what happen if you connect a 5 volt directly to a 3v battery?

-> Dunno... Some electrical magic for sure Will you tell me?
Are you talking about a problem based on different voltage levels or on batteries in parallel?
IF the latest, I thought putting batteries in parallel increases their capacity 'i.e. 2x1.5V 2500mAh will give 1.5V 5000 mAh... ?)

voltage regulator output, battery... yeah you may think battery don't produce noise.. well yes and no, they have internal resistor, everything that need juice will make the voltage drop a little bit, knowing there's a clock source somewhere and you will also drive something, the voltage will swing accordingly.

-> I'm not using oscilloscopes. I'm building a DIY lasergame project. I'm biochemist and filmmaker, that's why I'm asking so many dumb questions
I will use a capacitor on the power supply, good practice, for the reason you are giving above. Thank you for that.

Do a test, remove all capacitors, do a led blink (10mSec or so) then use your scope to monitor your Vdd rails... cool huh? now add the mandatory filtering caps... how better now?

-> Great.

Sounds right


No, it will be disabled. It's not a bad idea to have it enabled so far. Just look the datasheet to know it's threshold and what advantage it may give you.

-> Well, BOD resets the chip when the voltage drops too low if I remember well, but my system must not reset... I will have a look at the BOD feature in the datasheet this WE, just in case.

Don't worry, way to complicated....[/QUOTE]

-> Okay, the simpler the better anyway