dc42:
You need to add a 0.1uF capacitor between Avcc and ground.
Thanx! Yeah, this is shown in the datasheet on page 258 ... I just forgot to add it ...
I'm not sure whether C6 (between Vref and ground) is whrthwhile, the datasheet doesn't mention putting a capacitor there, although I can see that it might help filter out power supply noise.
The datasheet on page 250 says "The internal voltage reference may thus be decoupled by an external capacitor at the AREF pin to improve noise immunity" ... so I did.
PS - you should join up the ground plane traces much more. You seem to be trying to avoid having lops in the ground plane, but there is no need to do that. Aim to put ground plane almost everywhere there isn't anything else.
I tried to seperate analog and digital ... I realized there is no (major) analog (load), but with my limited knowledge I try to do it as "perfect" as possible for me. I saw some PCBs with copper all over - i.e. the UNO board. But is this really the "correct way"?
CrossRoads:
You Want ground plane under the cap, try for both sides if possible. See attached.
Thank you for posting that interesting document - I didn't know yet. But: I can't read / conclude which cap you exactly mean. (?)
nanohex:
On a side note, you need a good place to get these manufactured! I used seeedstudio for mine, and the boards came out pretty decent, especially for the price (although silkscreen wasn't amazing).
I already checked out seedstudio and also heard good things about, but as I am located in germany I will try someone nearer.
greets,
Marcel