#include <SPI.h> // use the Arduino SPI library

#define SPI_MASTER_SLAVE_SEL_PIN  (2)  // SPI Master/Slave Select pin
#define SS_PIN                   (10)  // SPI Slave Select pin

boolean is_spi_master;
volatile boolean slave_received  = false;
volatile uint8_t slave_recv_data = 0xff;

void setup() {
  Serial.begin( 115200 ); 
  pinMode( SPI_MASTER_SLAVE_SEL_PIN, INPUT_PULLUP );
  if ( digitalRead(SPI_MASTER_SLAVE_SEL_PIN) == LOW ) {
    is_spi_master = true;
  } else {
    is_spi_master = false;
  }
  if (is_spi_master) { // for SPI master/controller device
    pinMode( SS_PIN, OUTPUT );      // use the SS pin for output
    digitalWrite( SS_PIN, HIGH );   // output HIGH to the SS pin
    SPI.begin();                    // start the SPI master
    SPI.setDataMode( SPI_MODE0 );   // use SPI mode 0 (CPOL=0,CHPA=0)
    SPI.setBitOrder( MSBFIRST );    // use MSB-first bit order 
    // set clock divider for SCK 
    // use SPI_CLOCK_DIVx, where x=4,8,16,32,64,128
    SPI.setClockDivider( SPI_CLOCK_DIV16 ); // 16MHz/16 = 1MHz
    Serial.println( F("SPI Master...") );
    delay( 100 );
  } 
  else { // for SPI slave/peripheral device
    pinMode( MISO, OUTPUT );         // configure the MISO pin as output
    pinMode( SS_PIN, INPUT_PULLUP ); // enable internal pullup on SS input pin 
    SPDR  = 0xFF;                    // write 0xFF to SPDR
    // MSTR=0 (Slave mode), DORD=0 (MSB first), Mode 0 (CPOL=0, CPHA=0)
    SPCR &= ~_BV(MSTR);              // clear the MSTR bit (MSTR=0)
    SPCR &= ~_BV(CPOL);              // clear the CPOL bit (CPOL=0)
    SPCR &= ~_BV(CPHA);              // clear the CPHA bit (CPHA=0)
    SPCR &= ~_BV(DORD);              // clear the DORD bit (DORD=0)
    SPCR |=  _BV(SPE);               // set the SPE bit to enable SPI
    slave_received = false;          // clear the slave data reception flag
    SPI.attachInterrupt();           // enable the SPI interrupt
    Serial.println( F("SPI Slave...") );
  } 
}

void loop() {
  if ( is_spi_master ) { // SPI master operation
    static uint8_t send_data = 0x00, recv_data;
    Serial.print( F("Master> sent: 0x") );
    Serial.print( send_data, HEX );
    // pull the /SS pin LOW to activate the SPI bus
    digitalWrite( SS_PIN, LOW );
    // send-receive one data byte over the SPI bus
    recv_data = SPI.transfer( send_data ); 
    // pull the /SS pin HIGH to de-activate the SPI bus
    digitalWrite( SS_PIN, HIGH );
    Serial.print( F(", received: 0x") );
    Serial.println( recv_data, HEX ); 
    send_data += 1; // increment by 1
    delay( 1000 );
  } 
  else { // SPI slave operation
    if ( slave_received ) {
      // write the received data byte to SPDR
      SPDR = slave_recv_data;
      // clear the slave data reception flag
      slave_received = false;
      Serial.print( F("Slave: received 0x") );
      Serial.println( slave_recv_data, HEX );
    }
    delay(1);
  }
}

// ISR for 'SPI Serial Transfer Complete' (SPI Slave mode)
ISR(SPI_STC_vect) { 
  slave_recv_data = SPDR;
  slave_received  = true;
}