GATE Previous Questions on Asynchronous & Synchronous Counters with Solutions (1987 - Till Date)

1987

1.       A ripple counter using negative edge triggered D flip-flops is shown below. The flip-flops are cleared to ‘0’ at the R input. The feedback logic is to be designed to obtain the count sequence shown in the same figure. The correct feedback logic is : 

Answer:      A

                           Solution :  https://www.youtube.com/watch?v=qljzFR3B6TM

1990

1.       A 4 bit modulo-16 ripple counter uses JK flip-flops. If the propagation delay of each flip-flop is 50 nsec, the maximum clock frequency that can be used is equal to ……

                        a.       20 MHz

                        b.      10 MHz

                        c.       5 MHz

                        d.      4 MHz

Answer:     C
                            Solution :     https://www.youtube.com/watch?v=v4RIzGm0DGY

1993

1.       A pulse train with a frequency of 1 MHz is counted using a modulo 1024 ripple counter built with JK flip-flops. For proper operation of the counter, the maximum permissible propagation delay per flip flop stage is ____ nsec.

Answer:     50 nSec

Solution :   https://www.youtube.com/watch?v=VXYFD01hQDU

2.       A clocked sequential circuit has three states, A, B and C and one input X. As long as the input X is ‘0’, the circuit alternates between the states A and B. if the input X becomes ‘1’(either in state A or in state B), the circuit goes to state C and remains in state C as long as X continues to be 1. The circuit returns to state A if the input becomes ‘0’ once again and from then on repeats its behavior. Assume that the state assignments are A = 00, B = 01 and C = 10.

                        a.       Draw the state diagram of the circuit.

                        b.      Give the state table of the circuit.

                        c.       Draw the circuit using D flip flops.

Solution :    https://www.youtube.com/watch?v=uEwqg5Ho7Vo

1994

1.       Synchronous counters are ________ than the ripple counters.

Answer:     Faster

Solution :    https://www.youtube.com/watch?v=1qBwgRPMMU8

2.       A ring oscillator consisting of 5 inverters is running at a frequency of 1 MHz. the propagation delay per each gate is ________ nsec.

Answer:     100 nSec

Solution :    https://www.youtube.com/watch?v=aAm-0ZPJhAM

1996

1.   A state machine is required to cycle through the following sequence of states:

            ABC :  000  ->  010  ->  111  ->  100  ->  011  ->  101.

One possible implementation of the state machine is shown in figure. Specify what signals should be applied to each of the multiplexer inputs.

                              Solution :   https://www.youtube.com/watch?v=fNIP5XoARP0   

1997

1.   A sequence generator is shown in figure. The counter status (Q₀Q₁Q₂) is initialized to 010 using preset/clear inputs.

      The clock has a period of 50 ns and transitions take place at the rising clock edge.

                        a.       Give the sequence generates a tQ0 till repeats.

                        b.      What is the repetition rate of the generated sequence?

                 Solution :      https://www.youtube.com/watch?v=iOk6-PX-kY8

1998

1.   The figure is shows a mod – K counter, here K is equal to

                        a.       1

                        b.      2

                        c.       3

                        d.      4

Answer:    C

                  Solution :       https://www.youtube.com/watch?v=M521k-KIu3U  

2.   The mod – 5 counter is shown in the figure counts through states Q₂Q₁Q₀ = 000, 001, 010, 011 and 100.

            a.       Will the counter lockout if it happen to be in any one of the unused states?

            b.      Find the maximum rate at which the counter will operate satisfactorily. Assume the     propagation delays of flip-flop and AND gate to be t_FF and t_A respectively.

                  Solution :      https://www.youtube.com/watch?v=We99dI2DCJw    

1999

1.       The ripple counter shown in the given figure is works as a 

                        a.       Mod – 3 up counter

                        b.      Mod – 5 up counter

                        c.       Mod – 3 down counter

                        d.      Mod – 5 down counter

Answer:    D

                 Solution :      https://www.youtube.com/watch?v=JYOaavMXJzE 

2.       The circuit diagram of a synchronous counter is shown in the given figure. Determine the sequence of states of the counter assuming that the initial state is ‘000’. Give your answer in a tabular form showing the present state Q_A, Q_B, Q_C, J-K inputs (J_A, K_A, J_B, K_B, J_C, K_C) and the next state Q_A+, Q_B+, Q_C+. From the table, determine the modulus of the counter.

Answer:       Mod - 6

                 Solution :        https://www.youtube.com/watch?v=wnKE1bQlfhg

2000

1.   In the given figure, the J and K inputs of all the four flip-flops are made high. The frequency of the signal at output Y is

                        a.       0.833 KHz

                        b.      1.0 KHz

                        c.       0.91 KHz

                        d.      0.77 KHz

            Answer:    B

                 Solution :    https://www.youtube.com/watch?v=ed2xKY4lc2c

2001

1.       For the ring oscillator shown in teh figure, the propagation delay of each inverter is 100 pico sec. What is the fundamental frequency of the oscillator output?

                        a.       10 MHz

                        b.      100 MHz

                        c.       1 GHz

                        d.      2 GHz

Answer:    C

                        Solution :  https://www.youtube.com/watch?v=HefE3DzUfOo     

2.       The digital block in the figure, realized using two positive edge triggered D flip-flops. Assume that for t<t₀, Q₁ = Q₂ =0. The circuit in the digital block is given by 

Answer:      C

           Solution :       https://www.youtube.com/watch?v=gkkCKeFhQ-s

2002

1.       It is required to design a binary mod-5 synchronous counter using AB flip-flop, such that the output Q₂Q₁Q₀ changes as 000 -> 001 -> 010..... and so on. The truth table for the AB flip-flop is given in figure.

            a.       Write down the state table for the mod-5 counter

            b.      Obtain simplified SOP expressions for the inputs A₂, B₂, A₁, B₁, A₀ and B₀ in              terms of Q₂, Q₁, Q₀ and their complements.

            c.       Hence complete the circuit diagram for the mod-5 counter given in the figure using     minimum number of 2 input NAND gate only.

                 Solution :     https://www.youtube.com/watch?v=2HJzJK-C38Y

2003

1.       A 0 to 6 counter consists of 3 flip flops and a combination circuit of 2 input gate(s). The combination circuit consists of

                        a.       one AND gate

                        b.      one OR gate

                        c.       one AND gate and one OR gate

                        d.      two AND gates

Answer:     D
                  Solution :  https://www.youtube.com/watch?v=eQA5gMcQSuw

2.       A 4 bit ripple counter and a 4 bit synchronous counter are made using flip flops having a propagation delay of 10 ns each. If the worst case delay in the ripple counter and the synchronous counter be R and S respectively, then

                        a.       R = 10 ns, S = 40 ns 

                        b.      R = 40 ns, S = 10 ns

                        c.       R = 10 ns, S = 30 ns

                        d.      R = 30 ns, S = 10 ns

Answer:    B

                 Solution :  https://www.youtube.com/watch?v=cAStF7mPky8

2004

1.   In the modulo – 6 ripple counter shown in the given figure, the output of the 2 input gate is used to clear the JK flip flops.

The 2 input gate is

                a.       NAND gate

                b.      NOR gate

                c.       OR gate

                d.      AND gate

Answer:    C

                  Solution :  https://www.youtube.com/watch?v=2b1wBH220Oo

2005

1.       The given figure shows a ripple counter using positive edge triggered flip-flops. If the present state of the counter is Q₂Q₁Q₀ = 011, then its next state Q₂Q₁Q₀ will be

                        a.       010

                        b.      100

                        c.       111

                        d.      101

Answer:    B

                  Solution :  https://www.youtube.com/watch?v=iZWdwF2lfmw

2006

1.       Two D flip-flops are to be connected as a synchronous counter as shown below, that goes through the following Q₁Q₀ sequence 00 -> 01 -> 11 ->10 -> 00  ->………..

The inputs D₀ and D₁ respectively should be connected as 

 

Answer:    A

                                 Solution :     https://www.youtube.com/watch?v=u3fgelNywPs

2007

1.       For the circuit shown, the counter state (Q₁Q₀) follows the sequence

                        a.       00, 01, 10, 11, 00…..

                        b.      00, 01, 10, 00, 01…..

                        c.       00, 01, 11, 00, 01…..

                        d.      00, 10, 11, 00, 10…..

Answer:    B

               Solution :    https://www.youtube.com/watch?v=aLGSUebXUW4

2008

1.       For each of the positive edge triggered JK flip flop used in the following figure, the propagation delay is ΔT.

  Which of the following waveforms correctly represents the output at Q₁?

Answer:    B

 Solution :   https://www.youtube.com/watch?v=N7ZYplccPcA

 

2009

1.       What are the counting stages (Q₁, Q₂) for the counter shown in the figure below?

                        a.       11, 10, 00, 11, 10….

                        b.      01, 10, 11, 00, 01….

                        c.       00, 11, 01, 10, 00…..

                        d.      01, 10, 00, 01, 10…..

Answer:    A

               Solution :   https://www.youtube.com/watch?v=marTcMTzg0Y

2010

1.       Assuming that all flip-flops are in reset condition initially, the count sequence observed at Q_A in the circuit shown is

                        a.       0010111….

                        b.      0001011….

                        c.       0101111….

                        d.      0110100….

Answer:    D

               Solution :  https://www.youtube.com/watch?v=UsicxIAMqx0

2011

1.       When the output Y in the circuit below is ‘1’, it implies that data has

                        a.       Changed from ‘0’ to ‘1’

                        b.      Changed from ‘1’ to ‘0’

                        c.       Changed in either direction

                        d.      Not changed

Answer:    A

               Solution :  https://www.youtube.com/watch?v=amudsVzHy88

2.       The output of a 3 stage Johnson (twisted ring) counter is fed to a digital to analog (D/A) converter as shown in the figure below. Assume all states of the counter to be unset initially. The waveform which represents the D/A converter output V_o is 

Answer:    A

                Solution :  https://www.youtube.com/watch?v=g9vVeVizDIM

3.       Two D flip-flops are connected as a synchronous counter that goes through the following Q_B Q_A sequence 00 -> 11 ->  01 -> 10 -> 00-> ………. The connections of the inputs D_A and D_B are

Answer:    D

               Solution :  https://www.youtube.com/watch?v=_9k8tNpWn7w

2012

1.       The state transition diagram for the logic circuit shown is

Answer:    D

                 Solution : https://www.youtube.com/watch?v=S2ytlAcBkFk

2014

1.       Five JK flip flops are cascaded to form the circuit shown in figure. Clock pulses at a frequency of 1 MHz are applied as shown. The frequency (in KHz) of the waveform at Q3 is …………

Answer:    62.5

                 Solution :  https://www.youtube.com/watch?v=ID70DQCpDCg

2.       The digital logic shown in the figure satisfies the given state diagram when Q1 is connected to input A of the XOR gate. Suppose the XOR gate is replaced by the XNOR gate. Which one of the following options preserves the state diagram?

Answer:    D

                 Solution :  https://www.youtube.com/watch?v=tBqtKpt1oSE

3.       In the circuit shown, choose the correct timing diagram of the output (y) from the given waveforms W1, W2, W3 and W4.

                      a.       W1

                      b.      W2

                      c.       W3

                      d.      W4

  Answer:    C

                Solution :  https://www.youtube.com/watch?v=E3XeZjGgzqY

4.       The outputs of the two flip-flops Q1, Q2 in the figure shown are initialized to 0, 0. The sequence generated at Q1 upon application of clock signal is 

                        a.       01110….

                        b.      01010…..

                        c.       00110….

                        d.      01100…..

Answer:    D

                 Solution : https://www.youtube.com/watch?v=wNoctGWwcfs

5.       The circuit shown in the figure is a

                        a.       Toggle flip flop

                        b.      JK flip flop

                        c.       SR Latch

                        d.      Master – Slave D flip flop

Answer:    D

                 Solution : https://www.youtube.com/watch?v=AvKzOVpre_k