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I've been doing embedded work for many years, but have only very recently started working with the Raspberry Pi. I'm planning to build a project that uses a 5" TFT display and a Pi Zero W, and was considering using Adafruit's RA8875 board to drive it.

The RA8875 board uses SPI, however there is a warning in the technical details section that states that this board does not tri-state the MISO pin, with a link to a forum thread that discusses the topic at some length.

I will not have any other boards attached to the SPI connection, but I still would like to know if there are any devices on the Pi Zero W board itself that are connected to the MISO pin. The reason I want to be 100% certain is that with the Arduino device described in the forum thread, mention is made of an SD card, and of course the entire Pi family of boards relies on (micro)SD cards.

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None of the accessible GPIO pins are used for any other purpose, except for GPIO0 pin 27 & GPIO1 pin 28 which are used for HATs and are marked as Reserved.

See https://www.raspberrypi.org/documentation/usage/gpio/

There are a number of GPIO pins, not connected to the expansion header on the Pi family (CM excepted), which are used for other purposes.

The gpioinfo command can be used to list all pins, and identify those in use. See libgpiod

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The MISO line is connected to the SoC. It is not connected to anything else on any current model of the Raspberry Pi.

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Unfortunately, the Raspberry Pi hardware (and firmware) are proprietary. There are some partial schematics available, but they may or may not be up-to-date.

All of the RPi GPIO pins (except power & ground) have multiple functions. You may wish to take advantage of some of the GPIO tools available: pinout.xyz is useful, and there are other resources available on the RPi GitHub site.

To answer your question re the MISO pin on the RPi Zero W: I don't know how to determine with certainty whether or not that pin is connected to any other devices - other than to monitor it for "irregularities" with a scope or logic analyzer.

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