46. SPI Programming

What number will be printed on Arduino2’s and Arduino3’s serial monitor? (after executing the following code)

Arduino1 (master) code:

#include <SPI.h>

void setup() {
  SPI.begin();
  SPI.setClockDivider(SPI_CLOCK_DIV8);
  delay(2000);
}

void loop() {
  digitalWrite(SS, LOW);
  SPI.transfer(9);
  digitalWrite(SS, HIGH);
  
  while (1);
}

Arduino2 and Arduino 3 (slave’s) code:

#include <SPI.h>

uint32_t count = 0;
uint32_t start_ms = 0;

void setup() {
  Serial.begin(115200);

  pinMode(MISO, OUTPUT);
  SPCR |= _BV(SPE);
  pinMode(SS, INPUT_PULLUP);
  SPI.attachInterrupt();
  start_ms = millis();
}

void loop() {
}

ISR(SPI_STC_vect) {
  Serial.println(SPDR);
}