SPI test code for Arduino UNO R4

Arduino_UNO_R4_SPI_test.ino

#include <SPI.h>

SPISettings spi_setting(1000000, MSBFIRST, SPI_MODE0);

void setup() {
  Serial.begin(9600);
  while (!Serial)
    ;

  SPI.begin();
  pinMode(10, OUTPUT);  // <- this is required to make `digitalWrite(SS, LOW);` work on Arduino R4 Minima.
}

void loop() {
  uint8_t data[] = { 0x55, 0xAA, 0x55, 0xAA };

  SPI.beginTransaction(spi_setting);

  digitalWrite(SS, LOW);
  SPI.transfer(data, sizeof(data));
  digitalWrite(SS, HIGH);

  SPI.endTransaction();
}