A simple understanding of AFIO for beginners of STM32

When configuring the external line interrupt of the PC13 pin, the multiplexing function is used when configuring its external clock:

RCC_APB2PeriphClockCmd(RCC_APB2Periph_GPIOC|RCC_APB2Periph_AFIO,ENABLE);

The reason is that the first function pin of PC13 is GPIO, and the second function pin is TAMER-RTC1, which does not have the EXTI function. If you want to use it to generate an external line interrupt, you must configure the AFIO multiplexing function and map the EXTI function to the PC13 pin.

PC13 pin function diagram

Another example:

The CAN bus uses PB8 and PB9, but these two pins do not have CAN functions and need to be remapped:

RCC_APB2PeriphClockCmd(RCC_APB2Periph_AFIO | RCC_APB2Periph_GPIOB, ENABLE);

PB8, PB9 pin function diagram

CAN&485 TR pin diagram

Some configurations do not require multiplexing functions, for example, UASRT1 pins PA9 and PA10, whose second function pins are USART functions, so when using the UASRT function, the clock cannot be configured in AFIO.

When configuring the RCC external clock, you need to configure both the basic GPIO mode and the clock of its second function USART.

The second pin of PA9 is used, and one of the output modes is the multiplexed output mode, so it needs to be configured into the multiplexed output mode.

GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AF_PP;

Since both USART and GPIOA are externally connected to RCC_APB2, the parameter is a combination of the two values.

/* config USART1 clock */
RCC_APB2PeriphClockCmd(RCC_APB2Periph_USART1 | RCC_APB2Periph_GPIOA, ENABLE);