For CE6, the output depends only on the state of the system (not the
present input) and, thus, occurs after the desired input pattern has
occurred. Such a system is called a Moore model, named after
E. F. Moore. The output for the first input is shown as unknown
because we have no history of what happened before. (If the system
were initialized to indicate that no 1’s had yet occurred, then that output
would be 0.) After three consecutive inputs are 1, the system goes
to a state where the output is 1, and remains there as long as the input
remains 1.
Several designs are possible for this system. We will defer until
Chapter 7 our discussion of the techniques for designing a system from a
verbal description. At this point, we will introduce two tools for describing
sequential systems.
A state table shows, for each input combination and each state, what
the output is and what the next state is, that is, what is to be stored in
memory after the next clock.
A state diagram (or state graph) is a graphical representation of the
behavior of the system, showing for each input combination and each
state what the output is and what the next state is, that is, what is to be
stored in memory after the next clock.
Table 6.1 shows an example of a state table, one that does describe
CE6, although that is not obvious at this point.