Datasheet

ARef - the analog reference pin. Normally the reference voltage is the same as the chip logic voltage (3.3V)

but if you need an alternative analog reference, connect it to this pin and select the external AREF in your

firmware. Can't go higher than 3.3V!

SWCLK & SWDIO - These pads on the bottom are used to program the chip. They can also be connected to an

SWD debugger.