Digital Circuit Design with an Introduction to CPLDs and FPGAs

8.1: Introduction to Sequential Circuits

8.1 Introduction to Sequential Circuits

In the previous chapter a combinational logic circuit was defined as a logic circuit whose output(s) are strictly a function of its inputs. A sequential circuit is a logic circuit whose output(s) is a function of its input(s) and also its internal state(s). The (internal) state of a sequential logic circuit is either a logic 0 or a logic 1, and because of its ability to maintain a state, it is also called a memory circuit. Generally, sequential circuits have two outputs one of which is the complement of the other and multivibrators [*] fall into this category. Sequential circuits may or may not include logic gates.

Flip flops, also known as bistable multivibrators, are electronic circuits with two stable outputs one of which is the complement of the other. The outputs will change only when directed by an input command.

There are 4 types of flip flops and these are listed below.

  1. Set-Reset (SR) or Latch

  2. D Type (Data or Delay)

  3. JK

  4. T (Toggle)

[*] For a thorough discussion on multivibrator circuits, please refer to Electronic Devices and Amplifier Circuits, ISBN 0-9744239-4-7