In this tutorial we are going to start learning about a great protocol used for implementing communication between microcontrollers and their peripheral devices on a serial bus. This protocol is generally used for its high speed data transfer rate than asynchronous protocol communications such as CAN and UART. I guess the question that comes directly to your mind is: what is an SPI protocol ?
SPI protocol :
Serial
peripheral interface, or SPI, is a synchronous serial communication protocol
used by microcontrollers to communicate with one or more peripheral devices.
The main difference between the SPI protocol and the old asynchronous
communication protocol (ex: CAN bus) is that it has a clock signal that synchronizes
the communication between devices.
Pin description :
In SPI
communications, there is only one device that generates the clock speed which
is usually a microcontroller (the Arduino board in this tutorial), it is called
a Master. The other devices have to set up the same clock speed and they are
called Slaves.
 |
| SPI bus: single master and single slave |
- MOSI : Master Out Slave In is the line used to send data from Arduino to the peripheral device.
- MISO : Master In Slave Out is the line used to send from the peripheral devices to Arduino.
- SCK : This line is used to transmit the clock signal generated by the Master device (Arduino) to the Slave device’s clock to synchronize data transmission.
When
multiple peripheral devices are used, we add another line to select the device
which is :
CS : Chip
Select is the line that selects the slave device that the master wants to talk
to. All CS lines are set to high (logic 1) which disconnects the slave devices
from SPI bus. Before sending data, Arduino sets the desired CS line to low to activate the peripheral
device that it wants to communicate with.
 |
| SPI bus: master and three independent slaves |
For further information about the SPI protocol, you can visit these links to see more detailed illustrations :
Serial Peripheral Interface — Wikipédia
Serial Peripheral Interface — Wikipédia
SPI protocol implementation :
- SPI library :
It is a standard library which is included by default with the Arduino IDE, it allows you to use multiple functions used for establishing communication. You can add it by going to Sketch -> Import library -> SPI on your Arduino software, or by just adding this line to the beginning of your program :
#include <SPI.h>
Now that you included the SPI library into your program, you have to set up some functions to initiate communication :
- Hardware connexions :
For arduino Uno board, you have to connect the pins as follows :
MOSI : pin 11
MISO : pin 12SCK : pin 13
/Cs : You can use any digital pin and set it to output.
Example
: using SPI with 74HC595 shift registers :
In this
example, we are going to set up an SPI communication between the Arduino
microcontroller and the 74HC595 via its serial data input DS. If you are not
introduced to use shift registers you can check my previous article to see what
it is and how it works.
This is a
simple program that shows how to implement SPI communication, initialize it in
the setup function and then use it to send data to the shift register’s DS
input, the data sent is an 8-bit binary counter that increments every time the
Chip Select goes low.
//includes the functions necessary for SPI communication
#include <SPI.h>
const int CS_pin = 10; //Chip Select
void setup() {
pinMode(CS_pin, OUTPUT); //CS as output
SPI.begin(); //starts the SPI communication
SPI.setBitOrder(MSBFIRST); //data sent MSB first
}
void binary_counter(int cnt) {
digitalWrite(CS_pin, LOW); //Chip select low to select the slave device
SPI.transfer(cnt); //data transfer
digitalWrite(CS_pin, HIGH); //deselect the device by turning it to high
cnt++; //counter incrementation
delay(200);
}
void loop() {
int counter = 0;
binary_counter( counter); //calls the binary_counter function
}
