I2C acknowledge and NACK behavior in Embedded C - Step-by-Step Execution

Concept Flow - I2C acknowledge and NACK behavior

Start Condition

↓

Send 7-bit Address + R/W bit

↓

Wait for ACK from Receiver

↓

Continue

↓

Send/Receive Data

This flow shows how the I2C master sends an address and waits for an ACK from the slave. If ACK received, communication continues; if NACK, the master stops or retries.

Execution Sample

Embedded C

i2c_start();
i2c_write(address);
ack = i2c_wait_ack();
if (ack == 0) {
  i2c_write(data);
} else {
  i2c_stop();
}

This code sends a start, writes an address, waits for ACK, then writes data if ACK received; otherwise, it stops communication.

Execution Table

Step	Action	Signal Sent	ACK/NACK Received	Next Action
1	Send Start Condition	START	N/A	Send Address
2	Send Address + R/W bit	Address+R/W	Waiting	Wait for ACK
3	Wait for ACK	Address+R/W	ACK (0)	Send Data
4	Send Data Byte	Data	Waiting	Wait for ACK
5	Wait for ACK	Data	ACK (0)	Continue or Stop
6	Send Stop Condition	STOP	N/A	End Communication

💡 Communication ends after sending STOP or if NACK received at any wait step.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5	Final
ack	undefined	undefined	0 (ACK received)	undefined	0 (ACK received)	0

Key Moments - 2 Insights

Why does the master wait for an ACK after sending the address?

The master waits to confirm the slave received the address and is ready. If no ACK (NACK), the master knows the slave did not respond and stops communication (see execution_table step 3).

What happens if the master receives a NACK after sending data?

If the master receives a NACK after data, it means the slave cannot accept more data or an error occurred. The master should stop or retry (not shown in this trace but implied after step 5).

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'ack' after step 3?

A 1 (NACK received)

B 0 (ACK received)

C undefined

D -1 (error)

💡 Hint

Check the 'ACK/NACK Received' column at step 3 in execution_table.

At which step does the master send the STOP condition?

A Step 4

B Step 3

C Step 6

D Step 2

💡 Hint

Look at the 'Action' column for 'Send Stop Condition' in execution_table.

If the slave sends a NACK after the address, what should the master do next?

A Send STOP condition and end communication

B Send another address immediately

C Continue sending data

D Ignore and wait for ACK again

💡 Hint

Refer to concept_flow where NACK after address leads to STOP condition.

Concept Snapshot

I2C communication starts with a START condition.
Master sends 7-bit address + R/W bit.
Slave responds with ACK (0) or NACK (1).
ACK means continue; NACK means stop or retry.
Data bytes also require ACK from slave.
STOP condition ends communication.

Full Transcript

In I2C communication, the master begins by sending a START condition. Then it sends the 7-bit address plus a read/write bit. The slave must respond with an ACK signal (logic 0) to confirm it received the address. If the master receives an ACK, it continues by sending or receiving data bytes. Each data byte sent also requires an ACK from the slave. If the slave sends a NACK (logic 1) at any point, the master stops communication by sending a STOP condition. This process ensures both devices are synchronized and data is transferred correctly.