USB and Serial Communication - Exploring Arduino: Tools and Techniques for Engineering Wizardry - page 122

Figure 6-9: ASCII table
However, you'll often want to send numeric values to the Arduino. So how
do you do that? You can do so in a few ways. First, you can simply compare the
characters directly. If you want to turn an LED on when you send a 1 , you can
compare the character values like this: if (Serial.read() == '1') . Note that
the single quotes around the '1' indicate that it should be treated like a character.
A second option is to convert each incoming byte to an integer by subtracting
the zero-valued character, like this: int val = Serial.read() - '0' . However,
this doesn't work very well if you intend to send numbers that are greater than 9,
because they will be multiple digits. To deal with this, the Arduino IDE includes
a handy function called parseInt() that attempts to extract integers from a
serial data stream. The examples that follow elaborate on these techniques.
Sending Single Characters to Control an LED
Before your dive into parsing larger strings of multiple-digit numbers, start by
writing a sketch that uses a simple character comparison to control an LED.