Getting Up and Blinking with the Arduino - Exploring Arduino: Tools and Techniques for Engineering Wizardry - page 7

Figure 1-2: AVR ISP MKII programmer
General I/O and ADCs
The part of the Arduino that you'll care the most about during your projects is
the general-purpose I/O and ADC pins. All of these pins can be individually
addressed via the programs you'll write. All of them can serve as digital inputs
and outputs. The ADC pins can also act as analog inputs that can measure volt-
ages between 0 and 5V (usually from resistive sensors). Many of these pins are
also multiplexed to serve additional functions, which you will explore during
your projects. These special functions include various communication interfaces,
serial interfaces, pulse-width-modulated outputs, and external interrupts.
Power Supplies
For the majority of your projects, you will simply use the 5V power that is
provided over your USB cable. However, when you're ready to untether your
project from a computer, you have other power options. The Arduino can accept
between 6V and 20V (7-12V recommend) via the direct current (DC) barrel jack
connector, or into the V in pin. The Arduino has built-in 5V and 3.3V regulators:
■ 5V is used for all the logic on the board. In other words, when you toggle
a digital I/O pin, you are toggling it between 5V and 0V.
■ 3.3V is broken out to a pin to accommodate 3.3V shields and external
circuitry.