Wednesday, May 1, 2013

Constrained Random Verification flow strategy

The explosive growth of cellular market has affected the semiconductor industry like never before. Product life cycle have moved to an accelerated track to meet time to market. In parallel, engineering teams are in a constant quest to add more functionality on a given die size with higher performance and less power consumption. To manage this, the industry adopted reusability in design & verification. IPs & VIPs have carved out a growing niche market. While reuse happens either by borrowing from internal groups or buying from external vendors, the basic question that arises is, whether the given IP/VIP would meet the specifications of SoC/ASIC? To ensure that the IP serves requirement of multiple applications, thorough verification is required. Directed verification falls short in meeting this target and that is where Constrained Random Verification (CRV) plays an important role.
A recent independent research conducted by Wilson Research Group, commissioned by MentorGraphics revealed some interesting results on deployment of CRV.
In past 5 years –
- System Verilog as a verification language has grown by 271%
- Adoption of CRV increased by 51%
- Functional coverage by 65%
- Assertions by 70%
- Code coverage by 46%
- UVM grew by 486% from 2010 to 2012
- UVM is expected to grow by 46% in next 12 months
- Half of the designs over 5M gates use UVM
A well defined strategy with Coverage Driven Verification (CDV) riding on CRV can really be a game changer in this competitive industry scenario. Unfortunately, most of the groups have no answer to this strategy and pick adhoc approaches only to lose focus during execution. At a very basic level, focus of CRV is to generate random legal scenarios to weed out corner cases or hidden bugs not anticipated easily otherwise. This is enabled by developing a verification environment that can generate test scenarios under direction of constraints, automate the checking and provide guidance on progress. CDV on the other hand uses CRV as the base while defining Simple, Achievable, Measurable, Realistic and Time bound coverage goals. These goals are represented in form of Functional coverage, Code coverage or Assertions.
The key to successful deployment of CDV+CRV demands avoiding redundant simulation cycles while ensuring overall goals, defined (coverage) and perceived (verified design) are met. Multiple approaches to enable this further are in use –
- Run random regressions while observing coverage trend analysis till incremental runs aren’t hitting additional coverage. Analyze coverage results and feedback to the constraints to hit remaining scenarios.
- Run random regressions and use coverage grading to come up with a defined regression suite. Use this for faster turnarounds with a set of directed tests hitting the rest.
- Look for advanced graph based solutions that help you attain 100% coverage with most optimal set of inputs.
To define a strategy the team needs to understand the following –
- Size of design, coverage goals and schedule?
- Availability of HW resources (server farm & licenses)?
- Transition from simulator to accelerator at any point during execution?
- Turnaround time for regressions with above inputs?
- Room to run random regressions further after achieving coverage goals?
- Does the design services partner bring in complementing skills to meet the objective?
- Does the identified EDA tool vendor support all requirements to enable the process i.e. Simulator, Accelerator, Verification planner, VIPs, Verification manager to run regressions, coverage analysis, coverage grading, trend analysis and other graph based technologies.
A sample flow using CRV is given below -

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