AN312 USI Verständnissfrage

#pragma vector=USI_START_VECTOR

__interrupt void USI_Start_Condition_ISR(void)

{

     unsigned char tmpUSISR;                                         //

Temporary variable to store volatile

     tmpUSISR = USISR;                                               //

Not necessary, but prevents warnings

-- Ist klar

 // Set default starting conditions for new TWI package

     USI_TWI_Overflow_State = USI_SLAVE_CHECK_ADDRESS;

-- Wird hier schon die angesprochene Adresse eingelesen??

     DDR_USI  &= ~(1<<PORT_USI_SDA);                                 //

Set SDA as input

-- Wieso nocheinmal als Input??

     while ( (PIN_USI & (1<<PORT_USI_SCL)) & !(tmpUSISR & (1<<USIPF))

);   // Wait for SCL to go low to ensure the "Start Condition" has

completed.

-- !(tmpUSISR & (1<<USIPF)) was soll sich daran aendern??? 

-- Ist klar

                                                                   // If

a Stop condition arises then leave the interrupt to prevent waiting

forever.

     USICR   =   (1<<USISIE)|(1<<USIOIE)|                            //

Enable Overflow and Start Condition Interrupt. (Keep StartCondInt to

detect RESTART)

                 (1<<USIWM1)|(1<<USIWM0)|                            //

Set USI in Two-wire mode.

                 (1<<USICS1)|(0<<USICS0)|(0<<USICLK)|                //

Shift Register Clock Source = External, positive edge

                 (0<<USITC);

-- Clear FLAGS heisst Bit setzen??

      USISR  =   

(1<<USI_START_COND_INT)|(1<<USIOIF)|(1<<USIPF)|(1<<USIDC)|      //

Clear flags

                 (0x0<<USICNT0);                                     //

Set USI to sample 8 bits i.e. count 16 external pin toggles.

}