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
