In this article, we have listed Interview Questions and Answers for FPGA Engineer Job opportunities. These FPGA Engineer Interview Question Answers are divided into various categories which will help you crack Interviews and secure your job. All the categories and questions are listed below, click and explore the l/topic -
Interview Questions for FPGA Engineer Categories:
1. General FPGA Questions:
Q1: What is an FPGA?
A: FPGA (Field-Programmable Gate Array) is a reconfigurable semiconductor device consisting of logic blocks and interconnects that can be programmed to perform custom hardware functionality.
Q2: What are the advantages of using an FPGA over a microcontroller or ASIC?
A:
Reconfigurability – Can be reprogrammed after deployment.
Parallel Processing – Supports concurrent execution of tasks.
Low Latency – Faster than software-based implementations.
Custom Hardware Acceleration – Ideal for DSP, AI/ML, and cryptography.
Prototype-Friendly – Can be modified during development.
Q3: What are the main components of an FPGA?
A:
Configurable Logic Blocks (CLBs) – Implement logic functions.
Interconnects – Wires connecting different logic elements.
I/O Blocks – Interface with external components.
Block RAM (BRAM) – On-chip memory storage.
Digital Signal Processing (DSP) Blocks – Perform high-speed arithmetic operations.
Clock Management Units (CMUs) – Control clock signals.
Q4: What are the key steps in FPGA design flow?
A:
Specification & Architecture Design
RTL (Register Transfer Level) Design – Using VHDL/Verilog.
Simulation & Verification – Using testbenches.
Synthesis – Converting RTL to gate-level design.
Implementation – Placement & routing.
Bitstream Generation – Final FPGA programming file.
Programming & Debugging – Loading bitstream into FPGA.
Q5: What is the difference between Verilog and VHDL?
A:
Feature | Verilog | VHDL |
Syntax | C-like | Ada-like |
Usage | More popular in industry | Used in defense/aerospace |
Readability | Easier to learn | More structured |
Concurrency Handling | Weaker | Stronger |
Q6: What is the difference between an FPGA and a CPLD?
A:
FPGA (Field-Programmable Gate Array) – High logic density, RAM-based, flexible.
CPLD (Complex Programmable Logic Device) – Lower logic density, non-volatile, fast boot time.
Q7: What are the different types of FPGA architectures?
A:
SRAM-based – Reconfigurable, needs external memory.
Flash-based – Non-volatile, faster boot time.
Antifuse-based – One-time programmable, high security.
Q8: What are the main types of FPGA programming models?
A:
RTL-based (Register Transfer Level) – Using Verilog/VHDL.
High-Level Synthesis (HLS) – Using C/C++ to generate RTL.
Soft Processors – Implementing MicroBlaze/Nios II CPU cores inside FPGA.
Hard IP Cores – Built-in functional blocks (PCIe, Ethernet).
Q9: What are LUTs (Look-Up Tables) in FPGAs?
A: LUTs are small memory blocks inside FPGAs used to implement logic functions.Example: A 4-input LUT can store precomputed values for a logic function with 4 inputs.
Q10: What is pipelining in FPGA design, and why is it used?
A: Pipelining is a technique where a task is split into multiple stages, each executed in a different clock cycle.Benefits:
Increases throughput.
Reduces combinational logic delay.
Improves timing closure in high-speed designs.
Q11: What are the different verification methods used in FPGA design?
A:
Simulation – Using ModelSim, XSIM, or QuestaSim.
Hardware-in-the-Loop (HIL) Testing – Running test cases on actual FPGA.
Formal Verification – Mathematical proof-based correctness checking.
Timing Analysis – Checking setup and hold time violations.
Q12: How do you debug an FPGA design?
A:
Use Signal Taps/Chipscope (Xilinx) or Signal Analyzer (Intel).
Perform Functional Simulation before Synthesis.
Check Timing Constraints in Static Timing Analysis (STA).
Use JTAG Debugging for real-time monitoring.
Q13: What are some common communication protocols used in FPGA designs?
A:
UART (Universal Asynchronous Receiver-Transmitter) – Simple serial communication.
I2C (Inter-Integrated Circuit) – Used for low-speed peripherals.
SPI (Serial Peripheral Interface) – Fast serial communication.
PCIe (Peripheral Component Interconnect Express) – High-speed data transfer.
Ethernet – Network communication.
AXI (Advanced eXtensible Interface) – Used in high-performance FPGA systems.
Q14: How do you interface an FPGA with external memory?
A:
Use DDR/DDR2/DDR3/DDR4 Memory Controllers.
Follow AXI (Advanced eXtensible Interface) Bus Protocol.
Ensure proper timing constraints for read/write operations.
Q15: How do you optimize an FPGA design for better performance?
A:
Reduce Logic Levels – Minimize combinational logic depth.
Use Pipelining – Improve data processing throughput.
Optimize Clock Domains – Reduce clock domain crossing issues.
Use DSP Blocks – Offload complex calculations to dedicated DSP hardware.
Balance Resource Utilization – Efficiently use LUTs, BRAM, and registers.
Q16: What is timing closure in an FPGA, and how do you achieve it?
A:Timing closure means ensuring the design meets timing constraints for setup and hold times.Techniques to achieve timing closure:
Reduce Critical Path Delay – Optimize logic placement.
Use Timing Constraints in SDC Files – Define clock, input, and output delays.
Increase Clock Frequency Gradually – Test at different clock speeds.
Perform Static Timing Analysis (STA) – Identify failing paths.
Q17: What are some real-world applications of FPGAs?
A:
Telecommunications – 5G networks, Software-defined radios (SDR).
Aerospace & Defense – Radar processing, cryptography.
AI/ML Acceleration – Running deep learning models on FPGA.
Automotive – ADAS (Advanced Driver Assistance Systems).
Finance (HFT) – Low-latency trading systems.
Medical Devices – Real-time imaging and diagnostics.
Q18: What is Partial Reconfiguration in FPGAs?
A: Partial Reconfiguration (PR) allows modifying part of an FPGA design while the rest remains operational.Use Cases:
Adaptive hardware acceleration.
Power optimization.
Multi-functional FPGA applications.
Q19: What would you do if an FPGA fails to configure correctly?
A:
Check Power Supply & Clock Sources.
Verify Bitstream File.
Debug Configuration Interface (JTAG, SPI, etc.).
Use FPGA Vendor Debugging Tools (e.g., Xilinx Vivado, Intel Quartus).
Q20: What challenges do you face in FPGA-based development?
A:
Meeting timing constraints in high-speed designs.
Debugging synthesis vs. post-implementation mismatches.
Managing power consumption in large FPGAs.
Resource utilization balancing for optimal performance.
Q21: What is an FPGA ?
A: A field-programmable gate array is a semiconductor device containing programmable logic components called "logic blocks", and programmable interconnects. Logic blocks can be programmed to perform the function of basic logic gates such as AND, and XOR, or more complex combinational functions such as decoders or mathematical functions. In most FPGAs, the logic blocks also include memory elements, which may be simple flip-flops or more complete blocks of memory. A hierarchy of programmable interconnects allows logic blocks to be interconnected as needed by the system designer, somewhat like a one-chip programmable breadboard. Logic blocks and interconnects can be programmed by the customer or designer, after the FPGA is manufactured, to implement any logical function—hence the name "field-programmable". FPGAs are usually slower than their application-specific integrated circuit (ASIC) counterparts, cannot handle as complex a design, and draw more power (for any given semiconductor process). But their advantages include a shorter time to market, ability to re-program in the field to fix bugs, and lower non-recurring engineering costs. Vendors can sell cheaper, less flexible versions of their FPGAs which cannot be modified after the design is committed. The designs are developed on regular FPGAs and then migrated into a fixed version that more resembles an ASIC.
What logic is inferred when there are multiple assign statements targeting the same wire?
A: It is illegal to specify multiple assign statements to the same wire in a synthesizable code that will become an output port of the module. The synthesis tools give a syntax error that a net is being driven by more than one source.
However, it is legal to drive a three-state wire by multiple assign statements.
What do conditional assignments get inferred into?
A: Conditionals in a continuous assignment are specified through the “?:” operator. Conditionals get inferred into a multiplexor. For example, the following is the code for a simple multiplexor
assign wire1 = (sel==1'b1) ? a : b;
What value is inferred when multiple procedural assignments are made to the same reg variable in an always block?
When there are multiple nonblocking assignments made to the same reg variable in a sequential always block, then the last assignment is picked up for logic synthesis. For example
always @ (posedge clk) begin
out <= in1^in2;
out <= in1 &in2;
out <= in1|in2;
In the example just shown, it is the OR logic that is the last assignment. Hence, the logic synthesized was indeed the OR gate. Had the last assignment been the “&” operator, it would have synthesized an AND gate.
Few more important questions:
1) What is the minimum and maximum frequency of dcm in spartan-3 series fpga?
A: Spartan series dcm’s have a minimum frequency of 24 MHZ and a maximum of 248
2)Tell me some of the constraints you used and their purpose during your design?
A: There are lot of constraints and will vary for tool to tool ,I am listing some of Xilinx constraints
a) Translate on and Translate off: the Verilog code between Translate on and Translate off is ignored for synthesis.
b) CLOCK_SIGNAL: is a synthesis constraint. In the case where a clock signal goes through combinatorial logic before being connected to the clock input of a flip-flop, XST cannot identify what input pin or internal net is the real clock signal. This constraint allows you to define the clock net.
c) XOR_COLLAPSE: is a synthesis constraint. It controls whether cascaded XORs should be collapsed into a single XOR.
For more constraints detailed description refer to constraint guide.
3) Suppose for a piece of code equivalent gate count is 600 and for another code equivalent gate count is 50,000 will the size of bitmap change?In other words will size of bitmap change it gate count change?
A: The size of bitmap is irrespective of resource utilization, it is always the same,for Spartan xc3s5000 it is 1.56MB and will never change.
4) What are different types of FPGA programming modes?what are you currently using ?how to change from one to another?
A: Before powering on the FPGA, configuration data is stored externally in a PROM or some other nonvolatile medium either on or off the board. After applying power, the configuration data is written to the FPGA using any of five different modes: Master Parallel, Slave Parallel, Master Serial, Slave Serial, and Boundary Scan (JTAG). The Master and Slave Parallel modes
Mode selecting pins can be set to select the mode, refer data sheet for further details.
5) Tell me some features of the FPGA you are currently using?
A: I am taking an example of xc3s5000 to answer the question .
Very low cost, high-performance logic solution for
high-volume, consumer-oriented applications
- Densities as high as 74,880 logic cells
- Up to 784 I/O pins
- 622 Mb/s data transfer rate per I/O
- 18 single-ended signal standards
- 6 differential I/O standards including LVDS, RSDS
- Termination by Digitally Controlled Impedance
- Signal swing ranging from 1.14V to 3.45V
- Double Data Rate (DDR) support
• Logic resources
- Abundant logic cells with shift register capability
- Wide multiplexers
- Fast look-ahead carry logic
- Dedicated 18 x 18 multipliers
- Up to 1,872 Kbits of total block RAM
- Up to 520 Kbits of total distributed RAM
• Digital Clock Manager (up to four DCMs)
- Clock skew elimination
• Eight global clock lines and abundant routing
6) What is the gate count of your project?
A: Well mine was 3.2 million, I don’t know yours.!
7) Can you list out some of synthesizable and non synthesizable constructs?
A: not synthesizable->>>>
initial
ignored for synthesis.
delays
ignored for synthesis.
events
not supported.
real
Real data type not supported.
time
Time data type not supported.
force and release
Force and release of data types not supported.
fork join
Use nonblocking assignments to get the same effect.
user defined primitives
Only gate level primitives are supported.
synthesizable constructs->>
assign,for loop,Gate Level Primitives,repeat with constant value...
8)Can you explain what struck at zero means?
A:These stuck-at problems will appear in ASIC. Sometimes, the nodes will permanently tie to 1 or 0 because of some fault. To avoid that, we need to provide testability in RTL. If it is permanently 1 it is called stuck-at-1 If it is permanently 0 it is called stuck-at-0.
9) Can you draw the general structure of an fpga?
10) Difference between FPGA and CPLD?
A: FPGA:
a)SRAM based technology.
b)Segmented connection between elements.
c)Usually used for complex logic circuits.
d)Must be reprogrammed once the power is off.
e)Costly
CPLD:
a)Flash or EEPROM based technology.
b)Continuous connection between elements.
c)Usually used for simpler or moderately complex logic circuits.
d)Need not be reprogrammed once the power is off.
e)Cheaper
11) What are dcm's?Why are they used?
A: Digital clock manager (DCM) is a fully digital control system that
uses feedback to maintain clock signal characteristics with a
high degree of precision despite normal variations in operating
temperature and voltage.
That is, the clock output of DCM is stable over a wide range of temperature and voltage , and also the skew associated with DCM is minimal and all phases of input clock can be obtained . The output of DCM coming from the global buffer can handle more load.
12) FPGA design flow?
Also,Please refer to the presentation section synthesis ppt on this site.
13)What is slice,clb,lut?
A: I am taking example of xc3s500 to answer this question
The Configurable Logic Blocks (CLBs) constitute the main logic resource for implementing synchronous as well as combinatorial circuits.
CLB are configurable logic blocks and can be configured to combo,ram or rom depending on coding style
CLB consists of 4 slices and each slice consists of two 4-input LUT (look up table) F-LUT and G-LUT.
14) Can a clb configured as ram?
A: YES. The memory assignment is a clocked behavioral assignment, Reads from the memory are asynchronous, And all the address lines are shared by the read and write statements.
15)What is the purpose of a constraint file? What is its extension?
A: The UCF file is an ASCII file specifying constraints on the logical design. You create this file and enter your constraints in the file with a text editor. You can also use the Xilinx Constraints Editor to create constraints within a UCF(extension) file. These constraints affect how the logical design is implemented in the target device. You can use the file to override constraints specified during design entry.
16) What is the FPGA you are currently using and some of the main reasons for choosing it?
17) Draw a rough diagram of how the clock is routed throughout our FPGA?
18) How many global buffers are there in your current fpga,what is their significance?
A: There are 8 of them in xc3s5000
An external clock source enters the FPGA using a Global Clock Input Buffer (IBUFG), which directly accesses the global clock network or an Input Buffer (IBUF). Clock signals within the FPGA drive a global clock net using a Global Clock Multiplexer Buffer (BUFGMUX). The global clock net connects directly to the CLKIN input.
19) What is the frequency of operation and equivalent gate count of u r project?
20)Tell me some of the timing constraints you have used?
21)Why is the map-timing option used?
A: Timing-driven packing and placement is recommended to improve design performance, timing, and packing for highly utilized designs.
22)What are different types of timing verifications?
A: Dynamic timing:
a. The design is simulated in full timing mode.
b. Not all possibilities tested as it is dependent on the input test vectors.
c. Simulations in full timing mode are slow and require a lot of memory.
d. Best method to check asynchronous interfaces or interfaces between different timing domains.
Static timing:
a. The delays over all paths are added up.
b. All possibilities, including false paths, verified without the need for test vectors.
c. Much faster than simulations, hours as opposed to days.
d. Not good with asynchronous interfaces or interfaces between different timing domains.
23) Compare PLL & DLL ?
A: PLL:
PLLs have disadvantages that make their use in high-speed designs problematic, particularly when both high performance and high reliability are required.
The PLL voltage-controlled oscillator (VCO) is the greatest source of problems. Variations in temperature, supply voltage, and manufacturing process affect the stability and operating performance of PLLs.
DLLs, however, are immune to these problems. A DLL in its simplest form inserts a variable delay line between the external clock and the internal clock. The clock tree distributes the clock to all registers and then back to the feedback pin of the DLL.
The control circuit of the DLL adjusts the delays so that the rising edges of the feedback clock align with the input clock. Once the edges of the clocks are aligned, the DLL is locked, and both the input buffer delay and the clock skew are reduced to zero.
Advantages:
· precision
· stability
· power management
· noise sensitivity
· jitter performance.
24) Given two ASICs. One has a setup violation and the other has a hold violation. how can they be made to work together without modifying the design?
A: Slow the clock down on the one with setup violations..
And add redundant logic in the path where you have held violations.
25)Suggest some ways to increase clock frequency?
A:
· Check critical paths and optimize it.
· Add more timing constraints (over constrain).
· pipeline the architecture to the max possible extent keeping in mind latency req's.
26)What is the purpose of DRC?
A: DRC is used to check whether the particular schematic and corresponding layout(especially the mask sets involved) cater to a predefined rule set depending on the technology used to design. They are parameters set aside by the concerned semiconductor manufacturer with respect to how the masks should be placed , connected , and routed keeping in mind that variations in the fab process does not affect normal functionality. It usually denotes the minimum allowable configuration.
27)What are LVs and why do we do that? What is the difference between LVS and DRC?
A: The layout must be drawn according to certain strict design rules. DRC helps in layout of the designs by checking if the layout abides by those rules.
After the layout is complete we extract the netlist. LVS compares the netlist extracted from the layout with the schematic to ensure that the layout is an identical match to the cell schematic.
28)What is DFT ?
A: DFT means design for testability. 'Design for Test or Testability' - a methodology that ensures a design works properly after manufacturing, which later facilitates the failure analysis and false product/piece detection
Other than the functional logic,you need to add some DFT logic in your design.This will help you in testing the chip for manufacturing defects after it comes from fab. Scan,MBIST,LBIST,IDDQ testing etc are all part of this. (this is a hot field and with lots of opportunities)
29) There are two major FPGA companies: Xilinx and Altera. Xilinx tends to promote its hard processor cores and Altera tends to promote its soft processor cores. What is the difference between a hard processor core and a soft processor core?
A: A hard processor core is a pre-designed block that is embedded onto the device. In the Xilinx Virtex II-Pro, some of the logic blocks have been removed, and the space that was used for these logic blocks is used to implement a processor. The Altera Nios, on the other hand, is a design that can be compiled to the normal FPGA logic.
30)What is the significance of contamination delay in sequential circuit timing?
A: Look at the figure below. tcd is the contamination delay.
Contamination delay tells you if you meet the hold time of a flip flop. To understand this better please look at the sequential circuit below.
The contamination delay of the data path in a sequential circuit is critical for the hold time at the flip flop where it is exiting, in this case R2.
mathematically, th(R2) <= tcd(R1) + tcd(CL2)
Contamination delay is also called tmin and Propagation delay is also called tmax in many data sheets.
31)When are DFT and Formal verification used?
A: DFT:
· manufacturing defects like stuck at "0" or "1".
· test for set of rules followed during the initial design stage.
Formal verification:
· Verification of the operation of the design, i.e, to see if the design follows spec.
· gate netlist == RTL ?
· using mathematics and statistical analysis to check for equivalence.
32)What is Synthesis?
A: Synthesis is the stage in the design flow which is concerned with translating your Verilog code into gates - and that's putting it very simply! First of all, the Verilog must be written in a particular way for the synthesis tool that you are using. Of course, a synthesis tool doesn't actually produce gates - it will output a netlist of the design that you have synthesised that represents the chip which can be fabricated through an ASIC or FPGA vendor.
33)We need to sample an input or output something at different rates, but do I need to vary the rate? What's a clean way to do this?
A: Many, many problems have this sort of variable rate requirement, yet we are usually constrained with a constant clock frequency. One trick is to implement a digital NCO (Numerically Controlled Oscillator). An NCO is actually very simple and, while it is most naturally understood as hardware, it also can be constructed in software. The NCO, quite simply, is an accumulator where you keep adding a fixed value on every clock (e.g. at a constant clock frequency). When the NCO "wraps", you sample your input or do your action. By adjusting the value added to the accumulator each clock, you finely tune the AVERAGE frequency of that wrap event. Now - you may have realized that the wrapping event may have lots of jitter on it. True, but you may use the wrap to increment yet another counter where each additional Divide-by-2 bit reduces this jitter. The DDS is a related technique. I have two examples showing both an NCOs and a DDS in my File Archive. This is tricky to grasp at first, but tremendously powerful once you have it in your bag of tricks. NCOs also relate to digital PLLs, Timing Recovery, TDMA and other "variable rate" phenomena
Conclusion:
Be prepared and understand the architecture and digital logic fundamentals.
Final Tips for the Interview:
Be comfortable with Verilog/VHDL and simulation tools.
Understand FPGA architecture and digital logic fundamentals.
Practice writing synthesizable and optimized HDL code.
Stay updated with FPGA industry trends (AI acceleration, RISC-V, HLS).
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