Hardware Reference
In-Depth Information
5 Regular (Category 1) State Machines
5.1 Introduction
We know that, from a hardware perspective, state machines can be classii ed into two
types, based on their
input connections
, as follows.
1)
Moore machines:
The input, if it exists, is connected only to the logic block that
computes the next state.
2)
Mealy machines:
The input is connected to both logic blocks, that is, for the next
state and for the actual output.
In Section 3.6 we introduced a new classii cation, also from a hardware point of view,
based on the
transition types
and
nature of the outputs
, as follows (see i gure 5.1).
1)
Regular (category 1) state machines:
This category, illustrated in i gure 5.1a and
studied in chapters 5 to 7, consists of machines with only untimed transitions and
outputs that do not depend on previous (past) output values.
2)
Timed (category 2) state machines:
This category, illustrated in i gure 5.1b and
studied in chapters 8 to 10, consists of machines with one or more transitions that
depend on time (so they can have all four transition types: conditional, timed,
conditional-timed, and unconditional). However, all outputs are still independent
from previous (past) output values.
3)
Recursive (category 3) state machines:
This category is illustrated in i gure 5.1c and
studied in chapters 11 to 13. It can have all four types of transitions, but one or more
outputs depend on previous (past) output values. Recall that the outputs are produced
by the FSM's
combinational
logic block, so the current output values are “forgotten”
after the machine leaves that state; consequently, to implement a recursive (recurrent)
machine, some sort of extra memory is needed.
As seen in this and in upcoming chapters, the classii cations mentioned above
(no other classii cation is needed) will immensely ease the design of hardware-based
