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共同点

都能访问串口的SR寄存器

不同点

USART_GetFlagStatus(USART_TypeDef USARTx, uint16_t USART_FLAG)：*
该函数只判断标志位（访问串口的SR寄存器）。在没有使能相应的中断函数时，通常使用该函数来判断标志位是否置1
USART_GetITStatus(USART_TypeDef USARTx, uint16_t USART_IT) ：*
不仅会判断标志位是否置1（访问串口的SR寄存器），同时还会判断是否使能了相应的中断（访问串口的CR1寄存器）。所以在串口中断函数中，如果要获取中断标志位，通常使用该函数。USART_GetITStatus判定的是事件标志和使能控制位，这两个位同时为1返回值才为1

USART_GetITStatus函数详解

#define USART_IT_PE                          ((uint16_t)0x0028)
#define USART_IT_TXE                         ((uint16_t)0x0727)
#define USART_IT_TC                          ((uint16_t)0x0626)
#define USART_IT_RXNE                        ((uint16_t)0x0525)
#define USART_IT_ORE_RX                      ((uint16_t)0x0325) /* In case interrupt is generated if the RXNEIE bit is set */
#define USART_IT_IDLE                        ((uint16_t)0x0424)
#define USART_IT_LBD                         ((uint16_t)0x0846)
#define USART_IT_CTS                         ((uint16_t)0x096A)
#define USART_IT_ERR                         ((uint16_t)0x0060)
#define USART_IT_ORE_ER                      ((uint16_t)0x0360) /* In case interrupt is generated if the EIE bit is set */
#define USART_IT_NE                          ((uint16_t)0x0260)
#define USART_IT_FE                          ((uint16_t)0x0160)

ITStatus USART_GetITStatus(USART_TypeDef* USARTx, uint16_t USART_IT)
{uint32_t bitpos = 0x00, itmask = 0x00, usartreg = 0x00;ITStatus bitstatus = RESET;/* Check the parameters */assert_param(IS_USART_ALL_PERIPH(USARTx));assert_param(IS_USART_GET_IT(USART_IT)); /* The CTS interrupt is not available for UART4 and UART5 */ if (USART_IT == USART_IT_CTS){assert_param(IS_USART_1236_PERIPH(USARTx));} /* Get the USART register index */usartreg = (((uint8_t)USART_IT) >> 0x05);/* Get the interrupt position */itmask = USART_IT & IT_MASK;itmask = (uint32_t)0x01 << itmask;if (usartreg == 0x01) /* The IT  is in CR1 register */{itmask &= USARTx->CR1;}else if (usartreg == 0x02) /* The IT  is in CR2 register */{itmask &= USARTx->CR2;}else /* The IT  is in CR3 register */{itmask &= USARTx->CR3;}bitpos = USART_IT >> 0x08;bitpos = (uint32_t)0x01 << bitpos;bitpos &= USARTx->SR;if ((itmask != (uint16_t)RESET)&&(bitpos != (uint16_t)RESET)){bitstatus = SET;}else{bitstatus = RESET;}return bitstatus;  
}

以RXNE为例：

一开始是把USART_IT_RXNE 0x0505写成2进制，然后只要它的低八位，然后低八位右移5位，然后得到usartreg的值为0b001，然后单击鼠标可以找到IT_MASK的值，它与我们的USART_IT相与得到它的值为5，接下来就是IT_MASK等于0x01左移5位，为什么是5位，因为要判定RXNEIE,他就是bit5。我们得到了IT_MASK的第5位为1，然后和CR1寄存器相与，进而判定RXNEIE是不是1itmask &= USARTx->CR1;也就是判断是否打开了中断。接下来就是通过bitpos &= USARTx->SR;判断下面寄存器SR的第五位。

综上所述

USART_GetITStatus判定的是事件标志和使能控制位，这两个位同时为1返回值才为1，

USART_GetFlagStatus函数

#define USART_FLAG_CTS                       ((uint16_t)0x0200)
#define USART_FLAG_LBD                       ((uint16_t)0x0100)
#define USART_FLAG_TXE                       ((uint16_t)0x0080)
#define USART_FLAG_TC                        ((uint16_t)0x0040)
#define USART_FLAG_RXNE                      ((uint16_t)0x0020)
#define USART_FLAG_IDLE                      ((uint16_t)0x0010)
#define USART_FLAG_ORE                       ((uint16_t)0x0008)
#define USART_FLAG_NE                        ((uint16_t)0x0004)
#define USART_FLAG_FE                        ((uint16_t)0x0002)
#define USART_FLAG_PE                        ((uint16_t)0x0001)

FlagStatus USART_GetFlagStatus(USART_TypeDef* USARTx, uint16_t USART_FLAG)
{FlagStatus bitstatus = RESET;/* Check the parameters */assert_param(IS_USART_ALL_PERIPH(USARTx));assert_param(IS_USART_FLAG(USART_FLAG));/* The CTS flag is not available for UART4 and UART5 */if (USART_FLAG == USART_FLAG_CTS){assert_param(IS_USART_1236_PERIPH(USARTx));} if ((USARTx->SR & USART_FLAG) != (uint16_t)RESET){bitstatus = SET;}else{bitstatus = RESET;}return bitstatus;
}

假设判定的是RXNE 标志位：

#define USART_FLAG_RXNE ((uint16_t)0x0020) if ((USARTx->SR & USART_FLAG) != (uint16_t)RESET) {
bitstatus = SET; }

这不就是判定的SR寄存器的bit5-RXNE 标志位是不是1

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