Can I connect a 5V tolerant UART interface to 3.3V GPIO pins configured as a serial port?

Question

I want to connect the telemetry RX/TX pins of a flight controller running this microcontroller to a single-board computer, using the GPIO pins.

According to 1 (ArduPilot mapping), the UART port is 5V tolerant while the GPIO of the computer is using 3.3V and not 5V tolerant.

I am wondering, if I would connect the pins, would the result be

1. 5V provided by the flight controller, frying the single-board computer or
2. 3.3V provided by the single-board computer with the 5V tolerant flight controller adapting to it, enabling communication?

Answer 1

"5V tolerant I/O" is unfortunately a bit vague. While this means the pins will be happy interfacing with a 5V input, it doesn't actually clarify what logic level they output, though it would be quite reasonable to assume this as 3v3 logic.
I'd be looking at this thinking it's most likely that the flight controller is running 3v3, and therefore can be plugged directly into the single-board computer, but I would 100% check beforehand when I got the physical devices, and be prepared to put some level shifting in if needed.
If the flight controller puts out 5V logic and this in connected to a 3v3 logic input, that will likely cause damage.

Answer 2

The STM32 tolerates 5V as input but it can't use 5V as output. It will use 3.3V as output logic levels.

Only some IO pins are 5V tolerant, not all.