CSE 140L Discussion

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CSE 140L Discussion

• Finite State Machines

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Finite State Machines
Two types Mealy and Moore Mealy depends on both
the current state and the input Moore depends
only on the current state. (You can think of it
as so in a moore machine, the output is
associated with the state. In a mealy machine,
the output is associated with the
transitions). Typically, moore has more states,
but are easier to implement. Mealy machines have
less states, and also are faster in clocked
systems (the output happens right away when the
inputs are detected). Moore has to wait the clock
cycle to transition to the next state before the
output is changed. But Mealy is usually more
sensitive to race conditions.
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What you need to think about 1. All the
possible states you could be in 2. All the
outputs 3. All the inputs 4. The transitions
between the states based on the inputs Note All
inputs and outputs should be accounted for in all
the transitions, even when they are dont
cares.
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First step Draw out the state machine. Second
step From the state machine, figure out how many
bits are needed to encode all the states. If you
have 3 states, you need 2 bits. If you have 5
states, you need 3 bits. Encode each of the
states. For example, if you have 5 states, then
your first state can be 000, and your other
states would be 001, 010, etc. Third step Based
on your transitions, draw out the transition
table. This is a large table that will map the
outputs based on the inputs. You can and should
take advantage of dont cares.
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How to do this
Take the initial circuit we give you, which has
the D flip flops. The DFFs hold the current
state. When the output of the three DFFs is 000
for example, that means that our current state is
0. The inputs into the DFFs correspond to the
NEXT STATE. That is because whatever you send
into the DFF will be output on the next clock
cycle.
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Once you have figured out your finite state
machine, start on your transition table. This
should map all your inputs to outputs, your
inputs being present state and the actual inputs,
and the outputs being the segment display bits
and next state. From the table, you can derive
the combinational logic for each of the outputs.
Once you have finished your transition table,
complete the circuit by drawing out all the
outputs and connecting them to the current
state and inputs. For a moore machine, the
outputs only depend on the current state. For a
mealy, the outputs depend on both the current
state and the inputs.
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For a mealy machine, the outputs depend on both
the current state and
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Also create the circuit for all your next state
bits. In the case for our moore machine, there is
n0, n1, and n2.