How to Send Data Using SPI in Embedded C: Simple Guide

To send data using SPI in embedded C, you typically write the data byte to the SPI data register and wait for the transmission to complete by checking the status register. This process involves initializing the SPI peripheral, writing data, and optionally reading received data if full-duplex communication is used.

Syntax

The basic syntax to send data over SPI involves writing a byte to the SPI data register and waiting for the transmission to finish by checking the SPI status register.

• SPI_DR: SPI Data Register where you write the byte to send.
• SPI_SR: SPI Status Register to check if transmission is complete.
• TXE: Transmit buffer empty flag in SPI_SR indicating ready to send next byte.
• RXNE: Receive buffer not empty flag indicating data received.

c

SPI_DR = data_byte;  // Write data to SPI data register
while (!(SPI_SR & TXE)) {}  // Wait until transmit buffer is empty
// Optionally read received data
uint8_t received = SPI_DR;

Example

This example shows how to initialize SPI, send a byte, and wait for the transmission to complete on a generic microcontroller.

c

#include <stdint.h>
#include <stdbool.h>

// Mock SPI registers for demonstration
volatile uint8_t SPI_DR = 0;
volatile uint8_t SPI_SR = 0;
#define TXE 0x02  // Transmit buffer empty flag

void SPI_Init(void) {
    // Initialize SPI peripheral (mock)
    SPI_SR = TXE;  // Set TXE flag to indicate ready
}

void SPI_SendByte(uint8_t data) {
    while (!(SPI_SR & TXE)) {
        // Wait until transmit buffer is empty
    }
    SPI_DR = data;  // Send data
    SPI_SR &= ~TXE; // Clear TXE flag to simulate busy
    // Simulate transmission complete
    SPI_SR |= TXE;  // Set TXE flag again
}

int main(void) {
    SPI_Init();
    SPI_SendByte(0xAB);  // Send 0xAB over SPI
    return 0;
}

Common Pitfalls

Common mistakes when sending data over SPI include:

• Not waiting for the transmit buffer to be empty before sending new data, causing data loss.
• Forgetting to initialize the SPI peripheral properly before use.
• Ignoring the full-duplex nature of SPI and not reading received data, which can cause the receive buffer to overflow.

c

/* Wrong way: Sending data without checking if SPI is ready */
SPI_DR = data;  // May overwrite data if TXE not set

/* Right way: Wait for TXE flag before sending */
while (!(SPI_SR & TXE)) {}
SPI_DR = data;

Quick Reference

Tips for sending data using SPI in embedded C:

• Always initialize SPI before sending data.
• Check the transmit buffer empty flag before writing data.
• Read received data to clear the receive buffer.
• Use interrupts or DMA for efficient data transfer in complex applications.

Key Takeaways

Initialize the SPI peripheral before sending data.

Always wait for the transmit buffer to be empty before writing new data.

Read received data to avoid buffer overflow in full-duplex SPI.

Use status flags like TXE and RXNE to manage data flow.

For complex transfers, consider using interrupts or DMA.