Why I2C is used in Embedded C - Performance Analysis

Time Complexity: Why I2C is used

O(n)

Understanding Time Complexity

We want to understand how the time to communicate using I2C changes as we add more devices.

How does the communication time grow when more devices share the same I2C bus?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

// Send data to multiple I2C devices
for (int i = 0; i < num_devices; i++) {
    i2c_start();
    i2c_write(device_addresses[i]);
    i2c_write(data[i]);
    i2c_stop();
}
    

This code sends data to each device on the I2C bus one by one.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Loop over each device to send data.
• How many times: Once per device, so num_devices times.

How Execution Grows With Input

As the number of devices increases, the total communication time grows linearly.

Input Size (num_devices)	Approx. Operations
10	About 10 communication sequences
100	About 100 communication sequences
1000	About 1000 communication sequences

Pattern observation: Doubling the devices roughly doubles the communication time.

Final Time Complexity

Time Complexity: O(n)

This means the time to communicate grows directly with the number of devices on the I2C bus.

Common Mistake

[X] Wrong: "Adding more devices won't affect communication time much because I2C is fast."

[OK] Correct: Each device requires its own communication sequence, so more devices mean more total time.

Interview Connect

Understanding how communication time grows with devices helps you design efficient embedded systems and explain your choices clearly.

Self-Check

"What if we used a faster bus speed or combined data for multiple devices in one transmission? How would the time complexity change?"

Practice

1. Why is I2C commonly used in embedded systems?

easy

A. It uses wireless signals for communication.

B. It requires a separate wire for each device.

C. It only supports one device at a time.

D. It allows multiple devices to communicate using only two wires.

Solution

1. Step 1: Understand I2C wiring

   I2C uses two wires: SDA (data) and SCL (clock) to connect devices.

2. Step 2: Recognize multi-device communication

   Multiple devices share these two wires, communicating by unique addresses.

3. Final Answer:

   It allows multiple devices to communicate using only two wires. -> Option D

4. Quick Check:

   I2C uses 2 wires for many devices [OK]

Hint: I2C uses 2 wires for many devices, not one per device [OK]

Common Mistakes:

• Thinking each device needs its own wire
• Confusing I2C with wireless communication
• Believing only one device can connect

2. Which of the following is the correct way to describe I2C communication lines?

easy

A. Separate data and clock lines for each device

B. Only one wire for both data and clock

C. Single data line and single clock line shared by all devices

D. Wireless data and wired clock lines

Solution

1. Step 1: Identify I2C lines

   I2C uses two lines: SDA for data and SCL for clock.

2. Step 2: Understand line sharing

   Both lines are shared by all devices on the bus, not separate per device.

3. Final Answer:

   Single data line and single clock line shared by all devices -> Option C

4. Quick Check:

   I2C shares SDA and SCL lines [OK]

Hint: I2C shares one data and one clock line for all devices [OK]

Common Mistakes:

• Assuming separate lines per device
• Thinking data and clock share one wire
• Confusing wireless with wired lines

3. You want to connect 5 sensors to a microcontroller using I2C. What is the main advantage of using I2C here?

easy

A. Each sensor needs its own data and clock wires

B. You can use only two wires total for all sensors

C. Sensors communicate wirelessly with microcontroller

D. I2C requires no addressing for devices

Solution

1. Step 1: Understand wiring for multiple devices

   I2C allows multiple devices to share the same two wires: SDA and SCL.

2. Step 2: Recognize wiring advantage

   This saves pins and wiring complexity compared to separate wires per sensor.

3. Final Answer:

   You can use only two wires total for all sensors -> Option B

4. Quick Check:

   Two wires for many devices saves pins [OK]

Hint: I2C uses 2 wires total regardless of device count [OK]

Common Mistakes:

• Thinking each sensor needs separate wires
• Assuming I2C is wireless
• Believing devices don't need addresses

4. Consider this I2C communication snippet in embedded C:

i2c_start();
i2c_write(0x50); // device address
uint8_t data = i2c_read();
i2c_stop();
printf("%d", data);

What does this code do?

medium

A. Starts I2C, writes device address, reads one byte, stops, prints data

B. Starts I2C, reads device address, writes one byte, stops, prints data

C. Starts I2C, writes data, reads address, stops, prints data

D. Starts I2C, stops immediately, prints zero

Solution

1. Step 1: Analyze I2C function calls

   i2c_start() begins communication, i2c_write(0x50) sends device address.

2. Step 2: Understand data flow

   i2c_read() reads one byte from the device, i2c_stop() ends communication, printf prints the data.

3. Final Answer:

   Starts I2C, writes device address, reads one byte, stops, prints data -> Option A

4. Quick Check:

   Write address, read data, stop, print [OK]

Hint: Write address before reading data in I2C [OK]

Common Mistakes:

• Mixing read and write order
• Assuming stop happens before read
• Confusing address with data

5. This embedded C code tries to communicate over I2C but fails:

i2c_write(0x50);
i2c_start();
i2c_read();
i2c_stop();

What is the main error?

medium

A. Calling i2c_write before i2c_start

B. Missing device address in i2c_write

C. Not calling i2c_stop before i2c_start

D. Reading data before writing device address

Solution

1. Step 1: Check function call order

   i2c_start() must be called before i2c_write() to begin communication.

2. Step 2: Identify error

   Here, i2c_write() is called before i2c_start(), which is incorrect and causes failure.

3. Final Answer:

   Calling i2c_write before i2c_start -> Option A

4. Quick Check:

   Start communication before writing [OK]

Hint: Always call i2c_start() before i2c_write() [OK]

Common Mistakes:

• Writing before starting communication
• Ignoring function call order
• Assuming stop must come before start