Designing Embedded Hardware, 2nd Edition

Digital Signals

Being an electronic circuit, the operation of a computer is about voltages and current flow. Understanding the basic principles of voltages and current flow within the computer is mandatory if you're going to produce a working system. Common operating voltages inside a computer are normally either 5 V or 3.3 V. For some low-power or exceptionally fast computers, voltages may be as small as 1.8 V or lower.

An output pin of a digital device can be in one of three states. It can be high (logic 1), low (logic 0), or tristate (high-impedance, also known as floating). A logic high is defined as the output voltage at the pin being higher than a given threshold. When a device's pin is outputting a high, it is said to be sourcing current to that connection. Similarly, a logic low is where the output voltage is below a given threshold, and the device's pin is said to be sinking current. Typically, components can sink more current than they can source.

A tristate pin is outputting neither a high nor a low. Instead, it becomes high-impedance (high-resistance) such that current flow in or out of the pin is negligible. It is, in effect, invisible to other components to which it is connected. For example, within a computer system there may be several memory devices connected to the data bus. When a particular device is being read, its data outputs will be either high or low (corresponding to the bit pattern being read back). All other memory devices in the system, because ...