SPI-Master.ino

#include <SPI.h>

int button = 6;

bool command(int pin)
{
  return (digitalRead(pin) == HIGH) ? true : false;
}

char * dataFormattedString(double data)
{
  char *datastr = (char *) malloc(100 * sizeof(char));
  char angle[10];
  dtostrf(data,5,3,angle);
  sprintf(datastr,"a:%s\r",angle);
  return datastr;
}

void setup (void) {

   Serial.begin(115200); //set baud rate to 115200 for usart
   digitalWrite(SS, HIGH); // disable Slave Select
   SPI.begin();
   SPI.setClockDivider(SPI_CLOCK_DIV8);//divide the clock by 8

}

void loop (void) {
   char c;
   int count = 0;

   double commandAngle = 200;

   if(command(button))
   {
    digitalWrite(SS, LOW);

   for (const char *p = dataFormattedString(commandAngle); c = *p; p++)
    {
      //if(count++ > strlen(p)) { free(p); } // an attempt to prevent memory leak
      SPI.transfer(c);
      Serial.print(c);
   }
   
   digitalWrite(SS, HIGH);
   delay(2000);
   }
     
}

SPI-Slave.ino

#include <SPI.h>

char buff [100];
volatile byte indx;
volatile boolean process;
volatile double setpoint = 0;

int getAngle(char *str)
{
  int count = 0;
  char *token;
  const char *s = ":";
  token = strtok(str,s);
  while(token != NULL)
  {
      if(count == 1)
      {
        return atoi(token);
      }
      count++;
      //Serial.println(token);
      token = strtok(NULL,s);
  }
}

void setup (void) {

   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 (void) {
   if (process){
      process = false;        //reset the process

      int angle = getAngle(buff);
      Serial.println(2*angle);    
      indx= 0;              //reset button to zero
   }
}