The original post has been removed because it was out of date.
The original post contained input voltage and input current measurements taken on Raspberry Pi model A and model B boards. The GPIO (general purpose inputs/outputs) circuitry appears to have changed, probably since the introduction of the A+, B+ and subsequent models.
The original post argued that the measurement data indicated that the gpio, when operated as inputs, were probably 5V safe. The input current didn't show much of an increase (with input voltage increase) until the input reached about 5.8V. So my assertion is that there is probably input protection operating somewhere around 5.8V on these early models.
Therefore using pull-up resistors between inputs and 5V supply on model A & B Pi boards should be OK, and I have not had any failures.
What about newer Pi models?
I continue to use input pull-up resistors to 5V, but I've noticed that even relatively high resistance values result in almost the full 5V level appearing at the input.
So it looks like the input impedance is now much higher on the newer models (A+, B+, Pi2 & Pi3). I've noticed that the input current is much lower (approx 2uA against maybe 60uA for the earlier models).
As there are probably more of the newer design in circulation than the original models, I've crossed out much of the original text below (it is still readable, but remember it relates to the early models).
When I can find time to do a proper evaluation on a newer Pi, I'll come back an update this page.
My take on this
|This is a great illustration, but is it accurate?|With an input voltage near to the ideal 3.3V, the input current is approx 61uA. Increasing the input voltage to 4.7V only increases the input current by approximately 3uA. Therefore I conclude that the circuit does not include a diode clamp operating in the range 3.3 to 4.7V. Also, the small change in input current indicates that nothing bad is happening (i.e. nothing is conducting much more at 4.7V than it was at 3.3V, and the chip is not going to heat-up due to the extra 3uA).
Let's push it a bit further!
...but I'd be very interested to hear your interpretation!