.png){kind=logo}
In the fast-evolving world of high-speed data transfers, Peripheral Component Interconnect Express (PCIe) continues to be the backbone of modern electronic communication. However, with each new generation, verification of its compliance states becomes increasingly complex. In his paper, Deepak Kumar Lnu explores how pre-silicon verification methods are crucial for ensuring that the PCIe Polling Compliance state functions correctly before it is physically implemented.
PCIe operates through a complex state machine known as the Link Training and Status State Machine (LTSSM). Among its various substates, the Polling Compliance state stands out as it ensures the electrical conformity of devices in testing scenarios, not during regular operations. This state transmits predefined bit patterns that the test equipment uses to validate whether the transmitter and channel meet PCIe’s stringent electrical specifications. It’s a seldom-exercised state, which means that its verification poses unique challenges for engineers.
Pre-silicon verification is essential for ensuring design correctness before hardware implementation. It uses formal methods and simulations to identify issues early, unlike post-silicon validation, which relies on lab testing. Verifying rare states like the Polling Compliance state is particularly challenging, as flaws may lead to interoperability or compliance failures. The paper highlights how increasing PCIe generation complexities—from Gen1 to Gen7—introduce new encoding, pattern, and handshake challenges, requiring advanced verification methods to ensure system reliability.
PCIe has progressed significantly from Generation 1 to Generation 7 in terms of encoding schemes, data rates, and now the complexity of compliance patterns. PCIe moved to more sophisticated 128b/130b block coding methods that are used across the PCIe protocol specification, along with PAM4 modulation in Gen 6 and Gen 7. PCIe Generation 1 and Generation 2 compliance patterns were simple and used repeated 8b/10b symbols. PCIe Generation 3 was the first generation to include scrambled data blocks and training sequences as compliance patterns to verify and demonstrate compliance with the PCIe protocol specification. The introduction of PAM4 in PCIe Generation 6 added even more complexity with new compliance requirements including toggle compliance patterns, jitter compliance tests, and usable IP scaling with in the pre-silicon verification methods introduced to manage the ever evolving complexities of the PCIe protocol.
Guaranteeing the Polling Compliance state is a two-part process: formal verification and simulation. Formal verification can provide a mathematical proof to guarantee that the design follows the expected behaviors by verifying entry conditions, valid state transitions, and pattern generation logic. It can determine that the device only enters the Polling Compliance state when one of the valid triggers occurs, such as receiving a specific compliance request or proceeding after timing out in training.
Simulation will complement formal methods by being a more dynamic verification of pattern generating and state transitions over many different test cases. Simulations will clarify the data rate settings, the preset sequences, and whether the generated patterns were generated correctly. Simulation-based methods are also very useful to check the correctness of patterns required in Gen6 and Gen7 where moving to PAM4 encoding creates additional complexity.
The pre-silicon verification of the Polling Compliance state had very positive results. During formal verification and simulation, various issues were discovered, including minor bugs associated with handshake signals and preset sequence implementation, in time to avert expensive debugging in the post-silicon stage. Verification coverage had nearly reached 100%; every specification was met through Gen1 to Gen7 testing, not just confirming functional correctness, but also for risk mediation in future development stages; we were much more confident that the hardware would satisfy PCIe requirements in the end.
While PCIe is developing more and more robust data rates and more and more complex encoding schemes, the need for a diligent verification pre-silicon is paramount. We can find insight in the difficulties of compliance when dealing with the Polling Compliance state, especially as PCIe transitions to new generations, clearly requiring verification methods that become more sophisticated as PCIe evolves.
The sequence that the author outlined can act as a template to enhance future designs, ensuring that pre-silicon verification methods constantly advance to capture the growing diversity of PCIe. These advances will not only ensure interoperability and conformance to standards, but they will also help create devices that are more robust and dependable.
In conclusion, Deepak Kumar Lnu's research highlights the important role that pre-silicon verification played in validating rarely-exercised states such as Polling Compliance for PCIe. The extensive validation approach which included both formal verification and simulation techniques was deemed critical in eliciting potential problems early in the design cycle leading to better hardware that meets reliability and standards. As PCIe standards advance, the strategies shown in this research will still be important in addressing the difficulties posed by new generations of designs while maintaining backward-compatibility with previous generations as well as ensuring compliance to tomorrow's capabilities.
