I2C acknowledge and NACK behavior in Embedded C - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

I2C Master Transmit: What happens after sending address?

Consider this embedded C code snippet for an I2C master transmitting a slave address. What is the expected behavior if the slave acknowledges (ACK) the address?

i2c_start();
i2c_send_address(0x50, I2C_WRITE);
if (i2c_check_ack()) {
    // Slave acknowledged
    i2c_send_data(0xA5);
} else {
    // Slave did not acknowledge
    i2c_stop();
}

What happens after i2c_send_address if the slave sends an ACK?

Embedded C

i2c_start();
i2c_send_address(0x50, I2C_WRITE);
if (i2c_check_ack()) {
    i2c_send_data(0xA5);
} else {
    i2c_stop();
}

A The master immediately sends a stop condition without sending data.

B The master sends the data byte 0xA5 to the slave.

C The master waits indefinitely for a NACK from the slave.

D The master resets the I2C bus due to error.

💡 Hint

Remember, an ACK means the slave is ready to receive data.

When the slave acknowledges the address, the master proceeds to send data bytes. If the slave does not acknowledge, the master stops communication.

❓ Predict Output

intermediate

I2C Slave Receive: What does NACK indicate?

In an I2C slave device, after receiving a data byte from the master, the slave sends an ACK or NACK. What does it mean if the slave sends a NACK after a data byte?

Choose the correct interpretation.

A The slave is signaling it cannot accept more data or an error occurred.

B The slave is requesting the master to resend the last byte.

C The slave is initiating a bus reset.

D The slave is ready to receive more data bytes.

💡 Hint

NACK usually means stop or error in communication.

A NACK from the slave after a data byte means it cannot accept more data or there was a problem, so the master should stop or handle the error.

🔧 Debug

advanced

Why does this I2C master code hang after sending data?

Examine this I2C master code snippet. The code hangs after sending the first data byte. Identify the cause.

i2c_start();
i2c_send_address(0x68, I2C_WRITE);
if (!i2c_check_ack()) {
    i2c_stop();
    return;
}
i2c_send_data(0x12);
while (!i2c_check_ack()) {
    // wait for ACK
}
i2c_send_data(0x34);
i2c_stop();

A The master waits for an ACK after sending the first data byte, but the slave never sends it, causing an infinite loop.

B The start condition is not sent properly, so the slave never responds.

C The stop condition is sent too early, causing bus reset.

D The address sent is incorrect, so the slave ignores data bytes.

💡 Hint

Check the loop waiting for ACK after sending data.

The code waits indefinitely for an ACK after sending the first data byte. If the slave does not acknowledge, the loop never ends, causing a hang.

🧠 Conceptual

advanced

I2C NACK on last byte: What is the master’s role?

In an I2C read operation, the master reads multiple bytes from the slave. According to the I2C protocol, the master must send an ACK after each byte except the last one, after which it sends a NACK.

Why does the master send a NACK after the last byte?

A To reset the I2C bus for the next communication.

B To request the slave to resend the last byte.

C To indicate the master wants to end the read operation.

D To signal the slave to continue sending more data bytes.

💡 Hint

Think about how the master tells the slave it is done reading.

The master sends a NACK after the last byte to tell the slave it will not read more data, so the slave stops sending bytes.

❓ Predict Output

expert

What is the output of this I2C slave ACK/NACK simulation code?

This embedded C code simulates an I2C slave device sending ACK or NACK based on received data. What is the output?

#include <stdio.h>

int i2c_slave_receive_byte(int byte) {
    if (byte == 0xFF) {
        // Invalid data, send NACK
        return 0; // NACK
    } else {
        // Valid data, send ACK
        return 1; // ACK
    }
}

int main() {
    int data[] = {0x10, 0x20, 0xFF, 0x30};
    for (int i = 0; i < 4; i++) {
        int ack = i2c_slave_receive_byte(data[i]);
        if (ack) {
            printf("ACK ");
        } else {
            printf("NACK ");
        }
    }
    return 0;
}

Embedded C

#include <stdio.h>

int i2c_slave_receive_byte(int byte) {
    if (byte == 0xFF) {
        return 0; // NACK
    } else {
        return 1; // ACK
    }
}

int main() {
    int data[] = {0x10, 0x20, 0xFF, 0x30};
    for (int i = 0; i < 4; i++) {
        int ack = i2c_slave_receive_byte(data[i]);
        if (ack) {
            printf("ACK ");
        } else {
            printf("NACK ");
        }
    }
    return 0;
}

A ACK NACK ACK ACK

B NACK ACK ACK NACK

C NACK NACK NACK NACK

D ACK ACK NACK ACK

💡 Hint

Check which byte triggers NACK in the function.

Only the byte 0xFF causes the function to return NACK (0). All other bytes cause ACK (1). The output matches the sequence of ACKs and one NACK.