At the heart of all Arduinos is an Atmel microcontroller. This appendix does not summarize the features of all the microcontrollers in all the Arduinos, but it is a useful exercise to investigate an ATMega datasheet to get a better idea about how it works. Further, taking a look at the open source schematics for the Arduino Uno will make it easier to understand how an Arduino actually works.
One of the most important skills that you can develop as an engineer is the ability to read datasheets. Just about any electronic component that you can buy has an associated datasheet that contains info about the technical limits of the part, instructions on how to use its capabilities, and so forth.
Consider the datasheet for the Atmel ATMega 328p, for instance. Recall that the ATMega 328p is the microcontroller unit (MCU) used in the Arduino Uno and many Arduino clones. Finding a datasheet can often be the trickiest part. I recommend just doing a Google search for “ATMega 328p datasheet” and looking for the first PDF link from Atmel. The datasheets for the MCUs used in the Arduinos can also be found on the hardware page for each board on the www.Arduino.cc website. When you have the datasheet in hand, start by reviewing the first page (see Figure A-1). In most cases, the first page tells you all you need to know about the features of that MCU.