Datasheet

RST - this is the Reset pin, tie to ground to manually reset the ATSAMD21, as well as launch the bootloader

manually

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!

IORef - the digital reference voltage pin. This reference voltage is the same as the chip logic voltage (3.3V) and

cannot be changed.

Debug Interface