Digital Design Expert - By Rahul Jain

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rtl_design_lab - bash

$ compile rtl_top.v -f filelist.f

INFO elaboration complete - 0 errors

INFO timing met at 1 GHz (WNS +0.12)

INFO CDC clean - 0 violations

$ run signoff --all

RTL DESIGN // VERILOG // SYSTEMVERILOG // CDC // STATIC TIMING ANALYSIS // SYNTHESIS // SDC // UPF // AXI // AHB // PCIE // UVM // PHYSICAL DESIGN // LOW POWER // FPGA // ASIC // RTL DESIGN // VERILOG // SYSTEMVERILOG // CDC // STATIC TIMING ANALYSIS // SYNTHESIS // SDC // UPF // AXI // AHB // PCIE // UVM // PHYSICAL DESIGN // LOW POWER // FPGA // ASIC //

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Welcome to my digital design blog! I share in-depth articles on RTL design, timing analysis, ASIC/FPGA concepts, and interview preparation.

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Interview Prep

Top 10 Questions Asked in
Digital Design Synthesis Based Interviews

Q1

What are the various Design constraints used while performing Synthesis for a design?

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Q2

What are the various design changes you do to meet design power targets?

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Q3

What is meant by Library Characterizing?

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Q4

What is meant by Wireload Model?

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Q5

What are the measures to be taken to design for optimized area?

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Q6

What are the key aspects while performing floorplan aware synthesis?

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Q7

What are the measures in the Design taken for Meeting Signal-integrity targets?

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Q8

What is the difference between Timing Constraints and Physical Constraints in Synthesis?

Timing constraints define the functional timing requirements of the design — clock definitions, clock periods, input/output delays, false paths, multicycle paths, and min/max delays. These are provided through SDC (Synopsys Design Constraints) files and guide the synthesis tool to meet the performance targets.

Physical constraints, on the other hand, define physical placement and routing rules such as blockages, pin placements, layer assignments, density constraints, and area targets. These are typically handled in the floorplanning and P&R stages rather than synthesis. In synthesis, the tool primarily optimizes for timing, area, and power using timing constraints, while physical constraints become more relevant in downstream physical design flows.

Q9

What are False Paths and Multi-cycle Paths, and when do you use them?

False Paths: A false path is a path in the design where data does not actually need to meet timing constraints because the signals are either independent or the path is logically never exercised. For example, when an asynchronous FIFO's read and write clocks are independent, the path crossing between them should be marked as a false path using set_false_path. This tells the synthesis tool not to optimize for timing on that path, freeing up resources for critical paths.

Multi-cycle Paths: A multi-cycle path is a path that has more than one clock cycle to propagate its data. For example, if a signal is registered on the rising edge of the clock and consumed two cycles later, it has a 2-cycle path. This is specified using set_multicycle_path with the -setup and -hold flags. This tells the tool to relax the timing requirement, allowing the use of slower cells or longer routing paths, which can save power and area.

Q10

What is the role of a Wireload Model and how does it differ from Parasitic Extraction?

A Wireload Model is a statistical estimation of wire delay and capacitance based on fanout, used during synthesis when actual physical routing information is not yet available. It provides a rough estimate of interconnect effects so the tool can make logical decisions about cell sizing and path optimization.

On the other hand, Parasitic Extraction is performed after physical design (placement and routing) when actual wire geometries, lengths, and layer assignments are known. Parasitic extraction produces accurate RC (resistance and capacitance) values for every net, stored in SDF (Standard Delay Format) or OpenAccess format.

The key difference is that wireload models are estimates used during synthesis, while parasitic extraction provides accurate measurements used for final timing sign-off in STA.

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