# Chap. 5 Flip-Flops and Related Devices

Urban and Civil

Nov 15, 2013 (4 years and 8 months ago)

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Digital Systems

Korea Univ.. of Tech. & Edu.

Dept. of Info. & Comm.

Chap. 5 Flip
-
Flops and Related Devices

5
-
1

Chap. 5 Flip
-
Flops and Related Devices

Introduction

Combinational Circuit

The output levels at any instant of time are dependent on the levels present at the
inputs at that time

»
Any prior input
-
level conditions have no effect on the present outputs because
combinational logic circuits have no memory

Most Digital Systems = Combinational circuits + Memory elements

General digital system that combines combinational logic gates with memory
device :
Fig. 5
-
1

»
The external outputs are a function of both its external inputs and the information stored
in its memory elements

The most important memory element = Flip
-
Flop

F/F is made up of an assembly of logic gates

»
Even though a logic gate, by itself, has no storage capability, several can be connected
together in ways that permit information to be stored
(Refer to
Fig. 5
-
7
, p. 186).

Output State of F/F :
Fig. 5
-
2

Normal output (Q) :

0

or
1

1 = HIGH = Set

Inverted output(Q) :
1

or
0

0 = LOW = Clear = RESET

F/F = Latch = Bistable multivibrator
(Refer to
Slide 5
-
16
).

Digital Systems

Korea Univ.. of Tech. & Edu.

Dept. of Info. & Comm.

Chap. 5 Flip
-
Flops and Related Devices

5
-
2

5
-
1
NAND Gate Latch

NAND gate latch
(or

Latch
)

Constructed from two NAND gates
:
Fig. 5
-
3

Setting the Latch

Both cases Q ends up HIGH :

Fig. 5
-
4

Clearing the Latch

Both cases Q ends up LOW :

Fig. 5
-
5

Simultaneous Setting and Clearing

Set = Clear = 0

»
Q = = 1 : Undesired condition

Set = Clear = 1

»
No change

NAND Latch Summary

Fig. 5
-
6(a),(b)

Q
Q
SET
CLEAR
1

1

0

1

Q
Q
SET
CLEAR
1

1

1

0

Fig. 5
-
3

2 possible resting state when SET=RESET=1

Q
Q
SET
CLEAR
1

Q
Q
SET
CLEAR
1

1 0

1 0

0 1

1 0

1 1

0 0

Q

Q
Q
SET
CLEAR
1

Q
Q
SET
CLEAR
1

1 0

1 1

0 0

1 0

1 0

0 1

Fig. 5
-
4

Pulsing SET input to 0

Fig. 5
-
5

Pulsing CLEAR input to 0

Set
Clear
Output
1
1
No change
0
1
Q=1
1
0
Q=0
0
0
Invalid
Fig. 5
-
3
참고

Normal rest

Normally High Input

Digital Systems

Korea Univ.. of Tech. & Edu.

Dept. of Info. & Comm.

Chap. 5 Flip
-
Flops and Related Devices

5
-
3

Alternate Representations :
Fig. 5
-
7

Ex. 5
-
1)

Determine Q output in

Fig. 5
-
8

Ex. 5
-
2)

Switch debouncing circuit in

Fig. 5
-
9

5
-
2 NOR gate Latch

Ex. 5
-
3)

Determine Q output in
Fig. 5
-
11

Ex. 5
-
4)

what happen if the light beam is momentarily interrupted in
Fig. 5
-
12

Q will remain HIGH and the alarm will remain ON even if phototransistor return to
ON( Set=0, Clear=0 : no change)

F/F State on Power
-
Up

When power is on, not possible to predict the starting state of a F/F’s output

Output depend on factors such as internal propagation delays, parasitic

To start of in a particular state, activate
SET
/
CLEAR

input at the start of circuit.

Q
Q

S
R

Q
Q
SET
CLEAR
Set
Clear
Output
1
1
No change
0
1
Q=1
1
0
Q=0
0
0
Invalid
Fig. 5
-
7

Alternate Representation

Q
Q
SET
CLEAR
0

0

0

1

Set
Clear
Output
0
0
No change
1
0
Q=1
0
1
Q=0
1
1
Invalid
Q
Q

S
R

Resting Input

= 0

*
Invalid

Q = = 0

Q

Fig. 5
-
10

(a) NOR gate latch, (b) truth table, (c) simplified block symbol

*
Inactive Stage(Resting )

NAND latch : S=C=1

NOR latch : S=C=0

Digital Systems

Korea Univ.. of Tech. & Edu.

Dept. of Info. & Comm.

Chap. 5 Flip
-
Flops and Related Devices

5
-
4

5
-
3
Troubleshooting Case Study

Ex. 5
-
5)

Describe & analyze the circuit in
Fig. 5
-
13

Ex. 5
-
6)

what are the possible faults(refer to
Tab. 5
-
1
)

Possible faults(
Switch position A
에서

Q=1
이여야

)

»
Internal open at Z1
-
1 : 0

입력되지

않음

»
Component failure in NAND gate Z1

»
Internally shorted to ground at Z1
-
3, Z1
-
4, and Z2
-
2

5
-
4 Clock Signals and Clocked F/Fs

Async/Synchronous System

Asynchronous System : The output of logic circuits can change state any time
one or more of the input change

Synchronous System : The exact times at which any output can change states
are determined by a signal commonly called the
clock

»
Synchronous circuits are easier to design and troubleshoot because the circuit outputs
can change only at specific instants of time.

Clock Signal =
rectangular pulse train

or
square wave
(
Fig. 5
-
14
)

Positive
-
Going Transition(
PGT
), Negative
-
Going Transition(
NGT
)

The synchronizing action of the clock signals is accomplished through the use of
clocked flip
-
flops

Digital Systems

Korea Univ.. of Tech. & Edu.

Dept. of Info. & Comm.

Chap. 5 Flip
-
Flops and Related Devices

5
-
5

Clocked Flip
-
Flops :
Fig. 5
-
15

1. Clocked FFs have a
clock input
(
CLK
,
CK
, or
CP
)

»
In most clocked FFs, the CLK input is edge
-
triggered :
NGT

or
PGT

2. Clocked FFs have one or more
control inputs

»
The control inputs will have no effect on Q until the active clock transition
occurs(=
Synchronous control inputs
)

3. In summary,

»
The
control inputs

control the
WHAT

: Output state(
DATA 0 or 1
) will go to

»
The
clock input

determines the
WHEN
: actually triggers the change

Setup and Hold Times

Setup time
(5
-

50 ns)

»
minimum time that control input must remain at constant

value before the transition.

Hold time
(0
-

10 ns)

»
minimum time that control input must not change

after the positive transition

5
-
5 Clocked S
-
C F/F

Clocked S
-
C F/F

Waveform analysis in
Fig. 5
-
17

: positive going edge transition

t
s

t
h

Positive clock

transition

Control
Input

Clock
Input

Set
-
Clear F/F

50 %

Digital Systems

Korea Univ.. of Tech. & Edu.

Dept. of Info. & Comm.

Chap. 5 Flip
-
Flops and Related Devices

5
-
6

The clock input = Trigger input

Negative
-
going edge transition :
Fig. 5
-
18

Internal circuitry of the edge
-
triggered S
-
C F/F

Edge
-
triggered S
-
C F/F :
Fig. 5
-
19

»
1. NAND Latch

»
2. Pulse
-
steering : NAND gate

모두

1

입력되면

SET=0

되고

Q=1

»
3. Edge
-
detector :
Fig. 5
-
20

5
-
6 Clocked J
-
K F/F

Clocked J
-
K F/F :
Fig. 5
-
21

Toggle Mode : J = K = 1(
S
-
C F/F
에서는

Invalid
)

Negative
-
going edge transition :
Fig. 5
-
22

Internal circuitry of the edge
-
triggered J
-
K F/F :
Fig. 5
-
23

Q=0, = 1

상태에서

J=K=1

입력되면

»
NAND 1

입력은

모두

1
이고

따라서

출력은

0

되고

Q =1

Toggle

»
NAND 2

입력은

1, 1, 0
이고

따라서

출력은

1

되고

=0
으로

Toggle

5
-
7 Clocked D F/F

Clocked D F/F :
Fig. 5
-
24

Implementation of the D F/F :
Fig. 5
-
25

Parallel Data Transfer :
Fig. 5
-
26

Q

Q

Jack
-
King F/F

Data F/F

Digital Systems

Korea Univ.. of Tech. & Edu.

Dept. of Info. & Comm.

Chap. 5 Flip
-
Flops and Related Devices

5
-
7

5
-
8
D Latch : Transparent Latch

D Latch :
Fig. 5
-
27

Edge detector is not used : EN(Enable) input
사용

Ex. 5
-
7)

Determine waveform Q in
Fig. 5
-
28

5
-
9
Asynchronous Inputs

Asynchronous Inputs(=

override inputs
)

Used to set the FF to the 1 or clear the FF to the 0 state at any time, regardless
of the conditions at the other inputs

Clocked J
-
K F/F with asynchronous inputs :
Fig. 5
-
29

Designations for Asynchronous Inputs

PRE
(Preset),
CLR
(Clear)

S
D
(Direct SET),
R
D
(Direct RESET)

Ex. 5
-
8)

Determine the Q output in
Fig. 5
-
30

5
-
11 F/F Timing Considerations

Setup/Hold Time

Propagation Delays :
Fig. 5
-
33 (
Typ. MAX Few
-

100 ns
)

t
PLH

: Delay going from LOW to HIGH,
t
PHL

: HIGH to LOW

D Latch is not edge Triggered,

(Level Triggered)

Use the overbar to indicate

the active LOW

CLK

Q

t
PHL

t
PLH

Digital Systems

Korea Univ.. of Tech. & Edu.

Dept. of Info. & Comm.

Chap. 5 Flip
-
Flops and Related Devices

5
-
8

Maximum Clock Frequency :

f
MAX
(
Typ. Max 20 to 35 MHz
)

Clock Pulse HIGH and LOW Times :
Fig. 5
-
34(a)

The minimum time duration that the CLK must remain LOW before it goes HIGH
t
W
(L),

and HIGH before it returns LOW
t
W
(H)

Asynchronous Active Pulse Width :
Fig. 5
-
34(b)

The minimum time duration that a PRESET or CLEAR input must be kept in its
active state in order to reliably set or clear the FF

t
W
(L)

for
active
-
LOW

asynchronous inputs

Clock Transition Times

Manufacturer usually do not list a maximum transition time requirement

Generally less than 50 ns

for TTL, and less than 200 ns for CMOS

Actual ICs :
Tab. 5
-
2
(TTL : 7474, 74LS112, CMOS : 74C74, 74HC112)

Ex. 5
-
9)

Determine following from
Tab. 5
-
2

(a)
t
PLH

= 25 ns for 7474, (b)
t
PHL

= 41 ns for 74HC112, (c)
t
W
(L)

for 74LS112,
active
-
LOW CLR input
,

(d) 7474, Hold time is needed(
non
-
zero hold time
), (e) All
F/F, Setup time is needed(
No non
-
zero setup time
)

CLK

Digital Systems

Korea Univ.. of Tech. & Edu.

Dept. of Info. & Comm.

Chap. 5 Flip
-
Flops and Related Devices

5
-
9

5
-
12
Potential Timing Problem in FF Circuits

Potential Timing Problem :
Fig. 5
-
35

J2 input of Q2 will be changing as it receives the same NGT( ). This could
lead to an unpredictable response at Q2

해결책

:
t
PHL
must be greater than Q2’s hold time requirement

Hold time

적다

=
CLK
후에도

control input

계속

유지시킬

필요

없음

Fortunately, all modern edge
-
triggered FFs have hold time requirements that are
5 ns or less; most have
t
H

= 0(clock transition

동시에

control input

바뀌어도

상관이

없다
)

For these FFs, situation like that in Fig. 5
-
35 will not be a problem

가정

:
FF’s hold time requirement is short enough to respond reliably

The FF output will go to a state determined by the logic levels present at its
synchronous control inputs just prior to the active clock transition

»
if we apply this rule to Fig. 5
-
35,
J2 = 1
, K2 = 0

Ex. 5
-
10)

Determine the Q output in
Fig. 5
-
36

Clock transition

이전

입력

값을

갖는다

CLK
입력과

동시에

J2
에는

1(
Q1)

유지되어야

하지만

J1 = K1 = 1

따라

Toggle
되어

CLK
입력과

동시에

곧바로

J2 = 0

되어

J2

Hold time

만족

시킬

없다

-

현재

그림은

정상

동작

-

CLK
입력

전에

Q1 = 1
이며
,
CLK
입력과

동시에

J2 = 1
이고

따라서

Q2 =

1

0
1

Digital Systems

Korea Univ.. of Tech. & Edu.

Dept. of Info. & Comm.

Chap. 5 Flip
-
Flops and Related Devices

5
-
10

5
-
13
Master/Slave FFs

Master/Slave FF

2
개의

F/F

사용
(
Slave

Master F/F)
하며

negative
-
edge transition
사용

위와

같이

사용하는

이유

:
Timing Problem
해결
(
Sec 5
-
12
)

Timing Problem

해결

방법

Negative Edge triggered F/F :
현재

사용

Master/Slave F/F
사용

:
과거에

사용

예제

7470 :
J
-
K Edge triggered F/F

7471 : J
-
K Master/Slave F/F

5
-
14
FF Application

Unclocked FFs

Switch debouncing(
Ex. 5
-
2
), Event storage(
Ex. 5
-
4
)

Clocked FFs

We will briefly introduce the
more common applications in the following
sections

Digital Systems

Korea Univ.. of Tech. & Edu.

Dept. of Info. & Comm.

Chap. 5 Flip
-
Flops and Related Devices

5
-
11

5
-
15
FF Synchronization

Asynchronous signal input

A human operator’s actuating input switch at some random time

A FF can be used to synchronize the effect of an asynchronous input

Partial Pulse :
Fig. 5
-
37
(Ex. 5
-
11)

»
The operator actuates or releases the switch are essentially random, This can produce
partial clock pulses at output X

A method for preventing the appearance of partial pulses :
Fig. 5
-
38
(Ex. 5
-
11)

5
-
16 Detecting an Input Sequence

Detecting an Input Sequence :
Fig. 5
-
39

An output is to be activated only when the inputs are activated in a certain
sequence

»
HIGH output only if A goes HIGH and then B goes HIGH some time later

5
-
17 Data Storage and Transfer

Register

A data(binary number, BCD number,..) are generally stored in
groups of FFs

called
registers

FF

Synchronization

Digital Systems

Korea Univ.. of Tech. & Edu.

Dept. of Info. & Comm.

Chap. 5 Flip
-
Flops and Related Devices

5
-
12

Data Transfer

The data transfer involves the transfer of data from one FF or register to another

The logic value stored in
FF

A

is transferred to
FF

B

upon the NGT of the
TRANSFER pulse

Synchronous data transfer :
Fig. 5
-
40

Asynchronous data transfer :
Fig. 5
-
41

»
Transfer Enable = 0 : PRE=CLR=1,
통상적인

FF
으로

동작

»
Transfer Enable = 1 : A=1
이면

B=1, A=0
이면

B=0

Parallel Data Transfer :
Fig. 5
-
42

The contents of X1, X2, and X3 are transferred
simultaneously

into Y1, Y2, and
Y3(Upon application of the PGT of the TRANSFER pulse)

Parallel transfer does not change the contents of source register

5
-
18 Serial Data Transfer : Shift Registers

Shift Register :
Fig. 5
-
43

A group of FFs arranged so that the binary numbers stored in the FFs are shifted
from one FF to the next for every clock pulse

Hold Time Requirement

In shift register, the FFs must have a very small or zero hold time requirement

Sec. 5
-
12 Timing Problem

동일

Digital Systems

Korea Univ.. of Tech. & Edu.

Dept. of Info. & Comm.

Chap. 5 Flip
-
Flops and Related Devices

5
-
13

Serial Transfer between Registers :
Fig. 5
-
44

Ex. 5
-
12)

The contents of each FF after sixth shift pulse in
Fig. 5
-
44
?

The registers are filled up with zeros(zero inserted)

Shift
-
Left Operation

역으로

배치
(
Shift
방향에

따른

장단점은

없으며
,
응용

특성에

따라

선택
)

Parallel versus Serial Transfer

Parallel transfer :
Speed

»
All of the information is transferred simultaneously upon the occurrence of a single
transfer command pulse

Serial transfer :
economy

and
simplicity

»
The complete transfer of N bits requires N clock pulses

5
-
19 Frequency Division and Counting

3 bit binary counter :
Fig. 5
-
45

The FFs change state(toggle) whenever the pulses are applied

Each FF divides the frequency of its input by 2

Counting Operation :
Fig. 5
-
4
6
(State Table)

State Transition Diagram :
Fig. 5
-
4
7

Graphical representation of state table

»
Circle
(state),
Line
(transition),
I/O
(input/output)

여러

개의

Transmission wire
필요

N

FF

1/2
N
까지

분주

가능

01

11

1/0

clock

Digital Systems

Korea Univ.. of Tech. & Edu.

Dept. of Info. & Comm.

Chap. 5 Flip
-
Flops and Related Devices

5
-
14

MOD Number

MOD Number

indicates
the number of states

»
N Flip
-
flops = 2
N

different state, and count up to 2
N

-

1

Ex. 5
-
13)

What will be the state after 13 pulses(
현재는

101)
in
Fig. 5
-
45

Ex. 5
-
14)

6 Flip
-
flop arrangement of
Fig. 5
-
45

5
-
20 Microcomputer Application

Transfer binary data of
internal register

to
external register X

:
Fig. 5
-
48

1) Place the binary number onto its data output lines

3) Generate the clock pulse CP(
Write

signal)

Ex. 5
-
16)

a) What is address decode logic ? :
11111110

X = ? :
X will not change
(
그대로

0110)

5
-
21
Schmitt
-
Trigger Devices

Schmitt
-
Trigger Inverter :
Fig. 5
-
49

Schmitt
-
trigger type of input is designed to accept

slow
-
change signals and produce an oscillation
-

free output (
표시

:
Fig. 5
-
49b
)

STATE

1

0

V
T
-

V
T+

VOLT

Digital Systems

Korea Univ.. of Tech. & Edu.

Dept. of Info. & Comm.

Chap. 5 Flip
-
Flops and Related Devices

5
-
15

보통

0

에서

1

One
-
Shot

t
p

Quasi
-
stable

State

5
-
22
One
-
Shot(=Monostable Multivibrator)

One
-
Shot :
Fig. 5
-
50(a)

1) Once triggered by trigger input(T), Q = Opposite state

2) “1” remains for a fixed period of time t
p
(Determined by t
p

=
0.69RC
)

3) After a time t
p

Non
-
retriggerable One
-
Shot :
Fig. 5
-
50(b)

Retriggerable One
-
Shot :
Fig. 5
-
51

Actual Devices :
Fig. 5
-
52

74121/221 : Single/Dual non
-
retriggerable one
-
shot

74122/123 : Single/Dual retriggerable one
-
shot

5
-
23 Analyzing Sequential Circuits

Analyze a sequential circuits(
FFs + Gates
) in the following example

Ex. 5
-
16)

Determine the waveform at X, Y, Z, and W for 8 clock cycles

Counter stops counting at X=1, Y=0, and Z=0(
W=0 : no change
)

Digital Systems

Korea Univ.. of Tech. & Edu.

Dept. of Info. & Comm.

Chap. 5 Flip
-
Flops and Related Devices

5
-
16

5
-
24
Clock Generator Circuits

Multivibrator

Bi
-
stable multivibrator : Flip
-
flops have two stable state

Mono
-
stable multivibrator : One
-
shots have one stable state(“0”)

Astable = Free
-
running multivibrator : no stable state

Schmitt
-
Trigger Oscillator :
Fig. 5
-
54

555 Timer Used as an Astable Multivibrator :
Fig. 5
-
55

Ex. 5
-
17)

Calculate the frequency and the duty cycle of the 555 timer

Crystal
-
Controlled Clock Generators

Output frequency = Crystal’s resonant frequency

Clock Generator Circuit : 10 kHz

80 MHz
( refer to
7
Ed.

Fig. 5
-
58
)

»
Using TTL inverter : R = 300
-

1500 Ohm,
최대

20
MHz

»
Using CMOS inverter : R = 100 K Ohm,
최대

10
MHz

5
-
25 Troubleshooting FF circuits

Open Inputs :
Ex. 5
-
18

K
0

Open
되어

J
0

= K
0

= 1

Toggle

(
TTL open = 1)

1

=

Quasi
-
Stable State

* R depends on the type of
crystal used and its
frequency(Graph

제공됨
)

Digital Systems

Korea Univ.. of Tech. & Edu.

Dept. of Info. & Comm.

Chap. 5 Flip
-
Flops and Related Devices

5
-
17

Shorted Outputs :
Ex. 5
-
19
(
Fig. 5
-
57
)

D(Z2
-
2)

0

입력되며
,
따라서

Q(Z2
-
5) = 0
이어야

정상

Possible Circuit Faults

»
Z2
-
5 or Z1
-
4 is internally shorted to Vcc

»
Z2
-
5 or Z1
-
4 is externally shorted to Vcc

»
Z2
-
4 is internally or externally shorted to GROUND(
Preset : Q = 1
)

»
Z2 internal failure

In case of Z2 internal failure

»
1) Check Z2’s Vcc and GROUND : O.K.

»
2) Unsoler Z2, and Check it’s amplitude, frequency, pulse width, and transition times

(by using oscilloscope) : O.K.

»
3) Replace it with new one, but the new chip behaves in exactly the same way

»
4) Finally he detects a solder bridge between pins 6 and 7 of Z2

»
5) Remove the solder bridge and then the circuit functions correctly

Explain how this fault produced the operation observed

»
The Q and outputs are internally cross
-
coupled so that the level

on one will affect the other

»
A constant LOW at would keep a LOW at one input of NAND

gate so that Q would have to stay HIGH regardless of the J or K

현재는

Q = 1

Rule out

Q
Q
SET
CLEAR
Q

Q

1

0

Both outputs should be checked for faults, even those that are not connected to other devices

Digital Systems

Korea Univ.. of Tech. & Edu.

Dept. of Info. & Comm.

Chap. 5 Flip
-
Flops and Related Devices

5
-
18

Clock Skew

A clock signal arrives at the CLK inputs of different FFs at different
times(propagation delay

원인
)

The skew can cause a FF to go to a wrong state :
Fig. 5
-
58

»
Q2

CLOCK 1
에서

Q1=0

입력되어

계속

Q2=0

되어야

(
그러나

그림에서는

CLOCK 2
이후에

Q2=1

되어

오동작
)

해결

방법

»
Problems caused by clock skew can be eliminated by equalizing the delays(the active
transition arrives at each FF at approximately the same time)

각각의

Clock Input
에서의

Propagation Delay

계산

Digital Systems

Korea Univ.. of Tech. & Edu.

Dept. of Info. & Comm.

Chap. 5 Flip
-
Flops and Related Devices

5
-
19

5
-
26. Applications using PLD

CUPL syntax of NAND latch :
Fig. 5
-
59

Q

=
!SBAR # !QBAR
;

QBAR

=
!CBAR #!Q
;

CUPL syntax of D latch :
Fig. 5
-
27
(p.204)

Q

=
!SBAR # !QBAR

=
(D & EN) # !QBAR

;

QBAR

=
!CBAR # !Q

=
(!D & EN) # !Q

;

State transition example :
Fig. 5
-
49
(p.226)

sequence

[Q2, Q1, Q0] =
sequence

counter_out

»
Field

counter_out = [Q2, Q1, Q0] : field

SET
[Q2, Q1, Q0]

간결한

이름

CUPL state transition input file for 3
-
bit counter :
Fig. 5
-
60