RC522 RFID reader (SS 10, RST 9), on an Arduino Uno

// RC522 RFID reader: hold a card to it and see its ID.
//
//   RC522 SDA (SS) -> pin 10    RC522 SCK  -> pin 13
//   RC522 MOSI     -> pin 11    RC522 MISO -> pin 12
//   RC522 RST      -> pin 9     RC522 3.3V -> 3.3V   GND -> GND
//
// The reader talks SPI, the bus on pins 11, 12 and 13 that every SPI part on
// an Uno shares. SDA is the chip-select line that picks this reader. Every
// card carries a serial number, its UID, which the reader can read without
// knowing anything else about the card.
//
// The RC522 runs on 3.3 V. Never wire its supply to 5 V.
//
// Click the reader and pick a card to hold to it.

#include <SPI.h>
#include <MFRC522.h>

const int SS_PIN = 10;
const int RST_PIN = 9;

MFRC522 reader(SS_PIN, RST_PIN);

void setup() {
  Serial.begin(9600);
  SPI.begin();
  reader.PCD_Init();
  Serial.println("Mokxi Uno: RC522 on SS 10 and RST 9. Hold a card to the reader.");
}

void loop() {
  if (!reader.PICC_IsNewCardPresent()) return;
  if (!reader.PICC_ReadCardSerial()) return;
  Serial.print("Card UID:");
  for (byte i = 0; i < reader.uid.size; i++) {
    Serial.print(reader.uid.uidByte[i] < 0x10 ? " 0" : " ");
    Serial.print(reader.uid.uidByte[i], HEX);
  }
  Serial.println();
  reader.PICC_HaltA();
}