Q: Counter terminology: Evaluate the statement “Synchronous counters are clocked so that each flip-flop in the counter is triggered at the same time.” Determine whether this definition correctly distinguishes synchronous operation.

Introduction / Context: Counters are fundamental sequential circuits. The key distinction between synchronous and asynchronous (ripple) counters lies in how the flip-flops receive their clocking. This question checks whether the provided definition accurately captures that difference. Given Data / Assumptions: A counter consists of a chain of flip-flops. In synchronous counters, every flip-flop receives the clock directly. In asynchronous (ripple) counters, only the first stage is clocked by the external clock; subsequent stages are driven by preceding outputs. Concept / Approach: Synchronous counters are designed so that all flip-flops sample simultaneously on the same clock edge. Combinational logic determines which stages toggle on that edge, but the edge itself reaches each flip-flop at the same time (subject to skew). This coordinated triggering minimizes ripple delays and enables higher operating frequencies compared to ripple counters. Step-by-Step Solution: Define synchronous: common clock input to all flip-flops.Define asynchronous: cascaded clocking through outputs of preceding stages.Evaluate statement: it claims “each flip-flop is triggered at the same time,” which matches the synchronous definition.Conclusion: the statement is correct. Verification / Alternative check: Timing diagrams of synchronous counters show simultaneous edge application, with output transitions aligned to that edge (after propagation delays). Ripple counters exhibit serial, staggered transitions. Why Other Options Are Wrong: “Incorrect” misstates the accepted definition. Restrictions to “BCD counters” or “ripple counters” add irrelevant qualifiers and confuse clocking style with count code. Common Pitfalls: Ignoring clock skew: while skew exists physically, the design intent remains simultaneous triggering. Also, conflating synchronous design with code encodings (binary, BCD, Gray) is a common misunderstanding. Final Answer: Correct

Q: Ripple counters: Evaluate the statement “A ripple counter is an asynchronous counter.” Select whether this description correctly characterizes ripple counter clocking.

Introduction / Context: Ripple counters are widely used simple counters where clocking propagates stage-by-stage. This question checks whether calling a ripple counter “asynchronous” is appropriate and why. Given Data / Assumptions: Ripple counter: first flip-flop is clocked by the external clock; subsequent flip-flops are clocked by outputs of the previous ones. Asynchronous counters do not clock all flip-flops simultaneously from a common clock. Propagation causes a “ripple” of state changes through the chain. Concept / Approach: The hallmark of asynchronous (ripple) counters is that each stage toggles slightly after the preceding stage due to propagation delay. This produces staggered output transitions and limits maximum frequency because later stages see delayed clock edges derived from earlier outputs rather than the system clock itself. Step-by-Step Solution: Identify the clocking method: only the first stage uses the external clock.Observe that subsequent stages use other stage outputs as their clocks.Recognize the ripple of transitions—defining asynchronous operation.Conclude the statement is correct. Verification / Alternative check: Timing diagrams reveal stair-step transition sequences. Frequency limits are often specified lower than synchronous designs due to accumulated delay. Why Other Options Are Wrong: “Incorrect” contradicts standard definitions. Options tying this to bit-width or code type are unrelated to clocking style. Common Pitfalls: Assuming ripple counters are always acceptable at high speed; their cumulative delay can create decoding glitches and timing hazards without additional filtering or synchronizing logic. Final Answer: Correct

Q: Shift registers in digital systems: Assess the statement “Shift registers are used to store and transfer data.” Decide whether this summarizes their primary roles (temporary storage and serial/parallel data movement).

Introduction / Context: Shift registers are chains of flip-flops used to move data in a controlled manner. They are ubiquitous in digital interfaces, conversion between serial and parallel formats, and simple temporary storage applications. This question checks whether stating they “store and transfer data” is accurate. Given Data / Assumptions: A shift register consists of cascaded D (or JK) flip-flops with a shared clock. Data can enter serially or in parallel and exit serially or in parallel depending on the architecture (SISO, SIPO, PISO, PIPO). Flip-flops inherently store 1 bit each; together they form an n-bit register. Concept / Approach: Shift registers provide short-term storage (n bits) and controlled transfer (shift) of data with each clock edge. They are essential for serial communication (e.g., UART shift stages), data alignment, delay lines, and interface bridging (parallel-to-serial and serial-to-parallel conversion). Step-by-Step Solution: Identify storage: each flip-flop holds a bit; an n-stage register holds n bits.Identify transfer: on each clock, bits move to the next stage (serial movement) or load/read in parallel when enabled.Recognize common uses: buffering, timing alignment, data format conversion.Conclude the statement accurately reflects core functionality. Verification / Alternative check: Standard 74HC/74LS series data sheets (e.g., 74HC164, 74HC595) show storage capacity and shifting capability in block diagrams and truth tables. Why Other Options Are Wrong: Claiming “only transfer” ignores their inherent bit storage. Claiming “only storage” ignores the shift path and clocked movement of bits between stages. Common Pitfalls: Confusing shift registers with general RAM (random access); shift registers are sequential access in time and generally smaller, optimized for movement rather than arbitrary addressing. Final Answer: Correct

Q: J–K flip-flop excitation table: Evaluate the statement “An excitation table lists the present state, the next state, and the J and K levels required to produce each transition.” State whether this describes a standard excitation table correctly.

Introduction / Context: Excitation tables are design tools for determining input requirements that yield desired state transitions in sequential circuits. For the J–K flip-flop, the table specifies which J and K input combinations are needed to move from a current state to a target next state. This item checks your familiarity with that definition. Given Data / Assumptions: Present state (Q_n) and next state (Q_{n+1}) are considered. The goal is to compute required inputs (J, K) to cause the transition. J–K behavior: J=1,K=0→set; J=0,K=1→reset; J=0,K=0→no change; J=1,K=1→toggle. Concept / Approach: An excitation table inverses the usual truth table perspective. Instead of “given inputs, find next state,” it answers “given present and desired next state, what inputs are needed?” This is vital in designing sequential logic (e.g., counters or state machines) using J–K devices by mapping transitions to input equations. Step-by-Step Solution: List transitions: 0→0, 0→1, 1→0, 1→1.Map each to J,K: 0→0 requires J=0 (no set), K=X; 0→1 requires J=1, K=X; 1→0 requires J=X, K=1; 1→1 requires J=X, K=0 (where X means don’t care).Compile into an excitation table including present, next, and required inputs.Use table in logic minimization to derive J and K input equations. Verification / Alternative check: Compare with textbook excitation tables; results match standard definitions and are used in designing synchronous counters. Why Other Options Are Wrong: “Incorrect” contradicts the accepted structure. Options limiting the concept to T flip-flops or latches misunderstand the generality of excitation tables. Common Pitfalls: Confusing truth tables (inputs→next state) with excitation tables (transition→required inputs), and forgetting “don’t care” flexibility when minimizing logic. Final Answer: Correct

Q: Counter readout: Evaluate the statement “Counters are generally decoded in order to determine their count state.” Decide whether this describes typical practice for obtaining human- or logic-readable outputs.

Introduction / Context: Raw counter outputs are often in binary or Gray form and may not directly drive displays or control lines. Decoding maps counter states to meaningful outputs, such as segment drives for digits or enable lines for time-division multiplexing. This question asks whether decoding is a general step in using counters. Given Data / Assumptions: Counters produce multi-bit outputs representing state. Systems frequently need a specific “one-of-N” line active or a human-readable digit. Decoder ICs (e.g., 74xx138, 74xx154) or programmable logic often implement this mapping. Concept / Approach: Decoding means transforming the counter’s encoded state into a set of control or display signals. For example, a 4-bit binary count can be decoded to 10 active-low lines to drive digits 0–9, or to seven-segment patterns using a BCD-to-7-segment decoder. Decoding is not mandatory in every application but is common practice for interpreting the count state. Step-by-Step Solution: Identify counter encoding (binary, BCD, Gray).Select a suitable decoder (one-of-N, seven-segment, ROM/PLA) to map states.Generate outputs for displays, timing strobes, or control signals.Observe that most practical designs decode in some form to use the count meaningfully. Verification / Alternative check: Standard digital design texts show counters paired with decoders for timing sequences, scanning keyboards, or driving displays. Why Other Options Are Wrong: Limiting the statement to synchronous or ripple counters is unnecessary; both commonly use decoding. “Incorrect” ignores widespread usage patterns. Common Pitfalls: Overlooking hazards when decoding ripple-counter outputs (glitches) and forgetting to register decoded signals in high-speed contexts. Final Answer: Correct

Q: Compare counter types: Evaluate the statement “An asynchronous counter differs from a synchronous counter in the method of clocking.” Choose whether this distinction correctly captures their key difference.

Introduction / Context: Understanding how counters are clocked is crucial for predicting timing behavior, maximum frequency, and glitch susceptibility. This question asks whether the defining difference between asynchronous (ripple) and synchronous counters is indeed their method of clocking. Given Data / Assumptions: Asynchronous (ripple) counters: only the first flip-flop is clocked externally; subsequent stages are clocked by preceding outputs. Synchronous counters: all flip-flops receive the same external clock simultaneously. Other aspects (encoding, modulus) are independent of the clocking method. Concept / Approach: The clocking method drives the key behavioral differences: asynchronous counters exhibit ripple delays and potential decoding glitches; synchronous counters minimize skew between stages and support higher reliable speeds. Thus, “method of clocking” summarizes the core distinction accurately. Step-by-Step Solution: Define asynchronous counter clocking: cascaded clocking through Q outputs.Define synchronous counter clocking: parallel, common clock to all stages.Relate consequences: ripple delay vs. simultaneous updates.Conclude the statement is correct. Verification / Alternative check: Examine timing diagrams; asynchronous outputs change sequentially, synchronous outputs change together (after propagation delay from the same edge). Why Other Options Are Wrong: Tying the distinction to Johnson or BCD counters confuses code/modulus with clocking. “Incorrect” would deny a standard textbook definition. Common Pitfalls: Overlooking skew and routing in synchronous designs; although clocking is simultaneous in intent, physical skew must be controlled to maintain timing margins. Final Answer: Correct

Q: Using a J–K flip-flop in a design: Evaluate the statement “To determine the next outputs, we only need to consider the levels at J and K at the active clock edge.” Assess whether this captures the core edge-triggered behavior (ignoring asynchronous …

Introduction / Context: The J–K flip-flop is edge-triggered in most logic families, meaning the next state is determined by inputs sampled at a specific clock edge. This question evaluates whether considering J and K only at the active edge is sufficient to determine the next state under normal, synchronous operating conditions. Given Data / Assumptions: We are dealing with an edge-triggered J–K flip-flop. Asynchronous preset/clear, if present, are inactive. Setup and hold time requirements for J and K are met (no metastability). Concept / Approach: For an edge-triggered device, inputs are effectively sampled at the clock edge. The J–K characteristic table then determines the next state: J=0,K=0→no change; J=1,K=0→set; J=0,K=1→reset; J=1,K=1→toggle. Provided timing constraints are met, the values at that instant fully define the next output. Step-by-Step Solution: Ensure J and K are stable for at least t_setup before and t_hold after the active edge.At the active edge, sample J and K.Apply the J–K transition rules to compute the next state.Observe that values between edges (while the clock is stable) do not alter the stored state. Verification / Alternative check: Simulations and datasheets confirm that, absent violations and asynchronous actions, only edge-sampled values determine Q(n+1). Why Other Options Are Wrong: “Incorrect” would imply level-sensitive behavior, which contradicts edge-triggered operation. Limiting correctness to J=K or a specific edge polarity misunderstands the general rule. Common Pitfalls: Ignoring setup/hold, which can invalidate the assumption. Also, forgetting asynchronous set/clear can override edge-determined behavior if asserted. Final Answer: Correct

Question 1

Definition of a glitch in digital circuits: Assess the statement: “A glitch is a short, unintended pulse that can lead to an undesired result in a digital circuit due to logic hazards or timing.”

Accepted Answer

Introduction / Context: Glitches are abrupt, brief pulses appearing on digital signals when logic paths with different delays momentarily disagree. Even if the correct steady-state value is eventually reached, the transient pulse can trigger downstream elements, causing incorrect counts, spurious writes, or false interrupts. This question probes your understanding of what a glitch is and why it matters. Given Data / Assumptions: Digital gates have finite propagation delays. Multiple reconverging paths can create hazards (static or dynamic). Sequential circuits may respond to brief pulses if timing permits. Concept / Approach: When inputs transition, unequal path delays can produce a temporary output level that does not match the final logic evaluation. This momentary excursion is a glitch. Designers mitigate glitches by using synchronous sampling, hazard-free logic forms, proper gate balancing, or registering outputs before they drive sensitive destinations. Step-by-Step Solution: Recognize that hazards arise on reconvergent fanouts.Understand that unequal delays create brief pulses inconsistent with final logic.See that a flip-flop clock edge sampling a glitch may capture a wrong value.Therefore, a glitch is indeed a short undesired pulse that can cause errors. Verification / Alternative check: Waveform simulations show narrow spikes during input transitions; timing analysis identifies hazard conditions and recommends registering or re-timing. Why Other Options Are Wrong: Incorrect: Ignores the well-known definition of glitches.Only true for analog / only in asynchronous: Glitches occur in both asynchronous and synchronous systems; synchronous design merely reduces their impact via clocked boundaries. Common Pitfalls: Feeding combinational outputs directly into asynchronous controls; ignoring minimum pulse width specs of receiving circuits; assuming slow edges eliminate glitches. Final Answer: Correct

Question 2

Meaning of “synchronous” in digital systems: The statement says synchronous events do not occur at the same time. Decide whether “synchronous” actually implies coordination to a common clock or timing reference.

Accepted Answer

Introduction / Context: Digital systems favor synchronous design because it simplifies timing analysis, improves reliability, and eases scaling. “Synchronous” has a specific meaning related to a shared timing reference. This question tests whether you can correctly define the term. Given Data / Assumptions: A synchronous system uses a common clock or timing signal to coordinate state changes. Sequential elements sample inputs on defined clock edges or levels. Skew and jitter exist but are constrained by design to meet timing margins. Concept / Approach: “Synchronous” does not mean “not at the same time.” Instead, it means “occurring in lockstep with a reference,” typically a shared clock. While real hardware exhibits small skews, the design ensures that, logically, state transitions are treated as simultaneous at clock boundaries. Step-by-Step Solution: Define synchronous operation: all flip-flops update on clock edges.Acknowledge allowable skew, but within setup/hold margins it is equivalent to “same time.”Thus, the statement denying simultaneity contradicts standard usage.Conclude the statement is incorrect. Verification / Alternative check: Timing diagrams and STA (static timing analysis) model events as edge-aligned across the system clock domain, reflecting synchronous behavior. Why Other Options Are Wrong: Correct: Would invert the definition of synchronous.True only for multi-phase / Valid only in analog: These distract from the core concept that synchronization is to a reference, not “different times.” Common Pitfalls: Assuming asynchronous handshakes are equivalent to synchronous timing; ignoring clock domain crossing constraints where “synchronous” ceases to hold. Final Answer: Incorrect

Question 3

Counter modulus (MOD) definition: Evaluate the statement: “The modulus of a counter is the actual number of unique states in its counting sequence,” for example MOD-10 for a decade counter.

Accepted Answer

Introduction / Context: Modulus (often noted MOD or modulo) is a fundamental characteristic of counters. It tells you how many distinct states the counter cycles through before returning to its starting state. Recognizing modulus helps you design timing chains, frequency dividers, and numerical displays. Given Data / Assumptions: A counter produces a sequence of states (usually binary or coded). After a fixed number of states, the sequence repeats. The modulus equals the count of unique states per cycle. Concept / Approach: For a straight binary N-bit counter, the modulus is 2^N. For decade (BCD) counters, the modulus is 10 because the sequence resets after 10 states (0000 through 1001). Custom modulo counters use decoding to reset at a chosen terminal count, defining their modulus accordingly. Step-by-Step Solution: Consider an N-bit binary counter: states = 0 .. (2^N - 1).Number of states per cycle = 2^N → that is the modulus.For a MOD-10 BCD counter, states per cycle = 10 → modulus 10.Therefore, “modulus equals actual number of states” is correct. Verification / Alternative check: Datasheets and textbooks label counters as MOD-N to indicate division ratio and sequence length, matching observed behavior in timing diagrams. Why Other Options Are Wrong: Incorrect: Contradicts the formal definition used industry-wide.Only true for BCD / only for synchronous: Modulus applies equally to ripple, synchronous, and custom-coded counters. Common Pitfalls: Confusing modulus with maximum binary value; forgetting that illegal or skipped states in custom counters still define the modulus by the actual repeating sequence length. Final Answer: Correct

Question 4

Decade counters and BCD counters: Evaluate the claim “All decade counters are BCD counters.” Choose whether this statement is accurate in digital electronics, where a decade counter has 10 distinct states and a Binary-Coded Decimal (BCD) counter spe…

Accepted Answer

Introduction / Context: Decade counters and Binary-Coded Decimal (BCD) counters are related but not identical categories in digital electronics. A decade counter is any counter that sequences through ten distinct states before recycling, while a BCD counter is a specific decade counter that outputs the 8421 binary pattern for decimal digits 0 through 9. This question tests whether every decade counter must necessarily be a BCD counter. Given Data / Assumptions: A decade counter has 10 states per cycle. A BCD counter presents outputs corresponding to 0000 through 1001 (for 0–9) in 8421 coding. Other decade counters may use non-BCD state assignments (e.g., Johnson or ring-based sequences of length 10). Concept / Approach: All BCD counters are decade counters, but the reverse need not hold. A decade counter is defined by its modulus (10), whereas a BCD counter is defined by both modulus and the specific state encoding used (8421 for the 10 valid decimal digits). Non-BCD decade counters still count ten states but do not map their outputs to the standard BCD digit patterns. Step-by-Step Solution: Identify the definition of a decade counter: any mod-10 sequence.Identify the definition of a BCD counter: mod-10 sequence with states 0000–1001 only.Consider counter types such as Johnson counters that can be arranged with 10 states but produce non-BCD codes.Conclude that the statement “All decade counters are BCD counters” is not universally true. Verification / Alternative check: Inspect example datasheets/circuits: some decade counters specify “BCD outputs,” others specify “10-state sequence” without guaranteeing 8421 coding. Why Other Options Are Wrong: “Correct” conflates modulus with encoding. “True only for synchronous counters” and “True only for ripple counters” are irrelevant qualifiers; clocking style does not force BCD encoding. Common Pitfalls: Assuming “decade” implies BCD coding. In reality, decade only guarantees 10 states, not how those states are represented. Final Answer: Incorrect

Question 5

VHDL storage semantics: Evaluate the claim “When we want to remember a value in VHDL, it must be stored in a VARIABLE.” Decide whether this statement correctly reflects how synthesis infers storage (registers/latches).

Accepted Answer

Introduction / Context: Understanding how VHDL models storage is essential for writing synthesizable code. Designers often confuse VHDL variables with hardware registers. This item checks whether a remembered (stored) value must be implemented with a VARIABLE, or whether SIGNAL-based coding in clocked processes is the normal route for storage inference. Given Data / Assumptions: Synthesizable storage (flip-flops/latches) is typically inferred from SIGNAL assignments within processes using clock/enable conditions. VARIABLEs in synthesizable processes can be used for intermediate calculations but do not inherently imply hardware storage unless their usage and process structure force it. Asynchronous or synchronous resets, and edge detection, are commonly expressed around SIGNAL-based processes. Concept / Approach: In synthesizable VHDL, storage inference comes from the presence of clocked processes (if rising_edge(clk)) and signal assignment patterns that require state to be held between clock events. VARIABLES are convenient for temporary values in a process activation, while SIGNALs represent wires and registers across delta cycles and clock edges. Registers are created because of the sequential semantics and clock conditions, not because a VARIABLE is used. Step-by-Step Solution: Write a clocked process: if rising_edge(clk) then q <= d; end if;Synthesis infers a D flip-flop for SIGNAL q, even without any VARIABLES.Use a VARIABLE internally for combinational calculations; it will not by itself create a register unless code structure demands storage.Therefore, the statement claiming storage must be a VARIABLE is incorrect. Verification / Alternative check: Synthesis reports list inferred registers from SIGNAL assignments under clocked conditions, confirming that VARIABLES are not mandatory to “remember” values. Why Other Options Are Wrong: “Correct” reverses standard synthesis practice. “Applies only in testbenches” and “Applies only with shared variables” are misleading edge cases that do not define storage inference rules. Common Pitfalls: Confusing simulation convenience of VARIABLES with actual hardware registers; forgetting that registers arise from edge-guarded SIGNAL assignments and required state retention. Final Answer: Incorrect

Question 6

Counter terminology: Evaluate the statement “Synchronous counters are clocked so that each flip-flop in the counter is triggered at the same time.” Determine whether this definition correctly distinguishes synchronous operation.

Accepted Answer

Introduction / Context: Counters are fundamental sequential circuits. The key distinction between synchronous and asynchronous (ripple) counters lies in how the flip-flops receive their clocking. This question checks whether the provided definition accurately captures that difference. Given Data / Assumptions: A counter consists of a chain of flip-flops. In synchronous counters, every flip-flop receives the clock directly. In asynchronous (ripple) counters, only the first stage is clocked by the external clock; subsequent stages are driven by preceding outputs. Concept / Approach: Synchronous counters are designed so that all flip-flops sample simultaneously on the same clock edge. Combinational logic determines which stages toggle on that edge, but the edge itself reaches each flip-flop at the same time (subject to skew). This coordinated triggering minimizes ripple delays and enables higher operating frequencies compared to ripple counters. Step-by-Step Solution: Define synchronous: common clock input to all flip-flops.Define asynchronous: cascaded clocking through outputs of preceding stages.Evaluate statement: it claims “each flip-flop is triggered at the same time,” which matches the synchronous definition.Conclusion: the statement is correct. Verification / Alternative check: Timing diagrams of synchronous counters show simultaneous edge application, with output transitions aligned to that edge (after propagation delays). Ripple counters exhibit serial, staggered transitions. Why Other Options Are Wrong: “Incorrect” misstates the accepted definition. Restrictions to “BCD counters” or “ripple counters” add irrelevant qualifiers and confuse clocking style with count code. Common Pitfalls: Ignoring clock skew: while skew exists physically, the design intent remains simultaneous triggering. Also, conflating synchronous design with code encodings (binary, BCD, Gray) is a common misunderstanding. Final Answer: Correct

Question 7

Ripple counters: Evaluate the statement “A ripple counter is an asynchronous counter.” Select whether this description correctly characterizes ripple counter clocking.

Accepted Answer

Introduction / Context: Ripple counters are widely used simple counters where clocking propagates stage-by-stage. This question checks whether calling a ripple counter “asynchronous” is appropriate and why. Given Data / Assumptions: Ripple counter: first flip-flop is clocked by the external clock; subsequent flip-flops are clocked by outputs of the previous ones. Asynchronous counters do not clock all flip-flops simultaneously from a common clock. Propagation causes a “ripple” of state changes through the chain. Concept / Approach: The hallmark of asynchronous (ripple) counters is that each stage toggles slightly after the preceding stage due to propagation delay. This produces staggered output transitions and limits maximum frequency because later stages see delayed clock edges derived from earlier outputs rather than the system clock itself. Step-by-Step Solution: Identify the clocking method: only the first stage uses the external clock.Observe that subsequent stages use other stage outputs as their clocks.Recognize the ripple of transitions—defining asynchronous operation.Conclude the statement is correct. Verification / Alternative check: Timing diagrams reveal stair-step transition sequences. Frequency limits are often specified lower than synchronous designs due to accumulated delay. Why Other Options Are Wrong: “Incorrect” contradicts standard definitions. Options tying this to bit-width or code type are unrelated to clocking style. Common Pitfalls: Assuming ripple counters are always acceptable at high speed; their cumulative delay can create decoding glitches and timing hazards without additional filtering or synchronizing logic. Final Answer: Correct

Question 8

Shift registers in digital systems: Assess the statement “Shift registers are used to store and transfer data.” Decide whether this summarizes their primary roles (temporary storage and serial/parallel data movement).

Accepted Answer

Introduction / Context: Shift registers are chains of flip-flops used to move data in a controlled manner. They are ubiquitous in digital interfaces, conversion between serial and parallel formats, and simple temporary storage applications. This question checks whether stating they “store and transfer data” is accurate. Given Data / Assumptions: A shift register consists of cascaded D (or JK) flip-flops with a shared clock. Data can enter serially or in parallel and exit serially or in parallel depending on the architecture (SISO, SIPO, PISO, PIPO). Flip-flops inherently store 1 bit each; together they form an n-bit register. Concept / Approach: Shift registers provide short-term storage (n bits) and controlled transfer (shift) of data with each clock edge. They are essential for serial communication (e.g., UART shift stages), data alignment, delay lines, and interface bridging (parallel-to-serial and serial-to-parallel conversion). Step-by-Step Solution: Identify storage: each flip-flop holds a bit; an n-stage register holds n bits.Identify transfer: on each clock, bits move to the next stage (serial movement) or load/read in parallel when enabled.Recognize common uses: buffering, timing alignment, data format conversion.Conclude the statement accurately reflects core functionality. Verification / Alternative check: Standard 74HC/74LS series data sheets (e.g., 74HC164, 74HC595) show storage capacity and shifting capability in block diagrams and truth tables. Why Other Options Are Wrong: Claiming “only transfer” ignores their inherent bit storage. Claiming “only storage” ignores the shift path and clocked movement of bits between stages. Common Pitfalls: Confusing shift registers with general RAM (random access); shift registers are sequential access in time and generally smaller, optimized for movement rather than arbitrary addressing. Final Answer: Correct

Question 9

J–K flip-flop excitation table: Evaluate the statement “An excitation table lists the present state, the next state, and the J and K levels required to produce each transition.” State whether this describes a standard excitation table correctly.

Accepted Answer

Introduction / Context: Excitation tables are design tools for determining input requirements that yield desired state transitions in sequential circuits. For the J–K flip-flop, the table specifies which J and K input combinations are needed to move from a current state to a target next state. This item checks your familiarity with that definition. Given Data / Assumptions: Present state (Q_n) and next state (Q_{n+1}) are considered. The goal is to compute required inputs (J, K) to cause the transition. J–K behavior: J=1,K=0→set; J=0,K=1→reset; J=0,K=0→no change; J=1,K=1→toggle. Concept / Approach: An excitation table inverses the usual truth table perspective. Instead of “given inputs, find next state,” it answers “given present and desired next state, what inputs are needed?” This is vital in designing sequential logic (e.g., counters or state machines) using J–K devices by mapping transitions to input equations. Step-by-Step Solution: List transitions: 0→0, 0→1, 1→0, 1→1.Map each to J,K: 0→0 requires J=0 (no set), K=X; 0→1 requires J=1, K=X; 1→0 requires J=X, K=1; 1→1 requires J=X, K=0 (where X means don’t care).Compile into an excitation table including present, next, and required inputs.Use table in logic minimization to derive J and K input equations. Verification / Alternative check: Compare with textbook excitation tables; results match standard definitions and are used in designing synchronous counters. Why Other Options Are Wrong: “Incorrect” contradicts the accepted structure. Options limiting the concept to T flip-flops or latches misunderstand the generality of excitation tables. Common Pitfalls: Confusing truth tables (inputs→next state) with excitation tables (transition→required inputs), and forgetting “don’t care” flexibility when minimizing logic. Final Answer: Correct

Question 10

Counter readout: Evaluate the statement “Counters are generally decoded in order to determine their count state.” Decide whether this describes typical practice for obtaining human- or logic-readable outputs.

Accepted Answer

Introduction / Context: Raw counter outputs are often in binary or Gray form and may not directly drive displays or control lines. Decoding maps counter states to meaningful outputs, such as segment drives for digits or enable lines for time-division multiplexing. This question asks whether decoding is a general step in using counters. Given Data / Assumptions: Counters produce multi-bit outputs representing state. Systems frequently need a specific “one-of-N” line active or a human-readable digit. Decoder ICs (e.g., 74xx138, 74xx154) or programmable logic often implement this mapping. Concept / Approach: Decoding means transforming the counter’s encoded state into a set of control or display signals. For example, a 4-bit binary count can be decoded to 10 active-low lines to drive digits 0–9, or to seven-segment patterns using a BCD-to-7-segment decoder. Decoding is not mandatory in every application but is common practice for interpreting the count state. Step-by-Step Solution: Identify counter encoding (binary, BCD, Gray).Select a suitable decoder (one-of-N, seven-segment, ROM/PLA) to map states.Generate outputs for displays, timing strobes, or control signals.Observe that most practical designs decode in some form to use the count meaningfully. Verification / Alternative check: Standard digital design texts show counters paired with decoders for timing sequences, scanning keyboards, or driving displays. Why Other Options Are Wrong: Limiting the statement to synchronous or ripple counters is unnecessary; both commonly use decoding. “Incorrect” ignores widespread usage patterns. Common Pitfalls: Overlooking hazards when decoding ripple-counter outputs (glitches) and forgetting to register decoded signals in high-speed contexts. Final Answer: Correct

Question 11

Phototransistor behavior: Evaluate the claim “A phototransistor has a varying resistance from collector to emitter depending on how much light strikes it.” Decide whether this phrasing correctly describes the device operation.

Accepted Answer

Introduction / Context: Light-sensitive devices include photoresistors (LDRs), photodiodes, and phototransistors. Each has a distinct operating principle. This question probes whether describing a phototransistor as having a “varying resistance from collector to emitter” is an accurate statement of how it works. Given Data / Assumptions: A phototransistor is essentially a BJT where incident light generates base current. Collector current is proportional to light-induced base current times gain. Photoresistors (LDRs) are resistive sensors whose resistance changes with light intensity. Concept / Approach: A phototransistor is a current-controlled (actually light-controlled) device, not a variable resistor. Light generates carriers equivalent to base drive, causing a collector current that depends on illumination and transistor gain. While one could model the overall I–V behavior as an effective resistance in some operating region, that is not the correct nor standard description. The phrasing in the statement better matches an LDR, not a phototransistor. Step-by-Step Solution: Identify device class: phototransistor (BJT variant).Understand operation: light produces base current → increases collector current.Compare to LDR: resistance directly varies with light; this is a resistive element, not a transistor.Conclude statement is inaccurate for phototransistors. Verification / Alternative check: Datasheets specify collector current vs. irradiance, current gain, and V_CE characteristics—not “resistance” vs. light curves as a primary parameter, which are common for LDRs. Why Other Options Are Wrong: “Correct” confuses phototransistors with LDRs. “Correct only for LDRs” names a different device, so it does not validate the statement for phototransistors. “Correct only in saturation” still mischaracterizes the device’s fundamental control mechanism. Common Pitfalls: Treating all light sensors as interchangeable. Selecting a phototransistor when a linear resistive response is required can complicate interface design; conversely, using an LDR where fast response is needed is problematic. Final Answer: Incorrect

Question 12

Compare counter types: Evaluate the statement “An asynchronous counter differs from a synchronous counter in the method of clocking.” Choose whether this distinction correctly captures their key difference.

Accepted Answer

Introduction / Context: Understanding how counters are clocked is crucial for predicting timing behavior, maximum frequency, and glitch susceptibility. This question asks whether the defining difference between asynchronous (ripple) and synchronous counters is indeed their method of clocking. Given Data / Assumptions: Asynchronous (ripple) counters: only the first flip-flop is clocked externally; subsequent stages are clocked by preceding outputs. Synchronous counters: all flip-flops receive the same external clock simultaneously. Other aspects (encoding, modulus) are independent of the clocking method. Concept / Approach: The clocking method drives the key behavioral differences: asynchronous counters exhibit ripple delays and potential decoding glitches; synchronous counters minimize skew between stages and support higher reliable speeds. Thus, “method of clocking” summarizes the core distinction accurately. Step-by-Step Solution: Define asynchronous counter clocking: cascaded clocking through Q outputs.Define synchronous counter clocking: parallel, common clock to all stages.Relate consequences: ripple delay vs. simultaneous updates.Conclude the statement is correct. Verification / Alternative check: Examine timing diagrams; asynchronous outputs change sequentially, synchronous outputs change together (after propagation delay from the same edge). Why Other Options Are Wrong: Tying the distinction to Johnson or BCD counters confuses code/modulus with clocking. “Incorrect” would deny a standard textbook definition. Common Pitfalls: Overlooking skew and routing in synchronous designs; although clocking is simultaneous in intent, physical skew must be controlled to maintain timing margins. Final Answer: Correct

Question 13

Using a J–K flip-flop in a design: Evaluate the statement “To determine the next outputs, we only need to consider the levels at J and K at the active clock edge.” Assess whether this captures the core edge-triggered behavior (ignoring asynchronous …

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Introduction / Context: The J–K flip-flop is edge-triggered in most logic families, meaning the next state is determined by inputs sampled at a specific clock edge. This question evaluates whether considering J and K only at the active edge is sufficient to determine the next state under normal, synchronous operating conditions. Given Data / Assumptions: We are dealing with an edge-triggered J–K flip-flop. Asynchronous preset/clear, if present, are inactive. Setup and hold time requirements for J and K are met (no metastability). Concept / Approach: For an edge-triggered device, inputs are effectively sampled at the clock edge. The J–K characteristic table then determines the next state: J=0,K=0→no change; J=1,K=0→set; J=0,K=1→reset; J=1,K=1→toggle. Provided timing constraints are met, the values at that instant fully define the next output. Step-by-Step Solution: Ensure J and K are stable for at least t_setup before and t_hold after the active edge.At the active edge, sample J and K.Apply the J–K transition rules to compute the next state.Observe that values between edges (while the clock is stable) do not alter the stored state. Verification / Alternative check: Simulations and datasheets confirm that, absent violations and asynchronous actions, only edge-sampled values determine Q(n+1). Why Other Options Are Wrong: “Incorrect” would imply level-sensitive behavior, which contradicts edge-triggered operation. Limiting correctness to J=K or a specific edge polarity misunderstands the general rule. Common Pitfalls: Ignoring setup/hold, which can invalidate the assumption. Also, forgetting asynchronous set/clear can override edge-determined behavior if asserted. Final Answer: Correct

Question 14

Johnson counter MOD number: evaluate the claim “In a Johnson (twisted-ring) counter, the MOD number will always be equal to one-half the number of flip-flops in the counter.” Decide whether this statement is accurate, adding justification from count…

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Introduction / Context: A Johnson counter (also called a twisted-ring counter) is a common sequence generator used in digital electronics. Understanding how many unique states it produces for a given number of flip-flops is essential when selecting a counter for timing, sequencing, and divide-by-N applications. This item asks whether its MOD number equals one-half the number of flip-flops, which tests fundamental knowledge of counter state capacity. Given Data / Assumptions: We are dealing with an n-stage Johnson counter built from n edge-triggered flip-flops. The feedback is the inverted output from the last stage fed to the first stage (twisted-ring topology). All flip-flops are clocked synchronously by the same clock. Concept / Approach: For a conventional ring counter with n flip-flops and a single circulating 1, the MOD is n. A Johnson counter differs: it circulates a pattern of consecutive 1s followed by consecutive 0s created by feeding back the inverted last output. This structure produces 2n distinct states, not n, and certainly not n/2. Therefore, the MOD (the number of unique states before repetition) equals 2 * number_of_flipflops. Step-by-Step Solution: Let n = number of flip-flops.Johnson sequencing yields 2n valid codes (a run-length of 1s grows from 0 to n and then shrinks back to 0).Hence, MOD = 2 * n.The claim “MOD equals one-half the number of flip-flops” implies MOD = n/2, which contradicts the known result. Verification / Alternative check: Example: With n = 4, the Johnson counter generates 8 distinct states (0000 → 1000 → 1100 → 1110 → 1111 → 0111 → 0011 → 0001 → back to 0000), confirming MOD = 8 = 2n. Why Other Options Are Wrong: “Depends on propagation delay only” is irrelevant to the count of unique states. “Cannot be determined without the clock frequency” is incorrect because MOD depends on topology and n, not frequency. “Correct” contradicts the 2n property. Common Pitfalls: Confusing Johnson counters with ring counters; assuming MOD equals n; overlooking the role of the inverted feedback in doubling the count. Final Answer: Incorrect

Question 15

Synchronous counter behavior: evaluate the statement about clocking “In a synchronous (parallel-clocked) counter, each state transition is driven by the same clock pulse applied to every flip-flop.” Decide whether this statement holds in general pra…

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Introduction / Context: Counters come in two broad classes: asynchronous (ripple) and synchronous (parallel). The difference determines timing accuracy, maximum frequency, and ease of decoding. This question checks whether synchronous counters share a common clock for all internal flip-flops. Given Data / Assumptions: Synchronous counters are designed so that all flip-flops receive the same clock edge simultaneously. State transitions are determined by combinational logic feeding the flip-flop inputs. Propagation delays exist but are confined to the input logic paths, not a cascaded clock chain. Concept / Approach: In a synchronous counter, the defining feature is a common clock network to every stage. The next state is computed by combinational gates, often using enables or carry/borrow signals, but the actual switching of flip-flops occurs together on the same active clock edge. This reduces accumulated timing skew typical of ripple designs. Step-by-Step Solution: Identify counter type: synchronous.Check clocking scheme: one shared clock node to all flip-flops.Next-state logic prepares D, T, or J/K inputs prior to the edge.On the clock edge, all stages sample concurrently, producing a uniform state transition. Verification / Alternative check: Timing diagrams show simultaneous output updates at each clock edge (within flip-flop clock-to-Q delay), unlike ripple counters where edges propagate stage-by-stage. Why Other Options Are Wrong: “Incorrect” contradicts the defining characteristic. “Only true for ripple counters” reverses the truth. “Only true when using T flip-flops” is wrong; the property is independent of flip-flop flavor (D, T, or J-K) as long as they share the clock. Common Pitfalls: Confusing carry-enable chains (combinational) with clock chaining; assuming shared clock eliminates all skew (layout still matters). Final Answer: Correct

Question 16

Designing a divide-by-200 modulus counter using synchronous building blocks “Two 4-bit synchronous counters can be cascaded to make an 8-bit counter as a basis for a divide-by-200 design.” Evaluate this design statement.

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Introduction / Context: Divide-by-N counters are widely implemented using standard 4-bit synchronous counter ICs (or HDL modules) cascaded to expand the count range, then modified to reset at the desired terminal count. This question checks whether two 4-bit counters can serve as the foundation for a divide-by-200 solution. Given Data / Assumptions: Two 4-bit synchronous counters, when cascaded, form an 8-bit synchronous counter. An 8-bit counter provides a range of 0–255 (256 states). Divide-by-200 means the output period should be 200 input clock cycles. Concept / Approach: The approach is to use the 8-bit count to detect a terminal count (199 or 200, depending on design convention) and synchronously clear (or preset) back to zero. By decoding the desired count and feeding a synchronous reset, the sequence length becomes exactly 200, achieving a ÷200 ratio. Step-by-Step Solution: Cascade two 4-bit synchronous counters to obtain 8 bits: count range 0–255.Choose terminal count: typically reset when the count reaches 199 so the sequence length is 200 states (0 through 199).Design decode logic to assert a synchronous reset at count = 199 (1100 0111 for 199 decimal), or preset at an offset if preferred.Verify that all flip-flops share the same clock so reset occurs cleanly on the next edge. Verification / Alternative check: Simulation with a testbench counting cycles confirms that the output toggles or a pulse is produced every 200 clocks. Hardware implementations using common 74HC/SN74 synchronous counter families follow the same principle. Why Other Options Are Wrong: “Incorrect” ignores the standard modulus reduction technique. “Only correct for ripple counters” is backwards; synchronous counters are preferred. “Only correct if using gray-code counters” is unnecessary; binary counters suffice. Common Pitfalls: Using asynchronous clear mid-cycle causing glitches; decoding the wrong terminal count; not meeting setup/hold for the synchronous reset path. Final Answer: Correct

Question 17

Terminology check: up/down counters and bidirectional behavior “The term bidirectional is another way to describe up/down counters.” Assess this usage for correctness.

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Introduction / Context: Up/down counters can increment or decrement based on a control input. The vocabulary used in datasheets and textbooks often labels this capability as “bidirectional.” The question asks whether that synonym is appropriate. Given Data / Assumptions: Counters support two directions of counting: up (increment) and down (decrement). A mode or UP/ DOWN input selects the direction. Binary or BCD implementation details do not change the directional concept. Concept / Approach: “Bidirectional” simply means two directions of movement. For counters, it refers to the ability to move through the state space in ascending or descending numerical order on successive clocks. This is exactly what an up/down counter does; therefore, the terminology is accurate. Step-by-Step Solution: Identify the functional modes: count up vs. count down.Confirm that a dedicated control line selects between these modes.Conclude that the device inherently supports two directions, hence “bidirectional.” Verification / Alternative check: Review typical counter datasheets (e.g., 74HC191/193). They describe an UP/DOWN control and are often marketed as bidirectional counters. Why Other Options Are Wrong: “Incorrect” is contradicted by standard usage. “Only correct for synchronous designs” and “Only correct when using BCD counters” falsely restrict the concept; ripple or synchronous, binary or BCD, do not change the notion of direction. Common Pitfalls: Confusing bidirectional counting with bidirectional I/O pins; assuming bidirectional implies reversible data buses rather than count direction. Final Answer: Correct

Question 18

State diagram vs. state table: clarify the definition “A state diagram is a table of states.” Decide whether this statement correctly identifies the representation.

Accepted Answer

Introduction / Context: Finite-state machine (FSM) behavior can be represented in several ways, most commonly as a state diagram (graphical) or a state table (tabular). Distinguishing these forms is basic to digital design documentation and problem-solving. Given Data / Assumptions: State diagram: nodes (circles) represent states; directed edges represent transitions labeled by input/output conditions. State table: rows list present states, inputs, next states, and outputs in tabular form. Both describe the same machine but use different media. Concept / Approach: A diagram is not a table. A state diagram is a graph-based drawing; a state table is a matrix-style listing. The statement incorrectly equates the two. While they are equivalent descriptions in information content, they are distinct representations. Step-by-Step Solution: Identify the artifact: “state diagram.”Recall its elements: nodes and directed arcs with conditions.Contrast with “state table”: structured rows and columns enumerating transitions.Therefore, the statement is incorrect. Verification / Alternative check: Open any textbook’s FSM chapter: both are presented, with examples showing the same system rendered as a graph and as a table, explicitly contrasting the formats. Why Other Options Are Wrong: “Only correct for Mealy/Moore machines” is a red herring; the distinction between diagram and table is independent of machine type. Common Pitfalls: Assuming that because they are equivalent in content they are the same form; mixing symbols from diagrammatic notation into tables. Final Answer: Incorrect

Question 19

Asynchronous (ripple) counter timing: simultaneous switching? “In an asynchronous counter, all flip-flops change state at exactly the same time.” Evaluate this timing claim.

Accepted Answer

Introduction / Context: Asynchronous counters, also called ripple counters, are simple and popular for low-speed divide-by-N applications. However, their timing differs fundamentally from synchronous counters. This question probes whether all stages switch together in a ripple design. Given Data / Assumptions: Ripple counters chain the clock: one flip-flop’s Q output drives the next stage’s clock. Each flip-flop has a finite clock-to-Q propagation delay. Signals therefore propagate sequentially from LSB to MSB (or vice versa), not simultaneously. Concept / Approach: Because the clock is not common to all stages, transitions appear as a “ripple” through the chain. The LSB toggles first on the external clock edge; subsequent stages toggle only after receiving a (delayed) edge from the prior stage’s output. The visible effect is non-simultaneous switching and transient intermediate codes during transitions. Step-by-Step Solution: Identify architecture: asynchronous (ripple) clock chaining.Note finite t_CQ per stage → staggered toggles.Therefore, outputs do not change at exactly the same time.Conclusion: the statement is incorrect. Verification / Alternative check: Scope captures show a “stairstep” of output transitions for ripple counters. Data sheets warn about decoding glitches due to intermediate states during transitions. Why Other Options Are Wrong: “Correct” contradicts ripple behavior. Frequency does not make transitions simultaneous. Gray coding affects output codes, not clock distribution. Common Pitfalls: Using ripple counters in high-speed or glitch-sensitive decode paths; failing to register decoded outputs to remove hazards. Final Answer: Incorrect

Question 20

Definition of cascade in counter design “The term cascade means connecting the Q output of one flip-flop to the clock input of the next.” Determine whether this usage is acceptable in counter practice.

Accepted Answer

Introduction / Context: “Cascading” counters or flip-flops is common in building larger modulus dividers. The word can refer to several chaining methods, but a classic usage is in ripple counters where one stage’s Q drives the next stage’s clock input. This question checks the validity of that definition. Given Data / Assumptions: Ripple (asynchronous) counters often chain Q → CLK from stage to stage. Synchronous counters also cascade functionally but keep a common clock; their “cascade” uses carry/borrow or enable lines rather than Q → CLK. The statement focuses on one widely used cascading method. Concept / Approach: In many textbooks and datasheets, “cascade” in the specific context of ripple counters indeed means feeding the output of one flip-flop into the clock of the next to create divide-by-2, divide-by-4, etc. While the broader term can also describe synchronous expansion via carry chains, the stated definition is correct within the ripple-counter context. Step-by-Step Solution: Recognize ripple chaining: Q(n) drives CLK(n+1).This yields successive divide-by-2 stages (2^k overall).Thus, describing this as “cascading” is valid in standard practice. Verification / Alternative check: Examine application notes for classic TTL/CMOS counters (e.g., 74HC/T flip-flop chains); they use “cascade” terminology for Q-to-CLK connections in ripple configurations. Why Other Options Are Wrong: “Invalid” contradicts common usage. Restricting validity to synchronous or Johnson counters mischaracterizes the general term. Common Pitfalls: Assuming “cascade” only means ripple; forgetting that synchronous counters should share a common clock and use carry/enable for expansion. Final Answer: Valid

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