FLIP-FLOP AND FINITE STATE
MACHINE
By:
Radha Iyer
Prof: Dr. Sin-Min Lee
CS147 Computer Organization and Architecture
WHAT IS SEQUENTIAL CIRCUITS?
A Sequential Circuit can be defined as circuit
having sequential logic. Sequential logic is a type
of a logic circuit whose output depends not only on
current inputs but also on previous inputs.
WHAT ARE
FLIP-FLOPS?
In order to remember previous inputs, sequential
circuits must have some sort of storage element.
This storage element is called “flip-flop”.
Flip-flop depends on previous inputs to the
circuit.
The basic memory unit is called an SR flip-flop.
We can describe flip-flops using characteristic
table.
SR Flip-Flop operation (BUILT WITH NOR GATES)
Characteristic table
[6]
Excitation table
S
R
Action
Q(t)
Q(t+1)
S
R
Action
0
0
Keep
state
0
0
0
X
No
change
0
1
Q=0
0
1
1
0
reset
1
0
Q=1
1
0
0
1
set
1
Unstabl
e
1
combina
tion
0
race
conditio
n
1
1
X
UNSTABLE CIRCUIT
S-stands for set
R- stands for re-set
If both S and R are set to1 in the SR flip-flop at
the same time then it resulted in an unstable
circuit, so this combination is not allowed.
To take care of this problem JK flip-flops were
developed.
WHAT IS JK FLIP-FLOP?
It is a variation to SR flip-flop, but it takes care
of the S=R=1 combination.
(J=Set, K=Reset)
When it comes to the S=R=1 it toggles. It is a
great improvement over the SR flip-flops.
JK Flip Flop operation [6]
Characteristic table
Excitation table
Comm
Q
ent
Qnext
J
K
Comm
ent
0
hold
state
0
0
0
X
No
change
0
1
reset
0
1
1
X
Set
1
0
set
1
0
X
1
Reset
1
1
toggle
1
1
X
0
No
change
J
K
0
Qnext
WHAT ARE D FLIP-FLOPS?
Another modified version of SR flip-flops are D
flip-flops.
‘D’ stands for data flip-flops.
It represents physical computer memory.
The output always takes on the state of the D
input at the rising clock edge.
D flip-flops are very useful as they capture the
signal when the clock rises.
Flip-flops are very useful and are found in a lot of
electronic devices.
Truth table:
Clock
D
Q
Qprev
Rising edge
0
0
X
Rising edge
1
1
X
Non-Rising
X
Qprev
FINITE STATE MACHINE
It is a model of behavior composed of a finite
number of states, transitions between those
states, and actions, (wikipedia,2000).
A finite state machine is an abstract model of a
machine with a primitive internal memory.
It depicts the graphical part of the flip-flops.
MOORE/MEALY MACHINE’S
Moore machine represents the JK flip-flops
Output depends only on the state.
Example for the Moore machine model will be the
elevator door, which just open’s and closes
Mealy Machine depends on output and state.
Example of Mealy Machine will be Microwave
oven.
MEALY MACHINE FOR JK FLIP-FLOPS
CONCLUSION
We learned about flip-flops which are basic
storage elements.
There were different kinds of flip-flops versions
developed SR, JK, D flip-flops.
Then we learned about Finite State Machines
which are primitive internal memory.
We saw Moore Machine, Mealy Machine which
are types of Finite State machines.
SOURCES
http://en.wikipedia.org/wiki/Flip-flop_(electronics)
http://en.wikipedia.org/wiki/Finite-state_machine
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