VCCIO: +1.8V to +5.25V supply to the UART Interface and CBUS group pins (1...3, 5, 6, 9...14, 22, 23). In USB bus powered designs connect this pin to 3V3OUT pin to drive out at +3.3V levels, or connect to VCC to drive out at 5V CMOS level. This pin can also be supplied with an external +1.8V to +2.8V supply inorder to drive outputs at lower levels. It should be noted that in this case this supply should originate from the same source as the supply to VCC. This means that in bus powered designs a regulator which is supplied by the +5V on the USB bus should be used.
+3.3V to +5.25V supply to the device core. (See Note 1).Note1: The minimum operating voltage VCC must be +4.0V (could use VBUS=+5V) when using theinternal clock generator. Operation at +3.3V is possible using an external crystal oscillator.