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I am in need a lot of chip select pins for my circuit (8 pins) and I am worried about pins being multiplexed which might suddenly pull high/low mid operation. Ill only be using the HDMI port, Ethernet, and USB of the Raspberry Pi 3B+/4. So, if there are GPIO pins that are multiplexed for those devices I would like to know.
What GPIO should I look out for? are the sdio pins shared with the actual SD card or is this for another external SD card?
None of the GPIO pins are 'multiplexed' in the sense you mean. In other words, some of the GPIO pins have alternate functions, but these are functions that you maintain control of in your configuration, and in your code. You do not need to worry about the system commandeering a GPIO pin for some other purpose.
There are many sources of detailed information about the GPIO pins available online - here's one I like - and so I'll not repeat that here.
In the interest of adhering to absolute truth, I should add this caveat to my answer: the raspberry pi is not an open-source system. There are many hardware details that remain unpublished after all of these years, and the reaspberry pi firmware is not open source. The only point I am making here is that no one without access to those closed sources can give an absolute answer to your question. I can only tell you that there are no recognized uses for GPIO pins on the RPi - i.e. if you are building a life-support system, you'd be considered reckless if you used an RPi for that. For hobby usage, yeah - no multiplexing on GPIO pins. :)
No pins are "multiplexed" with ANYTHING.
Pins 27,28 (GPIO0,1) are reserved for HATs.
