AREF0 and AREF1 can also be used as analog inputs to any of the ADC channels.
For tinyAVR and megaAVR devices, where only AREF0 is connected to the device via the routing card, AREF1 can be connected to an ADC channel by connecting a cable from AREF1 on the AUX header and to the port pin header corresponding to the ADC channel. (Check the device data sheet for which port pin header to connect to.)
If the AVR device's internal voltage reference or an external voltage reference is used, the AREF0/AREF1 jumper must be removed. If this is the case, AREF0 or AREF1 can be used as analog inputs by connecting a cable from pin 1 on the AREF0/AREF1 header to the port pin header corresponding to the ADC channel.