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.