1);

Solution

1. Step 1: Identify correct function to read with NACK

   To signal no more data is needed, the read function must be called with NACK.

2. Step 2: Check options for correct usage

   i2c_read_byte(I2C1, NACK); uses i2c_read_byte with NACK, which is correct. Others either send ACK or write instead of read.

3. Final Answer:

   i2c_read_byte(I2C1, NACK); -> Option B

4. Quick Check:

   Read last byte with NACK to stop [OK]

Hint: Use read with NACK to stop reading last byte [OK]

Common Mistakes:

• Using ACK instead of NACK after last byte
• Calling write instead of read for NACK
• Confusing function names for ACK/NACK

3. Consider this code snippet reading 3 bytes from an I2C device:

for (int i = 0; i < 3; i++) {
  data[i] = (i < 2) ? i2c_read_byte(I2C1, ACK) : i2c_read_byte(I2C1, NACK);
}

What is the purpose of sending NACK on the last byte?

medium

A. To request the device to resend the last byte

B. To acknowledge the last byte and continue reading

C. To indicate the master will stop reading after this byte

D. To reset the I2C bus immediately

Solution

1. Step 1: Analyze the loop and ACK/NACK usage

   The loop reads 3 bytes; for first two bytes it sends ACK, for last byte it sends NACK.

2. Step 2: Understand NACK meaning on last byte

   NACK signals the master will stop reading after this byte, telling the slave no more data is needed.

3. Final Answer:

   To indicate the master will stop reading after this byte -> Option C

4. Quick Check:

   NACK on last byte = stop reading [OK]

Hint: NACK on last byte means stop reading [OK]

Common Mistakes:

• Thinking NACK requests resend
• Assuming NACK resets bus
• Confusing ACK and NACK roles

4. You wrote this code to read bytes from I2C but the slave device never stops sending data:

for (int i = 0; i < 4; i++) {
  data[i] = i2c_read_byte(I2C1, ACK);
}

What is the likely problem?

medium

A. You never sent NACK after the last byte to stop reading

B. You should use i2c_write_byte instead of i2c_read_byte

C. The loop should run 5 times instead of 4

D. You need to send a STOP condition before reading

Solution

1. Step 1: Check ACK/NACK usage in the loop

   The code sends ACK for every byte read, never sending NACK to signal stop.

2. Step 2: Understand effect of missing NACK

   Without NACK after last byte, slave keeps sending data, causing endless transmission.

3. Final Answer:

   You never sent NACK after the last byte to stop reading -> Option A

4. Quick Check:

   Missing NACK means no stop signal [OK]

Hint: Send NACK after last byte to stop slave [OK]

Common Mistakes:

• Using ACK for all bytes including last
• Confusing read and write functions
• Ignoring need for STOP condition after read

5. You want to write a function that reads a variable number of bytes from an I2C device and properly sends ACK for all bytes except the last one, which should send NACK. Which code snippet correctly implements this behavior?

hard

A. for (int i = 0; i < length - 1; i++) {
data[i] = i2c_read_byte(I2C1, ACK);
}
data[length - 1] = i2c_read_byte(I2C1, NACK);

B. for (int i = 0; i < length; i++) {
data[i] = i2c_read_byte(I2C1, ACK);
}

C. for (int i = 0; i < length; i++) {
data[i] = i2c_read_byte(I2C1, NACK);
}

D. data[0] = i2c_read_byte(I2C1, NACK);
for (int i = 1; i < length; i++) {
data[i] = i2c_read_byte(I2C1, ACK);
}

Solution

1. Step 1: Understand ACK/NACK pattern for multiple bytes

   All bytes except last must be read with ACK; last byte with NACK to stop.

2. Step 2: Analyze each option

   for (int i = 0; i < length - 1; i++) { data[i] = i2c_read_byte(I2C1, ACK); } data[length - 1] = i2c_read_byte(I2C1, NACK); reads all but last byte with ACK, then last byte with NACK, correctly signaling stop. Others send wrong signals or in wrong order.

3. Final Answer:

   for (int i = 0; i < length - 1; i++) { data[i] = i2c_read_byte(I2C1, ACK); } data[length - 1] = i2c_read_byte(I2C1, NACK); -> Option A

4. Quick Check:

   ACK all but last, NACK last byte [OK]

Hint: Send NACK only on last byte read [OK]

Common Mistakes:

• Sending NACK on all bytes
• Sending ACK on last byte
• Reading last byte first with NACK