Understanding Serial Peripheral Interface Communication Protocol

In this article we will learn in depth about the Serial Peripheral interface which is among the widely used communication protocol in Embedded and IOT world.

What is Serial Peripheral Interface - SPI?

SPI is a synchronous serial communication protocol that enables communication between microcontrollers, sensors, memory devices, and other peripheral devices. It allows for full-duplex communication, meaning data can be sent and received simultaneously.

Serial Peripheral Interface (SPI) offers advantages such as high-speed data transfer, simplicity, and versatility.

The serial peripheral interface (SPI) is a communication interaction protocol used to send data between multiple IoT Devices. The Serial Peripheral Interface (SPI) offers data exchange among multiple devices through a master-slave configuration. In SPI the master device begins communication, by sending action bits to the slave devices. In SPI protocol one device serves as the master, with the rest acting as slaves. These modules operate synchronously and SPI ensures simultaneous transmission and reception of data at high speeds. SPI proves efficient for inter-device communication, offering higher data transfer rates compared to alternative interfaces. Its ability to handle bidirectional data flow concurrently enhances efficiency. However, SPI requires more signal lines compared to alternative protocols.

Understanding SPI Communication

SPI typically involves one master device communicating with one or more slave devices over four signal lines:

MOSI (Master Out Slave In): This line carries data from the master to the slave.

MISO (Master In Slave Out): This line carries data from the slave to the master.

SCK (Serial Clock): This line carries the clock signal generated by the master to synchronize data transmission.

SS/CS (Slave Select/Chip Select): This line is used to select the slave device with which the master wants to communicate.

SPI Protocol Sequence

The master initiates communication by asserting the Slave Select (SS/CS) line corresponding to the intended slave device.

1. The master sends a clock signal (SCK) to synchronize data transmission.

2. The SS/CS pin gets switched low voltage state, which activates the connected slave

3. The master transmits data to the slave bit by bit through the MOSI line, while the slave promptly reads each incoming bit:

4. The slave sends data back to the master one bit at a time via the MISO line, with the master interpreting each received bit sequentially: 

Data is simultaneously transmitted from the master to the slave (MOSI) and from the slave to the master (MISO). Upon receiving data, the slave processes it and responds accordingly.

Let us now see the example of ESP32 SPI feature:

The pin mapping of SPI for ESP32 board is as follows:

To link multiple SPI devices, you can utilize the same ESP32 SPI bus provided that each peripheral employs a distinct CS (Chip Select) pin.

Sample ESP32 code to integrate BME280 (Pressure, Temperature, Humidity) SPI Sensor using Adafruit_BME280 library:

/*

 Rui Santos

 Complete project details at https://RandomNerdTutorials.com/esp32-spi-communication-arduino/

 Based on the Adafruit_BME280_Library example: https://github.com/adafruit/Adafruit_BME280_Library/blob/master/examples/bme280test/bme280test.ino

 Permission is hereby granted, free of charge, to any person obtaining a copy

 of this software and associated documentation files.

 The above copyright notice and this permission notice shall be included in all

 copies or substantial portions of the Software.

*/

#include <Wire.h>

#include <Adafruit_Sensor.h>

#include <Adafruit_BME280.h>

#include <SPI.h>

#define BME_SCK 25

#define BME_MISO 32

#define BME_MOSI 26

#define BME_CS 33

#define SEALEVELPRESSURE_HPA (1013.25)

//Adafruit_BME280 bme; // I2C

//Adafruit_BME280 bme(BME_CS); // hardware SPI

Adafruit_BME280 bme(BME_CS, BME_MOSI, BME_MISO, BME_SCK); // software SPI

unsigned long delayTime;

void setup() {

 Serial.begin(9600);

 Serial.println(F("BME280 test"));

 bool status;

 // default settings

 // (you can also pass in a Wire library object like &Wire2)

 status = bme.begin(); 

 if (!status) {

 Serial.println("Could not find a valid BME280 sensor, check wiring!");

 while (1);

 }

 Serial.println("-- Default Test --");

 delayTime = 1000;

 Serial.println();

}

void loop() { 

 printValues();

 delay(delayTime);

}

void printValues() {

 Serial.print("Temperature = ");

 Serial.print(bme.readTemperature());

 Serial.println(" *C");

 // Convert temperature to Fahrenheit

 /*Serial.print("Temperature = ");

 Serial.print(1.8 * bme.readTemperature() + 32);

 Serial.println(" *F");*/

 Serial.print("Pressure = ");

 Serial.print(bme.readPressure() / 100.0F);

 Serial.println(" hPa");

 Serial.print("Approx. Altitude = ");

 Serial.print(bme.readAltitude(SEALEVELPRESSURE_HPA));

 Serial.println(" m");

 Serial.print("Humidity = ");

 Serial.print(bme.readHumidity());

 Serial.println(" %");

 Serial.println();

}

Key Features of SPI

Full-Duplex Communication: SPI allows simultaneous data transmission and reception between the master and slave devices.

Master-Slave Architecture: One master device controls the communication and initiates data transfer to one or more slave devices.

Synchronous Communication: Data transfer in SPI is synchronized with a clock signal generated by the master device.

Variable Data Frame Format: SPI supports variable data frame formats, allowing flexibility in data transmission.

High-Speed Communication: SPI operates at high speeds, making it suitable for applications requiring rapid data transfer.

Advantages

• No need for start and stop bits, providing continuous streaming of data without interruptions. 

• Higher data transfer rates compared to I2C (almost twice as fast).

• Absence of a complex slave addressing system, unlike I2C. 

• Dedicated MISO and MOSI lines enabling simultaneous data transmission and reception.

Disadvantages

• Requires four wires for communication which increase the circuit size 

• Lacks acknowledgment of successful data reception (unlike I2C).

• Absence of error-checking mechanisms such as parity bit in UART.

Applications of SPI

• Interfacing with sensors such as accelerometers, gyroscopes, and temperature sensors.

• Memory devices like EEPROMs, flash memory, and SD cards.

• Communication between microcontrollers and peripheral devices.

• Display interfaces in TFT LCD displays and OLED displays.

• Networking peripherals such as Ethernet controllers and Wi-Fi modules.

Conclusion

Serial Peripheral Interface (SPI) is a versatile communication protocol widely used in embedded systems and IOT applications for its simplicity, high-speed data transfer, and flexibility. Understanding the fundamentals of SPI, its protocol sequence, applications, and best practices for implementation is essential for engineers and developers working on embedded systems projects. By mastering SPI communication, you can efficiently interface with a wide range of peripheral devices like displays, sensors, modules, microcontrollers and unleash the full potential of your embedded systems designs.

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