The reference voltage for the ADC (VREF) indicates the conversion range, which in this case is limited to 0V (VGND) and VREF = Vcc. Single ended channels that exceed VREF will result in codes close to 0x3FF.
VREF can be selected from VCC or internal reference. The internal voltage reference can be set to 1.1, 2.2, or 4.3V and is generated from the internal bandgap reference (VBG) through an internal amplifier. The first ADC conversion result after switching reference voltage source may be inaccurate, and the user is advised to discard this result.