SPI data transfer sequence in Embedded C

Introduction

SPI data transfer sequence shows how devices send and receive data step-by-step using SPI. It helps devices talk to each other quickly and clearly.

When connecting a microcontroller to a sensor to get data.

When sending commands from a microcontroller to a display screen.

When reading data from an external memory chip.

When controlling a motor driver with a microcontroller.

When communicating between two microcontrollers.

Syntax

Embedded C

1. Pull the chip select (CS) line low to start communication.
2. Send data byte by byte over the MOSI line.
3. Receive data byte by byte over the MISO line.
4. After all bytes are sent and received, pull CS line high to end communication.

The CS line tells the device when to listen or ignore data.

Data is sent and received at the same time, one byte per clock cycle.

Examples

Basic single byte send and receive using SPI.

Embedded C

CS = LOW; // Start communication
SPI_Transmit(data_byte); // Send one byte
received = SPI_Receive(); // Receive one byte
CS = HIGH; // End communication

Send and receive multiple bytes in a loop.

Embedded C

CS = LOW;
for (int i = 0; i < length; i++) {
    SPI_Transmit(buffer[i]);
    buffer[i] = SPI_Receive();
}
CS = HIGH;

Sample Program

This program simulates sending three bytes over SPI and receiving data back. It shows the chip select going low and high around the transfer.

Embedded C

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

// Mock functions for SPI hardware
void CS_Low() { printf("CS LOW\n"); }
void CS_High() { printf("CS HIGH\n"); }
uint8_t SPI_TransmitReceive(uint8_t data) {
    printf("Sent: 0x%02X\n", data);
    // For demo, just return data + 1 as received
    return data + 1;
}

int main() {
    uint8_t send_data[3] = {0x10, 0x20, 0x30};
    uint8_t receive_data[3];

    CS_Low();
    for (int i = 0; i < 3; i++) {
        receive_data[i] = SPI_TransmitReceive(send_data[i]);
    }
    CS_High();

    printf("Received bytes: ");
    for (int i = 0; i < 3; i++) {
        printf("0x%02X ", receive_data[i]);
    }
    printf("\n");

    return 0;
}

Important Notes

Always pull CS low before starting SPI transfer and high after finishing.

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

Clock polarity and phase settings must match between devices for correct data transfer.

Summary

SPI data transfer uses chip select to start and stop communication.

Data is sent and received byte by byte simultaneously.

Proper timing and settings are important for successful SPI communication.