Making Embedded Systems

Chapter 4. Outputs, Inputs, and Timers

From pin configuration registers, to debouncing buttons, to timers, this chapter will describe the most basic embedded concepts, focusing on the input and output aspects of the system. To keep it interesting, we’ll work through an example product, going through the phases of a project as the vision appears, changes, and finally crystallizes. Even the simple-sounding subject of making an LED blink is not immune to changes in product goals.

Toggling an Output

Marketing has come to you with an idea for a product. When you see through the smoke and mirrors, you realize that all they need is a light to blink.

Most processors have pins whose digital states can be read (input) or set (output) by the software. These go by the name of I/O pins, general-purpose I/O (GPIO), and occasionally general I/O (GIO). The basic use case is usually straightforward, at least when an LED is attached:

1. Initialize the pin to be an output (as an I/O pin, it could be input or output).

2. Set the pin high when you want the LED on. Set the pin low when you want the LED off. (Although the LED also can be connected to be on when the pin is low, but this example will focus on noninverted logic.)

Through this chapter, I’ll give you examples from three different user manuals so you get an idea of what to expect in your processor’s documentation. Atmel’s ATtiny AVR microcontroller manual describes an 8-bit microcontroller with plenty of peripherals. The TI MSP430x2xx User’s Guide describes ...