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.

>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.

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.

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.

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...

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.

I'd rather have it run on the lower voltage - generally easier to step down than buck up. Either way, the modules are pretty cheap, small, and easy to find.

How much tolerance does that have - can it run directly off a 3.7v lithium ion battery?

Yep, they explicitly call out that the onboard voltage regulator can work with a single lithium ion cell.

The regulator can take that, but as far as I can see it's only for DVDD, the core voltage of 1.1 V. You also need at least IOVDD, which should be between 1.8 V and 3.3 V. So you'll need to supply some lower voltage externally anyway.

I suppose the main draw of the regulator is that the DVDD rail will consume the most power. 1.1 V is also much more exotic than 3.3 V.