1查看datasheet，直接搜索USART3,就可以知道其映射关系了。

2代码

#define ISP_UART_PORT USART3
#define ISP_UART_IRQ USART3_IRQn
#define ISP_UART_RCC RCC_APB1Periph_USART3
#define ISP_UART_IRQHANDLER USART3_IRQHandler
#define ISP_UART_REMAP GPIO_FullRemap_USART3

#define ISP_RX_PORT GPIOD
#define ISP_RX_PIN GPIO_Pin_9

#define ISP_TX_PORT GPIOD
#define ISP_TX_PIN GPIO_Pin_8

static void isp_uart_init(uint32_t baudrate)
{
	GPIO_InitTypeDef  GPIO_InitStructure;
	USART_InitTypeDef USART_InitStructure;

	RCC_APB1PeriphClockCmd(ISP_UART_RCC, ENABLE);

	GPIO_InitStructure.GPIO_Speed = GPIO_Speed_50MHz;
	GPIO_InitStructure.GPIO_Pin   = ISP_RX_PIN;
	GPIO_InitStructure.GPIO_Mode  = GPIO_Mode_IPU;
	GPIO_Init(ISP_RX_PORT, &GPIO_InitStructure);
	GPIO_InitStructure.GPIO_Pin  = ISP_TX_PIN;
	GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AF_PP;
	GPIO_Init(ISP_TX_PORT, &GPIO_InitStructure);
	GPIO_PinRemapConfig(ISP_UART_REMAP, ENABLE);

	USART_InitStructure.USART_BaudRate            = baudrate;
	USART_InitStructure.USART_WordLength          = USART_WordLength_9b;
	USART_InitStructure.USART_StopBits            = USART_StopBits_1;
	USART_InitStructure.USART_Parity              = USART_Parity_Even;
	USART_InitStructure.USART_HardwareFlowControl = USART_HardwareFlowControl_None;
	USART_InitStructure.USART_Mode                = USART_Mode_Rx | USART_Mode_Tx;
	USART_Init(ISP_UART_PORT, &USART_InitStructure);  // 此处使能了CR1的TE和 RE

	USART_ITConfig(ISP_UART_PORT, USART_IT_RXNE, ENABLE);

	USART_Cmd(ISP_UART_PORT, ENABLE);

	NVIC_SetPriority(ISP_UART_IRQ, NVIC_EncodePriority(NVIC_GetPriorityGrouping(), 6, 0));
	NVIC_EnableIRQ(ISP_UART_IRQ);
}