[jayyhu]

You're right, after re-reading the Power section on the datasheet it seems connecting 5V to the VREG_VIN should suffice to power the digital domains, but if you want to use the ADC, you still need a external 3.3V source.

[dvdkon]

Maybe not even that:

> A separate, nominally 3.3 V, low noise supply (VREG_AVDD) is required for the regulator’s analogue control circuits.

It seems it would be painful trying to run this without 3.3 V.

[crote]

It's quite a bit more complicated.

The chip needs a) 1.1V to power the cores, b) 1.8V-3.3V to power IO, and c) 3.3V to properly operate USB and ADC.

The chip has one onboard voltage regulator, which can operate from 2.7V-5.5V. Usually it'll be used to output 1.1V for the cores, but it can be used to output anything from 0.55V to 3.3V. The regulator requires a 3.3V reference input to operate properly.

So yeah, you could feed the regulator with 4-5V, but you're still going to need an external 5V->3.3V converter to make the chip actually operate...

[snvzz]

See section on physical pin gpio electrical tolerances.

The TL;DR is that 3.3v must be fed into IOVDD for 5.5v tolerance to work.