Embedded Development in C: SPI Communication Protocol

Introduction

The SPI (Serial Peripheral Interface) is a synchronous serial communication protocol widely used for high-speed data transfer between microcontrollers and various peripherals. It was introduced by Motorola in the 1980s and has been widely adopted due to its simplicity, flexibility, and efficiency. SPI is typically used to connect devices such as sensors, memory, and displays.

This article will detail the basic concepts of the SPI communication protocol and demonstrate how to implement SPI communication in embedded systems using C language example code.

Working Principle of SPI

SPI uses a master-slave architecture, where one device acts as the master and the others as slaves. The master device is responsible for generating the clock signal and controlling data transfer, while the slave devices respond according to the master’s instructions.

SPI Signal Lines

1. MOSI (Master Out Slave In): The master device sends data to the slave device.
2. MISO (Master In Slave Out): The slave device sends data to the master device.
3. SCK (Serial Clock): The clock signal generated by the master device, used to synchronize data transfer.
4. SS/CS (Slave Select/Chip Select): Used to select a specific slave device, typically active low.

Data Transfer Process

1. The master pulls the CS pin of the selected slave low.
2. The master sends data through the MOSI line while generating SCK clock pulses.
3. The slave receives data on the rising edge of each SCK and can return corresponding data on the MISO line.
4. After the data transfer is complete, the master pulls the CS pin high to end the session.

Implementing SPI Communication in C

Below is a simple C language example code demonstrating how to configure and use SPI for basic data exchange. In this example, we assume the use of an STM32 series microcontroller, but most of the logic applies to other platforms with adjustments to hardware-specific parts.

Hardware Connections

• MOSI -> GPIO_PIN_X
• MISO -> GPIO_PIN_Y
• SCK -> GPIO_PIN_Z
• CS -> GPIO_PIN_CS

Example Code

#include "stm32f4xx.h" // Select header file based on specific chip 
void SPI_Init(void) {    // Enable GPIO and SPI clock    RCC->AHB1ENR |= RCC_AHB1ENR_GPIOAEN; // Assume using GPIOA    RCC->APB2ENR |= RCC_APB2ENR_SPI1EN;  // Enable SPI1 clock        // Configure GPIO pins to alternate function (AF)    GPIOA->MODER |= (0x02 << (5 * 2)); // PA5 - SCK     GPIOA->MODER |= (0x02 << (6 * 2)); // PA6 - MISO     GPIOA->MODER |= (0x02 << (7 * 2)); // PA7 - MOSI        // Set alternate function to AF5 (corresponding to SPI1)    GPIOA->AFR[0] |= ((5 << (5 * 4)) |                       (5 << (6 * 4)) |                       (5 << (7 * 4)));
    // Configure SPI parameters    SPI1->CR1 = SPI_CR1_MSTR |        // Set to master mode                 SPI_CR1_SSM |         // Software management of chip select                 SPI_CR1_SSI |         // Internal SS held high valid                   SPI_CR1_BR_0;         // Baud rate prescaler setting        SPI1->CR2 = SPI_CR2_SPE;               // Start SPI Module    }
uint8_t SPI_TransmitReceive(uint8_t data) {   while (!(SPI1->SR & SPI_SR_TXE));   // Wait for TXE flag to be set      SPI1->DR = data;                    // Write data to be sent to register      while (!(SPI1->SR & SPI_SR_RXNE));   // Wait for RXNE flag to be set      return(SPI1->DR);                     // Return received data  }
int main(void) {     uint8_t receivedData;          SystemInit();                     /* System initialization */     SPI_Init();                       /* Initialize SPI Module */          while(1) {          receivedData = SPI_TransmitReceive(0x55); /* Send byte to peripheral and receive return value */          for(int i=0;i<100000;i++); /* Simple delay loop */     }}

Conclusion

This article introduced the SPI communication protocol and its working principles, providing a simple implementation example based on C language. By understanding these fundamentals, you can start applying this powerful serial communication method in your embedded projects. We hope this article helps you better master the SPI protocol in C language embedded development.