#define USIBR _SFR_IO8(0x000)

USICR = (1 << USISIE) | (1 << USIOIE) | (1 << USIWM1 ) | (1 << USIWM0) | (1 << USICS1);

ISR(USI_START_vect)
{        
    USISR &=~(15);
    USISR |=(15);
    i2c_state = I2C_START_WAITING;    
    buf_add_byte(start);        
    USISR |= (1 << USISIF);
}

ISR(USI_OVERFLOW_vect)
{    
    USISR &=~(15);    
    if( i2c_state & 0x03 )
    {        
        USISR |= (14);
        //buf_add_byte(USIBR);
        buf_add_byte(USIDR);
        i2c_state = I2C_ACK_RECEIVING;                
    }    
    else if( i2c_state == I2C_ACK_RECEIVING )
    {
        //if( USIBR & 0x1 )
        if( USIDR & 0x1 )
            buf_add_byte(nack);
        else
            buf_add_byte(ack);
        i2c_state = I2C_DATA_RECEIVING;
    }
    else if( i2c_state ==  I2C_START_WAITING )
    {
        i2c_state = I2C_ADDR_RECEIVING;
    }
    USISR |= (1 << USIOIF);
}