Qwestrum Engineering360 · Computer & Hardware · Digital Logic Design
Programmable Logic Devices
PLDs implement digital logic through programmable interconnect rather than fixed ASIC wiring.
Exam tip: keep SI units consistent end-to-end, write the governing relation symbolically before substituting, and sanity-check magnitude and sign.
Key formulas & points
Skim these first — then read the full notes below.
- FPGA LUT n inputs gives 2^n SRAM cells
- Synthesis maps RTL to LUTs and flip-flops
- JTAG boundary scan for board test
Topic details
Introduction
Patterson and Hennessy discuss configurable logic as a practical path from design to prototype. Indian curriculum compares PAL, PLA, CPLD, and FPGA architecture/features.
Key relations & formulas
Formulas (Indian textbook notation)
Formulas (Indian textbook notation)
Formulas (Indian textbook notation)
Notation and sign conventions
Relation 1 —
Formulas (Indian textbook notation)
Write this relation with symbols exactly as in Digital Design — Morris Mano before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.
Relation 2 —
Formulas (Indian textbook notation)
Write this relation with symbols exactly as in Digital Design — Morris Mano before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.
Relation 3 —
Formulas (Indian textbook notation)
Write this relation with symbols exactly as in Digital Design — Morris Mano before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.
Concept in depth
PAL provides lower flexibility but faster programming, PLA offers full AND/OR programmability, and FPGA scales to system-level designs via LUTs, block RAM, and DSP slices. Logic synthesis converts HDL into technology primitives, and place-route maps them physically. Configuration memory defines actual circuit behavior at runtime.
Assumptions and validity limits
State assumptions explicitly before using any relation for programmable logic devices — steady state, uniform properties, linear elastic material, ideal gas, incompressible flow, etc., as applicable.
Wrong assumptions invalidate the entire solution even when the formula is correct. In Digital Logic Design viva and GATE descriptive questions, listing valid assumptions often earns separate marks.
Step-by-step problem approach
1. Read the question and list given data with SI units (common in Digital Logic Design papers).
2. Draw a neat labelled diagram where applicable — examiners in Indian universities award diagram marks even when arithmetic slips.
3. Identify which relation from this topic applies to programmable logic devices.
4. Use equation 1:
5. Use equation 2:
6. Substitute values, compute, and verify units and sign (direction).
7. State conclusion in one line — e.g. safe/unsafe, stable/unstable, feasible/infeasible.
2. Draw a neat labelled diagram where applicable — examiners in Indian universities award diagram marks even when arithmetic slips.
3. Identify which relation from this topic applies to programmable logic devices.
4. Use equation 1:
.
5. Use equation 2:
.
6. Substitute values, compute, and verify units and sign (direction).
7. State conclusion in one line — e.g. safe/unsafe, stable/unstable, feasible/infeasible.
Applications & exam relevance
Programmable Logic Devices appears in digital hardware and FPGAs. In Indian computer hardware curricula this topic is tested because it connects theory to combinational and sequential logic.
GATE and semester exams often combine programmable logic devices with earlier units — revise prerequisites before attempting mixed problems.
Industry interview panels sometimes ask: "Where did you use programmable logic devices?" — answer with a lab, mini-project, or plant visit example if possible.
Common mistakes in exams
Students often confuse CPLD macrocell architecture with FPGA LUT fabric and write generic answers. Mention architecture, configuration style, and suitable application to score full marks.
Quick revision checklist
Before attempting programmable logic devices problems, confirm you can:
1. FPGA LUT n inputs gives 2^n SRAM cells
2. Synthesis maps RTL to LUTs and flip-flops
3. JTAG boundary scan for board test
2. Synthesis maps RTL to LUTs and flip-flops
3. JTAG boundary scan for board test
Revise the solved examples in Digital Design — Morris Mano and one previous-year GATE or university paper for this unit.
Worked examples
Try the problem first — open the solution when you are ready to check.
Guided practice — Programmable Logic Devices
Problem
A standard Digital Logic Design numerical on programmable logic devices supplies given data in SI units. Using PAL: fixed AND array, programmable OR and PLA: programmable AND and OR arrays, find the unknown quantity and state whether the result is physically reasonable.
Solution
1. List all given quantities with units (convert to SI if needed).
2. Draw a neat labelled diagram — diagram marks are common in Indian B.Tech papers.
3. Select
4. Substitute values, compute, and attach correct units.
5. Sanity-check: magnitude, sign, and direction must match combinational and sequential logic.
2. Draw a neat labelled diagram — diagram marks are common in Indian B.Tech papers.
3. Select
and write it symbolically before substitution.
4. Substitute values, compute, and attach correct units.
5. Sanity-check: magnitude, sign, and direction must match combinational and sequential logic.
Cross-check with solved examples in your Digital Logic Design textbook.
Conceptual check — Programmable Logic Devices
Problem
In a Digital Logic Design semester or GATE paper you are asked: "State the main assumption, the governing relation, and one practical consequence of programmable logic devices." What should a complete answer include?
📖 Standard books (India)
Digital Design — Morris Mano
Read: Syllabus unit
Logic design and sequential circuits
Explore related topics
See real computer & hardware careers
After exams and interviews, see how engineers actually built careers — milestones and decisions from people in the field.