Connecting multiple I2C devices in Arduino - Time & Space Complexity

Time Complexity: Connecting multiple I2C devices

O(n)

Understanding Time Complexity

When connecting multiple I2C devices, the time to communicate depends on how many devices we talk to.

We want to know how the communication time grows as we add more devices.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

#include <Wire.h>

const int deviceCount = 3; // Example device count
const int deviceAddresses[] = {0x20, 0x21, 0x22}; // Example device addresses
const byte data = 0xFF; // Example data to send

void setup() {
  Wire.begin();
}

void loop() {
  for (int i = 0; i < deviceCount; i++) {
    Wire.beginTransmission(deviceAddresses[i]);
    Wire.write(data);
    Wire.endTransmission();
  }
  delay(1000);
}
    

This code sends data to multiple I2C devices one by one in a loop.

Identify Repeating Operations

• Primary operation: Loop over all devices to send data.
• How many times: Once per device each loop cycle.

How Execution Grows With Input

Each device adds one more communication step, so time grows steadily with device count.

Input Size (n)	Approx. Operations
10	10 transmissions
100	100 transmissions
1000	1000 transmissions

Pattern observation: Time increases directly with the number of devices.

Final Time Complexity

Time Complexity: O(n)

This means the time to send data grows in a straight line as you add more devices.

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 step, so total time adds up with each device.

Interview Connect

Understanding how communication time grows with devices helps you design efficient sensor networks and troubleshoot delays.

Self-Check

"What if we send data to all devices in parallel using multiple I2C buses? How would the time complexity change?"

Practice

1. What is the main reason multiple I2C devices can share the same two wires on an Arduino?

easy

A. They use different clock speeds to avoid conflicts

B. Each device uses a separate data wire

C. Each device has a unique address to identify it on the bus

D. They communicate one at a time using different power pins

Solution

1. Step 1: Understand I2C wiring basics

   I2C devices share two wires: SDA (data) and SCL (clock).

2. Step 2: Identify how devices are distinguished

   Devices use unique addresses so the master knows which device to talk to.

3. Final Answer:

   Each device has a unique address to identify it on the bus -> Option C

4. Quick Check:

   Unique addresses allow multiple devices on same wires [OK]

Hint: Remember: unique addresses let devices share wires [OK]

Common Mistakes:

• Thinking devices use separate wires
• Assuming different clock speeds separate devices
• Believing devices use different power pins to communicate

2. Which Arduino code snippet correctly initializes the I2C bus for multiple devices?

easy

A. Wire.init();

B. Wire.begin();

C. Wire.begin(0x3C);

D. Wire.start();

Solution

1. Step 1: Recall the correct function to start I2C

   The Arduino Wire library uses Wire.begin() to start the I2C bus as master.

2. Step 2: Check other options for correctness

   Wire.begin(0x3C) sets device as slave with address 0x3C, not for master init. Wire.init() and Wire.start() do not exist in Wire library.

3. Final Answer:

   Wire.begin(); -> Option B

4. Quick Check:

   Wire.begin() starts I2C master bus [OK]

Hint: Use Wire.begin() once to start I2C bus [OK]

Common Mistakes:

• Using Wire.begin(address) to start master
• Calling non-existent Wire.init() or Wire.start()
• Not calling Wire.begin() at all

3. Given this Arduino code snippet, what will be the output on the Serial Monitor if device at address 0x40 returns 25 and device at 0x41 returns 30?

Wire.begin();
Wire.beginTransmission(0x40);
Wire.write(0x00);
Wire.endTransmission();
Wire.requestFrom(0x40, 1);
int temp1 = Wire.read();
Wire.beginTransmission(0x41);
Wire.write(0x00);
Wire.endTransmission();
Wire.requestFrom(0x41, 1);
int temp2 = Wire.read();
Serial.print(temp1);
Serial.print(", ");
Serial.println(temp2);

medium

A. Compilation error

B. 30, 25

C. 0, 0

D. 25, 30

Solution

1. Step 1: Understand the communication sequence

   The code requests one byte from device 0x40, reads it as temp1 (25), then requests one byte from 0x41, reads it as temp2 (30).

2. Step 2: Check the Serial output commands

   Serial.print(temp1) prints 25, then ", ", then Serial.println(temp2) prints 30 with newline.

3. Final Answer:

   25, 30 -> Option D

4. Quick Check:

   Read values match device addresses in order [OK]

Hint: Match device address to read value order [OK]

Common Mistakes:

• Mixing up device addresses and values
• Assuming output is reversed
• Thinking code causes compile error

4. This Arduino code tries to read from two I2C devices but always reads zero. What is the likely error?

Wire.begin();
Wire.beginTransmission(0x50);
Wire.write(0x01);
Wire.endTransmission();
Wire.requestFrom(0x50, 1);
int val1 = Wire.read();
Wire.beginTransmission(0x50);
Wire.write(0x02);
Wire.endTransmission();
Wire.requestFrom(0x51, 1);
int val2 = Wire.read();
Serial.println(val1 + val2);

medium

A. Requesting from 0x51 instead of 0x50 causes no data

B. Missing Wire.begin() call

C. Using Wire.write() before Wire.requestFrom() is wrong

D. Serial.println() cannot print integers

Solution

1. Step 1: Analyze device addresses used

   The code writes to 0x50 twice but requests data from 0x50 first, then 0x51 second. If 0x51 device is not connected, no data is returned.

2. Step 2: Understand impact on read values

   Reading from 0x51 returns no data, so Wire.read() returns 0, causing sum to be zero or incorrect.

3. Final Answer:

   Requesting from 0x51 instead of 0x50 causes no data -> Option A

4. Quick Check:

   Address mismatch causes no data read [OK]

Hint: Check device addresses match in all calls [OK]

Common Mistakes:

• Confusing write and requestFrom addresses
• Forgetting to call Wire.begin() once
• Thinking Wire.write() before requestFrom() is invalid

5. You want to connect three I2C temperature sensors with addresses 0x48, 0x49, and 0x4A to an Arduino. Which of the following is the best approach to read all sensors reliably?

hard

A. Call Wire.begin() once, then read each sensor by its unique address with Wire.requestFrom()

B. Call Wire.begin() before reading each sensor separately

C. Use separate SDA and SCL wires for each sensor to avoid conflicts

D. Assign the same address to all sensors and read them one by one

Solution

1. Step 1: Understand I2C bus initialization

   Wire.begin() must be called once to initialize the bus as master before any communication.

2. Step 2: Read sensors by unique addresses on shared bus

   Each sensor has a unique address; read each by calling Wire.requestFrom() with that address on the same SDA and SCL lines.

3. Step 3: Evaluate other options

   Calling Wire.begin() multiple times is unnecessary. Using separate wires defeats I2C design. Assigning same address causes conflicts.

4. Final Answer:

   Call Wire.begin() once, then read each sensor by its unique address with Wire.requestFrom() -> Option A

5. Quick Check:

   One Wire.begin(), unique addresses, shared wires [OK]

Hint: Initialize once, read each device by unique address [OK]

Common Mistakes:

• Calling Wire.begin() multiple times
• Trying separate wires for each I2C device
• Using same address for multiple devices