Implementation for 5V tolerance on 3.3V MCU GPIO?

[bxh]

I've seen several IO pins on some devices which are 5V tolerant, and never really thought too much about it until now..

I have a rough understanding of how diodes are commonly used to protect IO pins from over/undervoltage, but how is 5V tolerance actually implemented without having too much of an effect on the capabilities of the IO pins?

If for some reason I wanted to make one of the 3.3V GPIO pins (on a development board let's say) 5V tolerant using external components, how could/should I achieve it?

[Fungus]

Put a resistor in front of the clamping diode to limit the current that can pass through that diode to something it can handle.

If the pin needs high-current output capability then you need an extra MOSFET to bypass the resistor when the pin is in output mode.

[station240]

I did find this page on how to interface the RPi with 5V IO devices.
http://tansi.info/rp/interfacing5v.html
Pretty much sums up the various circuits I'd use anyway.
Of note are the sparkfun level converters, remember their stuff is opens source hardware, so you can use it in your own designs.

If you're going to make an entire 8 bit port 5V tolerant, there are proper level converter chips that would be more practical to solder in.

[chicken]

Here's an app note from TI that discusses  several options to implement level conversion for 3.3V MCUs:
http://www.ti.com/lit/an/slaa148/slaa148.pdf