A critical aspect of any VIP, is the test suite. Without appropriate type of tests, it would be impossible to verify design optimally. One must take care to ensure that the tests cover all important areas of the design that are to be verified.
Let us start by asking - what is the difference between directed tests and constraint random tests?
Directed tests are simple tests, wherein a particular scenario is recreated for a known feature, and the expectations are set accordingly. Constraint random tests can cover a wide number of scenarios and/or multiple configurations. This is possible because the scenario generated is randomized with constraints, restricting parameters to values defined within the specification range.
Note: While an IP designer would wish to verify all the features using constraint random tests, a verification engineer would want to use as few constraint random tests as possible.
What are the trade-offs of using directed tests versus constraint random tests?