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Digital Logic Design

CS302 - Digital Logic & Design

Lesson No. 24

APPLICATIONS OF EDGE-TRIGGERED D FLIP-FLOP

1. Data Storage using D-flip-flop

A Multiplexer based Parallel-to-Serial converter needs to have stable parallel data at its

inputs as it converts it to serial data. Latches are used to maintain stable data at the input of

the multiplexer. The time required to convert Parallel data to Serial data depends upon the

number of parallel bits. A byte parallel data requires 8-bit storage and 8 clocks are required to

convert it into serial data. The demerit in a gated D-latch based circuit is the extended enable

time. During the time in which the D-latches are enabled data applied at the input of the

latches can change. D-latch is said to work in transparent Mode when the enable signal is

activated. D-latch operates in the latched mode when the enable signal is inactive. The

conversion should only start when the enable signal has been deactivated and the 8-bit data

has been stored in the latches. A better and a precise parallel to serial converter circuit uses

Edge triggered D-flip-flops. The 8-bit data to be converted into serial data is stored precisely at

the clock transition. Thus, if the data changes after the clock transition it has no effect on the

data stored in the D flip-flop. Figure 24.1

SET

CLR

SET

CLR

SET

CLR

SET

CLK

CLR

Figure 24.1 D-flip-flops used for Parallel Data Storage

In the timing diagram shown the data at inputs D0, D1, D2 and D3 is constantly

changing. At interval t1 the four D-flip-flops are reset to 0,0,0 and 0 by activating the clear

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input. In the timing diagram the outputs of the four D flip-flops are shown set to logic zero after

a slight delay. At interval t2 the clock transition from logic low to logic high latches in the in the

data at the inputs of the four D flip-flops. The Q output of all the four latches remains stable

after interval t1. Changes at the D inputs of the four latches do not change the Q outputs of the

flour D flip-flops.

In the transparent Mode, the changes in the data applied at the inputs of the latch are

seen at the output of the latch, where as in the latched mode changes in the input data are not

reflected at the output. Figure 24.2

transparent

latched

CLK

Figure 24.2

Timing diagram of D-Latch

2. Synchronizing Asynchronous inputs using D flip-flop

In synchronized digital systems all the circuits change their state with respect to a

common clock and all the input and output signals are synchronized. However, external inputs

that are applied to digital circuits through switches and keypads are not synchronized with the

clock. The asynchronous inputs can occur at any instant of time. Consider the circuit based on

a 2-input AND gate which has a clock signal connected to one of its inputs and the other input

is connected to an input de-bounced switch. Figure 24.3. An asynchronous input applied

through the switch can cause incomplete or partial pulses at the output of the AND gate.

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Figure 24.4. A D-flip-flop synchronizes the input asynchronous signal such that the output of

the AND gate has complete clock pulses. Figure 24.5. The timing diagram of the synchronized

input circuit is shown in figure 24.6.

Figure 24.3

AND Gate connected to external switch and clock

Figure 24.4

Timing Diagram of AND Gate connected to external switch and clock

Figure 24.5

D flip-flop used to synchronize the AND Gate output

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CLOCK

Input

Switch

Input

Output

CLOCK

Output

t1 t2

t3 t4

Figure 24.6

Timing Diagram of the synchronized switch input

3. Parallel Data Transfer using D flip-flop

Microprocessor use multi-bit flip-flops to store information. These multi-bit flip-flops are

known as registers. These registers for example, can store data generated at the output of the

ALU. The registers can also be used to exchange or copy data. Figure 24.7. A register is a set

of flip-flops connected in parallel to store multi-bit binary information. The clock inputs of all the

flip-flops are connected together, to allow simultaneous latching of the multi-bit input data.

Edge-Triggered J-K Flip-flop

The J-K flip-flop is widely used in digital circuits. Its operation is similar to that of the S-

R flip-flop except that the J-K flip-flop doesn't have an invalid state, instead it toggles its state.

The circuit diagram of a J-K edge-triggered flip-flop is similar to that of the edge-triggered S-R

flip-flop except that the Q and Q output of the J-K flip-flop are connected back to the input

NAND gates which have the K and J inputs respectively. Figure 24.8. The operation of the J-K

flip-flop for different combinations of inputs is described below.

1. J = 0 and K =0

With Q=1 and Q =0, on a clock transition the outputs of NAND gates 3 and 4 are set to

logic 1. With logic 1 value at the inputs of NAND gates 1 and 2 the output Q and Q remains

unchanged. Similarly, with Q=0 and Q =1, on a clock transition the outputs of the NAND gates

3 and 4 are set to logic 1. With logic 1 value at the inputs of NAND gates 1 and 2 the output Q

and Q remains unchanged. Thus when J=0 and K=0 the previous state is maintained and

there is no change in the output.

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8-bit

D flip-flop

Data

Store 1

8-bit

D flip-flop

Data

Store 2

8-bit

D flip-flop

Data

Store 3

8-bit

D flip-flop

Data

Store 4

8-bit

4-to-1 MUX

8-bit ALU

Figure 24.7

D-flip-flops used to store data

CLK

Figure 24.8

Edge-triggered J-K flip-flop

2. J = 0 and K =1

With Q=1 and Q =0, on a clock transition the output of NAND gate 3 is set to logic 1.

The output of the NAND gate 4 is set to 0 as all three of its inputs are at logic 1. The logic 1

and 0 at the inputs of the NAND gates 3 and 4 respectively resets the Q output to 0 and Q to

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1. With Q=0 and Q =1, on a clock transition the output of NAND gate 3 is set to logic 1. The

output of the NAND gate 4 is also set to 1 as the input of the NAND gate 4 is connected to

Q=0. Logic 1 at the inputs of the NAND gates 3 and 4 respectively retains the Q and Q to 0

and 1 respectively. Thus when J=0 and K=1 the J-K flip-flop irrespective of its earlier state is

rest to state Q=0 and Q =1.

3. J = 1 and K =0

With Q=1 and Q =0, on a clock transition the output of NAND gate 4 is set to logic 1.

The output of the NAND gate 3 is also set to 1 as its input connected to Q is at logic 0. Thus

inputs 1 and 1 at inputs of NAND gates 1 and 2 retain the Q and Q output to 1 and 0

respectively. With Q=0 and Q =1, on a clock transition the output of NAND gate 4 is set to

logic 1. The output of the NAND gate 3 is set to 0 as all its input are at logic 1. Thus inputs 0

and 1 at inputs of NAND gates 1 and 2 sets the flip-flop to Q=1 and Q =0. Thus when J=1 and

K=0 the J-K flip-flop irrespective of its output state is set to state Q=1 and Q =0.

4. J = 1 and K =1

With Q=1 and Q =0, on a clock transition the output of the NAND gates 3 and 4 depend

on the outputs Q and Q . The output of NAND gate 3 is set to 1 as Q is connected to its input.

The output of NAND gate 4 is set to 0 as all its inputs including Q is at logic 1. A logic 1 and 0

at the input of gates 1 and 2 toggles the outputs Q and Q from logic 1 and 0 to 0 and 1

respectively. With Q=0 and Q =1, on a clock transition the output of NAND gate 3 is set to 0 as

Q and the output of NAND gate 4 is set to 1. A logic 0 and 1 at the input toggles the outputs Q

and Q from logic 0 and 1 to 1 and 0 respectively.

In summary when J-K inputs are both set to logic 0, the output remains unchanged. At

J=0 and K=1 the J-K flip-flop is reset to Q=0 and Q =1. At J=1 and K=0 the flip-flop is set to

Q=1 and Q =0. With J=1 and K=1 the output toggles from the previous state. The truth tables

of the positive and negative edge triggered J-K flip-flops are shown in table 24.1. The logic

symbols of the J-K flip-flops are shown in figure 24.9. The timing diagrams of the J-K flip-flops

are shown in figure 24.10.

J-K

CLK

Flip-Flop

Figure 24.9

Logic Symbol of Positive and Negative edge triggered J-K flip-flops

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Input

Output

Input

Output

CLK

Qt+1

CLK

Qt+1

↑

↓

↑

↓

↑

↓

↑

↓

Table 24.1

Truth-Table of Positive and Negative Edge triggered J-K flip-flops

CLK

Figure 24.10a Timing diagram of a Positive Edge triggered J-K flip-flop

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CLK

Figure 24.10b Timing diagram of a Negative Edge triggered J-K flip-flop

Applications of Edge-Triggered J-K Flip-flop

1. J-K flip-flop used as sequence detector

Some digital applications require that the inputs be applied in a certain sequence to

activate an output. This is possible with J-K flip-flops. Figure 24.11

Figure 24.11a J-K flip-flop connected to respond to a particular input sequence

Figure 24.11b Timing diagram of the input sequence

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2. J-K flip-flop used as frequency divider

In digital circuit different parts of the circuit can operate at different frequencies

obtained from the master clock frequency. For example, three different parts of a digital

system might operate at 4 MHZ, 2 MHZ and 1 MHZ clock frequency respectively. Same clock

source should be used (instead of three separate clock sources) to maintain synchronization

between the three parts. A clock frequency can be divided by 2 using a J-K flip flop. The J-K

inputs of the flip-flop are connected to logic high (1). At each clock transition the output of the

flip-flop toggles to the alternate state. Figure 24.12. A 4MHz clock signal can be divided into 2

MHZ and 1 MHZ signal using two J-K flip-flops connected together. Figure 24.13.

Figure 24.12a J-K flip-flop connected as frequency divider

Figure 24.12b Timing diagram of J-K flip-flop frequency divider

Figure 24.13a J-K flip-flop connected as divide-by-4 frequency divider

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CLOCK

Input

F0 Output

F1 Output

Figure 24.13b Timing diagram of J-K divide-by-4 frequency divider

3. J-K flip-flop used as a shift register

Binary numbers can be multiplied or divided by a constant 2 by shifting the binary

numbers left or right by 1-bit respectively. Multiplication and Division by a factor of 2n, (where n

= 1, 2, 3, 4 ....) can be achieved by shifting the binary by n bits to the left or right respectively.

Binary numbers can be easily shifted in the left or right direction by using J-K flip-flop based

shift registers. figure 24.14.

Figure 24.14a 4-bit right shift register

4. J-K flip-flop used as a counter

Counters which count up or count down are commonly used in digital circuits. An up-

counter counts up from 0 to 10 increments to the next higher count value on the application of

each clock signal. Similarly, a down-counter counts down to the next lower count value on the

application of each clock pulse. Figure 24.15.

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Figure 24.14b Timing diagram of a 4-bit right shift register

Figure 24.15a 2-bit up-counter

CLOCK

Input

F0 Output

F1 Output

Figure 24.15b Timing diagram of a 2-bit up-counter

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