Hardware Reference
In-Depth Information
Figure 3.6
Mealy machine for a nonovelapping “010” detector. (a) State transition diagram. (b) Truth table
for both next state and output. (c) Corresponding Karnaugh maps and minimal Boolean expres-
sions. (d) Resulting circuit.
Karnaugh maps and optimal Boolean expressions of i gure 3.6c result. Finally, the
corresponding Mealy circuit is presented in i gure 3.6d.
Note that, contrary to the Moore case (previous section), here the expression for
the output (
y
) does include the input (
x
); that is, it is now
y
=
q
1
⋅
x
′
, whereas in the
Moore case it was
y
=
q
1
q
0
. This means that
y
can change asynchronously (that is, as
soon as
x
changes, independently from the clock), which occurs when
q
1
= '1', because
then
y
=
x
⋅
.
The decision on using or not using the extra register (step 5) is similar to that for
Moore machines. However, because Mealy machines are asynchronous, if a project
accepts this type of circuit, glitches are generally of no relevance. An interesting use
for out-registered (pipelined) Mealy machines is to implement glitch-free Moore-like
circuits (details are shown in the next section).
′
