Sequential Logic Notes

Digital logic circuits composed of components such as AND, OR and NOT gates and that do not contain loops are what we refer to as stateless. In other words, the output that the circuit produces only depends on the current inputs, and does not depend on any previous inputs. These types of circuits are also referred to as combinatorial circuits. However, as we design computing systems, we have a need for our circuits to maintain some record of what has happened before. For example, as you type letters into your favorite editor, you want your editor to remember what you have typed. In your computer, these data are stored in a variety of memories (including your disk drive and your Random Access Memory, or RAM). In general, a circuit that maintains a memory of the past is referred to as a sequential logic circuit. At the heart of these circuits are logic devices called latches and flip-flops that store a single bit of data until the bit is overwritten at a later (often arbitrary) time. The concepts outlined in the following sections will be important for our later understanding of several key ideas, including: