Module 3: Logic Circuit Structure

This module presents the basic structure of combinational logic circuits, and introduces the use of computer aided design (CAD) tools in modern circuit design. Combinational logic circuits use networks of logic gates to produce outputs that change in strict relation to input changes; that is, an output can only change state immediately after an input changes state. In a combinational circuit, some input signal changes propagate through the logic gates and interconnections and produce output signal changes, while some input changes may have no effect on outputs; further, the same input patterns will always produce the same outputs. In contrast, outputs from a “sequential circuit”, or a circuit that contains memory devices, can change irrespective of input signal changes, and the same input patterns applied at different times can produce different outputs (memory containing circuits are covered in a later module). All combinational circuits can be expressed in two forms: an OR’ing of AND’ed terms, or an AND’ing of OR’ed terms. Since these general forms, called “Sum of Products” (or SOP) and “Product of Sums” (or POS), can be used to express any combinational logic requirement, we will examine them in some detail.