Sequence Library in UVM

Looking into the history of verification we learn that the test bench and test cases came into picture when RTL was represented in form of Verilog or VHDL. As complexity grew, there was a need for another pair of eyes to verify the code and release the designer from this task that continues to transform into a humongous problem. The slow and steady pace of directed verification couldn’t cope up with the rising demand and constrained random verification (CRV) pitched in to fill the gap. Over the years industry played around with different HVLs and methodologies before squaring down to SV based UVM. The biggest advantage of CRV was auto generation of test cases to create scenarios that the human mind couldn’t comprehend. Coverage driven verification (CDV) further complimented CRV in terms of converging the unbounded problem. In the struggle to hit the user defined coverage goals, verification teams sometime forget the core strength of CRV i.e. finding hidden bugs by running random regressions. UVM provides an effective way to accomplish this aim through use of sequence library.

What is Sequence Library?

A sequence library is a conglomeration of registered sequence types derived from uvm_sequence. 

A sequence once registered shows up in the sequence queue of that sequence library. Reusability demands that a given IP should be configurable so as to plug seamlessly into different SoCs catering varied applications. The verification team can develop multiple sequence libraries to enable regressions for various configurations of the IP. Each of these libraries can be configured to execute sequences any no. of times in different order as configured by the MODE. The available modes in UVM include –

UVM_SEQ_LIB_RAND   : Randomly select any sequence from the queue

UVM_SEQ_LIB_RANDC :  Randomly select from the queue without repeating till all sequences exhausted

UVM_SEQ_LIB_ITEM    : Execute a single sequence item

UVM_SEQ_LIB_USER   : Call select_sequence() method the definition of which can be overridden by the user

Steps to setup a Sequence Library?

STEP 1 : Declare a sequence library. You can declare multiple libraries for a given verification environment.

STEP 2 : Add user defined & relevant sequences to the library. One sequence can be added to multiple sequence libraries.

STEP 3 : Select one of the 4 modes given above based on the context i.e. to run random or run a basic sequence item for sanity testing or a user defined mode as applicable like a set of sequences that test a part of the code for which bug was fixed.

STEP 4 : Select the sequence library as default sequence for a given sequencer from the test. This kind of depends on the STEP 3 i.e. the context.

Advantage

Sequence library is one of the most simple, effective but sparingly used mechanism in UVM. A user can plan to use sequence library by changing modes to achieve sanity testing, mini regression and constrained random regressions from the same test. Further, the library helps in achieving the goal of CRV in terms of generating complex scenarios by calling sequences randomly thereby finding those hidden bugs that would otherwise show up in the SoC verification or the field. 

As Albert Einstein rightly said "No amount of experimentation can ever prove me right; a single experiment can prove me wrong". It is important to run those experiments random enough so as to improve the probability of hitting that single experiment to prove that the RTL is not an exact representation of specification. Sequence library in UVM does this effectively!!!

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