Question 1

Why is Binary Coded Decimal (BCD) commonly used between computers and people? Pick the primary reason BCD is chosen in human-facing digital systems.

Accepted Answer

Introduction / Context: BCD encodes each decimal digit (0–9) as its own 4-bit binary pattern, making it ideal for interfaces where humans read or enter decimal numbers. This question clarifies the real motivation for BCD, countering common misconceptions about efficiency. Given Data / Assumptions: BCD maps exactly one decimal digit per 4-bit nibble. Human interfaces (keypads, seven-segment displays, printers) expect decimal digits. Arithmetic speed and storage efficiency are not BCD’s strengths. Concept / Approach: BCD simplifies the conversion between internal representation and human-readable decimal digits. It enables easy driving of decimal displays via simple decoders and straightforward acceptance of digit-wise input, avoiding complex binary-to-decimal conversions in real time. Step-by-Step Solution: Identify interface need: humans use base-10.Match representation: BCD represents each digit directly, simplifying decoding/encoding.Acknowledge trade-offs: BCD uses more bits than binary and arithmetic can be slower.Thus, the primary reason is user-facing alignment, not arithmetic efficiency. Verification / Alternative check: Consider a calculator display driver or seven-segment interface: BCD-to-7-segment ICs accept BCD directly. Why Other Options Are Wrong: It is too easy arithmetically: BCD arithmetic is typically more complex than binary.Minimizes storage: BCD is less space-efficient than pure binary.Maximizes speed: Binary arithmetic in ALUs is generally faster. Common Pitfalls: Assuming “ease of use by people” translates to “ease of arithmetic by machines”.Overlooking conversion/driver simplicity provided by BCD. Final Answer: It aligns directly with decimal digits for display and entry

Question 2

Monitoring eight sources with minimal MCU lines — how many input lines are needed using an 8-to-3 priority encoder? Assume standard enable logic is handled separately; focus on the data lines into the microcontroller.

Accepted Answer

Introduction / Context: Microcontrollers often have limited I/O. Encoders allow multiple sources to be represented as a smaller set of lines. An 8-to-3 priority encoder outputs a 3-bit code corresponding to the highest-priority active input, reducing the required MCU input pins. Given Data / Assumptions: There are eight distinct sources to monitor. An 8-to-3 priority encoder is used. Enable, valid, or interrupt lines (if any) are outside the “data bits” count for this question. Concept / Approach: An 8-to-3 encoder maps 8 inputs to a 3-bit binary index (since 2^3 = 8). With priority, if multiple inputs assert simultaneously, the highest-priority one determines the output code. The MCU reads the 3-bit value to know which source is active, conserving pins. Step-by-Step Solution: Determine bit width: log2(8) = 3 bits.Priority encoder provides these 3 bits on its outputs.MCU therefore needs 3 input lines to read the code.Additional housekeeping signals are optional and not counted here. Verification / Alternative check: Check typical parts (e.g., 74HC148): 8 inputs, 3 outputs representing the active input index. Why Other Options Are Wrong: 8: No encoder would be needed if you spent eight lines.4: More than required; 3 bits already cover 8 states.2: Insufficient to represent 8 distinct states. Common Pitfalls: Forgetting that additional lines (enable, GS, EO) do not change the core 3-bit code width. Final Answer: 3

Question 3

74154 characteristics — which statement is correct? Select the accurate description of the 74154 decoding behavior and interface.

Accepted Answer

Introduction / Context: The 74154 is a classic TTL part used for address decoding and selection tasks. Knowing its polarity and fan-out is critical when integrating with memory maps or enabling one of many devices. Given Data / Assumptions: 74154 has two enable inputs and 16 outputs. Outputs are active-LOW, one at a time, corresponding to the 4-bit input. It is a decoder/demultiplexer, not an encoder or display driver. Concept / Approach: A 4-bit binary input selects exactly one of 16 outputs to go LOW, with all others remaining HIGH. This is the definition of a 1-of-16 active-LOW decoder and is the key feature used in chip-select trees, memory decoding, or resource selection. Step-by-Step Solution: Recognize device function: decode 4-bit value → 1-of-16 line active.Note polarity: outputs are active-LOW (LOW = selected).Verify enablers: enable pins must be correctly asserted for outputs to respond.Therefore, statement A is correct. Verification / Alternative check: Typical datasheets label outputs Y0–Y15, active-LOW, with two enables (often one active-LOW and one active-HIGH). Why Other Options Are Wrong: 1-of-8, 3-bit, active-HIGH: That describes a different part (e.g., 74138 family).BCD to seven-segment: Requires a dedicated decoder/driver (e.g., 7447/4511), not 74154.Priority encoder: Opposite function; encoders compress inputs into codes. Common Pitfalls: Confusing 74154 with 74138 or display decoder drivers. Final Answer: It is a 1-of-16 decoder that accepts a 4-bit binary input and provides 16 active-LOW outputs

Question 4

Priority encoder behavior — what appears at the outputs when multiple inputs may be active? Choose the best description of the output code for a priority encoder.

Accepted Answer

Introduction / Context: Encoders compress multiple input lines into a smaller set of output lines. Priority encoders add arbitration: when more than one input is active, they choose the one with the highest priority and report its index as a binary code. Given Data / Assumptions: Multiple inputs may assert simultaneously. A static priority is predefined (e.g., input 7 over input 6, etc.). Only one encoded value is presented at a time. Concept / Approach: The function of a priority encoder is to output the binary number that identifies the highest-priority active input. Some devices also assert a “valid” output to indicate that at least one input is active, and may provide enable outputs for cascading. Step-by-Step Solution: Detect active inputs.Resolve conflicts via fixed priority order.Encode the index of the highest-priority active line.Drive outputs with that code; optionally assert “valid”. Verification / Alternative check: Check a part like 74HC148: output equals the index of the highest-numbered active input (with active-LOW conventions handled). Why Other Options Are Wrong: Lowest-priority: Opposite of specification.Population count: That is a different function (counting ones).Parity: Encoders do not compute parity unless explicitly designed to. Common Pitfalls: Forgetting about active-LOW input/output conventions and enable pins. Final Answer: Binary code corresponding to the highest-priority asserted input

Question 5

Output polarity decision in code-converter/decoder design — what must be decided up front? Identify the critical early choice about output active level that impacts device selection and wiring.

Accepted Answer

Introduction / Context: In designing decoders, encoders, and code converters, you must harmonize signal polarities across the system. Choosing the wrong active level can invert logic, break enables, or force unnecessary inversion stages late in the design. Given Data / Assumptions: Many standard logic parts offer active-LOW outputs (common in decoder ICs). Some subsystems expect active-HIGH enables; others expect active-LOW chip selects. Consistency reduces extra hardware (inverters) and confusion. Concept / Approach: Decide early if your design will use active-HIGH or active-LOW outputs. This governs part selection (e.g., 74154 active-LOW outputs) and how downstream loads interpret signals. Aligning polarities from the outset simplifies logic equations and minimizes glue logic. Step-by-Step Solution: Inventory subsystem expectations: which lines should assert HIGH vs. LOW.Select devices whose output polarity matches those expectations.Update truth tables and timing with the chosen polarity.Avoid late-stage fixes by planning polarity at the schematic stage. Verification / Alternative check: Cross-check with datasheets: many decoders and muxes explicitly state active-LOW outputs or enables. Why Other Options Are Wrong: Schmitt-trigger inputs: Useful for noisy signals but not the first system-wide decision.TTL vs. CMOS levels: Important, but polarity mismatches cause more pervasive logic errors.PCB color: Irrelevant to signal behavior. Common Pitfalls: Mixing active-HIGH and active-LOW conventions without clear documentation.Forgetting that some outputs default to active-LOW with inversion bubbles in schematics. Final Answer: Whether outputs are active-HIGH or active-LOW

Question 6

Digital magnitude comparator fundamentals: Evaluate the statement — “A basic binary comparator evaluates two multi-bit binary strings bit by bit and always outputs a logic 1.”

Accepted Answer

Introduction / Context: Binary magnitude comparators are widely used building blocks in digital systems for sorting, decision making, and address decoding. They compare two binary numbers and generate outputs that indicate the relationship between them, typically A > B, A = B, or A < B. This question checks whether you understand that a comparator does not “always output a 1,” but instead produces relationship-specific signals. Given Data / Assumptions: Two binary inputs (labeled A and B) of equal width are applied. The device is a standard digital magnitude comparator. Outputs are conventional comparator flags (A>B), (A=B), and (A Concept / Approach: A comparator determines relative magnitude by examining bits from the most significant bit downward. Once a definitive difference is found, lower-order bits no longer affect the outcome. The device asserts one of several indicator outputs, not a single line that is perpetually high. Therefore, “always outputs a 1” is a misunderstanding of comparator behavior. Step-by-Step Solution: Identify standard outputs: typically three lines show A>B, A=B, AUnderstand decision order: compare MSB first; if equal, proceed to the next bit.Recognize exclusivity: for a given pair, at most one indicator is active at a time (sometimes multiple are encoded).Conclude: there is no single output that is always 1; the statement is incorrect. Verification / Alternative check: Review a common device (e.g., a 7485-style comparator). When A = 0101 and B = 0110, the (A Why Other Options Are Wrong: Correct: Contradicts actual functionality; comparators do not keep one line constantly high. Outputs 1 only when A = B / only when A > B: Too specific; comparators have multiple relation outputs. Tri-states its output for unequal inputs: Standard comparators use active logic outputs, not tri-state by default. Common Pitfalls: Confusing comparators with simple equality detectors or with encoders; assuming a single “valid” line is always high; ignoring MSB-first decision logic. Final Answer: Incorrect

Question 7

Demultiplexer (DEMUX) function clarification: Evaluate the statement — “A demultiplexer accepts a dual (two-line) data input value and routes it to one of several outputs.”

Accepted Answer

Introduction / Context: Demultiplexers are essential for distributing a single data stream to one of many destinations under control of select lines. Confusing DEMUX behavior with multiplexers (which select one of many inputs) is a common beginner error. This item tests whether you can articulate the DEMUX direction of data flow correctly. Given Data / Assumptions: A DEMUX has one data input, multiple outputs, and select lines. Select inputs determine which single output line receives the input data. Nonselected outputs are typically forced low, high, or left inactive per implementation. Concept / Approach: By definition, a 1-to-2^n DEMUX takes one data input (not “dual”) and, based on n select lines, routes that input to exactly one of the 2^n outputs. The statement claiming a “dual input data value” incorrectly describes the DEMUX’s data interface; dual inputs relate more to differential signaling or to a 2-to-1 MUX, not to a DEMUX. Step-by-Step Solution: Define DEMUX: one data in → one of many outputs selected by control lines.Check the claim: “dual input” contradicts the standard definition.Therefore, the statement is incorrect. Verification / Alternative check: Consult a truth table for a 1-to-8 DEMUX. For each select combination, exactly one Yk mirrors the single data input D; all others are inactive. There is no “dual” data input. Why Other Options Are Wrong: Correct: Not correct because the DEMUX does not require dual data inputs. Correct only for 1-to-2 devices: Even 1-to-2 DEMUXs still have one data input, not two. Depends on the number of select lines: Select lines affect fan-out, not number of data inputs. True for time-division multiplexing only: TDM is a system-level concept, unrelated to the single-input DEMUX definition. Common Pitfalls: Mixing up MUX and DEMUX; assuming more outputs implies more data inputs; overlooking that select lines carry address information, not payload data. Final Answer: Incorrect

Question 8

Multiplexer control principle: Evaluate the statement — “In a multiplexer (MUX), the data-select control inputs determine which data input is forwarded to the single output line.”

Accepted Answer

Introduction / Context: A multiplexer (MUX) is a fundamental component that funnels one of many inputs to a single output. Understanding that the select lines act like a binary address to choose the active input is essential for designing bus interfaces, data acquisition systems, and communication paths. Given Data / Assumptions: Digital MUX with N inputs, one output, and log2(N) select lines. An enable pin may gate the device but does not change the core selection rule. All inputs are compatible with the MUX’s logic family. Concept / Approach: For a 2^n-to-1 MUX, the n select lines encode a number from 0 to 2^n - 1. That code chooses the corresponding input channel. This behavior holds across digital and analog multiplexers; enable simply allows or disables the output drive. Step-by-Step Solution: Interpret select lines as a binary index.Map the index to the chosen input (e.g., S = 011 selects I3).Output reflects the chosen input as long as the device is enabled.Therefore, the statement is correct. Verification / Alternative check: Examine a timing diagram for a 4-to-1 MUX. Changing select S1 S0 from 00 to 01 immediately routes I1 to the output. Oscilloscope measurements confirm the selection behavior. Why Other Options Are Wrong: Incorrect: Contradicts the fundamental purpose of a MUX. Only true for analog multiplexers: Digital MUXs work the same way regarding selection. Only true when enable is LOW: Enable polarity varies by device; selection logic is independent. Parity-protected inputs: Parity is unrelated to selection. Common Pitfalls: Confusing enable with selection; miswiring select order (endianness); ignoring MUX propagation delay and bandwidth limits. Final Answer: Correct

Question 9

Display system buses in multiplexed readouts: Evaluate the statement — “The digit bus and the display (segment) bus are common sets of conductors shared by the digit storage registers and the seven-segment display segments, respectively.”

Accepted Answer

Introduction / Context: Multiplexed display systems reduce wiring and driver count by sharing signals among digits. Two common buses exist: a digit-select bus (activating which digit is currently lit) and a segment (display) bus that carries segment data a–g (and possibly DP). Understanding these shared conductors is vital for designing efficient user interfaces on embedded systems. Given Data / Assumptions: Seven-segment LED or LCD digits are multiplexed. Digit-select lines enable one digit at a time. Segment lines are shared across all digits. Concept / Approach: In time-multiplexing, the controller presents segment data on a common segment bus while enabling exactly one digit through the digit bus (digit enables). Rapid scanning creates the illusion that all digits are lit simultaneously. Both sets of lines are shared: the digit bus distributes enables; the segment bus carries the segment pattern. Step-by-Step Solution: Identify two shared groups: digit enables and segment signals.During a scan slot, assert one digit enable on the digit bus.Drive required segment levels on the segment bus for that digit’s value.Repeat for all digits at a high refresh rate. Verification / Alternative check: Observe with an oscilloscope or logic analyzer: digit lines show periodic enables; segment lines show patterns synchronized with each digit’s time slice. Persistence of vision yields a stable multi-digit readout. Why Other Options Are Wrong: Incorrect: Contrary to well-established multiplexing practice. True only for LCDs: LEDs and LCDs both use shared buses in multiplexing. Depends on open-collector drivers: Driver type does not change the bus-sharing concept. Only valid with separate encoders: Encoders are optional; microcontrollers can drive segment data directly. Common Pitfalls: Insufficient refresh rate causing flicker; ghosting due to slow drivers; forgetting current-limiting resistors per segment; misordering segment labels. Final Answer: Correct

Code Converters and Multiplexers Questions