Hardware Reference
In-Depth Information
Figure 1.13
ASM chart for the “
abc
” detector of i gure 1.3b.
As in l owcharts, the main elements of ASM charts are rectangles (representing the
machine's states) and diamonds (representing condition checks). An example is pre-
sented in i gure 1.13, which is equivalent to the “
abc
” detector of i gure 1.3b.
In large and/or complex designs, ASM charts tend to be cumbersome. Moreover—
and more importantly—they do not convey the hardware aspects as clearly as state
transition diagrams. For these reasons, they are generally of limited use to hardware-
based designs.
1.10 When to Use the FSM Approach
Even though any sequential circuit can be modeled/designed using the FSM approach,
it is not always advantageous or necessary to do so. For example, if the circuit has
too many states (say, over 100), it might be not viable to represent it as a state ma-
chine. Also, if it has very few states (say two or three), it might happen that a direct
(experience-based) solution is straightforward. The number of control signals and the
number of transitions are also determinant factors in the decision on whether or not
to use the FSM approach.
Four candidates for the FSM approach are depicted in i gure 1.14. The i rst candi-
date, in i gure 1.14a, has only one (big) loop, with perhaps one control input. This is
the case, for example, of regular counters (possibly with an enable input), which, as
already mentioned, constitute a classical example of circuits for which the FSM tech-
nique is not needed.
The second candidate, in i gure 1.14b, has few states, but proportionally more con-
nections than the previous case due to more control inputs, which might even include
