SPI with external devices (sensors, displays) in Embedded C

Introduction

SPI helps microcontrollers talk quickly to sensors and displays using just a few wires.

Reading temperature or light data from a sensor.

Sending images or text to a small screen.

Controlling a memory chip to save data.

Communicating with a digital-to-analog converter.

Connecting multiple devices with one microcontroller.

Syntax

Embedded C

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

// Example to send and receive one byte over SPI

SPI usually uses 4 wires: MOSI, MISO, SCK, and CS (chip select).

SPI_Transfer sends a byte and receives a byte at the same time.

Examples

Initialize SPI and send 0x55, then get the response byte.

Embedded C

SPI_Init();
uint8_t received = SPI_Transfer(0x55);

Use chip select to talk to one device at a time.

Embedded C

CS_LOW(); // Select device
SPI_Transfer(0xA0); // Send command
CS_HIGH(); // Deselect device

Sample Program

This program shows how to initialize SPI, select a device, send a byte, and get a response.

Embedded C

#include <stdint.h>
#include <stdio.h>

// Dummy functions to simulate SPI hardware
void SPI_Init(void) {
    // Setup SPI pins and settings here
    printf("SPI initialized\n");
}

uint8_t SPI_Transfer(uint8_t data) {
    // Send data and return received byte (simulate echo)
    printf("Sent: 0x%02X\n", data);
    uint8_t received = data; // For demo, echo back
    printf("Received: 0x%02X\n", received);
    return received;
}

void CS_LOW(void) {
    printf("CS set LOW (device selected)\n");
}

void CS_HIGH(void) {
    printf("CS set HIGH (device deselected)\n");
}

int main(void) {
    SPI_Init();
    CS_LOW();
    uint8_t response = SPI_Transfer(0xA5); // Send command to device
    CS_HIGH();
    printf("Final response: 0x%02X\n", response);
    return 0;
}

Important Notes

Always set the chip select pin LOW before communication and HIGH after.

SPI sends and receives data at the same time, so you get a response with every byte sent.

Check your device datasheet for correct SPI settings like clock speed and polarity.

Summary

SPI is a fast way to connect microcontrollers with sensors and displays.

Use chip select to choose which device to talk to.

Sending data also receives data, so handle both in your code.