#include <SPI.h>

const int latchPin = 10;

void setup() {
  //SPI.begin();
  // can't change bit order...
  SPI.beginTransaction(SPISettings(15000000, LSBFIRST, SPI_MODE0));
  pinMode(latchPin, OUTPUT);
}

void loop() {
  for (int i = 0; i < 256; i++) {
    sendToShiftRegister(i);
    delay(250);
  }
  /*
  sendToShiftRegister(0b10101010);
  delay(1000);
  sendToShiftRegister(0b01010101);
  delay(1000);
  */
}

// Function to send one byte to the 74HC595 via SPI
void sendToShiftRegister(uint8_t data) {
  digitalWrite(latchPin, LOW);
  SPI.transfer(data);
  digitalWrite(latchPin, HIGH);
}