Writing data to I2C device in Embedded C - Step-by-Step Execution

Concept Flow - Writing data to I2C device

Start I2C Communication

↓

Send Device Address + Write Bit

↓

Wait for ACK from Device

↓

Send Data Byte

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Wait for ACK from Device

↓

Repeat Send Data Byte and Wait for ACK if more data

↓

Send Stop Condition

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End

The process starts communication, sends the device address with write intent, sends data bytes waiting for acknowledgments, and ends with a stop signal.

Execution Sample

Embedded C

i2c_start();
i2c_write(0x50 << 1 | 0);
i2c_write(0xA5);
i2c_stop();

This code writes one byte (0xA5) to an I2C device at address 0x50.

Execution Table

Step	Action	Data Sent	ACK Received	Next Step
1	Start I2C communication	Start condition	N/A	Send device address with write bit
2	Send device address + write bit	0xA0 (0x50 << 1 | 0)	Yes	Send data byte
3	Send data byte	0xA5	Yes	Send stop condition
4	Send stop condition	Stop condition	N/A	End communication

💡 Stop condition sent, communication ends.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4
i2c_state	Idle	Address sent	Data sent	Stopped
ack_flag	N/A	Received	Received	N/A

Key Moments - 3 Insights

Why do we shift the device address left by 1 and add 0?

The device address is 7 bits; shifting left makes room for the read/write bit (0 for write). See execution_table step 2 where 0x50 is shifted and combined with 0.

What happens if the device does not send an ACK after address or data?

The communication should stop or retry because the device did not acknowledge. In the execution_table, ACK is 'Yes' at steps 2 and 3, meaning communication continues.

Why is the stop condition important?

It signals the end of communication on the I2C bus. Without it, the device might wait for more data. See execution_table step 4 where stop condition ends the process.

Visual Quiz - 3 Questions

Look at the execution_table, what data is sent at step 2?

A 0xA0

B 0xA5

C 0x50

D Stop condition

💡 Hint

Check the 'Data Sent' column at step 2 in the execution_table.

At which step does the device acknowledge the data byte?

A Step 2

B Step 3

C Step 1

D Step 4

💡 Hint

Look at the 'ACK Received' column for the data byte send step in the execution_table.

If the device does not acknowledge at step 3, what should happen?

A Continue sending more data bytes

B Restart communication immediately

C Send stop condition and end communication

D Ignore and wait

💡 Hint

Refer to key_moments about handling missing ACK after data byte.

Concept Snapshot

I2C write sequence:
1. Start condition
2. Send 7-bit device address + 0 (write bit)
3. Wait for ACK
4. Send data byte(s), wait for ACK each
5. Send stop condition
Use shifting for address + write bit.
Stop signals end of communication.

Full Transcript

Writing data to an I2C device involves starting communication, sending the device address with the write bit, waiting for acknowledgment, sending data bytes with acknowledgments, and ending with a stop condition. The device address is shifted left by one bit to make space for the read/write bit. Each step waits for the device to acknowledge before continuing. If acknowledgment is missing, communication should stop to avoid errors. The stop condition signals the end of the data transfer on the I2C bus.