Power saving

[goc9000]

The main problem is the networking stuff. Keeping an Ethernet connection active prevents much ATMEGA idling and the ENC28J60 also draws a lot of power. Could implement some sort of "RJ45 plugged in" detection so that all networking components are shut down when there's nothing connected (the user would only visit the device with a laptop rather rarely anyway). With the networking components shut down, the ATMEGA would have very little work to do periodically and could be idle pretty much 99% of the time.