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LCD display (16x2) with I2C backpack in Raspberry Pi - Time & Space Complexity

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Time Complexity: LCD display (16x2) with I2C backpack
O(n)
Understanding Time Complexity

When working with an LCD display using an I2C backpack on a Raspberry Pi, it's important to understand how the time taken to update the display changes as the amount of data grows.

We want to know how the program's running time changes when we write more characters to the screen.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

from smbus2 import SMBus
import time

bus = SMBus(1)
address = 0x27

def write_string(string):
    for char in string:
        bus.write_byte(address, ord(char))
        time.sleep(0.01)

This code sends each character of a string one by one to the LCD via I2C, pausing briefly after each character.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

  • Primary operation: Loop over each character in the string to send it via I2C.
  • How many times: Once for every character in the input string.
How Execution Grows With Input

As the string gets longer, the number of I2C write operations grows directly with the number of characters.

Input Size (n)Approx. Operations
1010 writes + 10 pauses
100100 writes + 100 pauses
10001000 writes + 1000 pauses

Pattern observation: The time grows steadily and directly with the number of characters sent.

Final Time Complexity

Time Complexity: O(n)

This means the time to send data to the LCD increases in a straight line as the number of characters grows.

Common Mistake

[X] Wrong: "Sending one character or one hundred characters takes the same time because the I2C bus is fast."

[OK] Correct: Each character requires a separate write operation and a small delay, so more characters mean more time spent.

Interview Connect

Understanding how hardware communication time grows with data size helps you write efficient code and explain your design choices clearly.

Self-Check

"What if we removed the delay after each character? How would the time complexity change?"

Practice

(1/5)
1. What is the main advantage of using an I2C backpack with a 16x2 LCD on a Raspberry Pi?
easy
A. It powers the Raspberry Pi without an external source
B. It reduces the number of GPIO pins needed to control the LCD
C. It allows the LCD to display colors
D. It increases the LCD resolution beyond 16x2

Solution

  1. Step 1: Understand I2C backpack purpose

    The I2C backpack simplifies wiring by using only two data lines (SDA and SCL) instead of many GPIO pins.

  2. Step 2: Compare options

    Options A, B, and C are unrelated to I2C backpacks; only It reduces the number of GPIO pins needed to control the LCD correctly states the advantage.

  3. Final Answer:

    It reduces the number of GPIO pins needed to control the LCD -> Option B

  4. Quick Check:

    I2C reduces wiring = D [OK]
Hint: I2C means fewer wires needed for LCD control [OK]
Common Mistakes:
  • Thinking I2C changes LCD resolution
  • Assuming I2C adds color support
  • Confusing power supply with data interface
2. Which Python library is commonly used to control a 16x2 LCD with an I2C backpack on Raspberry Pi?
easy
A. NumPy
B. PIL
C. RPLCD
D. Matplotlib

Solution

  1. Step 1: Identify libraries purpose

    RPLCD is designed for controlling LCD displays, including those with I2C backpacks.

  2. Step 2: Eliminate unrelated libraries

    PIL is for images, NumPy for math arrays, Matplotlib for plotting; none control LCDs.

  3. Final Answer:

    RPLCD -> Option C

  4. Quick Check:

    LCD control library = RPLCD [OK]
Hint: RPLCD is the go-to library for LCDs on Raspberry Pi [OK]
Common Mistakes:
  • Confusing image or math libraries with LCD control
  • Using libraries without I2C support
  • Misspelling library names
3. What will be the output on the LCD after running this Python code snippet?
from RPLCD.i2c import CharLCD
lcd = CharLCD('PCF8574', 0x27)
lcd.write_string('Hello\nWorld!')
medium
A. Hello on first line, World! on second line
B. HelloWorld! on first line only
C. SyntaxError due to newline character
D. Nothing displayed because address is wrong

Solution

  1. Step 1: Understand write_string with newline

    The '\n' moves the cursor to the second line on the LCD.

  2. Step 2: Check address and syntax

    Address 0x27 is a common default; no syntax error in code.

  3. Final Answer:

    Hello on first line, World! on second line -> Option A

  4. Quick Check:

    Newline moves text to line 2 = C [OK]
Hint: Use '\n' to move to second line on LCD [OK]
Common Mistakes:
  • Thinking '\n' causes syntax error
  • Assuming text stays on one line
  • Using wrong I2C address without checking
4. Identify the error in this code snippet for initializing the LCD with I2C backpack:
from RPLCD.i2c import CharLCD
lcd = CharLCD(i2c_expander='PCF8574', address=0x3F)
lcd.write_string('Test')
medium
A. Missing 'port' parameter for CharLCD initialization
B. The parameter name should be 'address' not 'i2c_expander'
C. The I2C address 0x3F is invalid for this device
D. The parameter 'i2c_expander' should be 'i2c_expander' but is correct here

Solution

  1. Step 1: Check CharLCD parameters

    CharLCD requires 'i2c_expander', 'address', and 'port' parameters for I2C setup.

  2. Step 2: Identify missing parameter

    The code misses the 'port' parameter, which is needed to specify the I2C bus number.

  3. Final Answer:

    Missing 'port' parameter for CharLCD initialization -> Option A

  4. Quick Check:

    Missing required 'port' parameter = A [OK]
Hint: Always include 'port' when initializing CharLCD with I2C [OK]
Common Mistakes:
  • Assuming address 0x3F is always wrong
  • Confusing parameter names
  • Omitting required parameters
5. You want to display a scrolling message on a 16x2 LCD with I2C backpack using Python. Which approach is best?
hard
A. Write the full message once and rely on LCD auto-scroll
B. Change the I2C address dynamically during scrolling
C. Clear the LCD and write the full message repeatedly without delay
D. Use a loop to write substrings shifting one character at a time with delays

Solution

  1. Step 1: Understand scrolling on 16x2 LCD

    The LCD does not auto-scroll long messages; you must manually shift text.

  2. Step 2: Choose method to scroll

    Using a loop to write substrings with delays simulates scrolling smoothly.

  3. Step 3: Eliminate wrong options

    Write the full message once and rely on LCD auto-scroll is incorrect because LCD won't auto-scroll; C causes flicker; D is unrelated.

  4. Final Answer:

    Use a loop to write substrings shifting one character at a time with delays -> Option D

  5. Quick Check:

    Manual substring loop scrolls text = B [OK]
Hint: Scroll by shifting substrings in a timed loop [OK]
Common Mistakes:
  • Expecting LCD to auto-scroll long text
  • Clearing display too often causing flicker
  • Trying to change I2C address during scrolling
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