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7/1Introduction• Verification is the process of experimenting our design with possible test scenarios.• Verification contains many phases that includes Testcase generation, coverage, monitor ..,etc.• Before tape-out our design should be verified more than 90% successfully.
Verification methodology key features The key features associated with the verification methodology are: Assertion Based Verification Functional Coverage Driven Verification Constrained Random Verification Scoreboard & Checker
Key features..... Assertion Based Verification Assertion Based Verification helps to detect more functional bugs earlier in the verification process and enables short verification cycle times and faster debugging. Functional Coverage Driven Verification Functional Coverage Driven Verification is the process in which the result of functional coverage is used to drive the verification to completion. It also gives a more structured approach to verification, instead of writing more testcases.
Key features...... Constrained Random Verification It is well suited for designs that have diverse set of operations and sequences that are difficult to cover completely. Constrained random tests are faster to write and enables faster verification of the design. Scoreboard & Checker Scoreboard and Checker is a mechanism used to dynamically predict the response of the design and check the observed response against the predicted response. Usually it refers to the entire dynamic response-checking structure.
Verification Flow Design Specifications Verification plan Testcase Scenarios Testbench Environment1 Testcase generation Model and Testbench(handcrafted SCV/SV generated Devolopment Integration of RTL with Testbench Environment No RTL function Modify RTL verified YES Testsuit Regression Coverage NO Coverage Target Achieved? YES END
Verification Flow Design Specification Verification flow starts with understanding the architecture specification of the design under verification. Once the architecture is well understood then comes the verification plan. Verification Plan The verification plan comprises the preparation of test case scenarios and testbench environment
Testcase Scenarios The testcase scenarios document includes all the possible combinations to test the functionality of the design under test. Test Case Scenarios Self Checking Non-Self Checking
Self Checking Self checking test cases are written in such a way that it tests itself. Self Checking Corner Directed Coverage
Self Checking Corner Corner cases covers the scenarios which are prone to errors and non-trivial, where there are more possibilities of uncovering bugs. Directed Directed cases covers all the scenarios related to the general features and they are straight forward cases which are written in an orderly manner. Coverage Coverage cases are based on the coverage report and they contain the scenarios which are missedout in the other normal categories.
Non - Self CheckingNon - self checking test cases has no self checking code and they are coded such that if the program execution reaches the last line of the test case properly then a success code will be moved into a specific register. After the completion of execution that register can be checked for the success code to ensure the correctness of the design. Non-Self checking Constrained Application Random Stress Negative Random specific
Non - Self Checking Random Random cases cover all the possible combinations of data as well as scenarios in a random manner. Constrained Random Constrained Random cases cover the all possible combinations of data as well as scenarios in a random manner, constrained for valid combinations. Stress Stress cases exercise the logic with more number of combinations of scenarios as well as data for a long time to ensure the reliability of the design under stress conditions.
Non - Self Checking Negative Negative cases covers the scenarios which are not possible under normal cases. They are used to analyze the behavior of the design under error conditions. Application Specific High level cases targeted for a particular application normally written in “C” language.
Testbench EnvironmentThe testbench environment includes different types of environments to be developed for effectively verifying the design under verification. The testbench environments are of two types based on the testing strategies adopted. • Top Level • Block Level
Top LevelThe top level testing includes the testing of the design as a whole and the stimulus forced in this case is the memory image generated from the assembly level or high level test files. Based on the requirement for the verification, there are three types of environments. Top Level Static Dynamic Coverage
Static Performs functional verification in a static manner using functional model. The results of DUT and model are dumped and compared for equivalence once the execution is over. Dynamic Performs timing verification in a dynamic manner using the cycle accurate model. The results of DUT and model are compared for equivalence at each and every cycle dynamically. Coverage The following are considered : • Line coverage • Branch coverage • FSM coverage
Block LevelBlock level verification includes the verification of the different inner modules of the design for checking the scenarios which cannot be covered using the top level verification. It includes both static and dynamic environments and the stimuli in this case are inputs generated using tasks.The testbench provides different debugging options and also uses sentinels to ensure the error free flow of data.
SystemVerilog Based Testbench Development Sequencer Sequencer is an object that defines a set of transactions to be executed and controls the execution of other sequences. Driver It is the component responsible for executing or processing transactions and provides stimuli to the design-under-test (DUT). Monitor This block continuously monitors the DUT signals and bus functions. Scoreboard Driver requests are transferred to the scoreboard via monitor block.
Scoreboard components : Functional Coverage Accumulation Functional coverage is a measure of which design features have been exercised by the tests. Dynamic response checking mechanism The comparison of golden data with DUT data is performed dynamically. This block controls the overall verification environment such as reporting, violations and changing of the stimulus during run-time. Reporting mechanism Compiler directives to issue messages throughout the simulation. (Error, Info, Warning)
Integration,SimulationIntegration Once the RTL coding is over, it is hooked up in the testbench environment and then the verification process can be started.Simulation Design is simulated using the simulation tool. Any error in the simulation triggers an error message which is dumped into an error log.TestSuite Regression / Coverage The Regression Environment is developed using perl and shell scripts, to automate the regression run.
Bug Report MechanismBug reports are maintained in Google docs for all the projects in the verification phase. Once the bug is encountered it is updated in the bug report with detailed information. Status of the open bugs is updated regularly.