The function of the pins of STM32F1 after power-on reset is called the main function. The main function of most pins is to serve as a general-purpose input and output pin (GPIO). In addition to the main function, the pins also have a backup function. When the backup function is selected, it is used as an alternate input and output (AFIO). This 
can be found in the microcontroller data sheet (Table 5 Medium-capacity STM32F103xx pin definition). Some pins have two types of backup functions, one is the default multiplexing and the other is the redefined function. When using the default multiplexing function of the pin, AFIO does not need to be turned on; when using the redefined function, AFIO needs to be turned on. In another case, AFIO also needs to be turned on when using the EXTI external interrupt.
For example, if we use PA7 as the second channel of TIM3, or configure PB10 and PB11 as USART3, there is no need to operate AFIO. 
The main function of most pins is GPIO, but the main function of PB3 and PB4 is to serve as pins for program download and resetting the microcontroller. Therefore, it is necessary to turn on the redefined function of PB3 and PB4 to make them general-purpose input and output. The program is as follows:
RCC_APB2PeriphClockCmd(RCC_APB2Periph_AFIO,ENABLE);
GPIO_PinRemapConfig(GPIO_Remap_SWJ_JTAGDisable,ENABLE);
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