[Vcc, Vdd, Vss, etc.]
A neat little discussion of the symbols and where they came from.
[Powering Light Emitting Diodes (LEDs)]
Can you use one resistor for multiple parallel LEDs? No. Or, you can, but it's not the best idea.
[Notes on LEDs]
Talks about different types of LEDs, mentioning typical brightnesses, voltages, and other intersting stuff.
[All About Circuits : Reference]
Looks like a great collection of info; didn't get to read much of it.
[Effects of Varying I2C Pull-Up Resistors]
(http://dsscircuits.com/articles/effects-of-varying-i2c-pull-up-resistors.html) : article by Wayne Truchsess
Great discussion on the topic, with oscilloscope graphs. The practical takeaway is that 4.7kΩ is a good value for 100kHz I²C, and 2.2kΩ is good for 400kHz.
[comp.lang.c Frequently Asked Questions]
Really good information, if they happen to address the question you have.
[The C Preprocessor]
One of my main references.
[The GNU C Preprocessor]
A modified version on a different site. Has a few really useful things that I don't think I saw in the official documentation.
[C Operators/Expressions] (http://www.lix.polytechnique.fr/~liberti/public/computing/prog/c/C/CONCEPT/expressions.html) (on http://www.lix.polytechnique.fr/)
[Bitwise Operators in C and C++: A Tutorial] (http://www.cprogramming.com/tutorial/bitwise_operators.html) (on http://cprogramming.com/)
[AVR Tutorials - [TUT] [C] Bit manipulation (AKA "Programming 101")] (http://www.avrfreaks.net/index.php?name=PNphpBB2&file=printview&t=37871&start=0)
[In C++ is "const" after type ID acceptable?] (http://stackoverflow.com/questions/988069/in-c-is-const-after-type-id-acceptable) (on http://stackoverflow.com/)
[Arrays and pointers in C] (http://www.ibiblio.org/pub/languages/fortran/append-c.html) (on http://www.ibiblio.org/)
[how to use array of function pointers?] (http://stackoverflow.com/questions/252748/how-to-use-array-of-function-pointers) (on http://stackoverflow.com/)
[The Function Pointer Tutorials] (http://www.newty.de/fpt/index.html)
[C preprocessor and concatenation] (http://stackoverflow.com/questions/1489932/c-preprocessor-and-concatenation) (on http://stackoverflow.com/)
[gcc: why the -lm flag is needed to link the math library?] (http://stackoverflow.com/questions/4606301/gcc-why-the-lm-flag-is-needed-to-link-the-math-library) (on http://stackoverflow.com/)
[AVR Newbie guide]
by stu_san (on http://www.avrfreaks.net/)
Didn't read much of it, but it looks like a great collection of info.
[AVR Libc Library Reference]
One of my main references.
[AVR : Data in Program Space]
How to use '<avr/pgmspace.h>'.
List of registers and associated bit numbers for the ATmega32U4
[A Brief Tutorial on Programming the AVR without Arduino]
by Chris Kuethe
Goes over a bunch of stuff pretty generally. Useful to me because it was talking about exactly what I was trying to do (e.g. program the thing directly instead of messing around with gratuitous libraries).
[Optimisations of AVR programs using avr-gcc]
About compiling and stuff.
[Arduino I²C Expansion IO]
(from [Keith's Electronics Blog]
Didn't use this much, but it's nice (if you're using the Arduino 'Wire' library), and it has pictures.
[Arduino Playground :: Wire Library, Explored]
More about the Wire library. If you're not using interrupts, I think you'll be better off looking throught the datasheet and writing your own trivial library in C. It really is trivial, almost all the code is in the datasheet (at least for the ATmega32U4), and it'll make more sense to you that way :) .
About Keyboard [Scan Codes]
Interesting discussion. For practical purposes, if you're just dealing with USB, you can skip this and go straight to the keyboard usage page in the HID Usage Tables (see below).
zip: [Phantom Firmware from PrinsValium]
Pretty basic (unless I'm missing nuances, which is entirely possible). Helped me a lot when I was learning how to initialize stuff on the Teensy.
forked by [Pyrolistical / tmk_keyboard] (https://github.com/Pyrolistical/tmk_keyboard/tree/master/fourway)
forked by [riffautae / tmk_keyboard] (https://github.com/riffautae/tmk_keyboard)
github: [humblehacker / keyboard]
Looks like it has some cool ideas, but I haven't read through it well enough to know what's going on.
mentioned on [the humblehacker keyboard website] (http://humblehacker.com/keyboard/)
mentioned on [the humblehacker geekhack page] (http://geekhack.org/showwiki.php?title=Island:6292)
mentioned on [the designer's blog] (http://humblehacker.com/blog/)
[LUFA (2012) (Lightweight USB Framework for AVRs)]
Very well known USB library for AVRs. And it looks like it'd be really good too, but the documentation is hard to get into, and the examples don't seem to have enough explanation for me, since I don't have any background in what USB is doing in the first place.
zip: [PJRC: usb_keyboard]
From the [LUFA Library : Alternative USB AVR Stacks] (http://www.fourwalledcubicle.com/files/LUFA/Doc/110528/html/_page__alternative_stacks.html) page: "Not so much a complete stack as a collection of USB enabled demos, this library is specifically designed for the PJRC Teensy line of USB AVRs, and thus may need to be modified for other USB AVR chips. These minimal code samples shows the inner workings of the USB controller, without all the abstraction present in most other USB AVR stacks."
From the [LUFA Library : Alternative USB AVR Stacks] (http://www.fourwalledcubicle.com/files/LUFA/Doc/110528/html/_page__alternative_stacks.html) page: "Well regarded and complete USB 1.1 software stack for several AVR models, implementing Low Speed HID. Used in many commercial and non-commercial designs, with user-submitted projects available for viewing on the company's website. Uses C language code mixed with assembly for time-critical sections."
Also, it has really good documentation.
(by Peter Fleury, on [his homepage]
Good I2C library. Includes stuff both in assembly and C (though, I didn't look at the assembly code). I still think you're better off writing your own, it's not that hard and you'll understand it better. Also, this library is under the GPL (from what it says on his site; couldn't find a licence in the code).
forum: [Interfacing MCP23018 io expander via Arduino]
Contains a quick MCP23017 library in C++. Didn't end up looking at it too hard.
github: [maniacbug / Arduino / libraries / MCP23018]
Library in C++. Don't think I ended up looking through this one at all.
[Soarer's Converter (XT/AT/PS2/Terminal to USB Converter with NKRO)]
No code for the part that talks over USB though. Code only available for the tools that allow you to change the keymap and stuff, I think.
zip: [PJRC: blinky]
Little baby example project for the Teensy. Quite nice.
[Microchip: Analog & Interface Product Selector Guide] (http://ww1.microchip.com/downloads/en/DeviceDoc/21060z.pdf)
[MCP23017 / MCP23S17 Data Sheet] (http://ww1.microchip.com/downloads/en/DeviceDoc/21952b.pdf)
[MCP23018 / MCP23S18 Data Sheet] (http://ww1.microchip.com/downloads/en/DeviceDoc/22103a.pdf)
[Teensy 2.0 (ATMEGA32U4) Data Sheet] (http://pjrc.com/teensy/atmega32u4.pdf)
[Teensy++ 2.0 (AT90USB1286) Data Sheet] (http://pjrc.com/teensy/at90usb1286.pdf)
[Teensy 2.0 Pin Assignments, Using C] (http://pjrc.com/teensy/card2a.pdf)
[Teensy 2.0 Pin Assignments, Using Arduino] (http://pjrc.com/teensy/card2b.pdf)
[Cherry MX Series Keyswitches : Specifications] (http://www.cherrycorp.com/english/switches/key/mx.htm)
Includes 3 different tests (2 of which are web based) to see which keys are actually registering as pressed.
A necessity if your code is on github :) .
I like the tool. Didn't need the functionality just for a little circuit diagram though, so I forwent the learning curve and used Inkscape.
mentioned on the [circuit/block-diagram drawing] (http://stackoverflow.com/questions/6422603/circuit-block-diagram-drawing) question (on http://stackoverflow.com/)
[Example: Block diagram line junctions] (http://www.texample.net/tikz/examples/line-junctions/)
[Inkscape : Connector tool tutorial]
The open source vector graphics tool. Not the best for diagrams (though, see the link; as that tool improves it will hopefully become much better), but it works and I like it :) .