46. SPI Programming

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

Arduino1 (master) code:

#include <SPI.h>

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

void loop() {
  for(uint32_t i = 0; i < 10; i++)  {
    digitalWrite(SS, LOW);
    delay(10);
    digitalWrite(SS, HIGH);
  }
  while (1);
}

Arduino2 (slave) 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() {
  if (millis() - start_ms > 10000) {
    Serial.println(count);
    while (1);
  }
}

ISR(SPI_STC_vect) {
  count++;
}