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I have no idea about using multiple microcontrollers together. However, you should do a Google search about Arduino and charlieplexing - charlieplexing allows you to control more things (like LEDs) than what you have pins available for. It may be a solution for you.
You know, I pretty much based this tutorial off of another tutorial I found so I just did what they did. Now that I know a little bit more about this chip though, I think you bring up a good point. You may be able to omit connecting 5V to pin 21 depending on what you're doing. I suppose you could try not connecting it and see if something doesn't work. If everything is OK, then don't worry about it.If you're working with analog components and need a 5V (or other voltage) reference, then you would hook it up. I experimented with it a little bit a couple years ago. I would just do a Google search on Arduino (or ATMega329) AREF.
I'm not sure about what your question is...
Hi RyanKranz,This circuit isn't to (re)program the MCU. It is meant to allow your MCU to run it's program *after* uploading your code from the Arduino IDE...without an entire Arduino board such as the Duemilanove/Uno/etc. You need to add extra hardware to this circuit to be able to upload code to it. Or you can pull the chip and pop it in a Duemilanove/Uno/etc to upload your code then put it back on this circuit.
I'm sorry AbduA6, but I'm not sure what you're asking...
Circuit Board Design Class
Excellent Instructable, thank you for creating/posting it!
I haven't had to so far...Thanks for reading!