# Digital Circuits Questions and Answers – Counter Implementation and Applications

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This set of Digital Electronics/Circuits Multiple Choice Questions & Answers (MCQs) focuses on “Counter Implementation and Applications”.

1. A ripple counter’s speed is limited by the propagation delay of ____________
a) Each flip-flop
b) All flip-flops and gates
c) The flip-flops only with gates
d) Only circuit gates

Explanation: A ripple counter is something that is derived by other flip-flops. It’s like a series of Flip Flops. The output of one FF becomes the input of the next. Because ripple counter is composed of FF only and no gates are there other than FF, so only propagation delay of FF will be taken into account. Propagation delay refers to the amount of time taken in producing an output when the input is altered.

2. A 5-bit asynchronous binary counter is made up of five flip-flops, each with a 12 ns propagation delay. The total propagation delay (tp(tot)) is ________
a) 12 ms
b) 24 ns
c) 48 ns
d) 60 ns

Explanation: Since a counter is constructed using flip-flops, therefore, the propagation delay in the counter occurs only due to the flip-flops. Each bit has propagation delay = 12ns. So, 5 bits = 12ns * 5 = 60ns.

3. A 4-bit ripple counter consists of flip-flops, which each have a propagation delay from clock to Q output of 15 ns. For the counter to recycle from 1111 to 0000, it takes a total of ________
a) 15 ns
b) 30 ns
c) 45 ns
d) 60 ns

Explanation: Since a counter is constructed using flip-flops, therefore, the propagation delay in the counter occurs only due to the flip-flops. Each bit has propagation delay = 15ns. So, 4 bits = 15ns * 4 = 60ns.

4. A ripple counter’s speed is limited by the propagation delay of __________
a) Each flip-flop
b) All flip-flops and gates
c) The flip-flops only with gates
d) Only circuit gates

Explanation: A ripple counter is something that is derived by other flip-flops. Its like a series of Flip Flops. Output of one FF becomes the input of the next. Because ripple counter is composed of FF only and no gates are there other than FF, so only propagation delay of FF will be taken into account. Propagation delay refers to the amount of time taken in producing an output when the input is altered.
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5. What is the maximum delay that can occur if four flip-flops are connected as a ripple counter and each flip-flop has propagation delays of tPHL = 22 ns and tPLH = 15 ns?
a) 15 ns
b) 22 ns
c) 60 ns
d) 88 ns

Explanation: Maximum propagation delay is the longest delay between an input changing value and the output changing value. Hence, 22 * n = 22*4 (Since there are 4 FFs) = 88ns.

6. The main drawback of a ripple counter is that __________
a) It has a cumulative settling time
b) It has a distributive settling time
c) It has a productive settling time
d) It has an associative settling time

Explanation: The main drawback of a ripple counter is that it has a cumulative settling time (i.e. another bit is transmitted just after one consequently).

7. 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

Explanation: Since a counter is constructed using flip-flops, therefore, the propagation delay in the counter occurs only due to the flip-flops. Each bit has propagation delay = 50ns. So, 4 bits or FFs = 50ns * 4 = 200ns. Clock frequency = 1/200ns = 5 MHz.

8. As the number of flip flops are increased, the total propagation delay of __________
a) Ripple counter increases but that of synchronous counter remains the same
b) Both ripple and synchronous counters increase
c) Both ripple and synchronous counters remain the same
d) Ripple counter remains the same but that of synchronous counter increases

Explanation: In ripple counter, the clock pulses are applied to one flip-flop only. Hence, as the number of flip-flops increases the delay increases. In the synchronous counter, clock pulses to all flip-flops are applied simultaneously.

9. A reliable method for eliminating decoder spikes is the technique called ________
a) Strobing
b) Feeding
c) Wagging
d) Waving

Explanation: A reliable method for eliminating decoder spikes is the technique called strobing. A strobe signal validates the availability of data on consecutive parallel lines.

10. A glitch that appears on the decoded output of a ripple counter is often difficult to see on an oscilloscope because of __________
a) It is a random event
b) It occurs less frequently than the normal decoded output
c) It is very fast
d) All of the Mentioned

Explanation: A glitch is a transition that occurs before a signal settles to a specific value. A glitch that appears on the decoded output of a ripple counter is often difficult to see on an oscilloscope because it is a random event and very fast and it occurs less frequently than the normal decoded output.

11. Assume a 4-bit ripple counter has a failure in the second flip-flop such that it “locks up”. The third and fourth stages will __________
a) Continue to count with correct outputs
b) Continue to count but have incorrect outputs
c) Stop counting
d) Turn into molten silicon

Explanation: The ripple counter would stop counting because next flip-flop’s input depends on the output of the previous flip-flop.

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