Wire library for I2C in Arduino - Time & Space Complexity

Time Complexity: Wire library for I2C

O(n)

Understanding Time Complexity

When using the Wire library for I2C communication, it's important to understand how the time to send or receive data changes as the amount of data grows.

We want to know how the time cost grows when we send more bytes over I2C.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

#include <Wire.h>

void sendData(byte address, byte* data, int length) {
  Wire.beginTransmission(address);
  for (int i = 0; i < length; i++) {
    Wire.write(data[i]);
  }
  Wire.endTransmission();
}
    

This code sends a list of bytes to a device over I2C using the Wire library.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The for-loop that calls Wire.write() for each byte.
• How many times: Exactly length times, once per byte to send.

How Execution Grows With Input

As the number of bytes to send increases, the number of write operations grows the same way.

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

Pattern observation: The time grows directly in proportion to the number of bytes sent.

Final Time Complexity

Time Complexity: O(n)

This means the time to send data grows linearly with the number of bytes you send.

Common Mistake

[X] Wrong: "Sending more bytes takes the same time as sending just one byte."

[OK] Correct: Each byte requires a separate write operation, so more bytes mean more time spent sending.

Interview Connect

Understanding how communication time grows with data size helps you write efficient code and explain your reasoning clearly in technical discussions.

Self-Check

"What if we buffered all bytes first and sent them in one call? How would the time complexity change?"

Practice

1. What is the main purpose of the Wire.begin() function in Arduino's Wire library?

easy

A. To request data from a device on the I2C bus

B. To send data to a device on the I2C bus

C. To end the I2C communication

D. To initialize the I2C bus and join the I2C network

Solution

1. Step 1: Understand the role of Wire.begin()

   The Wire.begin() function sets up the Arduino as a master or slave on the I2C bus and starts the communication.

2. Step 2: Differentiate from other Wire functions

   Functions like Wire.beginTransmission() and Wire.requestFrom() handle sending and receiving data, not initialization.

3. Final Answer:

   To initialize the I2C bus and join the I2C network -> Option D

4. Quick Check:

   Wire.begin() starts I2C communication [OK]

Hint: Wire.begin() always starts I2C communication [OK]

Common Mistakes:

• Confusing Wire.begin() with data sending functions
• Using Wire.beginTransmission() to start communication
• Thinking Wire.begin() sends or receives data

2. Which of the following is the correct syntax to start sending data to an I2C device with address 0x3C using the Wire library?

easy

A. Wire.start(0x3C);

B. Wire.beginTransmission(0x3C);

C. Wire.send(0x3C);

D. Wire.requestFrom(0x3C);

Solution

1. Step 1: Identify the function to start transmission

   Wire.beginTransmission(address) is used to begin sending data to the device at the given address.

2. Step 2: Check other options for correctness

   Wire.requestFrom() is for receiving data, Wire.send() is deprecated, and Wire.start() does not exist.

3. Final Answer:

   Wire.beginTransmission(0x3C); -> Option B

4. Quick Check:

   Use Wire.beginTransmission(address) to send data [OK]

Hint: Use Wire.beginTransmission(address) to start sending [OK]

Common Mistakes:

• Using Wire.requestFrom() to send data
• Using deprecated Wire.send() instead of Wire.write()
• Trying to use non-existent Wire.start() function

3. What will be the output of the following Arduino code snippet using the Wire library?

Wire.begin();
Wire.beginTransmission(0x50);
Wire.write(0x01);
Wire.endTransmission();
Wire.requestFrom(0x50, 1);
int data = Wire.read();
Serial.println(data);

medium

A. Prints the byte received from device at 0x50

B. Prints the byte 0x01

C. Prints 0 because Wire.read() returns 0 by default

D. Compilation error due to missing Wire.read()

Solution

1. Step 1: Analyze the code flow

   The code initializes I2C, sends one byte (0x01) to device 0x50, then requests 1 byte back from the same device.

2. Step 2: Understand Wire.read() behavior

   Wire.read() reads the byte received from the device. It does not return the sent byte but the device's response.

3. Final Answer:

   Prints the byte received from device at 0x50 -> Option A

4. Quick Check:

   Wire.read() returns received data, not sent data [OK]

Hint: Wire.read() reads incoming data, not what was sent [OK]

Common Mistakes:

• Assuming Wire.read() returns the sent byte
• Thinking Wire.read() returns 0 by default
• Confusing Wire.write() and Wire.read() roles

4. Identify the error in this Arduino I2C code snippet:

Wire.begin();
Wire.beginTransmission(0x40);
Wire.write(0x10);
Wire.requestFrom(0x40, 1);
Wire.endTransmission();
int val = Wire.read();

medium

A. Calling Wire.requestFrom() before Wire.endTransmission()

B. Missing Wire.beginTransmission() before Wire.write()

C. Using Wire.read() without requesting data

D. Not calling Wire.begin() to start I2C

Solution

1. Step 1: Check the order of I2C functions

   Wire.endTransmission() must be called to end sending data before requesting data with Wire.requestFrom().

2. Step 2: Identify the incorrect sequence

   The code calls Wire.requestFrom() before Wire.endTransmission(), which is incorrect and causes communication errors.

3. Final Answer:

   Calling Wire.requestFrom() before Wire.endTransmission() -> Option A

4. Quick Check:

   End transmission before requesting data [OK]

Hint: Always call Wire.endTransmission() before Wire.requestFrom() [OK]

Common Mistakes:

• Swapping the order of endTransmission and requestFrom
• Forgetting to call endTransmission() after beginTransmission()
• Calling Wire.read() without data available

5. You want to read 3 bytes from an I2C sensor at address 0x68 and store them in an array. Which code snippet correctly does this using the Wire library?

hard

A. Wire.beginTransmission(0x68);
for (int i = 0; i < 3; i++) {
data[i] = Wire.read();
}
Wire.endTransmission();

B. Wire.beginTransmission(0x68);
Wire.requestFrom(0x68, 3);
for (int i = 0; i < 3; i++) {
data[i] = Wire.read();
}
Wire.endTransmission();

C. Wire.requestFrom(0x68, 3);
for (int i = 0; i < 3; i++) {
data[i] = Wire.read();
}

D. Wire.requestFrom(0x68, 3);
Wire.endTransmission();
for (int i = 0; i < 3; i++) {
data[i] = Wire.read();
}

Solution

1. Step 1: Understand how to receive data with Wire.requestFrom()

   Wire.requestFrom(address, quantity) requests bytes from the device. Then Wire.read() reads each byte.

2. Step 2: Check the correct sequence for reading data

   Only Wire.requestFrom() is needed to receive data. Wire.beginTransmission() and Wire.endTransmission() are for sending data, not receiving.

3. Final Answer:

   Wire.requestFrom(0x68, 3); for (int i = 0; i < 3; i++) { data[i] = Wire.read(); } -> Option C

4. Quick Check:

   Use requestFrom() then read() in loop to get bytes [OK]

Hint: Use requestFrom() then read() in loop to get bytes [OK]

Common Mistakes:

• Using beginTransmission() when only reading data
• Calling endTransmission() after requestFrom()
• Reading data before requesting it