Roast my schematic, autonomous data logger

[nachocpol]

I'm using the SAM D21 for small projects.  It has built-in USB interface, and an internal clock oscillator (no xtal needed). 

Thanks for the recommendation, definitely worth keeping in mind!

You have a resistive voltage divider for measuring the battery voltage of 10k and 33k, which suck a lot of current out of your battery, and you probably can't turn it off.

What about switching the lower end of the voltage divider with a GPIO -- like this:

https://www.eevblog.com/forum/projects/how-to-prevent-battery-over-discharge-while-running-unnattended-circuit/msg3232554/#msg3232554

Oh nice one, I would've never thought of that. 

[tszaboo]

Don't you need a couple of small caps on the XTAL?

Now that you mention this, the hardware design considerations uses caps on the crystal... Interesting that the Arduino NANO schematics (https://www.arduino.cc/en/uploads/Main/Arduino_Nano-Rev3.2-SCH.pdf) don't have it..

And now I see this. Here is a story for all engineers and hobbyist:
Please dont ever use the original Arduino schematic as an example. Or to prove a point. It's bad. It is really bad.The schematic is bad. The layout is bad.
If a circuit works, it doesnt mean it is properly designed.

[nachocpol]

Don't you need a couple of small caps on the XTAL?

Now that you mention this, the hardware design considerations uses caps on the crystal... Interesting that the Arduino NANO schematics (https://www.arduino.cc/en/uploads/Main/Arduino_Nano-Rev3.2-SCH.pdf) don't have it..

And now I see this. Here is a story for all engineers and hobbyist:
Please dont ever use the original Arduino schematic as an example. Or to prove a point. It's bad. It is really bad.The schematic is bad. The layout is bad.
If a circuit works, it doesnt mean it is properly designed.

Thanks for the heads up. I didn't know it was that bad, I'm not an EE, just a hobbyist trying to get things done .  Arduino being a commercial product that I have used, gives me a degree of confidence to at least have a reference.

[bsodmike]

And now I see this. Here is a story for all engineers and hobbyist:
Please dont ever use the original Arduino schematic as an example. Or to prove a point. It's bad. It is really bad.The schematic is bad. The layout is bad.
If a circuit works, it doesnt mean it is properly designed.

Do you have an example of this done right then?  Wouldn't ATmel chip docs be a better source for application circuitry?

[tszaboo]

And now I see this. Here is a story for all engineers and hobbyist:
Please dont ever use the original Arduino schematic as an example. Or to prove a point. It's bad. It is really bad.The schematic is bad. The layout is bad.
If a circuit works, it doesnt mean it is properly designed.

Do you have an example of this done right then?  Wouldn't ATmel chip docs be a better source for application circuitry?

Yes, absolutely. Atmel/Microchip has application notes on their website. And they provide reference designs.
Much better design resource: https://www.microchip.com/DevelopmentTools/ProductDetails/PartNO/ATMEGA168PB-XMINI
Notice, how they managed to place the bypassing capacitors next to the chip, where they belong? Or how the USB port has ESD protection? Or how they didnt place a bunch of electrolytic capacitors on the board, because why would you, if you have the same value in an MLCC?