ARef and AVCC


I was wondering if some could help me answer this possible very simple question. I am trying to build my own little arduino style board and trying to work out if i should i should connect the AREF to the AVCC as shown in

but looking at the eagle file of the Arduino Diecimila they don’t appear to be connected

I will be using a 3.3v TTL-232R if that changes this

Many Thanks

James Page 4:

1.1.7 AVCC AVCC is the supply voltage pin for the A/D Converter, PC3:0, and ADC7:6. It should be externally connected to VCC, even if the ADC is not used. If the ADC is used, it should be connected to VCC through a low-pass filter. Note that PC6..4 use digital supply voltage, VCC. 1.1.8 AREF AREF is the analog reference pin for the A/D Converter.

AREF should be left un-connected for later use. It sets the precision on your analogInput pins. You can use down to 1.1v to get better precision:

Hope this helps! :D

(short answer, don't connect the AREF to AVCC.)

Have a look at It shows an appropriate circuit.


Thanks Captain Obvious

I’ll read up on the info as well

Can i just check if i powering a LCD or LED i connect it to AVCC not AREF



You'd want to use AVCC rather than AREF.

AREF is only for voltage inputs from what I've read. Trying to pull voltage out of the AREF would... well, I'm not sure! But I don't feel like trying! :D

Thanks I though that was the case

also thanks dpharris thats a useful link



You shouldn't power something directly from the AVCC pin though, you'll be fine if it's connected to 5v as shown on the website, but if you don't have a voltage hooked up to it, and try power an LED from there, you're sure to have problems.


Cool I think i got it now

If you have the time would you mind checking out my mapping and see if i am not doing something stupid