Hardware Reference
In-Depth Information
Figure 11.1
State machine categories (from a hardware perspective).
equation results (i.e., the output is a function of itself). For example,
y
=
y
,
y
=
y
, and
y
=
y
+ 1 mean that
y
(which is an output) should keep in the present state the same
value that it had in the previous state, or the complement of that value, or the incre-
mented version of that value, respectively. Equivalently, one could write
y
new
=
y
old
,
y
new
=
y
old
′
, and
y
new
=
y
old
+ 1. Occasionally, an output might be a function of a past value
of another signal, like
y
=
z
(same as
y
new
=
z
old
).
The two fundamental decisions that must be made before starting a design are then
the following:
′
1) The state machine category (regular, timed, or recursive).
2) The state machine type (Moore or Mealy).
It is important to recall, however, that regardless of the machine category and type,
the state transition diagram must fuli ll three fundamental requisites (seen in section
1.3):
1) It must include all possible system states.
2) All state transition conditions must be specii ed (unless a transition is uncondi-
tional) and must be truly complementary.
3) The list of outputs must be exactly the same in all states (standard architecture).
11.2 Recursive (Category 3) State Machines
Figure 11.2 shows two examples of very special circuits. In i gure 11.2a a simplii ed
l owchart for a memory-write procedure is shown in which an address is set, the data
to be stored at that address is presented, then a write-enable pulse is applied to store
the data. Note the presence of an incrementer (gray block), responsible for setting the
