Why SPI is used in Embedded C - See It in Action

Why SPI is used

📖 Scenario: Imagine you have a small robot that needs to talk to many sensors and devices quickly and clearly. To do this, it uses a special way to send messages called SPI.

🎯 Goal: You will create a simple program that shows why SPI (Serial Peripheral Interface) is used to connect devices. You will set up data, configure a speed limit, send data using SPI, and then show the result.

📋 What You'll Learn

Create a variable called data_to_send with the value 0x3F

Create a variable called spi_speed and set it to 1000000 (1 MHz)

Write a function called spi_transfer that takes data and speed and returns the data sent

Print the result of sending data_to_send using spi_speed

💡 Why This Matters

🌍 Real World

SPI is used in many devices like sensors, displays, and memory chips to send data quickly and reliably.

💼 Career

Understanding SPI helps in embedded systems programming, hardware interfacing, and working with microcontrollers.

1

DATA SETUP: Create the data to send

Create a variable called data_to_send and set it to 0x3F.

Embedded C

# Create a variable called data_to_send and set it to 0x3F
# Your code here

Hint

Use unsigned char to store one byte of data.

2

CONFIGURATION: Set SPI speed

Create a variable called spi_speed and set it to 1000000 (1 MHz).

Embedded C

unsigned char data_to_send = 0x3F;
// Create a variable called spi_speed and set it to 1000000
# Your code here

Hint

Use unsigned int for the speed variable.

3

CORE LOGIC: Write SPI transfer function

Write a function called spi_transfer that takes unsigned char data and unsigned int speed and returns the data sent.

Embedded C

unsigned char data_to_send = 0x3F;
unsigned int spi_speed = 1000000;

// Write spi_transfer function below
# Your code here

Hint

The function should return the data it receives.

4

OUTPUT: Send data and print result

Call spi_transfer with data_to_send and spi_speed, store the result in sent_data, and print it using printf.

Embedded C

unsigned char data_to_send = 0x3F;
unsigned int spi_speed = 1000000;

unsigned char spi_transfer(unsigned char data, unsigned int speed) {
    // In real SPI, data is sent at speed. Here we just return data to simulate.
    return data;
}

// Call spi_transfer and print the sent data
# Your code here

Hint

Use printf("Sent data: 0x%X\n", sent_data); to print the result in hexadecimal.

Practice

1. Why is SPI commonly used in embedded systems for communication?

easy

A. Because it requires no clock signal

B. Because it uses only one wire for communication

C. Because it is the slowest communication protocol

D. Because it provides fast and reliable data transfer

Solution

1. Step 1: Understand SPI communication basics

   SPI uses multiple lines including clock, data in, data out, and select lines to transfer data quickly.

2. Step 2: Identify the main advantage of SPI

   SPI is known for its speed and reliability compared to other protocols like I2C or UART.

3. Final Answer:

   Because it provides fast and reliable data transfer -> Option D

4. Quick Check:

   SPI speed and reliability = Because it provides fast and reliable data transfer [OK]

Hint: SPI is fast and reliable due to separate data lines [OK]

Common Mistakes:

• Thinking SPI uses only one wire
• Confusing SPI with protocols that don't use clock
• Assuming SPI is slow

2. Which of the following is the correct set of lines used in SPI communication?

easy

A. MOSI, MISO, SCLK, SS

B. TX, RX, GND, VCC

C. SDA, SCL, VCC, GND

D. CLK, DATA, RESET, ENABLE

Solution

1. Step 1: Recall SPI signal names

   SPI uses Master Out Slave In (MOSI), Master In Slave Out (MISO), Serial Clock (SCLK), and Slave Select (SS) lines.

2. Step 2: Match options with SPI lines

   Only MOSI, MISO, SCLK, SS lists the correct SPI lines.

3. Final Answer:

   MOSI, MISO, SCLK, SS -> Option A

4. Quick Check:

   SPI lines = MOSI, MISO, SCLK, SS [OK]

Hint: Remember SPI lines: MOSI, MISO, SCLK, SS [OK]

Common Mistakes:

• Confusing SPI lines with UART or I2C lines
• Mixing up signal names
• Choosing generic signal names not used in SPI

3. Given the following SPI communication code snippet, what will be the output if the master sends 0xA5 and the slave responds with 0x5A?

uint8_t master_send = 0xA5;
uint8_t slave_response = 0x5A;
uint8_t received = spi_transfer(master_send); // spi_transfer sends and receives simultaneously
printf("0x%X", received);

medium

A. 0xA5

B. 0x5A

C. 0x00

D. Compilation error

Solution

1. Step 1: Understand spi_transfer behavior

   SPI sends data from master and simultaneously receives data from slave.

2. Step 2: Identify received data

   The function returns the data received from the slave, which is 0x5A.

3. Final Answer:

   0x5A -> Option B

4. Quick Check:

   Received data = 0x5A [OK]

Hint: SPI sends and receives data simultaneously [OK]

Common Mistakes:

• Assuming received data equals sent data
• Confusing master and slave data roles
• Expecting compilation error due to function call

4. Identify the error in this SPI initialization code snippet:

SPI_InitTypeDef SPI_InitStruct;
SPI_InitStruct.Mode = SPI_MODE_MASTER;
SPI_InitStruct.Direction = SPI_DIRECTION_2LINES;
SPI_InitStruct.DataSize = 8;
SPI_InitStruct.CLKPolarity = 0;
SPI_InitStruct.CLKPhase = 0;
SPI_InitStruct.NSS = SPI_NSS_SOFT;
SPI_InitStruct.BaudRatePrescaler = 0;
SPI_Init(&SPI_InitStruct);

medium

A. BaudRatePrescaler cannot be zero

B. DataSize should be 16 instead of 8

C. NSS must be set to hardware mode

D. CLKPolarity must be 1

Solution

1. Step 1: Check BaudRatePrescaler valid values

   BaudRatePrescaler must be a non-zero value representing division factor (e.g., 2, 4, 8).

2. Step 2: Identify invalid value

   Setting BaudRatePrescaler to 0 is invalid and will cause incorrect SPI speed or error.

3. Final Answer:

   BaudRatePrescaler cannot be zero -> Option A

4. Quick Check:

   BaudRatePrescaler ≠ 0 [OK]

Hint: BaudRatePrescaler must be a positive division factor [OK]

Common Mistakes:

• Setting BaudRatePrescaler to zero
• Confusing NSS software and hardware modes
• Changing CLKPolarity without reason

5. You need to design a system where multiple sensors send data to a microcontroller quickly and reliably. Why would SPI be a better choice than UART or I2C in this case?

hard

A. SPI does not require a clock signal, making it simpler

B. UART uses fewer wires and is faster than SPI

C. SPI supports multiple devices with separate select lines and faster data rates

D. I2C is faster and simpler than SPI for multiple devices

Solution

1. Step 1: Compare SPI with UART and I2C for multiple devices

   SPI uses separate slave select lines allowing multiple devices and supports higher speeds than UART and I2C.

2. Step 2: Identify why SPI is better for fast, reliable multi-device communication

   SPI's dedicated lines and clock signal enable fast, simultaneous data transfer with multiple sensors.

3. Final Answer:

   SPI supports multiple devices with separate select lines and faster data rates -> Option C

4. Quick Check:

   SPI multi-device speed advantage = SPI supports multiple devices with separate select lines and faster data rates [OK]

Hint: SPI uses select lines and clock for fast multi-device communication [OK]

Common Mistakes:

• Assuming UART is faster than SPI
• Thinking I2C is always simpler and faster
• Believing SPI works without a clock