Writing data to I2C device in Embedded C

Introduction

We write data to an I2C device to send commands or information so it can perform tasks or store values.

You want to turn on an LED connected to an I2C controller.

You need to send a configuration setting to a sensor over I2C.

You want to update the display content on an I2C screen.

You need to control a motor driver via I2C commands.

Syntax

Embedded C

i2c_start();
i2c_write((device_address << 1) | I2C_WRITE);
i2c_write(register_address);
i2c_write(data);
i2c_stop();

i2c_start() begins communication.

device_address is the 7-bit I2C address of the device.

Examples

Write 0xFF to register 0x00 of device at address 0x3C.

Embedded C

i2c_start();
i2c_write((0x3C << 1) | I2C_WRITE);
i2c_write(0x00); // register

i2c_write(0xFF); // data

i2c_stop();

Send 0x01 to register 0x10 on device 0x50.

Embedded C

i2c_start();
i2c_write((0x50 << 1) | I2C_WRITE);
i2c_write(0x10); // register

i2c_write(0x01); // data

i2c_stop();

Sample Program

This program simulates writing the value 0xA5 to register 0x00 of an I2C device at address 0x3C. It prints each step to show the communication sequence.

Embedded C

#include <stdio.h>

#define I2C_WRITE 0

// Mock functions for I2C communication
void i2c_start() {
    printf("I2C Start\n");
}

void i2c_write(unsigned char data) {
    printf("Write: 0x%02X\n", data);
}

void i2c_stop() {
    printf("I2C Stop\n");
}

int main() {
    unsigned char device_address = 0x3C; // Example device
    unsigned char register_address = 0x00;
    unsigned char data = 0xA5;

    i2c_start();
    i2c_write((device_address << 1) | I2C_WRITE);
    i2c_write(register_address);
    i2c_write(data);
    i2c_stop();

    return 0;
}

Important Notes

Always shift the device address left by 1 bit and add the write bit (0) when sending the address.

Check the device datasheet for correct register addresses and data format.

Make sure to call i2c_start() before writing and i2c_stop() after finishing.

Summary

Writing data to an I2C device involves starting communication, sending the device address with write bit, the register address, the data, and then stopping communication.

Use the device's 7-bit address shifted left by one, with the least significant bit set to 0 for writing.

Always follow the device datasheet instructions for correct register and data values.

Practice

1. What is the first step when writing data to an I2C device in embedded C?

easy

A. Read the device address

B. Start the I2C communication

C. Stop the I2C communication

D. Send the data byte

Solution

1. Step 1: Begin communication with the device

   Writing data to an I2C device always starts with initiating the communication by sending a start condition. After starting, the device address and data can be sent, but the first step must be the start condition.

2. Final Answer:

   Start the I2C communication -> Option B

3. Quick Check:

   Start communication = first step [OK]

Hint: Always start communication before sending data [OK]

Common Mistakes:

• Sending data before starting communication
• Stopping communication too early
• Skipping the start condition

2. Which of the following is the correct way to specify the 7-bit I2C device address for writing in embedded C?

easy

A. device_address << 1 | 0

B. device_address >> 1

C. device_address << 1 | 1

D. device_address << 1 & ~1

Solution

1. Step 1: Understand I2C address format

   The 7-bit device address is shifted left by one bit to make room for the read/write bit as the least significant bit. For writing, the least significant bit must be 0, so OR with 0 is correct.

2. Final Answer:

   device_address << 1 | 0 -> Option A

3. Quick Check:

   Write bit = 0, address shifted left = correct [OK]

Hint: Shift address left and set LSB to 0 for write [OK]

Common Mistakes:

• Setting LSB to 1 for write operation
• Shifting address right instead of left
• Not shifting address at all

3. Given the following code snippet, what will be the result of the I2C write operation?

uint8_t device_addr = 0x50;
uint8_t reg_addr = 0x10;
uint8_t data = 0xAB;

I2C_Start();
I2C_Write(device_addr << 1 | 0);
I2C_Write(reg_addr);
I2C_Write(data);
I2C_Stop();

medium

A. Only register address is sent, data is missing

B. Data 0x10 is written to register 0xAB of device 0x50

C. Write operation fails due to wrong device address

D. Data 0xAB is written to register 0x10 of device 0x50

Solution

1. Step 1: Analyze the address and data sent

   The device address is correctly shifted and write bit set to 0. Then register address 0x10 and data 0xAB are sent in order. The sequence Start -> Address+Write -> Register -> Data -> Stop is correct for writing data to a register.

2. Final Answer:

   Data 0xAB is written to register 0x10 of device 0x50 -> Option D

3. Quick Check:

   Correct sequence = data written properly [OK]

Hint: Check address shift and write bit, then data order [OK]

Common Mistakes:

• Mixing register and data order
• Not shifting device address
• Forgetting to send stop condition

4. Identify the error in the following I2C write code snippet:

uint8_t device_addr = 0x68;
uint8_t reg_addr = 0x20;
uint8_t data = 0x55;

I2C_Start();
I2C_Write(device_addr >> 1 | 0);
I2C_Write(reg_addr);
I2C_Write(data);
I2C_Stop();

medium

A. Device address is shifted right instead of left

B. Data byte is sent before register address

C. Stop condition is missing

D. Write bit is set to 1 instead of 0

Solution

1. Step 1: Check device address shifting

   The device address is incorrectly shifted right instead of left, which corrupts the address sent on the bus. Register and data are sent in correct order, and stop condition is present, so no other errors.

2. Final Answer:

   Device address is shifted right instead of left -> Option A

3. Quick Check:

   Address must be shifted left for I2C write [OK]

Hint: Always shift device address left by 1 bit [OK]

Common Mistakes:

• Shifting address right instead of left
• Sending data before register address
• Omitting stop condition

5. You need to write multiple bytes to consecutive registers starting at 0x30 on an I2C device with address 0x60. Which code snippet correctly performs this operation in embedded C?

uint8_t device_addr = 0x60;
uint8_t start_reg = 0x30;
uint8_t data[] = {0x01, 0x02, 0x03};

// Which snippet is correct?

hard

A. I2C_Start();
I2C_Write(device_addr << 1 | 0);
for (int i = 0; i < 3; i++) {
I2C_Write(start_reg + i);
I2C_Write(data[i]);
}
I2C_Stop();

B. I2C_Start();
I2C_Write(device_addr << 1 | 0);
for (int i = 0; i < 3; i++) {
I2C_Write(data[i]);
}
I2C_Stop();

C. I2C_Start();
I2C_Write(device_addr << 1 | 0);
I2C_Write(start_reg);
for (int i = 0; i < 3; i++) {
I2C_Write(data[i]);
}
I2C_Stop();

D. I2C_Start();
I2C_Write(device_addr << 1 | 1);
I2C_Write(start_reg);
for (int i = 0; i < 3; i++) {
I2C_Write(data[i]);
}
I2C_Stop();

Solution

1. Step 1: Start communication and send device address with write bit

   The device address is shifted left and write bit set to 0, then start register address is sent once. After sending the start register, consecutive data bytes are sent to consecutive registers automatically by the device.

2. Final Answer:

   Start register sent once, then data bytes sent in loop -> Option C

3. Quick Check:

   Send start reg once, then data bytes [OK]

Hint: Send start register once, then all data bytes [OK]

Common Mistakes:

• Sending register address before each data byte
• Using read bit instead of write bit
• Not sending start register at all