Add power control functions (#334)

* Update MFRC522.h
Add new power control functions : 
PCD_SoftPowerDown
PCD_SoftPowerUp

src/MFRC522.cpp

@@ -363,6 +363,35 @@ bool MFRC522::PCD_PerformSelfTest() {
 	return true;
 } // End PCD_PerformSelfTest()
 
+/////////////////////////////////////////////////////////////////////////////////////
+// Power control
+/////////////////////////////////////////////////////////////////////////////////////
+
+//IMPORTANT NOTE!!!!
+//Calling any other function that uses CommandReg will disable soft power down mode !!!
+//For more details about power control, refer to the datasheet - page 33 (8.6)
+
+void MFRC522::PCD_SoftPowerDown(){//Note : Only soft power down mode is available throught software
+	byte val = PCD_ReadRegister(CommandReg); // Read state of the command register 
+	val |= (1<<4);// set PowerDown bit ( bit 4 ) to 1 
+	PCD_WriteRegister(CommandReg, val);//write new value to the command register
+}
+
+void MFRC522::PCD_SoftPowerUp(){
+	byte val = PCD_ReadRegister(CommandReg); // Read state of the command register 
+	val &= ~(1<<4);// set PowerDown bit ( bit 4 ) to 0 
+	PCD_WriteRegister(CommandReg, val);//write new value to the command register
+	// wait until PowerDown bit is cleared (this indicates end of wake up procedure) 
+	const uint32_t timeout = (uint32_t)millis() + 500;// create timer for timeout (just in case) 
+	
+	while(millis()<=timeout){ // set timeout to 500 ms 
+		val = PCD_ReadRegister(CommandReg);// Read state of the command register
+		if(!(val & (1<<4))){ // if powerdown bit is 0 
+			break;// wake up procedure is finished 
+		}
+	}
+}
+
 /////////////////////////////////////////////////////////////////////////////////////
 // Functions for communicating with PICCs
 /////////////////////////////////////////////////////////////////////////////////////

src/MFRC522.h

@@ -359,6 +359,12 @@ public:
 	void PCD_SetAntennaGain(byte mask);
 	bool PCD_PerformSelfTest();
 	
+	/////////////////////////////////////////////////////////////////////////////////////
+	// Power control functions
+	/////////////////////////////////////////////////////////////////////////////////////
+	void PCD_SoftPowerDown();
+	void PCD_SoftPowerUp();
+	
 	/////////////////////////////////////////////////////////////////////////////////////
 	// Functions for communicating with PICCs
 	/////////////////////////////////////////////////////////////////////////////////////