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.
Define allowed states and label each state with an arbitary number
or letter along with a label showing the actual output
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.
Select each of the allowed states in turn.
For each state write down the next state (note: could be multiple
next states)
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.