Question 1

Programmable Array Logic (PAL): After a PAL device has been programmed (fuses blown to set the logic), which of the following statements is generally true for standard one-time programmable PALs?

Accepted Answer

Introduction / Context: PALs (Programmable Array Logic) were among the earliest widely used programmable logic devices. Understanding their programming model and limitations is important for historical context and for interpreting legacy designs that predate modern CPLDs and FPGAs. Given Data / Assumptions: Standard PALs use fuse-link technology for programming. Programming is typically one-time; blowing fuses configures the logic. We are not considering erasable/flash variants such as GALs or CPLDs. Concept / Approach: In classic PALs, the AND array is programmable and the OR array may be fixed. Programming physically opens fuse links to implement the desired product terms. Once a fuse is blown, the change is permanent, and the device cannot be returned to an unprogrammed state or reconfigured to a new logic function. This permanence differentiates OTP PALs from reprogrammable GALs and modern flash-based logic. Step-by-Step Solution: Program the PAL by applying programming waveforms that selectively blow fuses.The resulting connections define the sum-of-products logic realized internally.Because fuse links are physically destroyed, they cannot be restored.Therefore, the device is one-time programmable and not reprogrammable. Verification / Alternative check: Datasheets for classic PAL families (e.g., PAL16L8, PAL20R8) clearly state OTP behavior. Service manuals often recommend keeping spare unprogrammed parts because re-use is impossible once programmed. Why Other Options Are Wrong: Outputs only active HIGH/LOW: PAL outputs can typically be configured for active-high or active-low depending on macrocell and fuses.Logic capacity is lost: programming uses the capacity; it is not lost—just fixed. Common Pitfalls: Confusing PAL (OTP) with GAL/CPLD (erasable/flash). Always check the specific device family to know whether reprogramming is supported. Final Answer: it cannot be reprogrammed.

Question 2

In digital design, which type of programmable logic device (PLD) is most appropriate for implementing basic combinational logic functions and simple glue logic in small-scale designs?

Accepted Answer

Introduction / Context: Programmable logic devices (PLDs) allow designers to implement custom logic without fabricating ASICs. Among the PLD family, some devices are best suited to small, basic logic tasks, while others target larger, system-level functions. This question tests recognition of which device class is typically chosen for simple combinational logic or glue logic. Given Data / Assumptions: Goal: implement basic logic functions (AND/OR combinations, simple decoders, small state decoding). Device options span PLA, PAL, CPLD, SLD, and FPGA. Emphasis on simplicity, low resource count, and quick turnaround. Concept / Approach: PALs feature a programmable AND plane feeding a fixed OR plane. This structure is sufficient for many small-scale logic functions, is easy to program, and historically replaced multiple TTL packages in glue-logic roles. PLAs provide more flexibility (both AND and OR planes programmable) but were often used when additional product-term flexibility was needed. CPLDs and FPGAs scale to much larger systems and are overkill for very small tasks. Step-by-Step Solution: 1) Identify the design scope: basic combinational logic.2) Map device capability to need: PAL offers simple, efficient implementation with minimal overhead.3) Compare alternatives: PLA is more flexible but not necessary for most simple tasks; CPLD/FPGA are larger and more complex.4) Conclude: PAL is the most appropriate choice. Verification / Alternative check: Typical legacy designs replaced several 74xx TTL gates with a single PAL. If additional flexibility or density was required, designers moved up to a PLA or CPLD. Why Other Options Are Wrong: PLA: more flexible than necessary for basic tasks. CPLD/FPGA: suited for larger designs; unnecessary complexity for simple glue logic. SLD: not a standard mainstream category in this context. Common Pitfalls: Jumping directly to CPLD/FPGA for tiny tasks increases cost and complexity. Always match device class to problem size. Final Answer: PAL (Programmable Array Logic)

Question 3

A Complex Programmable Logic Device (CPLD) integrates multiple PLD-like logic blocks. These blocks are tied together internally by which architectural resource?

Accepted Answer

Introduction / Context: CPLDs scale up the concept of small PLDs by combining several logic macrocells or PLD blocks on one chip. Understanding how these blocks interconnect is crucial to grasping device architecture, timing, and routing limitations. Given Data / Assumptions: CPLD comprises multiple logic blocks. Need to identify the internal interconnect fabric that links blocks. Conceptual architecture, not vendor-specific minutiae. Concept / Approach: CPLDs use a predictable, coarse-grained architecture where several PLD-type function blocks are connected via a global interconnection matrix (or switch matrix). This matrix provides configurable routing paths among macrocells and I/O pins, enabling designers to map logic across blocks while maintaining deterministic timing compared with many FPGAs. Step-by-Step Solution: 1) Identify components: logic blocks (macrocells) and routing fabric.2) Recall that CPLDs feature a centralized, often hierarchical, interconnect network.3) The correct architectural term is a global interconnection matrix (switch matrix).4) Therefore, option (c) best describes the CPLD linkage. Verification / Alternative check: Vendor block diagrams (generic) show macrocells connected through global arrays or switch matrices, which is consistent across families. Why Other Options Are Wrong: Field-programmable switches: too generic; the organized fabric is the matrix. AND/OR arrays: these live inside blocks, not the global routing layer. Language compiler: software tool, not on-chip hardware. External crossbar: routing resides on-chip, not externally. Common Pitfalls: Confusing internal macrocell logic (product terms) with the top-level routing fabric. Keep block-internal logic distinct from inter-block connectivity. Final Answer: a global interconnection matrix

Question 4

PLAs, CPLDs, and FPGAs all belong to which broad category of reconfigurable digital hardware devices?

Accepted Answer

Introduction / Context: Digital designers use a spectrum of reconfigurable hardware devices. Although architectures differ, several share the fundamental characteristic that their logic can be programmed after manufacturing. This question asks you to recognize the umbrella term for such devices. Given Data / Assumptions: Device types mentioned: PLA, CPLD, FPGA. Focus on classification, not internal architecture details. Reconfigurable (field-programmable) behavior is the unifying trait. Concept / Approach: Programmable Logic Device (PLD) is the overarching category for devices whose logic functions can be configured by the user. PLAs are early forms, CPLDs aggregate multiple PLD blocks with a routing matrix, and FPGAs use fine-grained logic cells and programmable interconnects. All are PLDs, despite differing granularity and configuration technologies. Step-by-Step Solution: 1) Identify the shared characteristic: user-programmable logic functions.2) Note that PLA, CPLD, and FPGA each fit this trait.3) The umbrella category is PLD.4) Select option (b) as correct. Verification / Alternative check: Textbooks classify PLAs, PALs, CPLDs, and FPGAs under PLDs, with ASICs as mask-programmed alternatives. Why Other Options Are Wrong: SLD: not a standard mainstream umbrella term here. EPROM/SRAM: memory types, not logic device classes (though FPGAs may use SRAM cells to hold configuration). ASIC: fixed-function silicon, not field-programmable by the end user. Common Pitfalls: Confusing configuration memory technology (e.g., SRAM-based FPGAs) with the device classification itself. The category concerns programmability of logic, not the memory cell type. Final Answer: PLD (Programmable Logic Device)

Question 5

In a simple (early-generation) programmable logic device, what does the internal array primarily consist of for implementing user-defined sum-of-products logic?

Accepted Answer

Introduction / Context: Early PLDs implemented logic by allowing users to selectively connect or disconnect product terms in an array. Understanding what physically stores the configuration bits illuminates how these devices realize arbitrary sum-of-products expressions. Given Data / Assumptions: Reference is to simple PLDs (e.g., PALs/PLAs) rather than CPLDs/FPGAs. Technology eras included fuse, antifuse, or UV-erasable links. Goal is sum-of-products implementation. Concept / Approach: Simple PLDs rely on arrays of programmable connections—commonly realized as fusible links or antifuses. By blowing (or forming) links, designers enable specific product terms in the AND plane and combine them in the OR plane. This approach predates the fine-grained LUT logic found in modern FPGAs. Step-by-Step Solution: 1) Identify the function: implement sum-of-products logic.2) Recognize the storage of configuration: fuse-link array determines which terms are connected.3) Conclude that simple PLDs chiefly contain programmable fuse-link arrays.4) Therefore, option (a) is correct. Verification / Alternative check: Classic PAL/PLA datasheets describe programming by opening/closing fusible connections to create product terms and sums. Why Other Options Are Wrong: Thousands of gates or advanced sequential logic: that scale and richness are more typical of CPLDs/FPGAs. Embedded cores: appear in SoC FPGAs, not simple PLDs. Common Pitfalls: Assuming all PLDs are gate-rich like FPGAs. Early devices focused on configurable connections rather than large quantities of discrete gates. Final Answer: fuse-link arrays

Question 6

What is the key architectural difference between a PLA (Programmable Logic Array) and a PAL (Programmable Array Logic) device with respect to their logic planes?

Accepted Answer

Introduction / Context: PLAs and PALs are foundational PLDs that realize sum-of-products logic. Their capabilities depend on whether the AND and OR planes are fixed or programmable. Recognizing which planes are programmable determines flexibility and resource usage in logic synthesis. Given Data / Assumptions: Comparing PLA versus PAL architecture. Focus on AND and OR plane programmability. Sum-of-products implementation model. Concept / Approach: A PLA exposes programmability in both the AND plane and the OR plane, providing maximum flexibility to create any required product terms and sums. A PAL typically provides a programmable AND plane feeding a fixed OR plane, simplifying hardware and programming at the cost of flexibility. This distinction is standard and appears across classic device families. Step-by-Step Solution: 1) Define planes: AND plane creates product terms; OR plane sums those terms.2) PLA: AND programmable + OR programmable → high flexibility.3) PAL: AND programmable + OR fixed → simpler, less flexible.4) Therefore, option (a) correctly states the difference. Verification / Alternative check: Historical datasheets and textbooks consistently describe PLAs with both planes programmable and PALs with only the AND plane programmable. Why Other Options Are Wrong: Option (b) reverses the roles and is incorrect. Option (c) is not inherently true across all devices; capacity varies by part number. Option (d) is false; they are distinct. Option (e) is incorrect; both support combinational logic, and sequential features (like registers) are device-dependent add-ons. Common Pitfalls: Assuming all PALs can realize any sum-of-products like PLAs; OR-plane fixity can constrain mapping and require re-expression of logic. Final Answer: The PLA has a programmable OR plane and a programmable AND plane, while the PAL only has a programmable AND plane

Programmable Logic Devices (PLD) Questions