Ptuece.loremate.com Digital Electronics 8

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http://ptuece.loremate.com/die/node/4 March 7, 2012

Digital Electronics

Remember These:

1. Flip-flop is a sequential circuit or device which is used

to store single bit of information or data i e either 0 or 1 Single flip-flop stores single bit at atime

2. Flip-flop is a bistable multivibrator as it has two stable states

3. An unclocked flip-flop is called a latch A latch Is that circuit which is constructed using twocross-coupled NAND gates or NOR gates

4. NOR gate S-R latch is called an active High SR latch

5. NAND gate S-R latch is called an active Low SR latch.

6. Flip flops may be level triggered or edge-triggered level triggered are:

High level triggered

Low level triggered

Edge-triggered are Positive edge triggered

Negative edge triggered

RS, JK D and T type

1. D flip flop is also called as transparent latch as its output follows the input when the clock ishigh

2. J K flip flop is the most widely used of all flip flops

3. For SR flip-flop

When S = R 0 there is NO change condition

When, S= 1, R 0, there is SET state.

When, S = 0, R = 1, there is RESET state.

When S = R = 1, there is invalid state

For J-K flip-flop :.

When J = K = 0 there is NO change condition

When, J = 1, K 0, there is SET state.

When J = 0, K = 1, there is RESET state

When J K = 1 there is Toggle state

1. Toggle state is that in which the output changes from its previous state with every clock pulsewhen the inputs are J = K = 1

2. Master slave flip-flop overcomes the problem of race around condition which is common in J-
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K flip-flops using level clocks.

3. Register is a set of flip flops used to store binary data or information

4. The registers in which shifting of data takes place are called as shift registers

5. Types of shift registers are:

SISO i e Serial in Serial Out

SIPO i.e. Serial in Parallel Out.

PISO i e Parallel in Serial Out

PIPO i e Parallel in Parallel Out

1. Bidirectional shift register is used to shift data from left to right or right to left

2. Universal shift register performs similar function as that of bidirection shift register in additionto it data can be shifted in and out in serial form or in parallel form.

3. Counter is a device used to count clock pulses or number of events

4. Counter types are:

Asynchronous Counters

Synchronous Counters

1. Asynchronous counters a those in which clock is not simultaneous e output of 1st flip flop actsas clock to the next and so on

2. Synchronous counters are those in which clock is simultaneous i e common to all the flip flops

3. Modulus counters are those which passes through number of states before returning to thestarting state

4. A twisted ring counter is known as Johnson counter

5. Monostable multivibrator has only one stable state It is also called as one-shot multivibrator.

6. Astable multivibrator has no stable states It is called as free running multivibrator

Q 1. Explain T-flip-flop with suitable internal structure.

Ans. The functional block diagram of T flip-flop is as shown in fig.

T flip-flop is known as Toggle flip-flop

When, T = 1 both NAND Gates 1 and 2 gets high

A T flip-flop is designed by combining both the inputs of

Gate 1 and 2 together. Thus, when J = K = 0, Q havesame previous state i.e. hold state or no change state.When J = K = l, Q have toggle state i.e. invert theprevious state.

Q 2. Convert SR flip-flop to T flip-flop.


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Ans. Firstly write the truth table for SR to T flip-flop as shown:

K maps for S & R are:

For S:For R:

Implementation:

Q 3. Give applications of J-K flip-flops.

Ans.1. J-K flip-flops are used in shift registers.

2. J-K flip-flops are used in counters.

Q 4. Give difference between latch and flip-flop.

Ans.

Q 5. How race around condition can be eliminated?

Ans. Race around condition can be eliminated in JK latch by two ways

1. Using the edge triggered J-K flip-flop.

2. Using the master slave J-K flip-flop.

Q 6. What is a Glitch?

Ans. Glitch is a short duration pulse or spike thatappears in the outputs of a counter with MODnumber

Q 7. How many flip-flops are required to count 16clock pulses? Why?

Ans. To

count nclock pulsesm flip-flopsare required,where,

Thus, for 16 clock pulses to count, 4 flip-flops are required as 2 = 16.

Q 8. Give application of D and T flip-flops.

Ans. D flip-flops are delay flip flops and are extensively used for temporary storage of data inregisters. Hence, registers make use of D flip-flops.

T fli -flo s are to le fli -flo s and are used in counters. Hence, counter desi nin make use of T


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the or inputs of NAND latch. Thus, 0 becomes same as D. Thus,when enable is 1 the output Q will look exactly like D. Hence, the Dlatch is said to be transparent latch.

Q 13. A presettable counter has eight flip-flops. If the preset number is 125, what is themodulus?

Ans.

256 125 = 131

Thus, MOD 131.

Q 14. Differentiate between sequential andcombination circuits.

Ans.

Q 15. The clock frequency is 2MHz. How long will it take to serial load the eight shiftregister?

Ans.

n = 8

Time takento loadserially theeight bit will

be given by

Q 16. Whatis flip-flop?

Ans. Flip-flop : Flip-flop is a sequential circuit which is used tostore single bit of

information at a time i.e. either 1 or 0 at a time. It has twostable output states. It can stay in one of the two stable statesunless state is changed by applying external inputs. Thus, it as abasic memory element for storage of data in binary form.

There are various types of flip-flops

1. S-R flip flop

2. J-K flip-flop

3. D-type flip flop

4. T-type flip-flop

Q 17. Explain about multivibrator.


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Ans. Multivibrators : Most of the digital circuits or systems need some kind of a timing

waveform for example, all clocked sequential system required a source of trigger pulses. A timingcircuit which produces a rectangular waveforms are referred to as multivibrators. There are differenttypes of multivibrators such as:

1. Astable Multivibrator

2. Monostable Multivibrator

3. Bistable Multivibrator.

Q 18. What is shift register?

Ans. Shift register : A register capable of shifting its binary information either from

right to left or left to right is known as shift register. It consists of flip-flops connected is

cascade. All flip-flops receive a common clock pulse which causes the shift from one stage to thenext stage.

It is of four basis types:

1. Serial in serial out register

2. Serial in parallel out register

3. Parallel in serial out register

4. Parallel in parallel out register.

Bi-directional shift register and Universal shift registers are also used for different

applications.

Q 19. What is universal shift register?

Ans. Universal shift register performs similar operation and function as that of bidirectional shiftregister i.e. it can shift data from left to right or right to left, in addition to it data can be shifted in andout and in serial as well as in parallel form.

Q 20. Draw a logic symbol for a D-flip-flop and compare with RS flip-flop.

Ans.

In case of RS two inputs are therehence four possible combined areused. But in case of D flip-flop it hasonly one input so and twocombination are used. Also, in D flip-flop what ever we want to store will beput as input. If we want to store 1. 1is the input and if 0 is input, 0 will

be stored in D flip-flop.

Q 21. Differentiate between synchronous and asynchronous counters.

Ans.


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Q 22. Whatis bufferregister?

Ans. Bufferregisters arealso calledas storage

registers.They areprimarilyused fortemporarystorage, ofbinaryinformationor data. Theyprovideplace to hold

data until it isprocessed.That is whythey areknown asbuffer registers.

Q 23. Suggest four applications of shift registers.

Ans. Applications of shift registers

1. It is used in ring counters.

2. It is used in sequence generators.

3. It is used for data conversion in computers i.e. serial to parallel, parallel to serial etc.

4. It is used for time delays i.e. serial in serial out shift register are used for this purpose.

5. It is used in Johnson counter / twisted ring counter.

Q 24 Give the truth-table for each flip-flop type: (a) J-K ; (b) D ; and (c) T

Ans. Truth table of J-K Flip-flop:

2. D Flip-flop:

Q output follows D input with each other.

3. T Flip-flop:

Toggle means 0 changes to 1 and 1changes to 0 with the passage of eachclock

Q 25 Explain why there may be a race condition in a shift register.

Ans. There is no race condition in Shift register as it make use of D flip flop,R S flip flop as D and J K flip flop So the condition of J = K = 1 never comes.


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Q 26 What is a ripple counter?

Ans. Ripple Counter- Ripple counters are also known as asynchronouscounters. It is as shown in fig for two bits

For ripple counter input clock is given to first flip-flop and

the output of first flip-flop acts as a clock to next flip-flopand so on

Q 27. Differentiate between static and dynamic shiftregisters.

Ans.

Q 28. Whatis the Mod of6 bit Ring

Counter ?

Ans. Themod of 6 bitring counteris given by 2where n =number ofbits.

2 = 64. Thus MOD-64.

Q 29. Give expression for maximum frequency of operation of n-bit Asynchronous

and synchronous counters.

Ans.

Max. frequency of operation for asynchorous counter

It is same for synchronous counter unless no. of flip-flop, wherethe maximum frequency of synchronous counter is

Q 30. Compare binary counters with non-binary counters.

Ans.

Q 31. Draw logic circuit diagram for 3-bit synchronous up-down counter with clearinput, startinput anddone output.The counter

shouldproducedone outputaftercom letion of


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a enable.

Edge Triggered S-R Flip-flop: In edge-triggered S-R flip-flop, the change of state in flip-flop takes place only whenedge (either +ve or -ve) of clock pulse takes place.

Q 37. What is the difference between level and edgetriggering? Explain the

working of master slave J-K flip flop.

Ans.

Master slaveJK flip-flop: The masterslave flip-flopmay bedesignedusing R-S, Dand JK flip-flops.Followingfigure showsthe functional block diagram of master slave JK flip- flop:

In figure m is used for Master and S is used for Slave

Working:

Case 1: When positive clock pulsegoes on leading edge is applied,the CLKm is 1 and CLKs is 0, thendata transferred to Qm is held uptoCLK = 1

Case 2: When the clock pulsegoes negative, trailing edge isapplied, the CLKm = 0 and CLKs =1, then Qm and will be transferred to Q = and at thatduration the inputs at J and K should not change This isovercome by the use of data lockout.

Internal structures of master slave J-K flip-flop, Truthtable is as shown in fig

Q 38. Draw a master-slave J-K flip-flop system. Explainits operation and show that the race-around condition iseliminated.

Ans.

A master-slave J-K flip-flop is constructed from two flip-flops. One flip-flop acts as a master


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and the other as a slave and the overall circuit is thus, called as master-slave flip-flop. It makeuse of J-K master flip-flop and S-A slave flip-flop. The master is positive edge triggered andslave is negativeedge triggered.Therefore, masterresponds to J-Kinputs before theslave. V

If J = 1 and K = 0,the master sets onthe positive clockedge. The highoutput of the masterdrives the J input ofthe slave, so whenthe negative clockedge arrives, the slavesets, copying the

action of the master.

If J = 0 and K = 1, themaster resets on thepositive clock edge.The high output ofmaster i.e. goes to Rinput of the slave.Therefore, slaveresets on arrival ofnegative clock edge.

If J = K = 1 for master, ittoggles on positive clockedge and the slave themtoggles on the negativeclock edge.

When J = K = 1

Let clock = 1 then master isactive and slave is in active

Therefore, output of master

toggle. So S and R also will be inverted

When clock 0 Master becomes in active and slave is active Therefore, output of the

slave will toggle These changed outputs are again returned back to the master inputs as feedbackis connected in fig

But here clock is 0, the master is still in active So it does not respond to these changed

outputs

This voids multiple toggling which is responsible for Race Around Condition Hence

by using Master-Slave J K Flip-flop RACE AROUND CONDITION will be avoided or eliminated.


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Q 39 Explain what is universal shift register? Explain its working.

Ans. A universal shift register is one which can function in any of the SISO SIPO PISO or PIPOmodes of operation To operate the register universally it contains serial input serial output, parallelinputs, parallel outputs and must be able to serially shift data to the right or to the left, hold the dataor to reset Thus, it has bidirectional property also It is a 74194 IC Its internal structure is as shown infigure.

Operatingmodes of IC-741 94

Ptuece.loremate.com Digital Electronics 8

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