SPI with display modules in Raspberry Pi - Time & Space Complexity

Time Complexity: SPI with display modules

O(n)

Understanding Time Complexity

When using SPI to send data to display modules, it's important to understand how the time to send data grows as the amount of data increases.

We want to know how the communication time changes when we send more pixels or commands.

Scenario Under Consideration

Analyze the time complexity of the following SPI data sending code.

import spidev

spi = spidev.SpiDev()
spi.open(0, 0)

pixels = [0xFF, 0x00, 0xAA, 0x55] * 100  # Example pixel data

for pixel in pixels:
    spi.xfer([pixel])  # Send one byte per pixel

spi.close()

This code sends a list of pixel bytes one by one over SPI to a display module.

Identify Repeating Operations

• Primary operation: Sending one byte over SPI inside a loop.
• How many times: Once for each pixel in the list.

How Execution Grows With Input

Each pixel byte requires one SPI transfer, so the total time grows as we add more pixels.

Input Size (n)	Approx. Operations
10	10 SPI transfers
100	100 SPI transfers
1000	1000 SPI transfers

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

Final Time Complexity

Time Complexity: O(n)

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

Common Mistake

[X] Wrong: "Sending more pixels doesn't affect the total time much because SPI is fast."

[OK] Correct: Even though SPI is fast, each pixel still requires a separate transfer, so more pixels mean more time overall.

Interview Connect

Understanding how data transfer time grows with data size helps you design efficient communication with hardware, a useful skill in many projects and interviews.

Self-Check

"What if we send multiple pixels in one SPI transfer instead of one byte at a time? How would the time complexity change?"