Hardware Reference
In-Depth Information
Figure 8.11
(a) Mealy counterpart of the Moore machine of i gure 8.10a. (b) A corresponding timing diagram.
3) The output value while in state A depends on
x
, so two different values can occur:
y
= 5 if
x
= '0' or
y
= 6 if
x
= '1'.
4) Because
T
B
= 3, state B lasts exactly three clock cycles (the timer counts from
0 to 2).
5) Because
T
C
= 2 but the CA transition is conditional-timed, state C lasts at least two
clock cycles (the timer counts from 0 to 1). The “at least” restriction is due to the
x
=
'0' condition, which might not be true when the timer reaches the monitored
(
T
C
1 = 1) value. In this example
x
= '0' was already available, so state C did last
only two clock periods.
6) In the states where the timer is not needed (only state A in this example), the timer
was kept stopped at zero.
7) The states and the timer operate exactly as in the Moore case, but that is not true
for the output.
8) As already seen, contrary to the Moore case, the output value is not unique in
all states.
9) Contrary to the Moore case, the output does not change together with the state.
It changes earlier.
10) Contrary to the Moore case, the output values do not necessarily last as long
as the states. They last less than the associated state if the transition condition into
that state is asynchronous, or they last exactly the same as the associated state
if the transition condition into that state is synchronous (a timed transition, for
example). Note that the output value
y
= 6 lasts less than three clock periods (the
transition control signal into state B is
x
, which is asynchronous), but
y
= 7 lasts
exactly two clock periods (the transition control signal into state C is
t
, which is
synchronous).
−
In conclusion, in a Mealy machine the output changes earlier than the state,
either by a fraction of a clock period (if the transition condition into that state
is asynchronous) or by a full clock period (for synchronous conditions, such as
time). The duration of each output value can then be different from that of its
associated state.
