Physical Design Interview Questions(2024)
To help you prepare for a job interview, get VLSI physical design interview questions and answers for both experienced and fresh candidates.
How do you determine the FIFO depth for clock domain crossing (CDC) scenarios?
To determine FIFO depth for CDC scenarios, calculate based on the difference in clock frequencies and potential burst sizes of incoming data. Ensure the depth can accommodate the maximum difference in data rates between the two clock domains, plus some margin for jitter and metastability effects.
Can you explain what metastability is and how it affects digital circuits?
Metastability occurs when a flip-flop or latch receives input signals that change near the clock edge, causing uncertain or unpredictable outputs. This can lead to unreliable data propagation and synchronization issues in digital circuits.
How can you create a latch using a MUX?
To create a latch using a MUX, connect the data input to one MUX input and the latch output to the other. Use a control signal to select between the new data and the stored value, effectively maintaining the output when the control signal is inactive.
Why might you avoid using asynchronous circuits even though they are faster?
Avoid using asynchronous circuits because they complicate design verification and timing analysis. They can cause unpredictable timing issues, leading to increased difficulty in ensuring reliable and robust operation across all conditions.
What is fanout, and how does it affect propagation delay?
Fanout refers to the number of gate inputs driven by a single output. Higher fanout increases propagation delay due to greater capacitive load, requiring more drive strength and resulting in slower signal transitions.
How can skew be used to our advantage in physical design?
Skew can be used advantageously by deliberately adjusting clock arrival times at different registers. This technique can balance setup and hold time requirements, potentially improving overall timing margins and circuit performance.
What are setup and hold times, and how do they affect the maximum operating frequency of a circuit?
Setup time is the minimum period before the clock edge when data must be stable; hold time is the period after the clock edge during which data must remain stable. Violations of these times limit the maximum operating frequency by requiring longer clock periods to ensure data stability.
Can you explain how to perform basic timing analysis using STA tools?
Perform basic timing analysis with STA tools by defining the clock constraints, input and output delays, and specifying the timing paths. The tool then calculates the delays and checks for setup and hold violations, providing reports to analyze and fix timing issues.
How do pipelining and parallel processing improve latency and throughput in a design?
Pipelining divides a task into stages, each handled in a clock cycle, reducing latency for long operations. Parallel processing splits tasks across multiple units simultaneously, increasing throughput by performing multiple operations concurrently.
How do you decide the channel spacing between macros in a design, especially when they have multiple pins and specific routing layers?
Decide channel spacing by considering the number of routing layers, pin density, and congestion. Ensure adequate space for routing resources and minimize crosstalk and signal interference by adhering to design rules and guidelines.
Where should isolation cells and level shifters be placed in a design, and why?
Place isolation cells between power domains to prevent leakage when a domain is powered down. Position level shifters at boundaries between different voltage domains to ensure signal integrity and proper logic level translation.
What are some techniques to remove congestion in a physical design, and how do you address different types of congestion?
Remove congestion by optimizing placement, adjusting cell spacing, rerouting critical nets, and using congestion-aware algorithms. Address localized congestion with buffer insertion and layer assignment, while global congestion may require floorplan adjustments.
What are the basic specifications provided to CTS?
CTS specifications include clock source definition, skew constraints, insertion delay limits, clock tree structure preferences, and buffer/inverter options. These ensure balanced and efficient clock distribution across the design.
Why do you remove the “dont_touch” attribute on clock cells before CTS?
Remove the “dont_touch” attribute to allow the CTS algorithm to modify, move, or replace clock cells for optimal skew and insertion delay management, ensuring efficient and balanced clock tree synthesis.
How do you choose the appropriate drive strength for clock cells?
Choose drive strength based on the load and fanout requirements. Higher drive strength is needed for larger loads and higher fanout to maintain signal integrity and minimize delay.
What is useful skew, and how can clock pushing and pulling be used in physical design?
Useful skew is the intentional adjustment of clock arrival times to improve timing margins. Clock pushing and pulling adjust clock paths to meet setup and hold requirements, effectively balancing the timing constraints.
How do you address dynamic IR drop issues in a design?
Address dynamic IR drop by optimizing power grid design, using decoupling capacitors, and adjusting the placement of high-power cells. Ensure adequate power distribution and minimize voltage fluctuations during operation.
Can downsizing clock cells help in mitigating dynamic IR drop?
Downsizing clock cells can reduce dynamic IR drop by decreasing the overall power consumption and switching noise, leading to less voltage variation in the power grid.
Why is a signoff tool necessary, and why can’t timing closure be achieved solely in tools like Innovus?
A signoff tool is necessary to perform final, detailed analysis with higher accuracy and comprehensive checks that go beyond the capabilities of P&R tools like Innovus. It ensures design meets all timing, power, and reliability specifications.
Read also: Analog Layout Interview Questions(2024)
What is the difference between Path-Based Analysis (PBA) and Graph-Based Analysis (GBA) in timing analysis?
PBA focuses on individual timing paths, providing more accurate results by considering real path delays and constraints. GBA analyzes all paths simultaneously using a simplified model, offering quicker but less precise results.
How do you identify and fix electromigration (EM) violations in a design?
Identify EM violations using specialized tools to analyze current densities. Fix them by widening metal traces, using thicker metal layers, and redistributing current paths to reduce stress on any single conductor.
How do you use the get_db command to access database objects and their attributes in physical design tools?
Use the get_db command to query and manipulate database objects by specifying the object type and desired attributes. This allows for detailed inspection and modification of design elements within the tool.
What are the key components of a Power Distribution Network, and how do you ensure its reliability?
Key components include power rings, stripes, grids, and vias. Ensure reliability by optimizing the layout for uniform current distribution, minimizing IR drop, and using appropriate metal layers and widths.
What files are required for setting up Redhawk for IR drop analysis?
Required files include the design netlist, floorplan, power grid specifications, technology file, and current activity profiles. These provide the necessary information for accurate IR drop simulation.
What are the typical inputs required for running analysis in Voltus?
Typical inputs include the design netlist, power intent file, parasitic extraction data, clock and power constraints, and switching activity information. These inputs allow Voltus to perform detailed power and IR drop analysis.
Which physical verification tools have you used, such as ICV and Calibre?
I have used physical verification tools like ICV and Calibre for DRC, LVS, and metal fill checks. These tools ensure the design adheres to manufacturing rules and design specifications.
Which Logic Equivalence Checking (LEC) tools have you used, and what are their purposes?
I have used LEC tools like Formality and Conformal to verify that the synthesized netlist matches the RTL design. These tools ensure functional equivalence and consistency throughout the design process.
How do you approach floorplanning and placement when dealing with a large number of macros?
Approach floorplanning by first organizing macros based on functionality and connectivity, ensuring minimal wire length and congestion. Use hierarchical planning and iterative refinement to optimize placement.
What strategies do you use to handle congestion and optimize placement?
Handle congestion by adjusting cell placement, modifying floorplan, using buffer insertion, and optimizing routing layers. Employ congestion-aware placement algorithms and regularly analyze congestion maps.
What are the necessary inputs for the synthesis process?
Necessary inputs include RTL code, constraint files (SDC), technology libraries, and design-specific configuration files. These inputs guide the synthesis tool to generate an optimized netlist.
What sanity checks do you perform after synthesis, floorplanning, and place-and-route (PNR)?
Perform checks for DRC/LVS violations, timing analysis, power integrity, and signal integrity. Verify that the design meets all functional and performance requirements before proceeding to the next stage.
VLSI Design Interview Question
If you want to know following questions answers, please do copy and paste into search box, you will get exact answers in details!
10 Ways To Fix Setup and Hold Time Violations
Antenna Effect in VLSI – Causes and Solution.
Can virtual clock analysis accurately predict the timing of the physical implementation?
DIBL GIDL BTBT and Tunneling Effect in CMOS Devices
Difference Between Clock Skew and Uncertainty
Explain all stages of Physical Design
How can you reduce dynamic power?
How do you fix the static IR drop?
How does the virtual clock help in the physical design process?
How to decide channel width between macros?
How to fix Dynamic IR drop?
How to fix Dynamic IR drop?
How to fix Electromigration (EM)?
How to fix setup and hold violations at a time?
How to fix setup and hold violations?
How to fix setup violations?
Interview questions for experienced Physical Design Engineer