现象为USART2连485转USB，然后在下载程序和仿真时都会疯狂打印0，用示波器查看发现Rx有50hz波形，单步暂停时也有。从另一个没有问题的F103工程移植。DE_RE配置正确。
串口2初始化：
void USART2_Init(uint32_t BaudRate,uint16_t StopBits)
{
    NVIC_InitTypeDef NVIC_InitStructure;
    USART_InitTypeDef USART_InitStructure;
    GPIO_InitTypeDef  GPIO_InitStructure;
    //USART2 init
    RCC_APB1PeriphClockCmd(RCC_APB1Periph_USART2, ENABLE);
    RCC_APB2PeriphClockCmd(RCC_APB2Periph_GPIOA|RCC_APB2Periph_AFIO, ENABLE);
    USART_DeInit(USART2);
       
    //USART2_TX   GPIOA.2
    GPIO_InitStructure.GPIO_Pin = GPIO_Pin_2;
    GPIO_InitStructure.GPIO_Speed = GPIO_Speed_50MHz;
    GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AF_PP;
    GPIO_Init(GPIOA, &GPIO_InitStructure);

    //USART2_RX          GPIOA.3
    GPIO_InitStructure.GPIO_Pin = GPIO_Pin_3;
    GPIO_InitStructure.GPIO_Mode = GPIO_Mode_IN_FLOATING;
    GPIO_Init(GPIOA, &GPIO_InitStructure);

    USART_InitStructure.USART_BaudRate = BaudRate;
    USART_InitStructure.USART_WordLength = USART_WordLength_8b;
    USART_InitStructure.USART_StopBits = StopBits;
    USART_InitStructure.USART_Parity = USART_Parity_No;
    USART_InitStructure.USART_HardwareFlowControl = USART_HardwareFlowControl_None;
    USART_InitStructure.USART_Mode = USART_Mode_Rx | USART_Mode_Tx;
    USART_Init(USART2, &USART_InitStructure);

    USART_ITConfig(USART2, USART_IT_RXNE, ENABLE);

    NVIC_InitStructure.NVIC_IRQChannel = USART2_IRQn;
    NVIC_InitStructure.NVIC_IRQChannelPreemptionPriority = 6;
    NVIC_InitStructure.NVIC_IRQChannelSubPriority = 0;
    NVIC_InitStructure.NVIC_IRQChannelCmd = ENABLE;
    NVIC_Init(&NVIC_InitStructure);

    USART_Cmd(USART2, ENABLE);
}

给PA3配个上拉

示波器显示有50HZ波形，一定是示波器探头的地线断了。