Digital Design Using VHDL A Systems Approach

Factoring a state machine is the process of splitting the machine into two or more simpler machines. Factoring can greatly simplify the design of a state machine by separating orthogonal aspects of the machine into separate FSMs where they can be handled independently. The separate FSMs communicate via logic signals. One FSM provides input control signals to another FSM and senses its output status signals. Such factoring, if done properly, makes the machine simpler and also makes it easier to understand and maintain – by separating issues.

In a factored FSM, the state of each sub-machine represents one dimension of a multidimensional state space. Collectively the states of all of the sub-machines define the state of the overall machine – a single point in this state space. The combined machine has a number of states that is equal to the product of the number of states of the individual sub-machines – the number of points in the state space. With individual sub-machines having a few tens of states, it is not unusual for the overall machine to have thousands to millions of states. It would be impractical to handle such a large number of states without factoring.

We have already seen one form of factoring in Section 16.3 where we developed a state machine with a datapath component and a control component. In effect, we factored the total state of the machine into a datapath portion and a control portion. Here we generalize this concept by showing how the control portion itself can be factored.

In this chapter, we illustrate factoring by working two examples. In the first example, we start with a flat FSM and factor it into multiple simpler FSMs. In the second example we derive a factored FSM directly from the specification, without bothering with the flat FSM. Most real FSMs are designed using the latter method. A factoring is usually a natural outgrowth of the specification of a machine. It is rarely applied to an already flat machine.