Fundamentals of Digital Logic and Microcontrollers, 6th Edition

Chapter 5Sequential Logic

5.1 Basic Concepts

So far, we have considered the design of combinational circuits. The main characteristic of these circuits is that the outputs at a particular time t are determined by the inputs at the same time t. This means that combinational circuits require no memory. However, in practice, most digital systems contain combinational circuits along with memory. These circuits are called sequential.

In sequential circuits, the present outputs depend on the present inputs and the previous states stored in the memory elements. These states must be fed back to the inputs in order to generate the present outputs. There are two types of sequential circuits: synchronous and asynchronous.

In a synchronous sequential circuit, a clock signal is used at discrete instants of time to ensure that all desired operations are initiated only by a train of synchronizing clock pulses. A timing device called the clock generator produces these clock pulses. The desired outputs of the memory elements are obtained upon application of the clock pulses and some other signal at their inputs. This type of sequential circuit is also called a clocked sequential circuit. The memory elements used in clocked sequential circuits are called flip-flops. The flip-flop stores only one bit. A clocked sequential circuit usually utilizes several flip-flops to store a number of bits as required. Synchronous sequential circuits are also called state machines.

In an asynchronous sequential ...