Sunday, February 8, 2009

functional coverage

Code coverage metrics such as line coverage, fsm coverage, expression coverage, block coverage, toggle coverage and branch coverage is extracted automatically by the code coverage tool it gives us the picture of which sections of the RTL have been executed. Root cause analysis can be done on the code coverage holes and suitable test case can be added to cover the RTL functionality.Code coverage has a draw back of not identifying missing features in the RTL. There is no automatic way of getting the correlation between the functionality to be tested and the implementation of the functionality.Lot of manual effort has to put in to get this correlation.

Functional coverage is the determination of how much functionality of the design has been exercised by the verification environment. Functional coverage is a user defined coverage which maps every functionality to be tested (defined in the test plan) to a coverage point. When ever the functionality to be tested is hit in the simulation the functional coverage point is automatically updated. Functional coverage report can be generated which gives us the summary of how many coverage points where hit. Functional coverage metrics can be used as a feedback path to measure the progress of a verification effort.

Adding functional coverage to a verification environment involves three steps.

1. Identifying the functional coverage and cross coverage points.
( directly maps to your test plan )
2. Implementing the functional coverage monitors.
3. Running simulation to collect the functional coverage and functional coverage analysis.

Methodology of identifying functional coverage points and cross coverage points can be explained with a simple USB2.0 Bulk transfer example.

12 diffrent axis have been identified for a simple USB 2.0 bulk transfer. Total no of basic coverage points for this functional coverage group is 34 coverage points. Now we need to get the cross coverage points. So what is a cross coverage point ? Cross coverage is the valid combinations of the identified axis for example one cross coverage point can be

HS(speed) --> IN(direction) -->ACK(response) -->SMALL(length) --> No (data toggle error)

--> No (crc error) -->No ( pid error) --> SMALL (No of tokens per uframe)

--> No ( token error) --> ODD (payload) --> OFF (ping token)

We need to identify all the cross coverage points for this functional coverage group. Each of the cross coverage point is an test scenario. The way to find all the cross coverage point is to simply cross all the axis which will give you all the possible combinations, now eliminate the invalid combinations. Eliminating the invalid cross can be done using ignore construct or bad state construct in VERA/NTB.

example :: finding the cross coverage points for Axis1 and Axis2 alone. ( In the actual scenario all the axis should be taken in to account). possible cross coverage points

HS --> IN, HS -->OUT , FS --> IN , FS-->OUT

4 cross coverage points have been identifyed by crossing Axis 1 and Axis 2.

Now the identified functional coverage / cross coverage points need to be implemented in VERA/NTB/SV as a coverage group and integrated to the verification environment.

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