Now code does not fit here, please grab it from GitHub
Pin 6 : NRSTPD = reset and power-down input:power-down: enabled when LOW; internal current sinks are switched off, the oscillatoris inhibited and the input pins are disconnected from the outside worldreset: enabled
byte _resetPowerDownPin; // Arduino pin connected to MFRC522's reset and power down input (Pin 6, NRSTPD, active low)
8.6 Power reduction modes8.6.1 Hard power-downHard power-down is enabled when pin NRSTPD is LOW. This turns off all internal currentsinks including the oscillator. All digital input buffers are separated from the input pins andclamped internally (except pin NRSTPD). The output pins are frozen at either a HIGH orLOW level.8.6.2 Soft power-down modeSoft Power-down mode is entered immediately after the CommandReg register'sPowerDown bit is set to logic 1. All internal current sinks are switched off, including theoscillator buffer. However, the digital input buffers are not separated from the input pinsand keep their functionality. The digital output pins do not change their state.During soft power-down, all register values, the FIFO buffer content and the configurationkeep their current contents.After setting the PowerDown bit to logic 0, it takes 1024 clocks until the Soft power-downmode is exited indicated by the PowerDown bit. Setting it to logic 0 does not immediatelyclear it. It is cleared automatically by the MFRC522 when Soft power-down mode isexited.Remark: If the internal oscillator is used, you must take into account that it is supplied bypin AVDD and it will take a certain time (t osc ) until the oscillator is stable and the clockcycles can be detected by the internal logic. It is recommended for the serial UART, to firstsend the value 55h to the MFRC522. The oscillator must be stable for further access tothe registers. To ensure this, perform a read access to address 0 until the MFRC522answers to the last read command with the register content of address 0. This indicatesthat the MFRC522 is ready.8.6.3 Transmitter power-down modeThe Transmitter Power-down mode switches off the internal antenna drivers thereby,turning off the RF field. Transmitter power-down mode is entered by setting either theTxControlReg register's Tx1RFEn bit or Tx2RFEn bit to logic 0.