#include <SPI.h>
char buff [50];
volatile byte indx;
volatile boolean process;

void setup() {
  Serial.begin (115200);
  pinMode(MISO, OUTPUT); // have to send on master in so it set as output
  SPCR |= _BV(SPE); // turn on SPI in slave mode
  indx = 0; // buffer empty
  process = false;
  SPI.attachInterrupt(); // turn on interrupt
}

ISR (SPI_STC_vect) {// SPI interrupt routine
  byte c = SPDR; // read byte from SPI Data Register
  if (indx < sizeof buff) {
    buff [indx++] = c; // save data in the next index in the array buff
    if (c == '\r') //check for the end of the word
    process = true;
  }
}

void loop(){
  if (process) {
    process = false; //reset the process
    Serial.println (buff); //print the array on serial monitor
    indx= 0; //reset button to zero
  }
}