In general, is the output voltage from GPIO always regulated?

Hi,
In general, is the output voltage from a GPIO pin of most microcontrollers always regulated? Is it always 3.3V or whatever?

A lot of microcontrollers, the VIN is suppose to be above 3.3V with some bit of margin. And the digital output voltage from the GPIO pin seems to be just under 3.3V or so. But then I see BLE modules that use coin cells and the VIN can be 2.5 to 4.8V, and I don't know if the GPIO output floats down if VIN starts going down.

I don't really know what happens between VIN and the output pin, basically. What's all that stuff in there?

VCC/VIN (whatever you want to call it) is essentially what the GPIO output will be. Your digital logic is turning the pin on or off via a transistor. There is a protection diode, a switchable pull-up resistor, and some leakage in the transistor and therefore the output won't be exactly the input voltage, but similar and directly related.

Hello,

arusr:
In general, is the output voltage from a GPIO pin of most microcontrollers always regulated?

No. The output voltage is not regulated.

For AVR processors the effective resistance of the output driver is ~25 ohms at room temperature.

Well, you could say it’s as regulated as much as VCC is. Output is guaranteed to be within 0.8V of VCC and 0.9V of Gnd for light loads (<20mA) and Vcc = 5V, temp = up to 85C, moving away from VCC or Gnd as current flow increases or as temperature increases.
If Vcc has a lot of ripple on it, so will the high output.