SPI with external devices (sensors, displays) in Embedded C - Mini Project: Build & Apply

SPI Communication with External Devices

📖 Scenario: You are working on a small embedded system that communicates with external devices like sensors and displays using SPI (Serial Peripheral Interface). SPI is a way for your microcontroller to send and receive data to these devices quickly.In this project, you will write simple embedded C code to set up SPI communication, send commands to a sensor, and read data back.

🎯 Goal: Build a simple SPI communication program that initializes SPI, sends a command to a sensor, reads the sensor data, and prints the received value.

📋 What You'll Learn

Create an SPI data buffer with specific command bytes

Define a variable for the number of bytes to send

Write a function to send and receive SPI data

Print the received sensor data

💡 Why This Matters

🌍 Real World

SPI is widely used in embedded systems to connect microcontrollers with sensors, displays, memory chips, and other devices. Understanding SPI helps you build projects like weather stations, smart watches, and home automation devices.

💼 Career

Embedded software engineers often write SPI communication code to interface with hardware components. This skill is essential for developing firmware for IoT devices, automotive systems, and consumer electronics.

1

Create SPI data buffer

Create an array called spi_tx_buffer with these exact bytes: 0xAA, 0xBB, 0xCC. This buffer will hold the command bytes to send to the sensor.

Embedded C

# Create the SPI transmit buffer with command bytes
// Your code here

Hint

Use an array of unsigned char with exactly three values.

2

Define SPI data length

Create an integer variable called spi_data_length and set it to 3, which is the number of bytes to send.

Embedded C

unsigned char spi_tx_buffer[3] = {0xAA, 0xBB, 0xCC};
// Define the length of SPI data to send
// Your code here

Hint

Use an int variable to store the number 3.

3

Write SPI send and receive function

Write a function called spi_transfer that takes a pointer to tx_buffer, a pointer to rx_buffer, and an integer length. Inside the function, use a for loop with variable i from 0 to length - 1 to simulate sending and receiving SPI data by copying each byte from tx_buffer to rx_buffer. This simulates the sensor echoing back the data.

Embedded C

unsigned char spi_tx_buffer[3] = {0xAA, 0xBB, 0xCC};
int spi_data_length = 3;

// Write spi_transfer function below
// Your code here

Hint

Use a for loop to copy bytes from tx_buffer to rx_buffer.

4

Call SPI function and print received data

Declare an array spi_rx_buffer of size 3 to hold received data. Call spi_transfer with spi_tx_buffer, spi_rx_buffer, and spi_data_length. Then use a for loop with variable i from 0 to 2 to print each received byte in hexadecimal format using printf with 0x%X . End with a newline.

Embedded C

unsigned char spi_tx_buffer[3] = {0xAA, 0xBB, 0xCC};
int spi_data_length = 3;

void spi_transfer(unsigned char *tx_buffer, unsigned char *rx_buffer, int length) {
    for (int i = 0; i < length; i++) {
        rx_buffer[i] = tx_buffer[i];
    }
}

// Declare spi_rx_buffer, call spi_transfer, and print received bytes
// Your code here

Hint

Remember to include stdio.h for printf and print each byte in hex.

Practice

1. What is the main purpose of the Chip Select (CS) line in SPI communication with external devices like sensors or displays?

easy

A. To increase the SPI clock speed

B. To provide power to the SPI device

C. To reset the SPI device

D. To select which device the microcontroller communicates with

Solution

1. Step 1: Understand SPI device selection

   The Chip Select (CS) line is used to enable communication with one specific SPI device among multiple devices connected to the same SPI bus.

2. Step 2: Identify the role of CS

   When CS is active (usually low), the selected device listens and responds to SPI signals; other devices ignore the communication.

3. Final Answer:

   To select which device the microcontroller communicates with -> Option D

4. Quick Check:

   CS line = device selector [OK]

Hint: CS line chooses the device to talk to in SPI [OK]

Common Mistakes:

• Confusing CS with power supply line
• Thinking CS resets the device
• Assuming CS controls clock speed

2. Which of the following is the correct way to initialize SPI communication in embedded C for a sensor device?

easy

A. SPI_Init(); CS_Pin = LOW; SPI_Transmit(0xAA);

B. SPI_Init(); CS_Pin = HIGH; SPI_Transmit(0xFF);

C. SPI_Start(); CS_Pin = LOW; SPI_Receive();

D. SPI_Begin(); CS_Pin = HIGH; SPI_Receive(0x00);

Solution

1. Step 1: Check SPI initialization and CS pin state

   SPI_Init() is the standard function to initialize SPI. CS must be LOW to select the device before transmitting data.

2. Step 2: Verify data transmission call

   SPI_Transmit(0xAA) sends data to the device. This matches typical SPI usage.

3. Final Answer:

   SPI_Init(); CS_Pin = LOW; SPI_Transmit(0xAA); -> Option A

4. Quick Check:

   Init SPI + CS low + transmit data = correct setup [OK]

Hint: CS LOW means device selected before sending data [OK]

Common Mistakes:

• Setting CS HIGH before transmit disables device
• Using wrong SPI init function name
• Calling receive without selecting device

3. Consider this embedded C code snippet communicating with an SPI sensor:

CS_PIN = 0;  // Select device
uint8_t data = SPI_TransmitReceive(0x55);
CS_PIN = 1;  // Deselect device

What value will data hold after execution?

medium

A. The sensor's response byte received during transmission

B. Always 0x55, the sent byte

C. Undefined, because CS_PIN is set to 1 after transmit

D. An error code due to missing SPI initialization

Solution

1. Step 1: Understand SPI_TransmitReceive behavior

   This function sends a byte (0x55) and simultaneously receives a byte from the sensor during the SPI clock cycles.

2. Step 2: Analyze CS pin usage

   CS_PIN is set LOW before transmission to select the device and set HIGH after to deselect. This is correct usage and does not affect the received data.

3. Final Answer:

   The sensor's response byte received during transmission -> Option A

4. Quick Check:

   SPI sends and receives simultaneously = sensor response [OK]

Hint: SPI sends and receives data at the same time [OK]

Common Mistakes:

• Assuming received data equals sent data
• Thinking CS high before transmit affects data
• Ignoring simultaneous send/receive nature

4. This code snippet is intended to read data from an SPI display, but it does not work correctly:

CS_PIN = 1;
SPI_Transmit(0x0F);
CS_PIN = 0;
uint8_t response = SPI_Receive();
CS_PIN = 1;

What is the main error?

medium

A. CS_PIN should remain HIGH during receive

B. SPI_Transmit should be SPI_Receive

C. CS_PIN should be set LOW before transmitting

D. SPI_Receive requires a parameter

Solution

1. Step 1: Check CS_PIN state before transmission

   CS_PIN must be LOW to select the SPI device before sending commands or data.

2. Step 2: Identify the error in CS_PIN usage

   The code sets CS_PIN HIGH before transmit, which deselects the device, so the device ignores the command.

3. Final Answer:

   CS_PIN should be set LOW before transmitting -> Option C

4. Quick Check:

   CS LOW before transmit = device listens [OK]

Hint: Always set CS LOW before sending SPI data [OK]

Common Mistakes:

• Setting CS HIGH before transmit disables device
• Confusing transmit and receive functions
• Assuming CS state doesn't affect communication

5. You want to read temperature data from an SPI sensor that requires sending a command byte 0x01, then reading two bytes of data. Which code snippet correctly implements this sequence?

hard

A. CS_PIN = 0;
SPI_Transmit(0x01);
uint8_t high = SPI_Receive();
uint8_t low = SPI_Receive();
CS_PIN = 1;

B. CS_PIN = 0;
SPI_Transmit(0x01);
high = SPI_Transmit(0x00);
low = SPI_Transmit(0x00);
CS_PIN = 1;

C. CS_PIN = 1;
SPI_Transmit(0x01);
high = SPI_Receive();
low = SPI_Receive();
CS_PIN = 0;

D. SPI_Transmit(0x01);
CS_PIN = 0;
high = SPI_Receive();
low = SPI_Receive();
CS_PIN = 1;

Solution

1. Step 1: Select device before communication

   CS_PIN must be set LOW before sending commands or data to select the sensor.

2. Step 2: Send command and read data correctly

   SPI_Transmit sends a byte and simultaneously receives a byte. To read data, send dummy bytes (0x00) to clock out sensor data.

3. Step 3: Deselect device after communication

   CS_PIN is set HIGH after communication to end the session.

4. Final Answer:

   CS_PIN = 0; SPI_Transmit(0x01); high = SPI_Transmit(0x00); low = SPI_Transmit(0x00); CS_PIN = 1; -> Option B

5. Quick Check:

   Send command, then send dummy bytes to receive data [OK]

Hint: Send dummy bytes to receive SPI data after command [OK]

Common Mistakes:

• Reading without sending dummy bytes
• Setting CS high before command
• Not deselecting device after communication