Chapter 4. - State Diagrams & State Tables

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Designing and understanding of sequential logic circuits requires a way of representing the internal states. State diagrams and state tables are a way of understanding this.

At the beginning of a design we will need to define our required output states, and also of equal importance, we need the transisitions between the states to be defined. This definition process is a direct translation of a specification that should be made available or formulated at the beginning of the design process. The process is acheived by putting all the information required into a state diagram.

To make a state diagram, follow the method below.

  1. Define allowed states and label each state with an arbitary number or letter along with a label showing the actual output
  2. Define the transitions between related states and mark this on the diagram by arrows. Also mark near the line, the input that initiates the change

This method can then be demonstrated with a very simple example.
Example: Design a logic system that counts down from logic 3 to logic 0 with every clock pulse then rolls over to 3 after 0..

A quick note about state diagrams

The example and method we are dealing with are both showing how to provide an output level rather than an output pulse. If our specification requires output pulses instead of output levels, then the output state should be defined on (or near) the transition arrows on the state diagram.

After verifying that the state diagram is correct, the next stage is to derive the state table. A state table shows the present state the next state and the output state. The best way to show this process is to continue our simple example.

  1. Select each of the allowed states in turn.
  2. For each state write down the next state (note: could be multiple next states)
  3. For each state note down the output level (or pulse) at the current state

A way to draw the state diagram when dealing with output levels is the diagram above. To draw the same diagram when using output pulses, have a look at the diagram below.

<insert state table with pulses>

More examples of state diagrams and state tables

Example 2.
Design a state diagram and related state table that has two inputs and two output states. The design should able to provide an output pulse every time that input 1 follows input 2. At all other times it must not provide any output.

Example 3.

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