# FLIP-FLOPS

```2013.04.14.
DIGITAL TECHNICS II
Dr. B&aacute;lint Pőd&ouml;r
&Oacute;buda University,
Microelectronics and Technology Institute
3. LECTURE: ELEMENTARY SEQUENTIAL
CIRCUITS: FLIP-FLOPS, PART 2
1st year BSc course 2nd (Spring) term 2012/2013
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FLIP-FLOPS: PART 2
1. Flip-flops (RS, JK, and T) revision and emphasis
2. D (delay, data) and D-G (gated D) flip-flop
3. Flip-flops in practice
4. Mutual transformation of flip flops
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FLIP-FLOPS
The most important flip-flops are the following:
R-S (or S-R) flip-flop
J-K flip-flop
T flip-flop
D-G flip-flop
D flip-flop
All flip-flops listed above can function in synchronous or
clocked mode, the R-S and D flip-flops can operate in
asynchronous mode too.
The behaviour of a particular type can be described by
truth/characteristic table and the characteristic equation,
which gives the next output in terms of the input control
signals and the current output.
OPERATION OF FLIP-FLOPS
-The change of state of asynchronous flip-flops occurs
directly as a response to the change of the input/control
variable(s), after the appropriate time delay of the circuit.
-The change of state of synchronous (clock controlled) flipflops occurs only when the synchronizing signal (clock)
arrives tot heir appropriate input.
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FLIP-FLOPS: STATIC AND DYNAMIC
CONTROL
The control of flip-flops can be
either static or dynamic.
Static: appropriate logic 0 and/or 1 levels should be applied to
the static control inputs to initiate the state changes.
Dynamic: the change of state of the flip-flop occurs due the
change in the appropriate direction (10 or 01 transition) of
the signal applied to the dynamic control input (edgetriggered).
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R-S FLIP-FLOP: STATE TABLE
AND STATE TRANSITION GRAPH
SR
Qn
0
1
Qn+1
00 01 11
10
—————————
0
0
X
1
—————————
1
0
X
1
—————————
Qn+1
_
= S + R Qn
0X
X0
10
0
1
01
No oscillation occurs in any of the columns, therefore
operation can be asynchronous or synchronous.
(Red: stable states.)
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THE S-R (SET-RESET) FLIP-FLOP:
TIMING BEHAVIOUR
The S-R flip-flop is an active high (positive logic) device.
In the NOR gate realization for the logically not defined
S=R=1 input the output of both NOR gates is forced to 0
state. In this case the implicite condition of having
complementary states in the output is not fulllfilled.
J-K FLIP-FLOP: STATE TABLE
AND STATE TRANSITION GRAPH
Qn+1
JK
Qn
0
1
00 01 11
10
—————————
0
0
1
1
—————————
1
0
0
1
—————————
JK
0X
X0
1X
0
1
X1
No stable state exist in the JK= 1 column. The JK FF can
only be operated as a synchronous circuit. Cannot operate
without gating/clocking signal.
Qn+1
_
_
_
n
n
=JQ +KQ +JK
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T FLIP-FLOP: STATE TABLE
AND STATE TRANSITION GRAPH
Qn+1
T
Qn
0
1
0
1
—————
0
1
—————
1
0
—————
_
_
n+1
n
Q = T Q + T Qn
= T  Qn
T
0
0
1
0
1
1
No stable state exist in the T = 1 column.
The T flip-flop can only be a synchronous
sequential network.
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D FLIP-FLOP (1)
The state of the Q output of
the D (DELAY) flip-flop Q in
the next, (n+1)-th state will be
equal to the state of the D
control input in the pevious, nth state:
Qn+1 = Dn
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D FLIP-FLOP (2)
Truth table and characteristic equation
n-th
(n+1)-th
state
Qn+1 = D
D Qn Qn+1
————————
0
0
0
0
1
0
1
0
1
1
1
1
In fact the state in the (n+1)-th
state in fac t will nt depend on what
was the state of the FF in the n-th
state!
The D flip-flop does not remember
its previous stater
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D FLIP-FLOP: STATE TABLE AND
STATE TRANSITION DIAGRAM
D
Qn
0
1
Qn+1
0
1
—————
0
1
—————
0
1
—————
D
0
Characteristic equation:
1
1
0
1
0
Qn+1 = D
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A D FLIP-FLOP (3)
-The D flip-flop is mostly
used to as a component in
storage registers.
-E.g.to store the value
displayed by a digital
instruments, till the value of
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D FLIP-FLOP WITH CLOCK
Operation of D (DELAY) flip-flop with synchronizing clock.
If there is no clock signal (C=0) the output does not
change (Qn = Qn-1), if there is a clock signal (C=1) the
output will take the actual value of the input, i.e. Q n = D.
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GATED D FLIP-FLOP
Qn+1
G
G
Qn
0
0
1
0
1
1
1
0
D
D
_
Qn+1 = D G + G Qn + D Qn
3rd loop: hazard elimination
Operation can be asynchronous or synchronous.
TIME DIAGRAM OF GATED FLIP-FLOP
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CHARACTERISTIC EQUATIONS
OF FLIP-FLOPS: A SUMMARY
T
_
= S + R Qn
_
_
_
n+1
n
n
Q =JQ +KQ +JK
_
_
Qn+1 = T Qn + T Qn = T  Qn
D
Qn+1 = Dn
D-G
Qn+1
Qn+1
RS
JK
_
= D G +G Qn + D Qn
Note: The third terms in the equation of JK and D-G flip-flops
serve for the elimination of race hazards
FLIP-FLOPS IN PRACTICE
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NOTES ON HISTORY
The first electronic flip-flop (bistable circuit) was invented in
1919 by W. Eccles and F. W. Jordan (Eccles-Jordan trigger
circuit), and consisted of two active elements (radio-tubes).
The name flip-flop was later derived from the sound
produced on a speaker connected with one of the
backcoupled amplifiers output during the trigger process
within the circuit.
NOTES ON HISTORY
William Henry Eccles and Frank
Wilfred Jordan, Improvements in
logic relays British patent number:
GB 148582 (filed: 21 June 1918;
published: 5 August 1920).
W. H. Eccles and F. W. Jordan (19
September 1919) &quot;A trigger relay
utilizing three-electrode thermionic
vacuum tubes,&quot; The Electrician,
vol. 83, page 298. Reprinted in:
Radio Review, vol. 1, no. 3, pages
143–146 (December 1919).
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_
THE BASIC RS FLIP-FLOP
RS Q Q
Eccles-Jordan circuit with transistors
L L no change
L H H L
H L L H
H H L L
Q
•
•
T1
•
R
•
+Ucc
_
Q
T2
S
NOR gate
FLIP-FLOPS: ELECTRONICS
Construction of a bistable flip-flop from two-transistor
amplifying stages. Eccles-Jordan circuit with transistors.
In practice, flip-flops made of discrete transistors are rarely
used today.
Being constructed on the basis of different types of
integrated circuits, such as timer 555 (integrated circuit most22
commonly used for this purpose).
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FLIP-FLOPS: ELECTRONICS
Bistable flip-flop with driving transistors (setting resetting
inputs). This is the classical circuit with transistor amplifying
stages T1, T2, with driving transistors Ta1, Ta2.
The circuit has two outputs (main, Uq, complementary, /Uq)
and two driving inputs (setting and resetting).
If Us &gt;0.65 V) Ta1 is switched (saturating), T2 is blocked (cutoff), T1 is switched, Ta2 is blocked, Uq  UCC .
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MOS version: six-transistor SRAM cell.
D FLIP-FLOP IN PRACTICE:
EDGE-TRIGGERED D FLIP-FLOP
Logic structure of edge-triggered D flip-flop (logic diagram of
type 7474).
Auxiliary circuitry: asynchronous CLEAR and PRESET
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MASTER SLAVE J-K FLIP-FLOP
MASTER-SLAVE D FLIP-FLOP:
LOGIC DIAGRAM
Auxiliary circuitry: asynchronous CLEAR and PRESET
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MASTER-SLAVE D FLIP-FLOP
TIME DIAGRAM
MSI CIRCUIT EXAMPLE (1)
Vcc
14
13
12
D
11
10
CLR
Q
D
&gt;CK #Q
2
3
8
CLR
Q
&gt;CK #Q
PR
1
9
PR
4
5
6
7
GND
Two independent D flip-flops in one package
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MSI CIRCUIT EXAMPLE (2)
Vcc
20
1
19
18
17
16
15
14
13
12
D Q
D Q
D Q
D Q
&gt;CK
&gt;CK
&gt;CK
&gt;CK
CLR
CLR
CLR
CLR
CLR
&gt;CK
CLR
&gt;CK
CLR
&gt;CK
CLR
&gt;CK
D Q
D Q
D Q
D Q
2
3
4
5
6
7
8
9
11
10
GND
Eight D FFs with common CLK and CLEAR in one package: register
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SETUP AND HOLD TIMES
Cannot ignore effect of propagation delay.
Need to worry about critical timing region.
Setup time: minimum time that the signal D must be stable
prior to the edge of the clock.
Hold time: minimum time that the signal must be stable
after the edge of the clock.
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CMOS D FLIP-FLOP
• CMOS technology allows a very different approach to
flip-flop design and construction. Instead of using logic
gates to connect the clock signal to the master and slave
sections of the flip-flop, a CMOS flip-flop uses
transmission gates to control the data connections.
MUTUAL TRANSFORMATONS
OF FLIP-FLOPS
Any flip-flop can be implemented on the basis of any other
flip-flop.
Functional diagram: from the input control(s) of the flip-flop
to be implemented a combinational circuit produces the
signals necessary to drive the implementing flip-flop’s
inputs.
The output (state bit) is appropriately fed back to the input of
the whole system.
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CONVERSION OF FLIP-FLOPS
Combinational circuit
implementing flip-flop
y
X
CC
Z
FF
y
Flip-flop to be implemented
Clock
Each flip-flop type can be realized and implemented on the
basis of any other type.
D FLIP-FLOP REALIZATIONS
D

J
Q
CLK
CLK
CLK
K
_
Q
1
R
Q
CLK

D
1
S
_
Q
D
J
K
Qn+1
&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;
0
0
1
0
1
1
0
1
D
S
R
Qn+1
&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;
0
0
1
0
1
1
0
1
The D control excludes the occurrence of R =S = 1 excitation
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T FLIP-FLOP REALIZATIONS

T
J
Q
CLK
&amp;
T

K
_
Q
R
Q
CLK
S
&amp;
_
Q
T
0
1
J
0
1
K
0
1
Qn+1
Qn
Qn
T
0
0
Qn
0
1
S
0
0
R
0
0
1
0
1
0
1
1
0
1
Qn+1
Qn
Qn
_
1(Qn)
_
0(Qn)
IMPLEMENTATION OF T FLIP-FLOP
WITH RS FLIP-FLOP: SYNTHESIS OF
THE FEEDBACK NETWORK
Qn+1
S
Qn
Qn
0-0 1-1
T
0-1 1-0
R
T
0
x
1
0
_
S = T Qn
Qn
T
x
0
0
1
R = T Qn
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T FLIP-FLOP ON THE BASIS OF D FLIP-FLOP
_
_
n
D = T Q + T Qn
&amp;
T
1
D
Q
CLK
&amp;
_
Q
Clock
Note that T flip-flops are not available in the CMOS and TTL
logic families.
IMPLEMENTATION OF JK FLIP-FLOP
WITH D FLIP-FLOP
J
&amp;
1
K
D
Q
•
CLK
&amp;
_
Q
•
Clock
Note that D flip-flops are available in some PLA circuits.
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JK FLIP-FLOP BUILT WITH
RS FLIP-FLOP
J
&amp;
S
K
&amp;
Clock
Q
•
R = K Qn
CLK
R
_
S = J Qn
_
Q
•
The feedback ensures that the not allowed R=S=1 condition
can not occur.
&Aacute;LTAL&Aacute;NOS K&Ouml;VETKEZTET&Eacute;SEK &Eacute;S
&Ouml;SSZEFOGLAL&Aacute;S
Az elemi sorrendi h&aacute;l&oacute;zatok, azaz a flip-flopok megismert
tulajdons&aacute;gait &ouml;sszefoglalva a szinkron sorrendi h&aacute;l&oacute;zatokra
vonatkoz&oacute;an is levonhatunk n&eacute;h&aacute;ny &aacute;ltal&aacute;nos k&ouml;vetkeztet&eacute;st.
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FLIP-FLOPS: SUMMARY
T
_
Qn+1 = S + R Qn
_
_
_
n+1
n
n
Q =JQ +KQ +JK
_
_
n+1
n
Q = T Q + T Qn = T  Qn
D
Qn+1 = Dn
D-G
_
Qn+1 = D G +G Qn + D Qn
RS
JK
In the case of JK &eacute;s D-G flip-flops the third terms ensure
hazard free operation.
SUMMARY OF FLIP-FLOP TYPES
S=R=1 not
allowed
Most versatile
type
Most simple
Gated (DG)
Copies input to
output
Complements
output if activated
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STATE TRANSITION DIAGRAMS: SUMMARY
SR flip-flop
JK flip-flop
D flip-flop
T flip-flop
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EXCITATION TABLES OF FLIP-FLOPS
Qn
Qn+1
R
S
J
K
D
T
&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;
0
0
x
0
0
X
0
0
0
1
0
1
1
X
1
1
1
0
1
0
X
1
0
1
1
1
0
X
X
0
1
0
The excitation table lists the required inputs for a given
change of state, i.e. the input conditions that will cause the
transition form a given present state to the next state.
The excitation table is required and is used in the design
process.
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IMPLEMENTATION OF FLIP-FLOPS:
SOME PRACTCAL ASPECTS
• In the popular CMOS and TTL logic families T flip-flop is
not available. It should be build on the basis of an other
type of flip-flop.
• TTL, CMOS: basically JK and D flip-flops.
• PLA, PLD: only D flip-flop (!), the JK flip-flop should be
• RS flip-flop: in TTL it is based on NAND gates, in CMOS
it is based on NOR gates.
APPLICATION OF FLIP-FLOPS: SOME
PRACTICAL ASPECTS
• T flip-flops are well suited for straightforward binary
counters.
But may yield worst gate and pin counts.
• No reason to choose RS over JK FFs: it is a proper subset
of JK. RS FFs don’t exist anyway.
Tend to yield best choice for packaged logic where gate
count is the key.
• D FFs yield simplest design procedure. In many cases
give the best pin count.
D storage devices are very transistor efficient in VLSI.
Best choice where area/pin count is the key.
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SUMMARY OF SOME ASPECTS
OF FLIP-FLOPS
Development of D flip-flop
Level-sensitive is used in custom ICs
Edge-triggered uesd in programmable logic deices
Good choice for data storage register
Historically JK flip-flop was popular but noe never used
Similar to RS but wit 11 used to toggle output
Good in days of TTL/SSI (more complex input function)
Not a good choice for PALs/PLAs as it requires two inputs
Can always be implemented using D flip-flops
Preset and clear inputs are highly desirable on flip-flops
Used at start-up or to reset systems to a known state
SUMMARY OF SOME ASPECTS
OF FLIP-FLOPS
RS clocked latch:
Used as storage element in narrow width clocked systems
Its use is not recommended!
However fundamental building block of other flip-flop types
JK flip-flop:
Versatile building block
Can be used to implement D and T flip-flops
Usually requires least amount of logic to implement f(In, Q)
But has two inputs with increase wiring complexity
D flip-flops:
Minimizes wires, much preferred in VLSI technologies
Simplest design technique
Best choice for storage registers
T flip-flops:
Don’t really exist, constructed from JK flip-flops
Usually best choice for implementing counters
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