#include <SPI.h>
SPISettings mySettings(1000000, MSBFIRST, SPI_MODE0, SPI_PERIPHERAL);
void setup() {
 Serial.begin(115200);
 while (!Serial);
 SPI.begin();
}
void loop() {
 uint8_t buf[] = {0x01, 0x02, 0x03, 0x04};
 SPI.beginTransaction(mySettings);
 SPI.transfer(buf, sizeof(buf));
 SPI.endTransaction();
 
 Serial.println("Received:");
 for (int i=0; i<sizeof(buf); i++) {
 Serial.print("0x"); Serial.println(buf[0], HEX);
 }
 
 delay(100);
}