ATTiny25 USI-Fehler im AVR Studio Simulator?

#include <avr/io.h>

#include <stdint.h>

#include <avr/interrupt.h>

#define MESSAGEBUF_SIZE       4

// Sample TWI transmission commands

#define TWI_CMD_MASTER_WRITE 0x10

#define TWI_CMD_MASTER_READ  0x20

//Device Dependant Definitions

#define DDR_USI             DDRB

#define PORT_USI            PORTB

#define PIN_USI             PINB

#define PORT_USI_SDA        PORTB0

#define PORT_USI_SCL        PORTB2

#define PIN_USI_SDA         PINB0

#define PIN_USI_SCL         PINB2

ISR(USI_START_vect) 

{   

  //USI Code....

}

void USI_TWI_Slave_Initialise( unsigned char TWI_ownAddress )

{

  unsigned char  TWI_slaveAddress = TWI_ownAddress;

  PORT_USI |=  (1<<PORT_USI_SCL);      // Set SCL high

  PORT_USI |=  (1<<PORT_USI_SDA);      // Set SDA high

  DDR_USI  |=  (1<<PORT_USI_SCL);      // Set SCL as output    

  DDR_USI  &= ~(1<<PORT_USI_SDA);      // Set SDA as input    

  USICR    =  (1<<USISIE)|(0<<USIOIE)| // Enable Start Interrupt. Disable

                                       //Overflow Interrupt.

              (1<<USIWM1)|(0<<USIWM0)| // Set USI in Two-wire mode. No

                                       // USI  Counter overflow prior to 

                                       // first Start Condition

              (1<<USICS1)|(0<<USICS0)|(0<<USICLK)| // Shift Register Clock

                                        // Source = External, positive edge

                          (0<<USITC);

//Clear all flags and reset overflow counter

  USISR    = 0xF0; //BEIM TINY25 werden im Simulaor die Flags nicht

                   //gelöscht sondern gesetzt.

}

int main( void )

{    

  unsigned char messageBuf[MESSAGEBUF_SIZE], TWI_slaveAddress,   

        samplebyte=0;

  TWI_slaveAddress = 0x10;

  USI_TWI_Slave_Initialise( TWI_slaveAddress );

  sei();                          

  for(;;)

  {   

        if (messageBuf[0] == TWI_CMD_MASTER_WRITE)

  {              

           samplebyte=messageBuf[1];                            

  }

        if (messageBuf[0] == TWI_CMD_MASTER_READ)

        {

          messageBuf[0] = samplebyte;                             

          //USI_TWI_Start_Transceiver_With_Data( messageBuf,1 );           

        }

  }

}