3.3 Digital inputs and outputs and pull resistors

Before we can communicate digitally, we must be able to generate and detect digital signals. We will focus on single-board computers as the generator and detector of digital signals, but the principles also apply to many devices that generate and detect digital signals. Single-board computers for real-time computing commonly have several digital input/outputs (DIO) that each can be configured as either input or output.¹

For a digital output (DO), a digital signal, usually voltage \(V_O\)⁠, is asserted to the associated pin, as shown in figure 3.5a. \(V_O\) is always either high or low. The logic levels depend on the device, but they are often compatible with a \(5\)⁠, \(3.3\)⁠, or \(2.4\) V CMOS logic family. Most single-board computers can source and sink a small amount of current …