All questions of Sequential Circuits for Electrical Engineering (EE) Exam

A ripple counter is a type of digital counter circuit that uses flip-flops to sequentially count through a series of binary states. It is called a "ripple" counter because each flip-flop's output is connected to the clock input of the next flip-flop in the series, causing a ripple effect as each flip-flop changes state.

In a ripple counter, the first flip-flop is connected to an external clock signal, and each subsequent flip-flop is triggered by the output of the previous flip-flop. As the clock signal pulses, each flip-flop changes state and produces a binary output, which becomes the input for the next flip-flop. This sequential process creates a binary counting sequence.

The advantage of a ripple counter is its simplicity and low-cost design. However, it has a limitation in terms of speed, as the propagation delay from one flip-flop to the next accumulates with each stage, causing a time delay in the overall counting process.

Ripple counters are commonly used in applications where speed is not critical, such as simple digital displays, frequency dividers, or control circuits.

Propagation delay is the time it takes for a signal to propagate through a logic gate or flip flop. In the context of counter design, the propagation delay of flip flops plays a significant role in determining the speed of operation, especially in asynchronous (ripple) counters.

Asynchronous counters are designed using a series of flip flops, where the output of one flip flop serves as the clock input for the next flip flop in the chain. Each flip flop introduces a certain amount of propagation delay, which is the time it takes for the output of the flip flop to stabilize after a change in its inputs.

The main reason why the propagation delay of flip flops affects the speed of operation in asynchronous counters is due to the ripple effect. When the input to the first flip flop changes, it takes some time for the output of that flip flop to stabilize. This delay in stabilization propagates through the subsequent flip flops in the chain, causing a delay in the counting sequence.

The propagation delay can lead to several issues in asynchronous counters:

1. Skew: The propagation delay of flip flops can cause a skew in the counting sequence. Skew refers to the difference in arrival times of the clock signal at different flip flops. This can result in an inconsistent and unpredictable counting sequence.

2. Glitches: The propagation delay can also cause glitches in the counter output. A glitch is a temporary and unwanted change in the output of a logic circuit. Due to the ripple effect, glitches can occur when the outputs of the flip flops are changing during the stabilization period.

3. Limited frequency of operation: Asynchronous counters have a limited operating frequency due to the cumulative effect of propagation delays. If the input clock signal has a high frequency, the propagation delays can accumulate and result in errors or unpredictable behavior in the counter.

On the other hand, synchronous counters, both up and down, do not suffer from these issues because they use a common clock signal to synchronize all flip flops simultaneously. The propagation delay of individual flip flops still exists but does not affect the counting sequence or introduce glitches, as all flip flops are updated at the same time.

In conclusion, the propagation delay of flip flops largely affects the speed of operation in asynchronous (ripple) counters due to the ripple effect, which can lead to skew, glitches, and limitations on the operating frequency.

Basic S-R Flip-Flop

The basic S-R (Set-Reset) flip-flop is a type of sequential logic circuit that can store one bit of information. It is constructed by cross-coupling two NOR (Negative-OR) gates.

Cross-Coupling of NOR Gates

To understand why the cross-coupling of NOR gates is used to construct an S-R flip-flop, let's first examine the truth table of a NOR gate:

A	B	Output
0	0	1
0	1	0
1	0	0
1	1	0

Set and Reset Inputs

In an S-R flip-flop, the two inputs are called the Set and Reset inputs. When the Set input is high (1), it sets the flip-flop to the high (1) state. Conversely, when the Reset input is high (1), it resets the flip-flop to the low (0) state.

Construction of S-R Flip-Flop

To construct an S-R flip-flop using NOR gates, we connect the output of one NOR gate to the Set input of the other NOR gate, and vice versa. This creates a feedback loop between the two gates, allowing the flip-flop to store information.

Working Principle

1. Initially, both inputs are low (0), and the outputs of both NOR gates are high (1).
2. When the Set input is activated (high), the output of the first NOR gate goes low (0), which is fed back to the Set input of the second NOR gate. This causes the output of the second NOR gate to go high (1), setting the flip-flop to the high state.
3. If the Set input returns to low (0), the flip-flop remains in the high state.
4. Similarly, when the Reset input is activated (high), the output of the second NOR gate goes low (0), which is fed back to the Reset input of the first NOR gate. This causes the output of the first NOR gate to go high (1), resetting the flip-flop to the low state.
5. If the Reset input returns to low (0), the flip-flop remains in the low state.

Summary

In summary, the cross-coupling of NOR gates allows the construction of a basic S-R flip-flop. The Set and Reset inputs control the state of the flip-flop, and the feedback loop between the gates maintains the stored information. This flip-flop is a fundamental building block in digital systems, and its behavior can be further modified and enhanced using additional logic gates.

Explanation:

The T-flipflop is a type of flipflop that toggles its output based on the input signal. It has two inputs: T (toggle) and CLK (clock) and one output Q.

Given:
Input signal frequency = 200 Hz

Working:
When a T-flipflop receives a pulse at its CLK input, it toggles its output (Q) based on the value of the T input. If T = 0, the output remains the same, and if T = 1, the output flips.

In a cascade connection of three T-flipflops, the output of one flipflop is connected to the CLK input of the next flipflop. This means that the output of the first flipflop becomes the input to the second flipflop, and so on.

Since the input signal to the first T-flipflop is a 200 Hz signal, the output of this flipflop will toggle at 200 Hz. This means that the output will change its state (from 0 to 1 or from 1 to 0) 200 times per second.

Now, the output of the first flipflop becomes the input to the second flipflop. Since the output of the first flipflop toggles at 200 Hz, the input to the second flipflop will also toggle at the same frequency.

Similarly, the output of the second flipflop becomes the input to the third flipflop. So, the input to the third flipflop will also toggle at 200 Hz.

Therefore, the final output of the three T-flipflops in cascade will also toggle at 200 Hz.

Conclusion:
The final output of the three T-flipflops in cascade is 200 Hz.

Therefore, the correct answer is option 'A' (25 Hz).

S-R Flip-Flop Construction using NOR gates

To construct a basic S-R flip-flop, we can use a combination of NOR gates. The S-R flip-flop is a fundamental building block in digital circuits and is used to store a single bit of information. It has two inputs, S (set) and R (reset), and two outputs, Q and Q̅ (complement of Q).

Truth Table
The truth table of an S-R flip-flop is as follows:

S | R | Q | Q̅
--|---|---|---
0 | 0 | Q | Q̅
0 | 1 | 0 | 1
1 | 0 | 1 | 0
1 | 1 | - | -

Design using NOR gates
The cross-coupling of NOR gates allows us to implement an S-R flip-flop. Here is the construction using NOR gates:

1. Connect the S input to one input of a NOR gate and the R input to one input of another NOR gate.
2. Connect the Q output to one input of the NOR gate corresponding to the R input.
3. Connect the Q̅ output to one input of the NOR gate corresponding to the S input.
4. Connect the output of the NOR gate connected to the R input to the other input of the NOR gate connected to the S input.
5. Connect the output of the NOR gate connected to the S input to the other input of the NOR gate connected to the R input.

Explanation
When both S and R inputs are low (0), the NOR gates produce high outputs, maintaining the previous state of the flip-flop. This forms a stable state where Q and Q̅ remain unchanged.

When S is high (1) and R is low (0), the NOR gate connected to S input produces a low output, which is connected to the R input NOR gate. This results in a high output at Q and a low output at Q̅, setting the flip-flop to the state Q=1.

Similarly, when S is low (0) and R is high (1), the NOR gate connected to R input produces a low output, which is connected to the S input NOR gate. This results in a low output at Q and a high output at Q̅, resetting the flip-flop to the state Q=0.

Finally, when both S and R inputs are high (1), it creates a condition called the "forbidden" state, where the outputs of the NOR gates are unpredictable. Therefore, this state is avoided in practical designs.

Thus, the cross-coupling of NOR gates allows us to construct a basic S-R flip-flop that can store and manipulate a single bit of information.

Microseconds.

The number of natural states in a 4-bit ripple counter can be determined by considering the number of possible combinations of its 4 bits.

- Decimal System:
In the decimal system, we have 10 digits (0-9) to represent numbers. Similarly, in a binary system, we have 2 digits (0 and 1) to represent numbers.

- Binary Number Representation:
A 4-bit ripple counter has 4 bits, which means it can represent 2^4 = 16 different states. Each bit can be either 0 or 1, so the total number of possible combinations is 2^4 = 16.

- Counting the States:
To understand the counting of states, let's consider a simple example of a 3-bit ripple counter. In a 3-bit counter, we have 3 bits that can be either 0 or 1.

- Number of States:
The number of states can be calculated using the formula 2^n, where n is the number of bits. In the case of a 3-bit counter, the number of states is 2^3 = 8. Similarly, in a 4-bit counter, the number of states is 2^4 = 16.

- Explanation:
In a ripple counter, the output of one flip-flop serves as the clock input to the next flip-flop. Each flip-flop represents one bit, and the number of bits determines the number of possible states.

- Each Bit:
Each bit can independently toggle between 0 and 1. So, for a 4-bit counter, each bit has 2 possible states (0 or 1).

- Total States:
To calculate the total number of states, we multiply the number of states for each bit. In this case, since we have 4 bits, the total number of states is 2 * 2 * 2 * 2 = 16.

Therefore, the correct answer is option 'C' - 16.

Ms

To calculate the time delay (td) of an 8-bit serial in/serial out shift register, we need to consider the number of bits being shifted and the clock frequency.

The time delay can be calculated using the formula:

td = (Number of bits x Clock cycle time)

Given that we have an 8-bit register and a clock frequency of 4 MHz (4 x 10^6 Hz), we can calculate the time delay as follows:

td = (8 bits x 1/(4 x 10^6 Hz)) = 8 x 0.25 microseconds = 2 microseconds

Therefore, the time delay (td) of an 8-bit serial in/serial out shift register with a clock frequency of 4 MHz is 2 microseconds.

Internal Propagation Delay of Asynchronous Counter

The internal propagation delay of an asynchronous counter refers to the time delay between the input signal and the output signal of each flip-flop within the counter. Asynchronous counters are also known as ripple counters because the output of each flip-flop ripples through to the next flip-flop, causing a delay in the output signal.

Synchronous Counter

A synchronous counter, on the other hand, is a type of counter where all the flip-flops within the counter are clocked simultaneously by a common clock signal. This means that the output of each flip-flop is updated at the same time, eliminating the propagation delay between the flip-flops.

Elimination of Internal Propagation Delay

The internal propagation delay of an asynchronous counter can cause timing issues and glitches in the output signal, especially at higher frequencies. To overcome this issue, a synchronous counter is used. When all the flip-flops within the counter are clocked simultaneously, the internal propagation delay is eliminated.

Advantages of Synchronous Counter
- Elimination of Propagation Delay: The synchronous counter ensures that the output of each flip-flop is updated at the same time, eliminating the delay between the flip-flops. This results in a more accurate and synchronized output signal.
- Improved Timing: Synchronous counters offer better timing characteristics compared to asynchronous counters. The synchronous operation allows for precise control of the clock signal, ensuring accurate counting and timing.
- Higher Speed Operation: Due to the elimination of the propagation delay, synchronous counters can operate at higher clock frequencies without the risk of glitches or timing errors.
- Better Design Flexibility: Synchronous counters are easier to design and analyze because the timing characteristics are more predictable. This allows for more complex counter configurations and applications.

Conclusion

The internal propagation delay of an asynchronous counter is removed by using a synchronous counter. The synchronous operation ensures that all flip-flops within the counter are clocked simultaneously, eliminating the delay between the flip-flops and improving the overall timing and accuracy of the counter.

Output goes lowc)Q output remains unchangedd)Q output toggles between high and low

Master-slave flip flop is a type of sequential logic circuit that can store one bit of binary information. It is commonly used in digital electronics for various applications such as memory elements, counters, and registers. This type of flip flop is also referred to as a pulse-triggered flip flop because it changes its output state in response to a specific pulse.

The master-slave flip flop consists of two D flip flops, namely the master and the slave flip flops. These two flip flops are connected in series, with the output of the master flip flop connected to the input of the slave flip flop.

The operation of the master-slave flip flop can be divided into two phases: the master phase and the slave phase.

Master Phase:
During the master phase, the master flip flop is enabled, and the input signal is allowed to propagate through the circuit. The input signal is applied to the master flip flop, and its state is stored. However, the output of the master flip flop is not directly connected to the output of the circuit.

Slave Phase:
During the slave phase, the master flip flop is disabled, and the slave flip flop is enabled. The stored state of the master flip flop is transferred to the slave flip flop, which then becomes the output of the circuit. This transfer of data occurs when a clock pulse triggers the transition from the master phase to the slave phase.

The clock pulse acts as a trigger for the master-slave flip flop. It synchronizes the operation of the flip flop and ensures that the state transfer occurs at the appropriate time.

In summary, the master-slave flip flop is referred to as a pulse-triggered flip flop because it changes its output state in response to a clock pulse. This type of flip flop is widely used in digital electronics due to its reliable and synchronized operation.

Effect of SET input in S-R latch:
When the SET input of an S-R latch is made high, it sets the latch, causing the Q output to become high.

Explanation:
- When the SET input is activated, it overrides the RESET input and forces the Q output to be high.
- This behavior is due to the internal logic of the S-R latch, where the SET input triggers the latch to store a high output.
- The Q output will remain high until a reset signal is applied to the latch.

Conclusion:
In summary, when the SET input of an S-R latch is made high, the output Q will be set to a high state, which can be represented as logic level 1.

The correct answer is option 'C', 8 states.

Explanation:
To understand why the maximum states possible for the counter formed by three T flip-flops is 8, we need to understand the operation of a T flip-flop and how it can be used to build a counter.

1. T Flip-Flop:
A T flip-flop is a type of flip-flop that toggles its output based on the input signal. It has a single input (T) and a single output (Q). The output changes state (toggles) when the input is high (1) and remains the same when the input is low (0).

2. Counter Operation:
A counter is a digital circuit that counts the number of events or occurrences of a specific signal. In this case, we are using three T flip-flops to build a counter.

- Initially, all the flip-flops are reset to 0.
- When a clock pulse is applied, the first flip-flop (FF1) toggles its output.
- The output of FF1 is connected to the input of the second flip-flop (FF2).
- When the next clock pulse is applied, FF2 toggles its output based on the input from FF1.
- The output of FF2 is connected to the input of the third flip-flop (FF3).
- When the next clock pulse is applied, FF3 toggles its output based on the input from FF2.

3. Maximum States:
To determine the maximum states possible for the counter, we need to consider the number of unique combinations of outputs from the three flip-flops.

- Each flip-flop has two possible output states: 0 or 1.
- For three flip-flops, the total number of possible combinations is 2^3 = 8.

Therefore, the maximum states possible for the counter formed by three T flip-flops will be 8 (option 'C'). These states can be represented by the binary numbers from 000 to 111.

Indeterminate Output Condition in a Latch

Explanation:
When both inputs of a latch are high, meaning that both inputs are at logic level 1, the output of the latch becomes unpredictable. This condition is known as the indeterminate output condition.

Reason for Indeterminate Output:
The indeterminate output condition occurs because both inputs being high causes the latch to enter a metastable state. In this state, the outputs oscillate rapidly between logic 0 and logic 1, resulting in an unpredictable output.

Metastability:
Metastability is a phenomenon that occurs in digital circuits when a flip-flop or latch is in a state of uncertainty or oscillation between logic levels. It can happen when the inputs to the flip-flop or latch change close to the edge of a clock signal. Metastability can cause the output to remain in an unknown state for an indefinite period of time until it eventually stabilizes.

Consequences of Indeterminate Output:
The indeterminate output condition can have several consequences:
1. Data corruption: The unpredictable output can lead to errors in the data being processed or stored by the latch.
2. Timing issues: The indeterminate output can affect the timing of subsequent logic operations, causing timing violations and potential malfunctioning of the circuit.
3. System instability: The unpredictable output can propagate through the system, leading to unstable behavior and unreliable operation.

Preventing Indeterminate Output:
To prevent the indeterminate output condition in a latch, it is important to ensure that both inputs are not simultaneously high. This can be achieved by using proper timing and sequencing of input signals. It is also common practice to use edge-triggered flip-flops instead of latches in critical circuits, as flip-flops have built-in synchronization mechanisms to avoid metastability issues.

Conclusion:
The indeterminate output condition occurs in a latch when both inputs are high, causing the latch to enter a metastable state. This condition can lead to unpredictable outputs, data corruption, timing issues, and system instability. It is important to prevent this condition by carefully designing the timing and sequencing of input signals and considering the use of edge-triggered flip-flops instead of latches in critical circuits.

C) Because of cross-coupled connection

The circuits of NOR based S-R latch are classified as asynchronous sequential circuits primarily because of their cross-coupled connection. In a NOR based S-R latch, the output of one NOR gate is connected to the input of the other NOR gate, creating a feedback loop. This cross-coupling allows the latch to store a state or hold information, making it a sequential circuit.

The inverted outputs (d) and triggering functionality (b) are also characteristics of NOR based S-R latch but are not the main reasons for classifying it as an asynchronous sequential circuit.

S-R Flip-Flop as a Switch Debouncer

Switch debouncing is a process used to eliminate the noise or chatter caused by mechanical switches when they are being pressed or released. This noise can result in multiple false transitions, which can interfere with the proper functioning of digital circuits. One way to debounce a switch is by using an S-R flip-flop.

What is an S-R Flip-Flop?
A flip-flop is a sequential logic circuit that can store one bit of information. The S-R (Set-Reset) flip-flop is one of the most widely used types of flip-flops. It has two inputs, S (set) and R (reset), and two outputs, Q (output) and Q' (complement of the output).

Operation of an S-R Flip-Flop
- When the S input is HIGH and the R input is LOW, the flip-flop is set, and the Q output goes HIGH.
- When the R input is HIGH and the S input is LOW, the flip-flop is reset, and the Q output goes LOW.
- When both inputs are HIGH, the flip-flop is in an indeterminate state, which is not desirable for proper operation.

Switch Debouncing with an S-R Flip-Flop
To use an S-R flip-flop as a switch debouncer, the switch is connected to the S input, and its complement (inverse) is connected to the R input. The Q output of the flip-flop is then connected to the input of the digital circuit.

When the switch is pressed or released, it may generate multiple transitions due to mechanical bouncing. These transitions may cause the digital circuit to behave unexpectedly. However, the S-R flip-flop eliminates these unwanted transitions by ensuring that the Q output stabilizes at the final desired state.

Working Principle
- Initially, both the S and R inputs of the flip-flop are LOW.
- When the switch is pressed, the S input goes HIGH, causing the flip-flop to set and the Q output to go HIGH.
- As long as the switch remains pressed, the S input remains HIGH, but the flip-flop does not change its state due to the stable latch behavior.
- When the switch is released, the S input goes LOW, but the flip-flop maintains its state, and the Q output remains HIGH.
- Similarly, when the switch is pressed again, the R input goes HIGH, causing the flip-flop to reset and the Q output to go LOW.
- As long as the switch remains pressed, the R input remains HIGH, but the flip-flop does not change its state.
- When the switch is released again, the R input goes LOW, but the flip-flop maintains its state, and the Q output remains LOW.

Advantages of Using an S-R Flip-Flop for Switch Debouncing
- Simple and cost-effective solution for eliminating switch noise.
- Provides a stable output state unaffected by switch bouncing.
- Can be easily implemented using basic logic gates.
- Suitable for both manual and mechanical switches.

Conclusion
Using an S-R flip-flop as a switch debouncer is an effective way to remove noise and ensure the proper functioning of digital circuits. By connecting the switch to the S input and its complement to the R input, the flip-flop provides a stable output state

Explanation:
To understand the given question and the correct answer, let's first review the operation of a JK flip-flop.

A JK flip-flop is a sequential logic device that has two inputs, J (set) and K (reset), and two outputs, Q (output) and Q' (complement of the output). The flip-flop operates based on clock pulses, which determine when the inputs are accepted and when the outputs are updated.

The behavior of a JK flip-flop is defined by its truth table:

Truth Table:


In the given question, it is stated that the output Q of the JK flip-flop is initially 0. When a clock pulse is applied, the output changes to 1. Based on this information, we can deduce the values of the J and K inputs.

Analysis:

Since the initial output Q is 0 and changes to 1 on a clock pulse, we need to find the input combination that satisfies this behavior.

From the truth table, we observe that the output Q changes to 1 when J is 1 and K is 0. This is the only input combination that results in a change from 0 to 1 on the output Q.

Therefore, the correct answer is option 'A' - J = 1 and K = X (don't care).

Explanation:
The reason why the correct answer is J = 1 and K = X (don't care) can be explained as follows:

When J = 1 and K = X (don't care), the flip-flop operates in the following manner:
- When the clock pulse is applied, the flip-flop checks the values of J and K.
- Since J is 1, the flip-flop sets the output Q to 1, regardless of the value of K.
- As a result, the output Q changes from 0 to 1 on the clock pulse.

It is important to note that in this specific scenario, the value of K is not relevant because the J input takes precedence. When J is 1, the output Q will always change to 1, regardless of the value of K.

This behavior is specific to the JK flip-flop and is not applicable to other types of flip-flops, such as D or T flip-flops.

Conclusion:
In summary, the output Q of a JK flip-flop changes from 0 to 1 when a clock pulse is applied. This behavior is achieved when the input J is 1 and the input K is don't care (X). Therefore, the correct answer to the given question is option 'A' - J = 1 and K = X.

C) Shift register

Disadvantage of S-R Flip-Flop: Race Condition
The S-R (Set-Reset) flip-flop is a fundamental memory element in digital electronics. However, it comes with a significant disadvantage known as the race condition.
Understanding Race Condition
- Definition: A race condition occurs when the output of a flip-flop depends on the sequence or timing of input signals. In the case of an S-R flip-flop, this can lead to unpredictable behavior.
- Scenario: If both the Set (S) and Reset (R) inputs are activated simultaneously (S=1 and R=1), the flip-flop enters an indeterminate state. This situation can cause the outputs to toggle unpredictably, leading to system instability.
Implications of Race Condition
- Unstable Output: The primary outcome of a race condition is that the output may not settle to a stable state, which can cause errors in digital circuits.
- Design Complexity: To mitigate this issue, designers may need to implement additional circuitry, such as using clocked flip-flops or incorporating external controls, complicating the design process.
Comparison with Other Options
- No Enable Input: While the S-R flip-flop lacks an enable input, this is not inherently problematic for all applications.
- No Clock Input: The absence of a clock input can limit its functionality but is not as critical as the race condition.
- Single Output: Having only a single output is a design choice and does not represent a flaw in functionality.
In conclusion, while S-R flip-flops are useful in many applications, the presence of race conditions significantly undermines their reliability, making it a key disadvantage to consider in digital design.

Introduction:
Asynchronous counters are a type of digital circuit that can count events or perform other operations based on clock signals. While they offer advantages such as simplicity and low power consumption, they also have some drawbacks that limit their use in certain applications.

Explanation:
The major drawback of using asynchronous counters is that high-frequency applications are limited because of internal propagation delays. Let's understand this in detail:

1. Asynchronous Counters:
- Asynchronous counters, also known as ripple counters, are made up of a series of flip-flops connected in a cascaded manner.
- Each flip-flop in the counter divides the frequency of the clock signal by 2.
- The output of one flip-flop serves as the clock input for the next flip-flop in the sequence, creating a ripple effect.
- As a result, the outputs of the flip-flops change sequentially, resulting in a binary count.

2. Propagation Delay:
- Propagation delay refers to the time it takes for a signal to travel through a digital circuit from input to output.
- In the case of asynchronous counters, the propagation delay is the time it takes for a change in the input signal to propagate through the flip-flops and reach the output.
- This delay is caused by the internal logic gates used in the flip-flops, which introduce a certain amount of delay.

3. High-Frequency Limitation:
- The internal propagation delays in asynchronous counters create a limitation for high-frequency applications.
- As the clock frequency increases, the propagation delay becomes more significant.
- At high frequencies, the output of each flip-flop may not have enough time to settle before the next clock pulse arrives.
- This can result in incorrect counting and unpredictable behavior of the counter.

Conclusion:
In conclusion, the major drawback of using asynchronous counters is the limitation they impose on high-frequency applications due to internal propagation delays. While they are suitable for low-frequency counting applications, their use becomes less reliable and predictable at higher frequencies.

SR Flip Flop Stability

The SR flip flop is a basic type of flip flop that consists of two inputs, S (Set) and R (Reset), and two outputs, Q (Output) and Q' (Inverted Output). It is used to store one bit of information and can be triggered by a clock signal.

Stability of SR Flip Flop

The stability of an SR flip flop refers to its ability to maintain a stable output state when the inputs are changed. In other words, a stable flip flop should not change its output state when the inputs are changed.

Explanation of Options

a) S = 0, R = 1:
In this case, the Set input S is 0 and the Reset input R is 1. When the Set input is 0 and the Reset input is 1, the flip flop enters an unstable state. This is because both inputs are active, which violates the normal operation of the flip flop. The output state becomes unpredictable, and it may toggle between different states or remain in an undefined state.

b) S = 1, R = 0:
In this case, the Set input S is 1 and the Reset input R is 0. When the Set input is 1 and the Reset input is 0, the flip flop enters an unstable state. This is again because both inputs are active, violating the normal operation of the flip flop. The output state becomes unpredictable and may toggle between different states or remain in an undefined state.

c) S = 0, R = 0:
In this case, both the Set input S and the Reset input R are 0. When both inputs are 0, the flip flop remains in its current state. The output state does not change, and the flip flop remains stable.

d) S = 1, R = 1:
In this case, both the Set input S and the Reset input R are 1. When both inputs are 1, the flip flop enters an unstable state. This is because both inputs are active, violating the normal operation of the flip flop. The output state becomes unpredictable and may toggle between different states or remain in an undefined state.

Conclusion

The SR flip flop is unstable when both the Set and Reset inputs are 1 (Option D). In this case, the flip flop enters an unpredictable state, and the output may toggle or remain undefined. Therefore, the correct answer is option D.