Question 1

Shift registers — PISO behavior explained: A parallel-in, serial-out (PISO) shift register loads all data bits simultaneously (in parallel) and then shifts them out one bit per clock. Decide whether this description is correct.

Accepted Answer

Introduction / Context: Shift registers bridge parallel and serial domains. The PISO variant is commonly used to serialize multiple input lines (e.g., microcontroller port expanders, keypad scanning, or sensor arrays) so that fewer wires are needed to communicate data across a link. Given Data / Assumptions: PISO has n parallel inputs D0…Dn-1. A load control captures the parallel inputs into internal flip-flops simultaneously. Subsequent clock pulses shift bits toward a single serial output. Concept / Approach: The essence of PISO is two-step operation: first, parallel load; second, serial shift. After loading, the internal register contents are shifted out one bit per clock, starting from a defined end (MSB-first or LSB-first per device). This enables efficient data transmission over a single wire while preserving the snapshot of multiple signals taken at the load instant. Step-by-Step Solution: Assert LOAD (or SH/LD) to capture the parallel inputs into the register.Release LOAD and apply clock pulses to shift the stored word toward Q_out.Observe one bit exiting on each clock edge until all bits are transmitted.Therefore, the description given is correct. Verification / Alternative check: Typical 74HC165 timing shows SH/LD controlling capture, then CLK moving data to the QH/QH’ outputs serially; logic analyzer traces confirm bit-by-bit serialization after the parallel snapshot. Why Other Options Are Wrong: Incorrect: Contradicts standard PISO operation. True only for Johnson counters: Johnson counters are a different structure. Valid only with synchronous load: Some devices use asynchronous load; the principle remains parallel load then serial shift. Depends on LSB being 1: Bit values do not affect the mechanism. Common Pitfalls: Confusing PISO with SIPO (which outputs in parallel); forgetting to latch inputs before shifting; mixing up MSB/LSB order when interfacing protocols. Final Answer: Correct

Question 2

Counter terminology — modulus vs maximum count value: For a digital counter, the modulus (mod-N) equals the number of unique states it cycles through (N). Is it correct to say the modulus is the same as the counter’s maximum count capacity N (with s…

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Introduction / Context: Students frequently mix up two related ideas: the modulus N (the number of distinct states) and the highest numeric code reached (which is N-1 for a simple up-counter starting at 0). Clarifying this relationship helps when designing divide-by-N frequency counters and decoding logic for terminal counts. Given Data / Assumptions: A mod-N counter cycles through N distinct states. For a natural binary up-counter, states typically enumerate 0, 1, …, N-1. On reaching N-1, the next clock wraps back to 0, completing the cycle. Concept / Approach: By definition, the modulus equals the period in states before the sequence repeats. Therefore a mod-10 counter repeats every 10 states (0 through 9). While the maximum numeric code observed is N-1, the counter’s “maximum count capacity” in terms of how many unique states it will visit per cycle is indeed N, which is its modulus. This definition is technology-independent and applies to synchronous and asynchronous counters alike. Step-by-Step Solution: Define modulus: number of unique states in one complete counting cycle.Example: mod-8 → states 0..7 → eight unique states.Maximum numeric code value = N-1 for up-counters starting at 0.Hence modulus N equals the counter’s capacity in unique states. Verification / Alternative check: Frequency division verifies the same: a mod-N counter divides the input clock by N (one output cycle per N input clocks), matching the definition that N is the number of states per cycle. Why Other Options Are Wrong: Incorrect: Confuses maximum numeric code (N-1) with the count of states (N). True only for synchronous counters: Applies equally to ripple and synchronous counters. Valid only when counting down: Direction does not change modulus. Depends on unused states being masked: Unused states are a design detail; modulus still counts the recurring legal states. Common Pitfalls: Equating the largest displayed number with modulus; forgetting that 0 is a valid state; assuming modulus depends on implementation type rather than the state machine cycle length. Final Answer: Correct

Question 3

Counter families — is a ripple counter asynchronous? Identify whether a ripple counter is, by definition, an asynchronous counter, considering how its stages are clocked.

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Introduction / Context: Terminology in digital design can be confusing early on. “Ripple counter” and “asynchronous counter” are commonly used interchangeably. This item checks whether you recognize that both names describe the same clocking approach and corresponding timing implications. Given Data / Assumptions: Ripple counters clock only the first stage from the external clock. Later stages are clocked by the output transitions of preceding flip-flops. Non-zero propagation delay exists for each stage. Concept / Approach: Because each stage waits for the previous stage to toggle before it sees an effective “clock,” a chain of delays propagates. This is asynchronous operation: not all flip-flops are triggered by the same global clock edge. The ripple of state changes causes skew and potential decoding glitches, placing an upper bound on speed relative to synchronous counters, which clock all stages simultaneously. Step-by-Step Solution: Define ripple counter: downstream stages clocked by upstream Q outputs.Identify asynchronous hallmark: no common simultaneous clock edge to all stages.Consequently, “ripple counter” = “asynchronous counter.”Therefore, the statement is correct. Verification / Alternative check: Standard texts label mod-N counters built from cascaded T or JK flip-flops (toggled by the previous stage) as asynchronous; timing diagrams show sequential rather than simultaneous transitions, confirming asynchronicity. Why Other Options Are Wrong: Incorrect: Contradicts conventional definitions. Only true for 4-bit designs: Bit-width is irrelevant. Valid at low frequencies only: Frequency affects reliability, not the definition. Depends on master clock duty cycle: Duty cycle does not change clock distribution topology. Common Pitfalls: Assuming “asynchronous” means “no clock at all”; ignoring hazards when decoding ripple counts at intermediate transitions. Final Answer: Correct

Question 4

Asynchronous vs synchronous counters — key difference: Does an asynchronous counter differ from a synchronous counter specifically in how its flip-flops are clocked? Choose the correct statement.

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Introduction / Context: Counter taxonomy centers on clock distribution. The distinction between synchronous and asynchronous counters informs timing, maximum frequency, and ease of decoding. This question asks you to pinpoint the primary differentiator between the two families. Given Data / Assumptions: Synchronous counters route one global clock to all stages. Asynchronous (ripple) counters propagate clock information from stage to stage. Flip-flop type (T, JK, D) can be used in either architecture. Concept / Approach: The essential difference is clocking methodology. Synchronous counters apply the clock edge simultaneously to every flip-flop, with combinational logic determining which stages toggle. Asynchronous counters rely on the previous stage’s output transition to clock the next stage, producing cascaded delays. This affects propagation timing and often dictates when decoding can safely occur without glitches. Step-by-Step Solution: Identify synchronous: all stages share a common clock input.Identify asynchronous: later stages clocked by earlier stage outputs.Thus, the counters differ specifically in clocking methods.Therefore, the statement is correct. Verification / Alternative check: Timing diagrams of binary up-counters show simultaneous transitions (synchronous) vs staggered transitions (asynchronous). Design notes link these to different maximum clock rates and hazard considerations. Why Other Options Are Wrong: Incorrect: The principal distinction is indeed clocking, not number base. True only for decimal counters / T flip-flops: Base and flip-flop choice are orthogonal to clock distribution. Depends on fan-out: Fan-out limits practical clock distribution but does not define the architecture. Common Pitfalls: Attributing differences to count direction or modulus; assuming one flip-flop family mandates one counter type. Final Answer: Correct

Question 5

Terminology — asynchronous counters are often called what? Fill in the blank: Asynchronous counters are often called ________ counters because clock transitions ripple through the stages.

Accepted Answer

Introduction / Context: Digital vocabulary matters. Multiple names for the same concept can hinder learning if their equivalence is not recognized. This question checks the common synonym for asynchronous counters used in textbooks and datasheets. Given Data / Assumptions: An asynchronous counter clocks its first stage from the external source; subsequent stages are clocked by upstream outputs. Clock information propagates, or “ripples,” through the chain. Non-zero propagation delay exists for each stage. Concept / Approach: The term “ripple counter” vividly describes the sequential toggling of stages caused by cascaded clocking. Because these toggles occur slightly later at each stage, the apparent clock edge ripples along the register, hence the name. Thus, “asynchronous counter” and “ripple counter” are interchangeable labels for the same structure. Step-by-Step Solution: Map architecture to behavior: asynchronous → ripple effect.Identify common synonym: “ripple counter.”Confirm usage across educational and vendor sources.Select the correct fill-in: ripple. Verification / Alternative check: Vendor datasheets and logic family guides (e.g., 74HC/74LS) explicitly label parts as “ripple-carry binary counters,” reinforcing the synonymy. Why Other Options Are Wrong: toggle: Describes a flip-flop action, not a counter class. binary: Many counters are binary, including synchronous ones; not distinctive. flip-flop: A building block, not the counter type. Johnson: A different counter topology (twisted ring), not synonymous with asynchronous. Common Pitfalls: Assuming “ripple” is a special mode rather than the core asynchronous mechanism; conflating counter base (binary/decimal) with clocking strategy. Final Answer: ripple

Question 6

Shift registers — which type can shift left or right? Fill in the blank: A _________ shift register can shift stored data either left or right under control, enabling bidirectional bit movement.

Accepted Answer

Introduction / Context: Shift registers are versatile building blocks. Some can shift in one direction only, while others support both left and right shifts. Recognizing which name corresponds to bidirectional capability helps when designing arithmetic shifters, serial interfaces, or data alignment logic. Given Data / Assumptions: Shift operation is clocked and direction-controlled. Register supports moving bits either toward MSB (left) or toward LSB (right). Parallel load may or may not be provided, depending on the specific device. Concept / Approach: A register that supports both directions is commonly called a “bidirectional shift register.” Some parts are called “universal shift registers,” which add parallel load and often three modes (shift left, shift right, parallel load). However, the minimal correct fill-in for left/right capability alone is “bidirectional.” Step-by-Step Solution: Identify required features: left and right shifting under control.Match terminology: “bidirectional shift register.”Note that “universal” usually implies additional parallel load.Choose the concise correct term: bidirectional. Verification / Alternative check: Datasheets for 74HC194/195 list modes including shift left/right and parallel load; literature commonly calls devices with two-direction shifting “bidirectional,” and those plus parallel load “universal.” Why Other Options Are Wrong: tri-state: Describes output enable behavior, not shift direction capability. universal: Often correct for parts that add parallel load, but the most direct fill-in for left/right shifting alone is “bidirectional.” bidirectional universal: Redundant phrasing not used as a standard category. ring: A different structure that circulates a single 1 or 0; not general left/right shifting. Common Pitfalls: Assuming “universal” is required terminology for any bidirectional register; overlooking that some devices offer only one shift direction. Final Answer: bidirectional

Question 7

Ring counters — correct initialization to start operation: To operate correctly, how must a ring counter be initialized at start-up? Select the statement that describes the required preset/clear pattern.

Accepted Answer

Introduction / Context: A ring counter is a shift register with its serial output fed back to the input, circulating a fixed pattern through the stages. For correct operation, it must begin with a valid one-hot state (exactly one flip-flop set and the rest cleared). This question checks whether you know the necessary start-up condition to avoid lock-up or all-zero states that would never generate a circulating 1. Given Data / Assumptions: n-stage ring counter implemented from edge-triggered flip-flops. Feedback path feeds Q_last back to the serial input of the first stage. No self-correcting bias unless additional logic is provided. Concept / Approach: In a simple ring counter, the only valid circulating pattern is a single 1 (or a single 0, depending on design) moving around the register. If all flip-flops are cleared, the pattern remains all zeros and will not evolve into a one-hot state. Therefore, the register must be initialized by presetting exactly one flip-flop and clearing all others, creating a one-hot word that will circulate with each clock pulse. Step-by-Step Solution: Identify required pattern: one-hot (1000…0) or equivalent.Apply asynchronous preset to one stage → sets its Q to 1.Apply asynchronous clear to all other stages → sets their outputs to 0.On each clock, the single 1 shifts to the next stage, producing a valid ring sequence. Verification / Alternative check: Timing diagrams show the single 1 marching through successive stages; simulation confirms that the all-zero state is absorbing without extra injection logic, requiring explicit initialization in hardware or during reset. Why Other Options Are Wrong: Clearing one and presetting the rest: Produces an (0111…1) state, not one-hot. Clearing all: Leaves an all-zero lock state that will not start the ring. Presetting all: Creates all ones; also not a one-hot state. Leaving floating: Unsafe and non-deterministic; may violate design rules. Common Pitfalls: Assuming rings self-start; forgetting to incorporate a reset network; confusing ring counters with Johnson counters, which can self-correct under some feedback schemes. Final Answer: Presetting one flip-flop and clearing all the others

Question 8

Shift registers used as counters — to make a shift register function as a circulating counter (e.g., ring or Johnson type), how should its connections be arranged between serial output and serial input?

Accepted Answer

Introduction / Context: A shift register can be repurposed as a counter by recirculating data through its stages. This question asks what connection enables a shift register to generate a repeating sequence of states, as used in ring and Johnson counters for timing and sequence generation. Given Data / Assumptions: A multi-bit shift register with clocked shifting. Desire to obtain a repetitive count/sequence of outputs. Proper initial seeding (e.g., one-hot or suitable pattern) is available. Concept / Approach: In a ring counter, a single 1 shifts through the register stages. In a Johnson (twisted-ring) counter, the inverted serial output is fed back to the serial input to create a 2N-state sequence for an N-bit register. In both cases, the essential idea is feedback from the output end of the shift register back to its input, which circulates a pattern deterministically with each clock pulse. Step-by-Step Solution: Identify shift operation → data moves one stage per clock.Choose a seed → ring: 1000... ; Johnson: 0000 with complemented feedback.Apply feedback → connect serial out to serial in (optionally inverted for Johnson).Observe sequence → pattern repeats cyclically, acting as a counter/timer. Verification / Alternative check: Examine the state diagram: with feedback, the register cycles through a fixed sequence length (N for ring, 2N for Johnson) before returning to the seed, which is the hallmark of a counter-like behavior. Why Other Options Are Wrong: Use serial in as counter input and serial out as output: Without feedback, no closed cycle is produced. Parallel inputs as clocked sources with serial output: Misuses the parallel load function; does not create a cyclic sequence. Only SISO without feedback: A SISO shifts data out and loses it; no counting loop forms. Common Pitfalls: Forgetting to seed the register (all-zeros ring locks up), or omitting the inversion required for Johnson counters. Also, confusing shift-direction conventions can flip left/right notions but the feedback principle remains the same. Final Answer: Connect the serial output back to the serial input (with proper seeding/inversion as required).

Question 9

Synchronous counters — using synchronous construction, the overall propagation delay can be reduced to (approximately) the delay of what element(s)?

Accepted Answer

Introduction / Context: Counter speed is limited by cumulative propagation delay. Ripple (asynchronous) counters suffer from stage-by-stage delays. Synchronous counters clock all flip-flops simultaneously and use combinational gating to determine the next state, greatly reducing the worst-case timing path. This item asks you to identify what the effective delay reduces to in synchronous design. Given Data / Assumptions: Synchronous counter architecture. All storage elements share the same clock edge. Next-state logic (gates) drives the flip-flop inputs. Concept / Approach: In synchronous counters, the critical path typically goes through a small amount of combinational logic (for toggling conditions) into the D/T/J-K inputs of a flip-flop, then through the flip-flop itself to the outputs. Thus, the worst-case delay resembles the sum of one gating network and one flip-flop propagation delay rather than an accumulative chain of multiple flip-flops as in ripple designs. Step-by-Step Solution: All flip-flops clocked together → no serial flip-flop delays accumulate.Next-state logic determines which flip-flops toggle.Critical path ≈ gate delay + flip-flop delay.Therefore, overall delay ≈ one flip-flop plus one gate network. Verification / Alternative check: Timing diagrams show outputs settling after one clock edge by an amount governed by the maximum input-combinational delay plus flip-flop Q delay, confirming the characterization. Why Other Options Are Wrong: All flip-flops and gates: Describes ripple behavior, not synchronous. All flip-flops and gates after a 3-count: Arbitrary restriction; synchronous delay does not depend on the count value like this. A single gate only: Ignores flip-flop propagation delay which always exists. Common Pitfalls: Assuming synchronous counters are “instantaneous.” Clock-to-Q and gate delays remain; they are simply not chained across stages. Final Answer: The delay is approximately that of one gating path plus one flip-flop.

Question 10

Counter taxonomy — ring and Johnson counters belong to which general class of counters when implemented with a common clock?

Accepted Answer

Introduction / Context: Ring and Johnson (twisted-ring) counters are widely used to generate timing sequences and nonbinary state patterns. They are constructed from shift registers with feedback and clocked synchronously. This question asks which class they belong to under standard digital design classifications. Given Data / Assumptions: Use of a shared clock for all stages. Feedback from serial output to serial input (inverted for Johnson). Focus on implementation style, not count radix. Concept / Approach: A counter is “synchronous” when all storage elements receive the same clock edge, and state changes occur simultaneously rather than rippling through successive devices. Ring and Johnson counters are built from synchronously clocked flip-flops that shift their content on each clock pulse; therefore, they are synchronous counters. They are not “true binary” counters because their state sequences are not binary-encoded numeric progressions; instead, they are one-hot (ring) or two-hot/overlapping (Johnson) patterns. Step-by-Step Solution: Identify clocking: all flip-flops share one clock → synchronous behavior.Check sequence: ring one-hot cycles; Johnson 2N-state pattern for N bits.Confirm category: synchronous counters (not ripple, not binary-coded numeric).Select “Synchronous counters.” Verification / Alternative check: Timing analysis shows no clock-to-clock ripple across flip-flops; outputs change together at the active edge, matching the synchronous definition. Why Other Options Are Wrong: Asynchronous counters: imply ripple propagation, which ring/Johnson do not use. True binary counters: those count in binary coding (000... to 111...); ring/Johnson do not. Both asynchronous and true binary: incorrect on both counts. Common Pitfalls: Equating “counter” with “binary counter”; many counters are sequence generators rather than pure binary incrementers. Final Answer: Synchronous counters.

Question 11

Pulse generation — which digital counter type is commonly used to generate a sequence of equally spaced timing pulses (one pulse per stage in a repeating pattern)?

Accepted Answer

Introduction / Context: Timing circuits often need a train of equally spaced pulses distributed across multiple outputs. Certain counter structures naturally produce such patterns, simplifying timing generation without decoding logic. This question asks which counter is especially suited for this purpose. Given Data / Assumptions: Multiple outputs required, each pulsing in turn. Simple, predictable, nonbinary state sequence desired. Clock provides uniform step timing. Concept / Approach: A ring counter circulates a single logic 1 through N flip-flops (for an N-stage register). Each stage output goes high exactly once per N clocks, producing N equally spaced pulses over one full cycle. This “one-hot” property makes the ring counter ideal for sequential timing (stepper control, multiplexing, strobes) with minimal decoding. Johnson counters also yield evenly distributed transitions but create two-hot patterns and 2N states, not single-pulse-per-stage timing by default. Binary counters require decoding to create isolated pulses for each count. Step-by-Step Solution: Pick desired output behavior → one pulse per stage per cycle.Map to structure → ring counter produces one-hot circulating pattern.Confirm equal spacing → each stage advances on every clock, repeating every N clocks.Therefore select “Ring counter.” Verification / Alternative check: Simulate a 4-stage ring: outputs Q0, Q1, Q2, Q3 each go high in turn every four clocks; pulses are evenly spaced in time. Why Other Options Are Wrong: Johnson counter: Generates two-hot sequences; not a single-pulse output per stage without extra logic. Binary up counter: Needs decoding to derive per-stage pulses. Ripple counter: Describes asynchronous clocking, not the specific output pattern needed. Common Pitfalls: Confusing “equally spaced pulses” at one output with “one pulse per stage.” A ring counter inherently gives the latter without extra decoding. Final Answer: Ring counter.

Question 12

Cascading counters — when two modulus-N counters are cascaded, the overall modulus (mod number) of the combined counter equals the ______ of the individual mod numbers.

Accepted Answer

Introduction / Context: Complex counting ranges are achieved by cascading counters. Understanding how the combined modulus relates to the individual moduli is fundamental for designing frequency dividers, timers, and sequence generators. This question asks for that relationship. Given Data / Assumptions: Two counters connected in cascade (output of one drives the count-enabling or clock of the next stage appropriately). Individual moduli are M and N. Each counter cycles through its full count before advancing the next stage appropriately. Concept / Approach: In a properly cascaded arrangement, the first counter advances through M states; for each full M-cycle, it increments the second counter by one state, which cycles through N states. The total unique state combinations are M * N, so the overall modulus is the product of the individual moduli. Step-by-Step Solution: Let Counter A be mod M and Counter B be mod N.For every M clocks, Counter A wraps and triggers Counter B to advance by one.Counter B requires N such wraps to return to zero.Total counts per overall cycle = M * N → overall modulus = product. Verification / Alternative check: Example: Cascading mod-10 and mod-6 yields mod-60 division, common in clock/timer designs (seconds 0–59). Why Other Options Are Wrong: Sum: Would imply mod (M + N), not consistent with state space combinations. Logarithm/Reciprocal: Not meaningful for counting modulus in digital logic. Common Pitfalls: Incorrect gating between stages or misinterpreting “ripple carry” signals can break the ideal product relationship; proper cascade wiring restores the M * N behavior. Final Answer: Product.

Question 13

Terminology — a mod-10 counter (counts 0 through 9 repeatedly) is commonly referred to as a ______ counter.

Accepted Answer

Introduction / Context: Digital counters are often named by their modulus and by conventional terms. A counter that cycles through ten states (0–9) is encountered in BCD decade divisions and timekeeping. This question asks for the standard common name used for a modulus-10 counter. Given Data / Assumptions: Counter cycles through 10 distinct states per full period. Typical application: BCD digit counting for displays or stopwatch seconds. We are selecting the widely accepted generic term. Concept / Approach: The conventional name for a mod-10 counter is “decade counter.” While many decade counters are indeed used to produce BCD digits (0–9), the umbrella term “decade” describes the modulus itself without implying a specific internal encoding or topology. Other named counters (ring, Johnson) describe different sequence structures and are not inherently mod-10 unless specially configured. Step-by-Step Solution: Identify modulus → 10 states: 0 to 9.Map to common terminology → “decade counter.”Note usage → frequently employed in BCD chains for numeric displays.Therefore choose “decade.” Verification / Alternative check: Datasheets for classic TTL/CMOS devices (e.g., 74xx90/160/162 families, CD4017) routinely label mod-10 devices as decade counters. Why Other Options Are Wrong: Strobing: Refers to timing technique, not a modulus name. Ring: One-hot circulating register; not inherently mod-10. Johnson: Twisted-ring with 2N states; not inherently mod-10. Common Pitfalls: Assuming “BCD” as a universal synonym; while many decade counters produce BCD, “decade” is the standard generic term for modulus 10. Final Answer: Decade.

Question 14

PISO (parallel-in serial-out) behavior — a 4-bit PISO shift register loads 4 bits in parallel; on each active clock edge it shifts the data to the ______ by ______ position(s) per clock.

Accepted Answer

Introduction / Context: Shift registers convert between parallel and serial forms of data. A PISO register loads a multi-bit word and then shifts it out serially. Designers must know the shift direction and how many bit positions change per clock to interface correctly with serializers and communication lines. This question targets that understanding. Given Data / Assumptions: 4-bit PISO register with typical right-shift convention (MSB first). One active clock edge causes a single shift event. No bidirectional or dual-edge special modes are assumed. Concept / Approach: Most PISO ICs are designed to shift one position per clock edge, moving the stored bits toward the serial output. The default convention in many families is right shifting (moving MSB toward the serial-out pin). Although some devices can be configured for left shift, the standard reference case is one position per clock to the right, serializing one bit at a time. Step-by-Step Solution: Parallel-load 4 bits into Q3..Q0.On each clock, Q3→Q2, Q2→Q1, Q1→Q0, and Q0→SerialOut.Movement is one position per clock in the right-shift sense.Thus, “right, one” matches the typical operation. Verification / Alternative check: Reference timing diagrams of standard parts (e.g., 74HC165) confirm one-bit-per-clock serialization with a right-shift convention. Functional truth tables illustrate the movement clearly. Why Other Options Are Wrong: Right, two / Left, one / Left, three: Do not reflect the standard single-position-per-clock behavior and/or shift direction of common PISO devices. Common Pitfalls: Confusing PISO with SIPO (serial-in parallel-out) parts; mixing up MSB-first vs LSB-first conventions between different device families or protocols. Final Answer: Right, one.

Sequential Logic Circuits Questions