Sunday, December 29, 2013

Sequence Layering in UVM

Another year passed by and the verification world saw further advancements on how better to verify the rising complexity. At the core of verification there exist two pillars that have been active in simplifying the complexity all throughout. First is REUSABILITY at various levels i.e. within project from block to top level, across projects within an organization and deploying VIPs or a methodology like UVM across the industry. Second is raising ABSTRACTION levels i.e. from signals to transaction and from transaction to further abstraction layers so as to deal with complexity in a better way. Sequence layering is one such concept that incorporates the better of the two.
What is layering?
In simple terms, the concept of layering is where one encapsulates a detailed/complex function or a task and moves it at a higher level. In day to day life layering is used everywhere e.g. instead of calling a human being as an entity with 2 eyes, 1 nose, 2 ears, 2 hands, 2 legs etc. we simply refer to it as human being. When we go to a restaurant & ask for an item from the menu say barbeque chicken, we don’t explain the ingredients & process to prepare it. Latest is apps on handhelds where by just a click of an icon everything relevant is available. In short, we avoid the complexity of implementation & details associated while making use of generic stuff.
What is sequence layering?
Applying the concept of layering to sequences in UVM (any methodology) improves the code reusability by developing at a higher abstraction level. This is achieved by adding a layering agent derived from uvm_agent. The layering agent doesn’t have a driver though. All it has is a sequencer & monitor. The way it works is that there is a high level sequence item now associated with this layering sequencer. It would connect to the sequencer of the lower level protocols using the same mechanism as used by sequencer & driver in an uvm_agent. The lower level sequencer would have only 1 sequence (translation sequence that takes the higher level sequence item & translates it into lower level sequence item) running as a forever thread. Inside this sequence we have a get_next_item similar to what we do in a uvm_driver. The item is received from the higher level sequencer. It is translated by this lower level sequence & given to its driver. Once done, the same item_done kind of response is passed back from this lower level sequence to the layered sequencer indicating that it is ready for the next item.
Figure 1 : Concept diagram of the layering agent
On the analysis front, the same layering agent can also have a monitor at higher level. This monitor is connected to the monitor of the lower layer protocol. Once a lower layer packet is received it is passed on to this higher level monitor wherein it is translated into a higher level packet based on the configuration information. Once done, the higher level packet is given to the scoreboard for comparison. So we need only 1 scoreboard for all potential configurations of an IP & the layering agent monitor does the job of translation.
Example Implementation
I recently co-authored a paper on this subject at CDNLive 2013, Bangalore and we received the Best Paper Award! In this paper we describe the application of Sequence Layering where our team was involved in verifying a highly configurable memory controller supporting multiple protocols from the processor side and a no. of protocols on the DRAM memory controller front. A related blog post here.
I would like to hear from you in case you implemented the above or similar concepts in any of your projects. If you would like to see any other topic covered through this blog do drop in an email to (Anonymity guaranteed if requested).
Wish you all a happy & healthy new year!!!


  1. Hey Gaurav, Good one!! I had implemented the layering concept at the sequence and sequencer side. But, this post caught my attention on the monitor and the scoreboard side. That sounds interesting to translate and have a single scoreboard.

  2. LinkedIn group : Coffee with SoC Design-Verification experts

    Thanks for the article. Is it "sequence layering" the only way to stack protocol in UVM ?
    By Tony Gladvin George

  3. Sequence layering is one of the ways typically useful when you want to create a generic abstraction layer. If it is a layered protocol like MPCIe/SSIC/CSI3 etc. you may use a mix of VIPs to create a verification environment. While the protocol conversion still happens from one layer to another, the abstraction is specific to a protocol & not generic.