Sequential Circuit Analysis

Tom Kelliher, CS 240

Apr. 6, 2012



Exam on 4/13, covering 4.1-5.4, including material covered in class on addition.


Read 5-5.

From Last Time

Flip-Flops and waveforms.


  1. Reverse engineering a sequential circuit.

  2. Input equations, state tables, and state diagrams.

  3. Example problems.

Coming Up

Sequential circuit design.

Reverse Engineering Sequential Circuits

Analysis = reverse engineering.

One doesn't ordinarily do this, but doing so will help with sequential circuit design.

Input Equations, State Tables, and State Diagrams

Consider the following sequential circuit:


What does it do -- high level?

What happens if we use !Q as the input to the NEXOR rather than Q?

Mealy (this circuit) vs. Moore machines.

Input Equations

What are the equations for O and D (input equation)?

Do these differ that much from anything we've already seen?

State Tables

State tables are similar to truth tables, with two additions:

  1. Present state inputs -- flip-flop outputs.

  2. Next state outputs -- derived from input equations for flip-flops.

Present State I Next State O
0 0 0 1
0 1 1 0
1 0 1 0
1 1 0 1

State Diagrams

Conveys same information as state table, in a visual form.

State diagram for our example (Mealy):


How do I read this?

Moore machine example:



Determine input and output equations, state table, and state diagram for this circuit:


Is this Mealy or Moore?

Thomas P. Kelliher 2012-04-05
Tom Kelliher