Hello,
FMC as Flexible Memory Controller and it manages parallel signals. It's used to connect external parallel devices like memories, LCDs etc. The interface has three types of signals: Data, Address and Control.
The following is an example of a connection of an SRAM and a NOR flash to STM32L476 device (STM32L476G-EVAL board).

So no relation with other protocols or comm interfaces such as SPI or UART ..
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