Kann mir jemand erklären was in diesem Code passiert (Beispiel Atmel AtXmega128A1 TWI)

Hallo,

ich habe hier diesen Beispiel Code von Atmel. Habe versucht ihn zu 
verstehen, bin aber noch sehr unerfahren und komme nicht weiter. Kann 
mir hier einer helfen?

Danke im Vorraus

Gruß Denis

#include "avr_compiler.h"
#include "twi_master_driver.h"
#include "twi_slave_driver.h"

/*! Defining an example slave address. */
#define SLAVE_ADDRESS    0x55

/*! Defining number of bytes in buffer. */
#define NUM_BYTES        8

/*! CPU speed 2MHz, BAUDRATE 100kHz and Baudrate Register Settings */
#define CPU_SPEED       2000000
#define BAUDRATE  100000
#define TWI_BAUDSETTING TWI_BAUD(CPU_SPEED, BAUDRATE)


/* Global variables */
TWI_Master_t twiMaster;    /*!< TWI master module. */
TWI_Slave_t twiSlave;      /*!< TWI slave module. */


/*! Buffer with test data to send.*/
uint8_t sendBuffer[NUM_BYTES] = {0x55, 0xAA, 0xF0, 0x0F, 0xB0, 0x0B, 
0xDE, 0xAD};


/*! Simple function that invert the received value in the sendbuffer. 
This
 *  function is used in the driver and passed on as a pointer to the 
driver.
 */
void TWIC_SlaveProcessData(void)
{
  uint8_t bufIndex = twiSlave.bytesReceived;
  twiSlave.sendData[bufIndex] = (~twiSlave.receivedData[bufIndex]);
}


/*! /brief Example code
 *
 *  Example code that reads the key pressed and show a value from the 
buffer,
 *  sends the value to the slave and read back the processed value which 
will
 *  be inverted and displayed after key release.
 */
int main(void)
{
  /* Initialize PORTE for output and PORTD for inverted input. */
  PORTE.DIRSET = 0xFF;
  PORTD.DIRCLR = 0xFF;
  PORTCFG.MPCMASK = 0xFF;
  PORTD.PIN0CTRL |= PORT_INVEN_bm;
//      PORTCFG.MPCMASK = 0xFF;
//      PORTD.PIN0CTRL = (PORTD.PIN0CTRL & ~PORT_OPC_gm) | 
PORT_OPC_PULLUP_gc;

//Enable internal pull-up on PC0, PC1.. Uncomment if you don't have 
external pullups
PORTCFG.MPCMASK = 0x03; // Configure several PINxCTRL registers at the 
same time
PORTC.PIN0CTRL = (PORTC.PIN0CTRL & ~PORT_OPC_gm) | PORT_OPC_PULLUP_gc; 
//Enable pull-up to get a defined level on the switches



  /* Initialize TWI master. */
  TWI_MasterInit(&twiMaster,
                 &TWIC,
                 TWI_MASTER_INTLVL_LO_gc,
                 TWI_BAUDSETTING);

  /* Initialize TWI slave. */
  TWI_SlaveInitializeDriver(&twiSlave, &TWIC, TWIC_SlaveProcessData);
  TWI_SlaveInitializeModule(&twiSlave,
                            SLAVE_ADDRESS,
                            TWI_SLAVE_INTLVL_LO_gc);

  /* Enable LO interrupt level. */
  PMIC.CTRL |= PMIC_LOLVLEN_bm;
  sei();

  uint8_t BufPos = 0;
  while (1) {
                while(!PORTD.IN); /* Wait for user to press button */

    switch(PORTD.IN){
      case (PIN0_bm):  BufPos = 0; break;
      case (PIN1_bm):  BufPos = 1; break;
      case (PIN2_bm):  BufPos = 2; break;
      case (PIN3_bm):  BufPos = 3; break;
      case (PIN4_bm):  BufPos = 4; break;
      case (PIN5_bm):  BufPos = 5; break;
      case (PIN6_bm):  BufPos = 6; break;
      case (PIN7_bm):  BufPos = 7; break;
      default:    break;
    }

    /* Show the byte to send while holding down the key. */
    while(PORTD.IN != 0x00){
      PORTE.OUT = sendBuffer[BufPos];
    }

    TWI_MasterWriteRead(&twiMaster,
                        SLAVE_ADDRESS,
                        &sendBuffer[BufPos],
                        1,
                        1);


    while (twiMaster.status != TWIM_STATUS_READY) {
      /* Wait until transaction is complete. */
    }

    /* Show the sent byte received and processed on LEDs. */
    PORTE.OUT = (twiMaster.readData[0]);

                while(PORTD.IN); /* Wait for user to release button */
  }
}

/*! TWIC Master Interrupt vector. */
ISR(TWIC_TWIM_vect)
{
  TWI_MasterInterruptHandler(&twiMaster);
}

/*! TWIC Slave Interrupt vector. */
ISR(TWIC_TWIS_vect)
{
  TWI_SlaveInterruptHandler(&twiSlave);
}