Table of Contents
Intro:
We know that SPI stands for the Serial Peripheral Interface. SPI developed by Motorola in the mid-1980.
It is a one of the serial communication protocol for connect low-speed devices.
It is commonly used for communication with flash memory, ram disk, sensor, real-time clock (RTC), analog-to-digital converters, and more.
SPI is a full-duplex synchronous serial communication, which means that data can be simultaneously transmitted from both directions.
The main advantage of the SPI is to transfer the data without any interruption.
SPI Interface:
Basically SPI protocol uses the four wires for the communication purpose.
- SCL Serial clock
- MISO Master In Slave Out
- MOSI Master Out Slave In
- SS Slave Select
(Note : For Communication SS Must be ‘0’ OR Low Similar For Stop Communication SS Must be ‘1’ OR High)
In SPI Communication Protocol number of Slave select increase with n number of slave so we use another method Daisy Chain Method
Daisy Chain Method
There four operating modes in this method depending on Rising and Falling Edge of clock pulse
| Operation Mode Clock pulse | Rising Edge | Falling Edge |
| Mode 0 | 0 | 0 |
| Mode 1 | 0 | 1 |
| Mode 2 | 1 | 0 |
| Mode 3 | 1 | 1 |
Sample C code for SPI Communication in Embedded
Master Slave Arduino communication:
//Master Arduino Code:
//SPI MASTER
//SPI COMMUNICATION BETWEEN TWO ARDUINO
//CIRCUIT DIGEST
#include<SPI.h> //Library for SPI
#define LED 7
#define ipbutton 2
int buttonvalue;
int x;
void setup (void)
{
Serial.begin(115200); //Starts Serial Communication at Baud Rate 115200
pinMode(ipbutton,INPUT); //Sets pin 2 as input
pinMode(LED,OUTPUT); //Sets pin 7 as Output
SPI.begin(); //Begins the SPI commnuication
SPI.setClockDivider(SPI_CLOCK_DIV8); //Sets clock for SPI communication at 8 (16/8=2Mhz)
digitalWrite(SS,HIGH); // Setting SlaveSelect as HIGH (So master doesnt connnect with slave)
}
void loop(void)
{
byte Mastersend,Mastereceive;
buttonvalue = digitalRead(ipbutton); //Reads the status of the pin 2
if(buttonvalue == HIGH) //Logic for Setting x value (To be sent to slave) depending upon input from pin 2
{
x = 1;
}
else
{
x = 0;
}
digitalWrite(SS, LOW); //Starts communication with Slave connected to master
Mastersend = x;
Mastereceive=SPI.transfer(Mastersend); //Send the mastersend value to slave also receives value from slave
if(Mastereceive == 1) //Logic for setting the LED output depending upon value received from slave
{
digitalWrite(LED,HIGH); //Sets pin 7 HIGH
Serial.println("Master LED ON");
}
else
{
digitalWrite(LED,LOW); //Sets pin 7 LOW
Serial.println("Master LED OFF");
}
delay(1000);
}
------------------------------------
//Slave Arduino Code:
//SPI SLAVE (ARDUINO)
//SPI COMMUNICATION BETWEEN TWO ARDUINO
//CIRCUIT DIGEST
//Pramoth.T
#include<SPI.h>
#define LEDpin 7
#define buttonpin 2
volatile boolean received;
volatile byte Slavereceived,Slavesend;
int buttonvalue;
int x;
void setup()
{
Serial.begin(115200);
pinMode(buttonpin,INPUT); // Setting pin 2 as INPUT
pinMode(LEDpin,OUTPUT); // Setting pin 7 as OUTPUT
pinMode(MISO,OUTPUT); //Sets MISO as OUTPUT (Have to Send data to Master IN
SPCR |= _BV(SPE); //Turn on SPI in Slave Mode
received = false;
SPI.attachInterrupt(); //Interuupt ON is set for SPI commnucation
}
ISR (SPI_STC_vect) //Inerrrput routine function
{
Slavereceived = SPDR; // Value received from master if store in variable slavereceived
received = true; //Sets received as True
}
void loop()
{ if(received) //Logic to SET LED ON OR OFF depending upon the value recerived from master
{
if (Slavereceived==1)
{
digitalWrite(LEDpin,HIGH); //Sets pin 7 as HIGH LED ON
Serial.println("Slave LED ON");
}else
{
digitalWrite(LEDpin,LOW); //Sets pin 7 as LOW LED OFF
Serial.println("Slave LED OFF");
}
buttonvalue = digitalRead(buttonpin); // Reads the status of the pin 2
if (buttonvalue == HIGH) //Logic to set the value of x to send to master
{
x=1;
}else
{
x=0;
}
Slavesend=x;
SPDR = Slavesend; //Sends the x value to master via SPDR
delay(1000);
}
}
Conclusion
In this article we saw basics of SPI Communication with different operating modes
