STM32 microcontrollers are one of the most popular MCUs in the world. There are many reasons for that. The Input/Output flexibility is not one of them. All SMT32 microcontrollers have a limited IO matrix that allows general inputoutput pins to work only with selected peripherals. There is no easy swap of pin functions. Some pins can be only UARTs, some only SPI, and some work only with 1 or 2 timers. This is called Alternate Functions.

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Each pin in each inputoutput port can have only one assigned Alternate Function at a time and no Alternate Functions are available.
This is extremely important during the hardware planning phase, as it might be hard to change the originally created pin assignment.