SPI master-slave architecture in Embedded C - Mini Project: Build & Apply

SPI Master-Slave Architecture

📖 Scenario: You are working on a simple embedded system where a master device communicates with a slave device using SPI (Serial Peripheral Interface). The master sends commands and data to the slave, and the slave responds accordingly.This project will guide you through setting up basic SPI communication in C, simulating the master and slave roles.

🎯 Goal: Build a simple SPI master-slave communication program in C where the master sends a byte to the slave, and the slave receives and stores it.

📋 What You'll Learn

Create a variable to represent the data byte to send from the master

Create a variable to represent the received data byte in the slave

Simulate the SPI transfer function that sends data from master to slave

Print the received data byte in the slave to verify communication

💡 Why This Matters

🌍 Real World

SPI is a common way microcontrollers and sensors communicate in devices like smart home gadgets, wearables, and industrial machines.

💼 Career

Understanding SPI communication is essential for embedded systems engineers working on hardware interfacing and device drivers.

1

Create SPI data variables

Create a variable called master_data and set it to 0x3A. Also create a variable called slave_data and initialize it to 0x00.

Embedded C

# Create variables master_data and slave_data
# Your code here

Hint

Use unsigned char type for 8-bit data bytes.

2

Create SPI transfer function prototype

Create a function prototype called spi_transfer that takes an unsigned char parameter called data_out and returns an unsigned char.

Embedded C

unsigned char master_data = 0x3A;
unsigned char slave_data = 0x00;

// Declare spi_transfer function prototype
# Your code here

Hint

Function prototype declares the function signature without the body.

3

Implement SPI transfer function

Implement the spi_transfer function so that it assigns the input data_out to the global variable slave_data and returns slave_data.

Embedded C

unsigned char master_data = 0x3A;
unsigned char slave_data = 0x00;

unsigned char spi_transfer(unsigned char data_out) {
    // Your code here
}

Hint

Assign the input to slave_data and return it.

4

Send data and print received value

Call the spi_transfer function with master_data and store the result in a variable called received. Then print the value of received in hexadecimal format using printf.

Embedded C

unsigned char master_data = 0x3A;
unsigned char slave_data = 0x00;

unsigned char spi_transfer(unsigned char data_out) {
    slave_data = data_out;
    return slave_data;
}

int main() {
    // Your code here
    return 0;
}

Hint

Use printf("Received data: 0x%02X\n", received); to print the hexadecimal value.

Practice

1. In SPI master-slave architecture, who controls the clock signal?

easy

A. The master device controls the clock signal.

B. The slave device controls the clock signal.

C. Both master and slave share clock control equally.

D. No clock signal is used in SPI communication.

Solution

1. Step 1: Understand SPI roles

   In SPI, the master initiates communication and controls timing.

2. Step 2: Identify clock control

   The master generates the clock signal to synchronize data transfer.

3. Final Answer:

   The master device controls the clock signal. -> Option A

4. Quick Check:

   Master controls clock = A [OK]

Hint: Master always controls clock in SPI [OK]

Common Mistakes:

• Thinking slave controls clock
• Assuming no clock is needed
• Believing clock is shared equally

2. Which of the following is the correct way to select a slave device in SPI using embedded C?

easy

A. Send a special data byte to select the slave.

B. Set the slave select (SS) pin HIGH to select the slave.

C. Toggle the clock pin to select the slave.

D. Set the slave select (SS) pin LOW to select the slave.

Solution

1. Step 1: Recall SPI slave selection method

   In SPI, the master selects a slave by pulling its SS pin LOW.

2. Step 2: Understand pin logic

   Setting SS LOW activates the slave; HIGH deselects it.

3. Final Answer:

   Set the slave select (SS) pin LOW to select the slave. -> Option D

4. Quick Check:

   SS LOW selects slave = A [OK]

Hint: SS pin LOW means slave selected [OK]

Common Mistakes:

• Setting SS HIGH to select slave
• Using clock pin to select slave
• Sending data to select slave

3. Given this embedded C code snippet for SPI master:

SPI_SS_PIN = 0; // Select slave
SPI_Write(0xA5); // Send data
SPI_SS_PIN = 1; // Deselect slave

What happens on the SPI bus during this code execution?

medium

A. The master selects the slave, sends 0xA5, then deselects the slave.

B. The slave sends 0xA5 to the master.

C. The master sends 0xA5 without selecting any slave.

D. The slave select pin stays HIGH, so no communication occurs.

Solution

1. Step 1: Analyze slave select pin changes

   Setting SPI_SS_PIN to 0 selects the slave before sending data.

2. Step 2: Understand data transmission

   SPI_Write(0xA5) sends data from master to selected slave.

3. Step 3: Deselect slave after sending

   Setting SPI_SS_PIN to 1 ends communication by deselecting slave.

4. Final Answer:

   The master selects the slave, sends 0xA5, then deselects the slave. -> Option A

5. Quick Check:

   SS LOW + send data + SS HIGH = D [OK]

Hint: SS LOW before send, HIGH after send [OK]

Common Mistakes:

• Assuming slave sends data
• Ignoring slave select pin state
• Thinking data sends without slave selected

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

SPI_SS_PIN = 1; // Deselect slave
SPI_Write(0x3C); // Send data
SPI_SS_PIN = 0; // Select slave

medium

A. Data 0x3C is invalid for SPI communication.

B. Slave select pin is toggled correctly; no error.

C. Slave is deselected before sending data, so data won't be received.

D. SPI_Write function should be called after deselecting slave.

Solution

1. Step 1: Check slave select pin timing

   Slave is deselected (SS=1) before sending data, so slave ignores data.

2. Step 2: Correct order for SPI communication

   Slave must be selected (SS=0) before sending data to receive it.

3. Final Answer:

   Slave is deselected before sending data, so data won't be received. -> Option C

4. Quick Check:

   SS must be LOW before send = C [OK]

Hint: Select slave before sending data [OK]

Common Mistakes:

• Sending data before selecting slave
• Assuming data sends regardless of SS pin
• Misunderstanding SPI_Write timing

5. You want to communicate with two SPI slaves using one master. Which approach correctly manages slave selection in embedded C?

hard

A. Connect both slaves to the same slave select pin and set it LOW to select both.

B. Use separate slave select pins for each slave and set only one LOW at a time.

C. Toggle the clock pin rapidly to switch between slaves.

D. Send a unique data byte to select each slave without changing slave select pins.

Solution

1. Step 1: Understand multiple slave selection

   Each slave needs its own slave select pin controlled by the master.

2. Step 2: Select only one slave at a time

   Set only one slave select pin LOW to activate that slave; others stay HIGH.

3. Step 3: Avoid simultaneous selection

   Selecting multiple slaves at once causes bus conflicts and data errors.

4. Final Answer:

   Use separate slave select pins for each slave and set only one LOW at a time. -> Option B

5. Quick Check:

   Separate SS pins, one LOW at a time = B [OK]

Hint: One SS pin LOW per slave communication [OK]

Common Mistakes:

• Using one SS pin for all slaves
• Trying to select slaves by clock toggling
• Selecting multiple slaves simultaneously