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Hi, I'm making my second venture in to custom pcbs, and I'm trying to make versions of a really bare bones surface mount arduino using the ATmega32u4 (leonardo) and I'm basing it on the adafruit style arduino micro http://arduino.cc/en/Main/ArduinoBoardMicro. Since I still l want to program this smd chip, the usb is not optional, which complicates things for my use a puts my brain in a loop of questions.

I plan on maintaining this post with eagle files and such, so I'll list my project goals first, and my questions below the large line of dashes.

Goals.
1) several versions, starting with the simplest, cheapest 2 layer bare bones + usb programming.
1) I'd like to only allow power via the usb connector, removing the power selector and the VIN option.
2) a 3v only version and a 5v only version - 3v is priority.
3) minimal components - no niceties like current overload fuses, or anything like that.
4) 2 layer, parts all on one side - which will be on bottom so the top can have the i/o  pins and a mini breadboard.
5) version with nrf24l01+ on board with chip or pcb antenna - seriously all arduinos should have built in wireless, or ethernet by now.
6) version configured to mount led matrix - possibly rgb matrix
7) version with embedded ethernet, following this : http://www.instructables.com/id/A-credit-card-sized-Ethernet-Arduino-compatable-co/

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QUESTIONS: (they all reference this schematic of the arduino micro : http://arduino.cc/en/uploads/Main/arduino-micro-schematic.pdf

1) considering the 3v only version, and usb communication. Is that going to work? Can I go from usb src to a 3v regulator to the arduino and still have the usb data pins connected directly to the arduino at 5v? removing a auto selector and 5v reg might cut about 25% off the complexity and bom, but if i have to use logic converters to support 3v, i'll just stick with 5v only version, but it'll kinda suck.

2) I read that there are current protection circuits and noticed a bunch of extra "stuff" on the left side of atmega. Unfortunately, I have no idea what they do. Anyone care to share some insight on what is easily removable?

3) concerning making a rgb led matrix mountable version. the matrix needs 24 pins.the atmel spec list 26 configurable i/o pins, arduino pulls out 20. what 4 pins would you suggest are able to be hijacked for my purposes? i could add an i/o expander, but it takes me in the opposite direction i'd like to go.

4) I was thinking of removing the usb communication and just keeping the usb power, and programming it via isp only. i've done this with tiny45s and a 168 using the arduino ide, but i don't know where to get the library files or settings to do this with a 32u4. Would someone point me in the right direction? Learning to do this for any chip would be pretty awesome, but i'll take the fish or the fishing lessons.


Any and all comments are very much appreciated.
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So you are after something like my Leonardino (Little Leonardo), about 1/2 way down the page here, perhaps as a starting point?
http://www.crossroadsfencing.com/BobuinoRev17/

"5) version with nrf24l01+ on board with chip or pcb antenna - seriously all arduinos should have built in wireless, or ethernet by now."
wireless/ethernet is not needed for many projects.
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Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
Arduino for Teens available at Amazon.com.

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"wireless/ethernet is not needed for many projects."

Crossroads, I completely agree from a engineering, practical perspective. But, for me it's not about need, it's about the next step in capability. The arduino wasn't needed. People got along fine with a blank atmel and assembly or C. But it's transformative. Make and arduino - for the same price or cheaper - than the standard arduino but with a built in transceiver (really should be wifi) and you'll change everything.

Anyway, I look at your board and circuit. You have already done most of what I wanted. And I learned a lot while looking at it. If its alright with you, I'll use it as a reference, with mine. I've come to realize that - as much as I reinventing the wheel is bad - it's highly educational, and I think I'm doing myself a disservice by copying so much directly.

I noticed you are available for design services. Does that mean I can contract you to take a solid look at my eagle files and get some feedback and some sage like advice on changes? You have a wicked high karma rating, and I am totally up for paying if it shortens the time to learn and yields a higher quality project.
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Sure. Would be easy to widen up the card, move bottom components to the top, and add header are for nrf24l01+ module.
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Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
Arduino for Teens available at Amazon.com.

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