How to Interface SPI Device with Microcontroller in Embedded C

To interface an SPI device with a microcontroller in Embedded C, configure the SPI pins (MOSI, MISO, SCK, SS), initialize the SPI peripheral with correct settings, and use SPI data transfer functions to send and receive data. Control the SS pin to select the device before communication and deselect it after.

Syntax

SPI communication involves configuring the SPI peripheral and controlling the pins:

• MOSI: Master Out Slave In - data sent from master to slave
• MISO: Master In Slave Out - data sent from slave to master
• SCK: Serial Clock - clock signal from master
• SS: Slave Select - selects the SPI device

Typical SPI functions in Embedded C:

void SPI_Init(void);  // Initialize SPI peripheral with settings
uint8_t SPI_Transfer(uint8_t data);  // Send and receive one byte
void SPI_Select(void);  // Pull SS low to select device
void SPI_Deselect(void);  // Pull SS high to deselect device

c

void SPI_Init(void);
uint8_t SPI_Transfer(uint8_t data);
void SPI_Select(void);
void SPI_Deselect(void);

Example

This example shows how to initialize SPI, select the device, send a byte, receive a response, and deselect the device.

c

#include <stdint.h>
#include <avr/io.h>  // Example for AVR microcontroller

#define SS_PIN 2

void SPI_Init(void) {
    DDRB |= (1 << PB3) | (1 << PB5) | (1 << PB2); // MOSI, SCK, SS as output
    DDRB &= ~(1 << PB4); // MISO as input
    SPCR = (1 << SPE) | (1 << MSTR) | (1 << SPR0); // Enable SPI, Master mode, clk/16
    PORTB |= (1 << SS_PIN); // Set SS high (deselect)
}

uint8_t SPI_Transfer(uint8_t data) {
    SPDR = data; // Start transmission
    while (!(SPSR & (1 << SPIF))) ; // Wait for transmission complete
    return SPDR; // Return received data
}

void SPI_Select(void) {
    PORTB &= ~(1 << SS_PIN); // Pull SS low
}

void SPI_Deselect(void) {
    PORTB |= (1 << SS_PIN); // Pull SS high
}

int main(void) {
    uint8_t received;
    SPI_Init();
    SPI_Select();
    received = SPI_Transfer(0x55); // Send 0x55 and receive response
    SPI_Deselect();
    while (1) {
        // Main loop
    }
    return 0;
}

Common Pitfalls

Common mistakes when interfacing SPI devices include:

• Not configuring the SPI pins correctly (MOSI, MISO, SCK, SS)
• Forgetting to set the microcontroller as SPI master or slave properly
• Not controlling the SS pin to select/deselect the device, causing bus conflicts
• Ignoring SPI clock polarity and phase settings required by the device
• Not waiting for the SPI transfer to complete before reading data

Always check your device datasheet for correct SPI mode and timing.

c

/* Wrong: Not controlling SS pin */
SPI_Transfer(0xAA); // May cause bus conflict

/* Right: Control SS pin properly */
SPI_Select();
SPI_Transfer(0xAA);
SPI_Deselect();

Quick Reference

Tips for SPI interfacing:

• Configure SPI pins and peripheral before communication
• Use SS pin to select the device before transfer
• Match SPI clock polarity and phase to device requirements
• Wait for transfer complete flag before reading data
• Use full-duplex transfer: sending data also receives data

Key Takeaways

Always initialize SPI pins and peripheral with correct settings before use.

Control the SS pin to select and deselect the SPI device during communication.

Match SPI clock polarity and phase to the device datasheet requirements.

Wait for SPI transfer completion before reading received data.

Use SPI_Transfer function to send and receive data simultaneously.