Question 1

NOR gate behavior with mixed inputs A two-input NOR gate has one input HIGH (logic 1) and the other input LOW (logic 0). What will be the logic level at its output?

Accepted Answer

Introduction / Context: NOR is a fundamental logic gate defined as the complement of OR. Understanding its response to different input combinations is essential because NOR is functionally complete, meaning any logic function can be built using only NOR gates. Given Data / Assumptions: Two inputs: A and B. Input levels: one is 1 (HIGH), the other is 0 (LOW). Ideal Boolean behavior is assumed (no analog thresholds considered). Concept / Approach: By definition, NOR performs NOT(A + B). The OR of 1 and 0 equals 1. Taking the NOT of 1 gives 0. Therefore, regardless of which input is HIGH, the output is LOW for this mixed-input case. Step-by-Step Solution: 1) Compute OR: A + B = 1 OR 0 = 1.2) Apply NOT: NOT(1) = 0.3) Conclude: NOR output is 0 (LOW).4) Symmetry: the result is the same if A=0,B=1. Verification / Alternative check: Truth table snippet for NOR: inputs 10 or 01 → output 0; only inputs 00 → output 1; inputs 11 → output 0. This matches the computed result. Why Other Options Are Wrong: Output HIGH: incorrect; OR of mixed inputs is 1, so NOR must be 0. Functions as an AND: gate identity does not change; behavior still NOR. Will not function: digital logic gates function for all valid inputs. Common Pitfalls: Mixing up NOR with NAND; remember NOR is NOT of OR, while NAND is NOT of AND. Final Answer: will output a LOW

Question 2

AHDL vs. VHDL — understanding language scope and ownership What is the basic difference between AHDL (Altera Hardware Description Language) and VHDL in the context of PLD/FPGA design?

Accepted Answer

Introduction / Context: Hardware description languages specify digital hardware behavior and structure. VHDL is an IEEE-standard language used across vendors, whereas AHDL was created by Altera (now part of Intel) for their tools. Knowing which language is standardized versus proprietary helps when choosing a design flow that is portable across devices. Given Data / Assumptions: AHDL originates from a single vendor ecosystem. VHDL is an IEEE standard (IEEE 1076). Target devices include PLDs and FPGAs. Concept / Approach: Proprietary languages are controlled by one company and may be supported primarily in that company's toolchain. Standardized languages are maintained by standards bodies and widely supported across EDA tools and vendors. Therefore, AHDL is proprietary to Altera's environment, while VHDL is a vendor-neutral, standardized HDL. Step-by-Step Solution: 1) Identify ownership: AHDL → vendor-originated; VHDL → IEEE standard.2) Assess portability: VHDL designs generally port among multiple vendors' tools; AHDL is limited.3) Conclude distinction: AHDL is proprietary, VHDL is not.4) Choose option reflecting this: “AHDL is proprietary.” Verification / Alternative check: Tool documentation historically lists AHDL support within Altera/Intel Quartus; VHDL is supported in nearly all mainstream FPGA workflows. Why Other Options Are Wrong: AHDL or VHDL used in all PLDs: exaggerated and incorrect. VHDL is proprietary: false; it is an open standard. Common Pitfalls: Assuming “used everywhere” equals “standard”; vendor adoption does not change IEEE standard status. Final Answer: AHDL is proprietary.

Question 3

VHDL execution model inside a process block In VHDL, how are the statements written between BEGIN and END within a process executed (i.e., what is their evaluation order)?

Accepted Answer

Introduction / Context: VHDL has two execution domains: concurrent statements at the architecture level and sequential statements inside processes, procedures, and functions. Designers must understand this distinction to predict signal updates and avoid simulation/synthesis mismatches. Given Data / Assumptions: The focus is on statements placed between BEGIN and END of a process. We are not referring to concurrent statements outside a process. Standard VHDL semantics apply (signal assignment scheduling, delta cycles, etc.). Concept / Approach: Within a process, statements execute in program order, one after another, as if in a conventional programming language. Although signal updates may take effect after delta cycles, the evaluation order of statements themselves is sequential. Outside processes, statements are concurrent and conceptually active at all times. Step-by-Step Solution: 1) Identify context: inside a process block bounded by BEGIN and END.2) Apply VHDL rule: process statements execute in sequence (top to bottom).3) Remember that signals assigned within the process update after a simulation delta cycle, but the execution order remains sequential.4) Therefore, choose “Sequentially.” Verification / Alternative check: Compare with architecture-level concurrent statements (e.g., conditional signal assignments) which are evaluated concurrently; this contrast reinforces that process internals are sequential. Why Other Options Are Wrong: Constantly/Simultaneously: describe concurrent behavior, not process internals. Concurrently: correct for architecture statements, not for statements inside a process. Common Pitfalls: Expecting sequential variable-like behavior for signals without accounting for delta cycles; mixing up concurrent architecture code with sequential process code. Final Answer: Sequentially

Question 4

Logic Gates — Understanding the NAND Function A standard NAND gate is the inversion of an AND gate. Considering conventional (active-HIGH) logic signaling, which description best matches the input sense and the output sense of a NAND gate?

Accepted Answer

Introduction / Context: A NAND (Not-AND) gate is one of the fundamental building blocks in digital electronics. It outputs the logical complement of the AND function. Understanding what “active-HIGH inputs” and “active-LOW output” mean clarifies how NAND behaves in truth tables, logic design, and real circuits. Given Data / Assumptions: Conventional binary logic with logic 1 = HIGH voltage and logic 0 = LOW voltage. NAND implements Y = NOT(A * B * ...), where * denotes logical AND. “Active-HIGH input” means a logic 1 is the asserted/true state for the input. “Active-LOW output” means the output asserts (true condition) as a LOW level in response to the input condition being met. Concept / Approach: The AND gate produces HIGH only when all inputs are HIGH. The NAND gate inverts that result, producing LOW only when all inputs are HIGH. Thus, the inputs are interpreted in the normal active-HIGH sense, and the output is LOW (active-LOW) in the unique case that all inputs are asserted. Step-by-Step Solution: 1) Consider an AND gate: for inputs A, B, the output is A * B.2) Invert that to obtain NAND: Y = (A * B)' = NOT(A * B).3) With active-HIGH inputs, the “all-ones” condition (A = 1 and B = 1) makes AND = 1.4) NAND then produces Y = 0 only in that condition; otherwise Y = 1.5) Therefore, inputs are active-HIGH, and the asserted condition at the output is a LOW (active-LOW) when all inputs are HIGH. Verification / Alternative check: Truth table: 00→1, 01→1, 10→1, 11→0 for 2-input NAND. The output is LOW exactly when both inputs are HIGH, matching the “active-HIGH inputs, active-LOW output” description. Why Other Options Are Wrong: active-LOW inputs and an active-HIGH output: Inputs are not interpreted as active-LOW for a standard NAND symbol. active-LOW inputs and an active-LOW output: Double mismatch with standard logic interpretation. active-HIGH inputs and an active-HIGH output: Describes an AND gate, not NAND. Common Pitfalls: Confusing AND with NAND; overlooking that “active-LOW output” simply means the output’s asserted condition is LOW (the bubble at the output in logic symbols). Final Answer: active-HIGH inputs and an active-LOW output.

Question 5

Digital Logic Documentation — Naming the Table What is the standard format used to present every possible input combination to a logic gate together with the corresponding output value?

Accepted Answer

Introduction / Context: In digital electronics, we often need a complete, unambiguous description of how a logic device responds to all possible input combinations. The universally accepted way to present this information is with a specific tabular format used in textbooks, datasheets, and design documents. Given Data / Assumptions: The device is a logic gate or a small combinational circuit. Inputs and outputs take on binary values (0 or 1) under conventional logic. The goal is to list input combinations and the corresponding output(s). Concept / Approach: The standard table that enumerates every possible input pattern and the resultant output is known as a truth table. It provides an exhaustive mapping from inputs to outputs and serves as the reference for deriving Boolean expressions and verifying implementation correctness. Step-by-Step Solution: 1) Identify the number of inputs, say n, leading to 2^n possible input combinations.2) Layout a table with n input columns and one or more output columns.3) Fill the table with all input combinations (e.g., binary count from 00…0 to 11…1).4) For each row, determine the output according to the device’s logic function and record it.5) The resulting table is the “truth table.” Verification / Alternative check: Compare the truth table with the Boolean expression or logic symbol. For example, an AND gate’s table will show a HIGH output only in the all-ones row; a NAND gate is the complement of that pattern. Why Other Options Are Wrong: input logic function: A vague phrase; not the standard name of the table. Boolean constant: A single fixed value (0 or 1), not a table. Boolean variable: A symbol that takes values 0 or 1, not a table of combinations. Common Pitfalls: Confusing the truth table with timing diagrams or Karnaugh maps. A truth table is purely combinational mapping, without timing information. Final Answer: truth table.

Question 6

OR Gate Evaluation — Substituting Concrete Inputs A 4-input OR gate receives A = 1, B = 1, C = 0, and D = 0. Which equation correctly evaluates the OR operation for these inputs and shows the correct result?

Accepted Answer

Introduction / Context: The OR operation outputs 1 if at least one input is 1. Evaluating a concrete input set helps reinforce the rule and avoid common mistakes such as thinking that “1 + 1” might produce a multi-bit sum in Boolean algebra. In logic, the plus symbol represents OR, not arithmetic addition. Given Data / Assumptions: Inputs: A = 1, B = 1, C = 0, D = 0. Operator “+” denotes logical OR, not numeric addition. Binary logic levels 1 (true/HIGH) and 0 (false/LOW). Concept / Approach: In Boolean algebra, X + Y = 1 if X = 1 or Y = 1 (or both). For multiple inputs, OR evaluates to 1 as soon as any single input is 1. Therefore, with at least one 1 among the inputs, the final result must be 1. Step-by-Step Solution: 1) Evaluate A + B: 1 + 1 = 1 (in Boolean logic, not arithmetic).2) Include C: 1 + 0 = 1.3) Include D: 1 + 0 = 1.4) Final expression: 1 + 1 + 0 + 0 = 1. Verification / Alternative check: Truth table of OR confirms that any row with at least one 1 yields an output of 1. Here there are two ones, so the output is definitely 1. Why Other Options Are Wrong: “= 01” or “= 00”: Suggests multi-bit arithmetic notation, not Boolean OR. “= 0”: Incorrect because OR of at least one 1 cannot be 0. Common Pitfalls: Mixing arithmetic with Boolean algebra; in Boolean algebra, 1 + 1 remains 1 (idempotent law), not 2. Final Answer: 1 + 1 + 0 + 0 = 1

Question 7

HDL to PLD Flow — Naming the Translation Tool In a programmable logic design workflow, which special software tool translates hardware description language (HDL) code into the 1/0 configuration (e.g., a JEDEC or bitstream) that can be loaded into a …

Accepted Answer

Introduction / Context: Designing with PLDs and FPGAs involves writing HDL (VHDL/Verilog/SystemVerilog) and then converting that description into a device-specific configuration of logic elements and interconnects. The software that performs synthesis, fitting, placement, and routing ultimately emits the 1/0 pattern used to program the device. Given Data / Assumptions: Target device is a PLD/FPGA. Input is HDL source. Output is a configuration file (e.g., JEDEC file, bitstream). Concept / Approach: EDA vendors commonly refer to the overall translation toolchain as a “compiler.” Internally it runs synthesis (HDL to netlist), technology mapping, and place-and-route to produce a final programming file. While different phases have specific names, the umbrella term in tool GUIs and documentation is typically “compiler.” Step-by-Step Solution: 1) Parse HDL to build an abstract hardware model.2) Synthesize logic into a gate-level or LUT-level netlist.3) Map, place, and route into the device fabric.4) Generate the 1/0 configuration (JEDEC/bitstream) for programming. Verification / Alternative check: Major toolchains (e.g., Quartus “Compiler”, Vivado “Synthesis/Implementation” runs) use similar terminology; user workflows often refer to “compiling” a design to produce a programming file. Why Other Options Are Wrong: formatter.: Not a standard term in PLD/FPGA flows. programmable wiring.: Describes a concept, not a software tool. CPU.: Hardware processor, not the EDA software. Common Pitfalls: Confusing individual phases (synthesis, fitter) with the name of the entire translation process; in many teaching contexts, “compiler” is the accepted high-level term. Final Answer: compiler.

Question 8

VHDL Fundamentals — Port Modes and What They Define In VHDL entity declarations, the mode of a port (e.g., in, out, inout, buffer) determines directionality. Which of the following is something that the port mode does not define?

Accepted Answer

Introduction / Context: VHDL modules (entities) expose ports that connect to other modules and signals. Each port has a mode that specifies whether it is read, written, or both. Separately, each port has a data type that specifies the kind of signal (e.g., std_logic, std_logic_vector, integer). Distinguishing these concepts avoids common compilation errors. Given Data / Assumptions: Typical VHDL port modes include in, out, inout, and buffer. Port types might be std_logic, std_logic_vector(n downto 0), integer, etc. The question asks specifically what a port mode does not define. Concept / Approach: Port mode defines signal direction and driving/reading rules, while the port’s subtype or type defines its value set and resolution semantics. Therefore, “mode” does not specify the data type; it only declares how the signal is used directionally by the entity. Step-by-Step Solution: 1) Identify meanings: in (read-only), out (write-only from the entity), inout (bidirectional), buffer (special output with feedback semantics).2) Recognize types: std_logic vs integer vs vectors define width and allowable values.3) Conclude that mode ≠ type; mode handles direction, type handles representation.4) Therefore, “the TYPE of the bit” is not defined by mode. Verification / Alternative check: Consider two declarations: port(a : in std_logic; b : out std_logic); Modes differ (in vs out) but both use the same type std_logic. Conversely, you could have in std_logic and in integer; same mode, different types. Why Other Options Are Wrong: an input.: Mode in defines an input. an output.: Mode out defines an output. both an input and an output.: Mode inout allows bidirectional use. Common Pitfalls: Assuming that declaring mode implicitly sets type; in VHDL, type must be specified explicitly even after choosing mode. Final Answer: the TYPE of the bit.

Question 9

An OR gate with inverted inputs functions as:

Accepted Answer

a NAND gate.

Question 10

Boolean Algebra — Associative Law (Addition/OR) Which expression correctly states the associative law for Boolean addition (logical OR), showing that grouping does not change the result?

Accepted Answer

Introduction / Context: The associative law is one of the cornerstone identities in Boolean algebra. It states that when combining three or more variables with the same operator, the way they are grouped does not affect the final outcome. For OR (Boolean addition), this greatly simplifies expression manipulation and logic reduction. Given Data / Assumptions: Operator “+” represents logical OR. Variables A, B, C take binary values 0 or 1. Standard Boolean algebra axioms apply. Concept / Approach: The associative law for OR says (A + B) + C = A + (B + C). This means parentheses can be moved without changing the result. The same property holds for AND: (AB)C = A(BC). Recognizing these equivalences helps restructure expressions and match available hardware gates. Step-by-Step Solution: 1) Evaluate X1 = A + (B + C). If any of A, B, or C is 1, X1 is 1.2) Evaluate X2 = (A + B) + C. Again, if any of A, B, or C is 1, X2 is 1.3) Since X1 and X2 are identical for all eight input combinations, the expressions are equivalent.4) Therefore, A + (B + C) = (A + B) + C is the correct associative-law statement for OR. Verification / Alternative check: Create a truth table with all combinations of A, B, C. The columns for A + (B + C) and (A + B) + C match row by row, proving associativity. Why Other Options Are Wrong: A + (B + C) = A + (BC): Mixes OR and AND; not an associative form. A(BC) = (AB) + C: Left is all-AND, right introduces OR; not equivalent. ABC = A + B + C: AND of all three is not equal to their OR; only true in trivial specific cases. Common Pitfalls: Confusing associativity (moving parentheses) with distributivity (mixing operators), and commutativity (swapping order). Final Answer: A + (B + C) = (A + B) + C

Question 11

Digital logic fundamentals — identify the gate with output HIGH when any one input is HIGH Which logic gate produces a HIGH (logic 1) at its output if any one of its inputs is HIGH?

Accepted Answer

Introduction / Context: In digital electronics, each basic logic gate implements a specific truth relationship between its inputs and output. The question asks you to recognize the gate whose output becomes HIGH (logic 1) when any one (or more) of its inputs is HIGH. This is a core identification skill used in circuit analysis, schematic reading, and logic design, especially when replacing functional blocks with equivalent gates or when predicting waveform behavior through combinational circuits. Given Data / Assumptions: Inputs are standard binary levels: 0 (LOW) and 1 (HIGH). We consider ideal, non-inverting gates with conventional positive logic. We compare the behavior of OR, AND, NOR, and the NOT operation. Concept / Approach: The defining property of the OR function is inclusivity: if at least one input is 1, the output is 1. Formally, for two inputs A and B, the OR output is Q = A + B (using the standard symbol ‘’+’’ for logical OR). By contrast, AND requires all inputs to be 1 (Q = A * B), NOT is a unary operation that inverts a single input, and NOR is the complement of OR (Q = NOT(A + B)), producing 1 only when all inputs are 0. Step-by-Step Solution: Define the target behavior: output HIGH if any one input is HIGH.Match to canonical gate definitions: OR gate is HIGH when A = 1 or B = 1 or both.Exclude AND: AND outputs HIGH only when all inputs are HIGH.Exclude NOT: NOT is a single-input inverter, not a multi-input ‘’any-one’’ function.Exclude NOR: NOR is HIGH only when all inputs are LOW; it is the exact opposite of “any-one HIGH → HIGH.” Verification / Alternative check: Use a quick truth table comparison for two inputs A and B. OR yields outputs 0, 1, 1, 1 for input pairs (00, 01, 10, 11) respectively. This directly satisfies the requirement that any single HIGH input forces the output HIGH. For three or more inputs, the same rule extends: Q = 1 if any input = 1. Why Other Options Are Wrong: NOR gate: Produces 1 only when all inputs are 0; it is the logical complement of OR and contradicts the described behavior. AND gate: Requires all inputs to be 1; a single HIGH input alone is insufficient to drive the output HIGH. NOT operation: Inverts a single input and does not aggregate multiple inputs; it cannot fulfill the “any-one HIGH” multi-input condition. Common Pitfalls: Confusing OR with NOR due to the similar symbol names; overlooking that ‘’NOT operation’’ is not itself a multi-input gate; assuming AND behaves like OR for “any-one HIGH,” which it does not. Always recall: OR is inclusive and flexible, AND is strict, NOR is the negation of OR, and NOT is a single-input inversion. Final Answer: OR gate

Question 12

Boolean algebra — identify the correct distributive law identity. Which of the following expressions correctly states the distributive law used in simplifying logic (algebra over {0,1})?

Accepted Answer

Introduction / Context: The distributive law is one of the foundational identities in Boolean algebra, just as in ordinary arithmetic. It allows us to distribute multiplication over addition, or addition over multiplication, enabling systematic simplification of logic expressions and reduction of gate counts in digital circuits. Given Data / Assumptions: We are working with Boolean variables A, B, C taking values in {0, 1}. Boolean '+' denotes logical OR, and adjacency (AB) denotes logical AND. We want the standard form of the distributive law that is most commonly applied in logic simplification. Concept / Approach: In Boolean algebra, both distributive identities hold: A(B + C) = AB + AC and A + BC = (A + B)(A + C). The first is the direct analog of arithmetic distribution of multiplication over addition and is used constantly to expand or factor expressions into sum-of-products or product-of-sums forms. Step-by-Step Solution: Start with A(B + C).Distribute A across the OR: AB + AC.Rewrite with Boolean notation: AB + AC.Therefore, A(B + C) = AB + AC is the correct distributive identity. Verification / Alternative check: Construct a small truth table for B + C and then AND with A. Compare its output bit-by-bit with AB + AC. You will observe identical outputs for all eight combinations of A, B, and C, confirming the equivalence. Why Other Options Are Wrong: (A + B) + C = A + (B + C): this is associativity of OR, not distribution. A + (B + C) = AB + AC: mixes OR on the left with AND on the right; not a valid identity. A(BC) = (AB) + C: incorrectly replaces an AND with an OR. (A + B)C = AC + BC: this is actually another valid distributive identity; however, the classic statement asked for in many basics questions is A(B + C) = AB + AC. If multiple correct forms are permitted, both apply; here we select the canonical version provided in option A. Common Pitfalls: Confusing association (grouping) with distribution (expanding across different operators). Accidentally changing operators during expansion (e.g., turning AND into OR). Final Answer: A(B + C) = AB + AC

Question 13

Logic gate notation — what does the small circle (bubble) on a gate output indicate? Choose the operation represented when a bubble is drawn on the output of a logic symbol.

Accepted Answer

Introduction / Context: Schematic symbols in digital logic often include a small circle, called a bubble, on an input or output. Understanding bubble notation is crucial when reading data sheets and designing with NAND/NOR logic, because bubbles denote inversion points and change the logic function when De Morgan’s transformations are applied. Given Data / Assumptions: The small circle is placed on a gate output (or sometimes input) in the logic symbol. Standard ANSI/IEEE logic symbols are assumed. Concept / Approach: A bubble explicitly represents a logical NOT (inversion). When a bubble appears on the output of an AND symbol, the device is a NAND. When it appears on the output of an OR symbol, the device is a NOR. Similarly, bubbles on inputs swap active polarity (active-LOW inputs). Step-by-Step Solution: Recognize the bubble as an inversion indicator.Map: AND + output bubble ⇒ NAND; OR + output bubble ⇒ NOR.Therefore, the bubble corresponds to a NOT operation applied at that node. Verification / Alternative check: Compare the truth table of an AND gate to a NAND gate. A NAND is simply the complement of AND for every input combination, matching the meaning of an output inversion bubble. Why Other Options Are Wrong: Comparator operation: comparators are analog/digital interface components; the bubble does not indicate comparison. OR operation / AND operation: the bubble is not a basic binary operator; it is an inversion modifier. Exclusive-OR operation: XOR has its own distinctive symbol; a bubble does not indicate XOR. Common Pitfalls: Assuming a bubble indicates an active-LOW output without realizing it also changes the gate’s logic family (e.g., NAND instead of AND). Final Answer: NOT operation (inversion)

Question 14

The complement of 1 is 0.

Accepted Answer

True

Question 15

The Boolean expression for a three-input AND gate is X = ABC.

Accepted Answer

True

Question 16

The inputs to an AND gate are: A = 1, B = 0, C = 1. The output will be LOW.

Accepted Answer

True

Question 17

In a text-based language, the circuit being described must be given a name.

Accepted Answer

True

Question 18

A LOW placed on the input of an inverter will produce a HIGH output.

Accepted Answer

True

Question 19

A minimum of three universal NOR gates would be required to perform the logical operation of a 2-input AND gate.

Accepted Answer

True

Question 20

In an expression containing both AND and OR operations, the AND operations are performed first (unless parentheses indicate otherwise).

Accepted Answer

True

Describing Logic Circuits Questions