stm8L PB1读IIC总线时设置成GPIO_Mode_In_PU_No_IT，会进外部中断，这是什么情况？

unsigned char I2C_Read(void)   
{   
    unsigned char  dat=0;   
    unsigned char i = 8;   
    I2C_SDA1;        //在读取数据之前，要把SDA拉高
    GPIO_Init(GPIOB,GPIO_Pin_1,GPIO_Mode_In_PU_No_IT );
    for (i=0; i<8; i++)   
    {   
        I2C_SCL1;   
        I2C_Delay();   
        dat <<= 1;   
        if (I2C_SDA)   
                dat |= 0x01;
        I2C_Delay();   
        I2C_SCL0;   
        I2C_Delay();  
    }   
    GPIO_Init(GPIOB,GPIO_Pin_1, GPIO_Mode_Out_PP_High_Fast);//PE_DDR |= 0x04; //output
    return (dat);   
}   

传感器SDA应答时会进入外部中断
INTERRUPT_HANDLER(EXTI1_IRQHandler, 9)
{
    /* In order to detect unexpected events during development,
       it is recommended to set a breakpoint on the following instruction.
    */
EXTI_ClearITPendingBit(EXTI_IT_Pin1);
}