<* LED와 Switch를 이용한 SPI 통신 *>
• Master에서 Swich를 누르면, Slave에서 LED가 동작한다.
• Slave에서 Swich를 누르면, Master의 LED가 동작한다.

---- < SPI 통신 LED와 Switch  : Master 소스 코드 > --------
#include<SPI.h>

#define LED_Pin        8         
#define Button_Pin  2

int ButtonValue;
int ButtonStat;

void setup() {
  Serial.begin(9600);                  //Starts Serial Communication at Baud Rate 9600
  pinMode(Button_Pin,INPUT);                //Sets pin 2 as input
  pinMode(LED_Pin,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);           

}

void loop() {
  byte Master_Send, Master_Receive;         
  ButtonValue = digitalRead(Button_Pin); 
  if(ButtonValue == HIGH)               
  {
    ButtonStat = 1;
  } else {
    ButtonStat = 0;
  }

  digitalWrite(SS, LOW);                  //Starts communication with Slave connected to master
   
  Master_Send = ButtonStat;                           
  Master_Receive=SPI.transfer(Master_Send); //Send the mastersend value to slave also receives value from slave
  if(Master_Receive == 1)                  //Logic for setting the LED output depending upon value received from slave
  {
    digitalWrite(LED_Pin,HIGH);              //Sets pin 7 HIGH
    Serial.println("Master LED ON");
  }
  else
  {
  digitalWrite(LED_Pin,LOW);              //Sets pin 7 LOW
  Serial.println("Master LED OFF");
  }
  delay(1000);

}

---- < SPI 통신 LED와 Switch  : Slave 소스 코드 > --------
#include<SPI.h>

#define LED_Pin    8
#define Button_Pin  2

volatile boolean received;
volatile byte Slave_Received,Slave_Send;
int ButtonValue;
int Button_Stat;

void setup() {
  Serial.begin(9600);
 
  pinMode(Button_Pin,INPUT);              // Setting pin 2 as INPUT
  pinMode(LED_Pin,OUTPUT);                // Setting pin 7 as OUTPUT
  pinMode(MISO,OUTPUT);                  //Sets MISO as
  SPCR |= _BV(SPE);                      //Turn on SPI in Slave Mode
  received = false;

  SPI.attachInterrupt(); 

}

ISR (SPI_STC_vect) {                      //Inerrrput routine function

  Slave_Received = 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 (Slave_Received==1) {
        digitalWrite(LED_Pin,HIGH);        //Sets pin 7 as HIGH LED ON
        Serial.println("Slave LED ON");
      }else {
        digitalWrite(LED_Pin,LOW);          //Sets pin 7 as LOW LED OFF
        Serial.println("Slave LED OFF");
      }
     
      ButtonValue = digitalRead(Button_Pin);  // Reads the status of the pin 2
     
      if (ButtonValue == HIGH) {              //Logic to set the value of x to send to master
        Button_Stat=1;
       
      } else {
        Button_Stat=0;
      }
     
  Slave_Send=Button_Stat;                           
  SPDR = Slave_Send;                          //Sends the x value to master via SPDR
  delay(1000);
  }

}