Slave.cpp

#include <MFRC522.h>

#define RST_PIN 22 // Configurable, see typical pin layout above
#define SS_PIN 21  // Configurable, see typical pin layout above

MFRC522 mfrc522(SS_PIN, RST_PIN); // Create MFRC522 instance

void setup()
{
  Serial.begin(115200); // Initialize serial communications with the PC
  while (!Serial)
    ;                                // Do nothing if no serial port is opened (added for Arduinos based on ATMEGA32U4)
  SPI.begin();                       // Init SPI bus
  mfrc522.PCD_Init();                // Init MFRC522
  delay(4);                          // Optional delay. Some board do need more time after init to be ready, see Readme
  mfrc522.PCD_DumpVersionToSerial(); // Show details of PCD - MFRC522 Card Reader details
  Serial.println(F("Scan PICC to see UID, SAK, type, and data blocks..."));
}

void loop()
{
  // Reset the loop if no new card present on the sensor/reader. This saves the entire process when idle.
  if (!mfrc522.PICC_IsNewCardPresent())
  {
    return;
  }

  // Select one of the cards
  if (!mfrc522.PICC_ReadCardSerial())
  {
    return;
  }

  // Dump debug info about the card; PICC_HaltA() is automatically called
  mfrc522.PICC_DumpToSerial(&(mfrc522.uid));
}