# Logic Design Questions and Answers – General Models for Sequential Circuits

This set of Logic Design Multiple Choice Questions & Answers (MCQs) focuses on “General Models for Sequential Circuits”.

1. Which of the following devices serve as a memory for the sequential circuits?
a) A multiplexer
b) A logic gate
c) A flip-flop
d) A decoder

Explanation: Sequential circuits give outputs based on the present input as well as the previous outputs. That’s why we need a memory block. Flip flops especially D and JK flip-flops are used as a memory for sequential circuits as they have the capacity to hold a bit value. Hence, the answer is a flip-flop.

2. A combinational circuit is built using a D flip-flop with a propagation delay of 20ns and setup time of 30ns. Another multiplexer used has a combinational circuit delay of 10ns. What is the minimum clock period should be applied to the circuit to operate properly?
a) 40ns
b) 50ns
c) 10ns
d) 60ns

Explanation: The minimum clock period is given as
t(clk)min = t(p) + t(c) + t(su)
here, t(p) = 20ns
t(c) = 10ns
t(su) = 30ns
Hence, t(clk)min = (20 + 10 + 30)ns = 60ns

3. Which of the following is not used to build the combinational logic in any sequential circuit?
b) Programmable Logic Array
c) Logic gates
d) Flip-flop

Explanation: Flip-flops are used to build the memory part of any sequential circuit, not the combinational logic. Hence, the right answer is flip-flops. The others are generally used to build combinational logic.

4. For which purpose a clock circuit is used with a sequential circuit?
a) To activate the circuitry
b) To synchronize the operations of all flip-flops
c) To overcome a definite propagation delay
d) To shift data through used shift registers

Explanation: Sequential circuits give outputs based on the present input as well as the previous outputs. That’s why we need a memory block. Flip flops especially D and JK flip-flops are used as a memory for sequential circuits as they have the capacity to hold a bit value. Hence, the answer is a flip-flop. But there are several flip-flops used in a circuit. So, we need a common exciter to work all the flip-flops simultaneously. Clock does actually that job by giving exactly same oscillation as input to all the flip-flops.

5. Which one of the following is used as a memory of a sequential circuit?
a) D flip-flop
b) JK flip-flop
c) T flip-flop
d) Master Slave flip-flop

Explanation: A sequential circuit can be divided conveniently into two parts—the flip-flops which serve as memory for the circuit and the combinational logic which realizes the input functions for the flip-flops and the output functions. The combinational logic may be implemented with gates, with a ROM, or with a PLA. Figure 13-17 illustrates the general model for a clocked Mealy sequential circuit with m inputs, n outputs, and k clocked D flip-flops used as memory.

6. A finite time is required for a flip-flop to get their output changed after the input is driven. What is the time delay known as?
a) Timing delay
b) Propagation delay
c) Circuit delay
d) Estimation delay

Explanation: The gates (or other logic) in the combinational subcircuit have finite time delays, so when the inputs to the circuit are changed, a finite time is required before the flip-flop inputs reach their final values. This delay is termed as propagation delay.

7. When does the flip-flops change their states after a clock pulse is arrived?
a) In the middle of the clock pulse
b) At the rising or falling edge of the clock pulse
c) After the clock pulse is completed
d) At the beginning of the clock pulse

Explanation: Clock pulses are generally received as square wave pulses. Thus every pulse has a rising and a falling edge. Flip-flops change their states at the rising or the falling edge. Not before, after or at the beginning of the pulse.

8. Sequential circuit contains which of the following?
a) No memory element
b) At least one memory element
c) All inputs applied simultaneously
d) All inputs applied one by one

Explanation: A sequential circuit can be divided conveniently into two parts—the flip-flops which serve as memory for the circuit and the combinational logic which realizes the input functions for the flip-flops and the output functions. So, the sequential circuits can contain at least one memory element.

9. Schmitt trigger can be used as which one of the following?
a) Triangular wave generator
b) Square wave generator
c) Sine wave generator
d) Sawtooth wave generator

Explanation: A Schmitt trigger is a comparator (not exclusively) circuit that makes use of positive feedback (small changes in the input lead to large changes in the output in the same phase) to implement hysteresis (a fancy word for delayed action) and is used to remove noise from an analog signal while converting it to a digital one. This circuit produces square waves.

10. T flip-flop can be used as which one of the following?
a) Delay switch
b) Toggle switch
c) Data switch
d) Differentiator

Explanation: T flip-flops toggle their output between 1 and 0 after setting both the inputs at logic 1.
Thus, it can be used as a toggle switch.

11. An astable multivibrator has which of the following conditions?
a) 0 stable states
b) 1 stable states
b) 2 stable states
c) 3 stable states

Explanation: Regenerative switching circuits such as Astable Multivibrators are the most commonly used type of relaxation oscillator because not only are they simple, reliable and ease of construction they also produce a constant square wave output waveform. These circuits have no stable state. Hence the correct answer is 0 stable states.

Sanfoundry Global Education & Learning Series – Logic Design.

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