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Q: 1 The number of unused states in a 4-bit switch tail ring counter, (Johnson Counter) are

  1. 2

  2. 4

  3. 8

  4. 12

[ Option C ]

In a 4-bit Johnson counter (also called a switch-tail ring counter), the total number of possible states is 24=16. However, a Johnson counter produces only 2n unique states, where n is the number of flip-flops. For n=4, the number of valid states is 2×4=8. Therefore, the number of unused or invalid states is 16−8=8.

Q: 2 In a 4-stage ripple counter, the propagation delay of a flip-flop is 50ns. If the pulse width of the strobe is 30ns, find the maximum frequency at which the counter operates reliably.

  1. 10.0 MHz

  2. 20.0 MHz

  3. 7.5 MHz

  4. 4.35 MHz

[ Option D ]

Q: 3 How many unique states can a K-bit switch tail ring counter generate?

  1. 2K

  2. Kn

  3. 2K

  4. K

[ Option A ]

Q: 4 How many decade counters are connected in cascade to count from 0 to 999?

  1. 5

  2. 6

  3. 9

  4. 3

[ Option D ]

Q: 5 Which of the following statements is/are true about Ripple Counter?
1. It is an asynchronous counter.
2. Constitutes of Logic Gates and Flip-flops.
3. It is affected by propagation delay of Flip-flops.
4. Constitutes only Flip-flops.
5. It is affected by propagation delay of Flip-flops and Logic gates.

  1. 1,2,5 only

  2. 1,3,4 only

  3. 2 and 5 only

  4. All are true

[ Option B ]

A Ripple Counter is a type of counter in which the output of one flip-flop serves as the clock input for the next flip-flop. Because the clock signal does not reach all flip-flops simultaneously, it is called an asynchronous counter.

  • Ripple counter is Asynchronous because, only the first flip-flop receives the external clock. Other flip-flops are triggered by the previous flip-flop output.
  • A basic ripple counter is made using only Flip-Flops connected in series. No additional combinational logic gates are required in its basic form.
  • In a ripple counter each flip-flop is triggered by the output of the previous flip-flop rather than by a common clock. Due to this sequential triggering, the propagation delay of one flip-flop adds to the next, causing delay to accumulate stage by stage. This cumulative delay produces the ripple effect and limits the overall operating speed of the counter.

Q: 6 A MOD-16 ripple counter using J—K flip-flop has a current state 1001. How many clock pulses are required to get the state 0000?
(a)    8
(b)    6
(c)    7
(d)    5

  1. 8

  2. 6

  3. 7

  4. 5

[ Option C ]

A MOD-16 ripple counter is a 4-bit binary counter that counts from 0000 (0) to 1111 (15) and then repeats. Each clock pulse increases the count by 1.
The current state is 1001, which is 9 in decimal. To reach 0000, the counter must first count up to 1111 (15) and then roll over to 0000.

Counting sequence from 1001 (9):

Clock PulsesCounter StateDecimal Value
Current State10019
1101010
2101111
3110012
4110113
5111014
6111115
700000

Therefore, 7 clock pulses are required to reach 0000.

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