I2C vs SPI decision matrix in Embedded C - Practice Questions

Challenge - 5 Problems

❓ Predict Output

intermediate

I2C vs SPI: Which bus speed is faster?

Consider the following code snippets initializing I2C and SPI buses with typical speeds. What is the output when comparing their speeds?

Embedded C

#include <stdio.h>

int main() {
    int i2c_speed = 400000; // 400 kHz
    int spi_speed = 10000000; // 10 MHz

    if (spi_speed > i2c_speed) {
        printf("SPI is faster\n");
    } else {
        printf("I2C is faster\n");
    }
    return 0;
}

A SPI is faster

B I2C is faster

C Both have the same speed

D Compilation error

💡 Hint

Compare the numeric values of spi_speed and i2c_speed.

SPI typically runs at higher clock speeds than I2C. Here, 10 MHz (SPI) is greater than 400 kHz (I2C), so SPI is faster.

🧠 Conceptual

intermediate

I2C vs SPI: Number of wires used

Which option correctly states the number of wires used by I2C and SPI respectively?

A I2C uses 3 wires; SPI uses 3 wires

B I2C uses 2 wires; SPI uses 4 or more wires

C I2C uses 4 wires; SPI uses 2 wires

D I2C uses 1 wire; SPI uses 2 wires

💡 Hint

Recall that I2C uses SDA and SCL lines.

I2C uses two wires: SDA (data) and SCL (clock). SPI uses at least four wires: MOSI, MISO, SCLK, and SS (slave select).

❓ Predict Output

advanced

SPI data transfer behavior

What is the output of this SPI data transfer simulation code?

Embedded C

#include <stdio.h>

int main() {
    unsigned char master_tx = 0xA5; // Master sends 0xA5
    unsigned char slave_tx = 0x3C;  // Slave sends 0x3C
    unsigned char master_rx = slave_tx; // Master receives slave data
    unsigned char slave_rx = master_tx; // Slave receives master data

    printf("Master received: 0x%X\n", master_rx);
    printf("Slave received: 0x%X\n", slave_rx);
    return 0;
}

A

Master received: 0xA5
Slave received: 0x3C

B Compilation error

C

Master received: 0x00
Slave received: 0x00

D

Master received: 0x3C
Slave received: 0xA5

💡 Hint

SPI is full-duplex: master and slave exchange data simultaneously.

In SPI, the master sends data to the slave while simultaneously receiving data from the slave. Here, master receives 0x3C from slave, and slave receives 0xA5 from master.

🧠 Conceptual

advanced

I2C addressing modes

Which statement correctly describes I2C addressing modes?

A I2C supports 7-bit and 10-bit addressing modes

B I2C supports only 8-bit addressing mode

C I2C supports 4-bit and 8-bit addressing modes

D I2C supports only 16-bit addressing mode

💡 Hint

Think about how many bits are used to identify devices on the bus.

I2C devices can be addressed using either 7-bit or extended 10-bit addresses, allowing more devices on the bus.

🚀 Application

expert

Choosing between I2C and SPI for a sensor array

You have a sensor array with 8 identical sensors that need to send data to a microcontroller. The sensors require moderate speed and minimal wiring complexity. Which bus would be the best choice and why?

A SPI, because it supports longer cable lengths and fewer devices

B SPI, because it uses fewer wires and supports multiple devices without addressing

C I2C, because it uses only two wires and supports multiple devices with unique addresses

D I2C, because it requires a separate chip select line for each device

💡 Hint

Consider wiring complexity and device addressing for multiple sensors.

I2C uses only two wires regardless of the number of devices and supports addressing each sensor uniquely, making it ideal for multiple sensors with minimal wiring.

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