To clarify: You can connect a 5V power source by connecting it to the VSYS pin which leads into the on-board voltage regulator.
But the µC itself runs on 3.3V and is not totally 5V-capable. You'd need level converters to interface with 5V.
But the µC itself runs on 3.3V and is not totally 5V-capable. You'd need level converters to interface with 5V.
Part of the GPIOs are CMOS are 5v-tolerant, and TTL considers 2v HIGH, thus it is possible to interface some 5v hardware directly.
> 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.
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...
The TL;DR is that 3.3v must be fed into IOVDD for 5.5v tolerance to work.