I2C vs SPI decision matrix in Embedded C - Visual Side-by-Side Comparison

Concept Flow - I2C vs SPI decision matrix

Start: Need to choose communication protocol

↓

Check number of devices

↓

Check speed

↓

Choose I2C

Start by checking device count, speed needs, and bus complexity to decide between I2C and SPI.

Execution Sample

Embedded C

if (num_devices <= 2) {
  if (speed < 400000) {
    protocol = I2C;
  } else {
    protocol = SPI;
  }
} else {
  protocol = I2C;
}

This code chooses I2C if devices are few and speed is low, otherwise SPI.

Execution Table

Step	num_devices	speed (Hz)	Condition Checked	Decision	Protocol Chosen
1	1	100000	num_devices <= 2? True	Check speed	None
2	1	100000	speed < 400000? True	Choose I2C	I2C
3	3	100000	num_devices <= 2? False	Choose I2C	I2C
4	1	1000000	num_devices <= 2? True	Check speed	None
5	1	1000000	speed < 400000? False	Choose SPI	SPI

💡 Decision made based on device count and speed conditions.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5
num_devices	undefined	1	1	3	1	1
speed	undefined	100000	100000	100000	1000000	1000000
protocol	undefined	undefined	I2C	I2C	undefined	SPI

Key Moments - 3 Insights

Why do we check the number of devices first?

Because the number of devices affects bus complexity and protocol suitability, as shown in execution_table step 1 and 3.

Why is speed compared to 400000 Hz?

400000 Hz is a common I2C fast mode limit; speeds above this often require SPI, as seen in steps 2 and 5.

Why choose I2C when devices are more than 2?

I2C supports multiple devices on the same bus easily, so for more than 2 devices, I2C is preferred (step 3).

Visual Quiz - 3 Questions

Look at the execution_table, what protocol is chosen at step 5?

A I2C

B SPI

C None

D Error

💡 Hint

Check the 'Protocol Chosen' column at step 5 in execution_table.

At which step does the condition 'num_devices <= 2' become false?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Condition Checked' column for 'num_devices <= 2? False' in execution_table.

If speed was 200000 Hz at step 4, what protocol would be chosen?

A I2C

B SPI

C None

D Error

💡 Hint

Refer to the speed check condition in execution_table steps 2 and 5.

Concept Snapshot

I2C vs SPI decision matrix:
- Check number of devices first
- If devices <= 2 and speed < 400kHz, choose I2C
- If speed > 400kHz or bus complexity high, choose SPI
- I2C supports multi-device easily
- SPI is faster but needs more wires

Full Transcript

This visual execution shows how to decide between I2C and SPI communication protocols. First, check how many devices you have. If two or fewer, check the speed needed. If speed is less than 400000 Hz, choose I2C. If speed is higher, choose SPI. If more than two devices, choose I2C because it handles multiple devices well. The execution table traces these decisions step-by-step with variable values. Key moments clarify why device count and speed matter. The quiz tests understanding of these steps. The snapshot summarizes the decision rules simply.

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