Introduction to Digital Systems: Modeling, Synthesis, and Simulation Using VHDL

3.5 BASIC LOGIC GATES AND TRUTH TABLES

Logic expressions describe an output as a function of the input and are called logic functions. In the digital world, logic gates are used to implement logic functions. There are seven types of logic gates: NOT, AND, OR, NAND, NOR, XOR, and NXOR. These circuit diagrams are symbolic representations of their corresponding logic functions (Figures 3.1 and 3.2).

The logic gates of AND, OR, XOR, NAND, NOR, and NXOR may have more than two inputs. The circuits in Figure 3.3 illustrate multi-input AND and OR logic gates.