I2C vs SPI decision matrix in Embedded C - Hands-On Comparison

I2C vs SPI Decision Matrix

📖 Scenario: You are working on a small embedded system project. You need to decide whether to use I2C or SPI communication protocol to connect sensors and devices.To help with this, you will create a simple decision matrix in C that stores key features of I2C and SPI and then selects the best protocol based on user requirements.

🎯 Goal: Build a C program that stores the features of I2C and SPI in a structured way, sets a requirement threshold, compares the protocols against the threshold, and prints which protocol is better for the given requirements.

📋 What You'll Learn

Create a struct to hold protocol features

Create two variables for I2C and SPI with exact feature values

Create a threshold variable for minimum required speed

Write a function or logic to compare protocols based on speed

Print the name of the protocol that meets or exceeds the threshold

💡 Why This Matters

🌍 Real World

Embedded systems often need to choose between I2C and SPI for device communication based on speed and wiring complexity.

💼 Career

Understanding how to compare hardware protocols and implement decision logic is useful for embedded software engineers and firmware developers.

1

Create protocol feature structures

Create a struct Protocol with const char* name and int speed_kbps. Then create two variables: i2c with "I2C" and speed 400, and spi with "SPI" and speed 10000.

Embedded C

# Define struct Protocol and create variables i2c and spi
# Your code here

Hint

Use a struct with two fields: name and speed_kbps. Initialize i2c and spi with the exact values.

2

Set minimum speed requirement

Create an int variable called min_speed and set it to 1000 to represent the minimum required speed in kbps.

Embedded C

struct Protocol {
    const char* name;
    int speed_kbps;
};

struct Protocol i2c = {"I2C", 400};
struct Protocol spi = {"SPI", 10000};

// Create min_speed variable
# Your code here

Hint

Just create an integer variable named min_speed and assign 1000.

3

Compare protocols based on speed

Write an if statement to check if i2c.speed_kbps is greater than or equal to min_speed. If yes, set a const char* variable best_protocol to i2c.name. Otherwise, set best_protocol to spi.name.

Embedded C

struct Protocol {
    const char* name;
    int speed_kbps;
};

struct Protocol i2c = {"I2C", 400};
struct Protocol spi = {"SPI", 10000};

int min_speed = 1000;

// Compare speeds and assign best_protocol
# Your code here

Hint

Use an if-else to compare speeds and assign best_protocol accordingly.

4

Print the best protocol

Write a printf statement to print "Best protocol: %s\n" with the variable best_protocol.

Embedded C

struct Protocol {
    const char* name;
    int speed_kbps;
};

struct Protocol i2c = {"I2C", 400};
struct Protocol spi = {"SPI", 10000};

int min_speed = 1000;

const char* best_protocol;

if (i2c.speed_kbps >= min_speed) {
    best_protocol = i2c.name;
} else {
    best_protocol = spi.name;
}

// Print the best protocol
# Your code here

Hint

Use printf to show the best_protocol string with a newline.

Practice

1. Which communication protocol uses only 2 wires and is suitable for connecting many devices with simple wiring?

easy

A. UART

B. SPI

C. CAN

D. I2C

Solution

1. Step 1: Recall wire count for I2C

   I2C uses 2 wires: SDA (data) and SCL (clock).

2. Step 2: Compare with SPI wiring

   SPI uses 4 wires, so it is not the protocol with only 2 wires.

3. Final Answer:

   I2C -> Option D

4. Quick Check:

   2 wires = I2C [OK]

Hint: I2C always uses 2 wires, SPI uses 4 wires [OK]

Common Mistakes:

• Confusing SPI as 2-wire protocol
• Thinking UART or CAN use 2 wires for multi-device
• Mixing up data and clock lines

2. Which of the following is the correct way to describe SPI wiring in embedded C context?

easy

A. SPI uses 3 wires: TX, RX, and CLK

B. SPI uses 2 wires: SDA and SCL

C. SPI uses 4 wires: MOSI, MISO, SCLK, and SS

D. SPI uses 1 wire for data

Solution

1. Step 1: Identify SPI signal lines

   SPI uses 4 wires: MOSI (Master Out Slave In), MISO (Master In Slave Out), SCLK (clock), and SS (slave select).

2. Step 2: Eliminate incorrect options

   Options B, C, and D do not list the correct 4 SPI wires.

3. Final Answer:

   SPI uses 4 wires: MOSI, MISO, SCLK, and SS -> Option C

4. Quick Check:

   SPI = 4 wires (MOSI, MISO, SCLK, SS) [OK]

Hint: SPI always has MOSI, MISO, SCLK, and SS wires [OK]

Common Mistakes:

• Confusing SPI with I2C wire names
• Forgetting slave select (SS) wire
• Assuming SPI uses only 2 or 3 wires

3. Given the following scenario: You need to connect 8 sensors to a microcontroller and require moderate speed communication. Which protocol is best suited?

medium

A. I2C, because it supports multiple devices with simple wiring

B. SPI, because it supports many devices easily

C. SPI, because it uses only 2 wires

D. I2C, because it is faster than SPI

Solution

1. Step 1: Analyze device count and wiring complexity

   Connecting 8 sensors means many devices; I2C supports multiple devices on 2 wires easily.

2. Step 2: Consider speed and wiring

   I2C is moderate speed but simpler wiring; SPI is faster but wiring gets complex with many devices.

3. Final Answer:

   I2C, because it supports multiple devices with simple wiring -> Option A

4. Quick Check:

   Many devices + simple wiring = I2C [OK]

Hint: Many devices + simple wiring = I2C [OK]

Common Mistakes:

• Choosing SPI for many devices ignoring wiring complexity
• Assuming SPI uses 2 wires
• Thinking I2C is faster than SPI

4. You wrote code to communicate with a sensor using SPI but the data is corrupted. Which of these is a likely cause?

medium

A. Using only 2 wires instead of 4 for SPI

B. Connecting multiple devices on I2C bus

C. Using I2C speed settings for SPI communication

D. Using pull-up resistors on SPI lines

Solution

1. Step 1: Check wiring requirements for SPI

   SPI requires 4 wires; using only 2 wires can cause data corruption.

2. Step 2: Evaluate other options

   Options A and C relate to I2C or speed mismatch, less likely cause of SPI data corruption; pull-ups on SPI lines are usually not needed.

3. Final Answer:

   Using only 2 wires instead of 4 for SPI -> Option A

4. Quick Check:

   SPI needs 4 wires, missing wires cause errors [OK]

Hint: SPI needs all 4 wires connected for reliable data [OK]

Common Mistakes:

• Using I2C settings for SPI communication
• Ignoring missing wires in SPI
• Adding pull-up resistors unnecessarily on SPI

5. You want to design a system with 3 devices requiring very fast data transfer and minimal wiring complexity. Which protocol should you choose and why?

hard

A. I2C, because it uses fewer wires and supports fast speed

B. SPI, because it supports fast data transfer with fewer devices

C. I2C, because it supports many devices even if slower

D. SPI, because it uses only 2 wires and is simple

Solution

1. Step 1: Analyze device count and speed needs

   3 devices is a small number; SPI is better for fast data transfer with fewer devices.

2. Step 2: Consider wiring complexity

   SPI uses 4 wires but with fewer devices wiring is manageable; I2C is slower though uses 2 wires.

3. Final Answer:

   SPI, because it supports fast data transfer with fewer devices -> Option B

4. Quick Check:

   Few devices + fast speed = SPI [OK]

Hint: Few devices + fast speed = SPI [OK]

Common Mistakes:

• Choosing I2C for fast speed needs
• Assuming SPI uses only 2 wires
• Ignoring device count impact on wiring