Lesson-8 SPI COMMUNICATION

Definition

The Serial Peripheral Interface (SPI) protocol is a synchronous serial communication protocol used for exchanging data between multiple devices, such as microcontrollers, sensors, displays, and other peripheral devices. In SPI, data is sent and received over a master-slave communication link, where one device (the master) initiates and controls the data transfer and the other devices (the slaves) respond to commands from the master.

In a SPI communication system, one device is typically designated as the “master” and the other devices are designated as “slaves”. The master device initiates communication and controls the data transfer, while the slave devices respond to commands from the master.

Hardware Requirement

The SPI protocol uses four lines for communication:

SCLK: The serial clock signal, which is generated by the master device and is used to synchronize data transfer between the master and the slave devices.

MOSI: The Master Out Slave In signal, which is used to transmit data from the master device to the slave device.

MISO: The Master In Slave Out signal, which is used to transmit data from the slave device to the master device.

SS: The Slave Select signal, which is used to select the slave device with which the master device wants to communicate.

Protocol

The SPI protocol is based on a master-slave relationship, where the master device initiates the communication and controls the data transfer by generating a clock signal and selecting the slave device to communicate with using the SS line.

Here is the basic protocol of SPI communication:

1. The master device asserts the SS line for the slave device it wants to communicate with.
2. The master device generates a clock signal on the SCK line.
3. The master device sends data to the slave device on the MOSI line, and the slave device sends data back to the master on the MISO line.
4. Both the master and slave devices shift the data bits in and out of their respective shift registers on each clock cycle.
5. After the transfer is complete, the master device deasserts the SS line to end the communication with the slave device.
6. The clock phase and polarity can be configured to support different communication modes, including SPI mode 0, 1, 2, and 3, which have different clock polarities and phase relationships between the clock signal and data signals.
7. In some cases, additional control lines or signals can be used to support features like bidirectional data transfer or to provide additional synchronization or error checking mechanisms.

Example

Now, let’s take an example of SPI communication using an Arduino board. In this example, we will connect an Arduino board to an SPI peripheral device, specifically an SPI-based LCD display.

Here are the steps to set up and communicate with an SPI LCD display using an Arduino board:

1. Connect the LCD display to the Arduino board as follows:
   • VCC to 5V
   • GND to GND
   • SCK to digital pin 13 (SCK)
   • MOSI to digital pin 11 (MOSI)
   • CS to digital pin 10
   • RST to digital pin 9
   • D/C to digital pin 8
2. Include the SPI library in your Arduino sketch by adding the following line at the top of your sketch:

#include <SPI.h>

3. Initialize the SPI interface by calling the SPI.begin() function in the setup() function of your sketch:

void setup() {
  // Initialize SPI communication
  SPI.begin();
}

4. Set the pin modes for the CS, RST, and D/C pins by calling the pinMode() function:

void setup() {
  // Initialize SPI communication
  SPI.begin();

  // Set pin modes
  pinMode(10, OUTPUT); // CS
  pinMode(9, OUTPUT);  // RST
  pinMode(8, OUTPUT);  // D/C
}

5. To send data to the LCD display, you first need to select the device by setting the CS pin to low. You can do this by calling the digitalWrite() function:

void loop() {
  // Select LCD display
  digitalWrite(10, LOW);

  // Send data over SPI
  SPI.transfer(0x01); // Example data

  // Deselect LCD display
  digitalWrite(10, HIGH);

  // Wait for some time
  delay(1000);
}

6. Finally, to send commands to the LCD display, you need to set the D/C pin to low before sending the data. To send data, you need to set the D/C pin to high. You can do this by calling the digitalWrite() function:

void loop() {
  // Select LCD display
  digitalWrite(10, LOW);

  // Set D/C pin to low to send command