doc added, mega port definition

main.cpp

@@ -11,9 +11,24 @@
 //from the core and we need to include it explicitly
 #include <SPI.h>
 
-//Define which port to use for the SPI Chip Select
+// Define which port to use for the SPI Chip Select by using the register
+// definitions. The data direction register is assigned as well.
+// The ports can be looked up on the official Arduino pinout schematics.
+#ifdef ARDUINO_AVR_MEGA2560
+// Defines for the Arduino Mega
+#define CS_PORT PORTK
+#define CS_DDR DDRK
+#elif defined(ARDUINO_AVR_UNO)
+#define CS_PORT PORTD
+#define CS_DDR DDRD
+#elif defined(__SAM3X8E__)
+// Define for the Arduino Due
 #define CS_PORT PORTC
 #define CS_DDR DDRC
+#else
+#define CS_PORT PORTC
+#define CS_DDR DDRC
+#endif
 
 static const uint8_t COMMAND_TRANSFER_SPI = 1;
 
@@ -45,9 +60,13 @@ void sendData(uint8_t *data, size_t len) {
 
 void transferSPI(uint8_t address, uint8_t *data, size_t datalen) {
 	SPI.beginTransaction(SPISettings(14000000, MSBFIRST, SPI_MODE3));
+	// The specified address is the bit where the last bit is port 0
+	// and the first bit is port 7. It is inverted because the SPI protocol
+	// requires the slave select to be driven low.
 	CS_PORT = ~address;
 	SPI.transfer(data, datalen);
 	delay(100);
+	// Pull the slave select high again.
 	CS_PORT = 0xff;
 	SPI.endTransaction();
 
@@ -157,7 +176,10 @@ void serialEvent() {
 }
 
 void setup() {
+    // Set data direction of selected port to output.
 	CS_DDR = 0xff;
+	// Drive all the slave selects high as required for the SPI protocol
+	// if no transfer is going on.
 	CS_PORT = 0xff;
 	Serial.begin(BAUD_RATE);
 	Serial.println("Setting up Arduino IO interface board.");