Hardware Reference
In-Depth Information
8 Timed (Category 2) State Machines
8.1 Introduction
We know that 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, additional classii cation, also from a hardware
point of view, based on the
transition types
and
nature of the outputs
, as follows (see
i gure 8.1).
1)
Regular (category 1) state machines
: This category, illustrated in i gure 8.1a and
studied in chapters 5 to 7, consists of machines with only untimed transitions and
outputs that do not depend on previous (past) values, so none of the outputs need to
be registered for the machine to function.
2)
Timed (category 2) state machines
: This category, illustrated in i gure 8.1b 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) values.
3)
Recursive (category 3) state machines
: This category is illustrated in i gure 8.1c and
studied in chapters 11 to 13. It can have all four types of transitions, but one or more
outputs depend on previous (past) values, so such outputs must be stored in auxiliary
registers for the machine to function.
The two fundamental decisions before starting a design in hardware are then the
following:
1) The state machine category (regular, timed, or recursive).
