I2C acknowledge and NACK behavior in Embedded C - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the ACK (Acknowledge) signal in I2C communication?

The ACK signal confirms that the receiver has successfully received a byte of data and is ready for the next byte. It helps ensure reliable data transfer between devices.

beginner

What does a NACK (Not Acknowledge) signal indicate in I2C communication?

A NACK signal means the receiver did not acknowledge the byte sent. This can indicate the end of data transfer, an error, or that the receiver is not ready to receive more data.

intermediate

How does the master device detect a NACK from the slave in I2C?

After sending a byte, the master releases the SDA line during the 9th clock pulse. If the slave pulls SDA low, it sends an ACK; if SDA stays high, it sends a NACK, which the master detects.

intermediate

In embedded C, how can you handle a NACK received after sending a byte over I2C?

You can check the status register or flag indicating NACK, then decide to retry, stop communication, or handle the error accordingly in your code.

intermediate

Why is it important to send a NACK after the last byte when reading data from a slave device?

Sending a NACK after the last byte tells the slave that the master will stop reading. This prevents the slave from sending more data and allows the master to generate a STOP condition safely.

What does the ACK signal in I2C indicate?

A Receiver successfully received data

B Receiver wants to stop communication

C Master is busy

D Data error occurred

ACK means the receiver got the data and is ready for the next byte.

When does the master detect a NACK from the slave?

A When SCL line is low

B When SDA line is pulled low during the 9th clock pulse

C When SDA line stays high during the 9th clock pulse

D When the slave sends a STOP condition

If SDA stays high during the 9th clock pulse, it means the slave sent a NACK.

What should the master do after sending a NACK during a read operation?

A Send a STOP condition to end communication

B Send more data bytes

C Ignore and continue reading

D Reset the slave device

A NACK signals the end of data; the master should stop communication.

Why is the 9th clock pulse important in I2C?

A It starts the data transfer

B It resets the bus

C It signals the STOP condition

D It is when the ACK/NACK bit is sent

The 9th clock pulse is used to send the ACK or NACK bit.

In embedded C, how can you detect a NACK after sending a byte?

A Check the data register for zero

B Check the I2C status register or flag

C Wait for a timeout

D Read the slave address again

The status register or flag indicates if a NACK was received.

Explain the role of ACK and NACK signals in I2C communication and how they help ensure data integrity.

Think about how devices say 'I got it' or 'stop' during talking.

Describe how you would handle a NACK received during an I2C data transfer in embedded C code.

Consider what your program should do if the other device says 'no' to your data.

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