DDRx Design and Verification

DDR interfaces contain multiple groups of signals, each with unique signal quality requirements. They also have relative timing relationships between signal groups that need to be satisfied. All of the signals in all of the groups need to be analyzed to ensure that the design will work as intended. In the image shown here, there are over 64 signals, including clock, command/address, data, data strobe and status. A signal quality or timing problem with any single signal has the potential to render the entire interface inoperable.

Fortunately, DDR interfaces are associated with JEDEC specifications that document interface requirements - but only for the DRAM side of the interface. JEDEC does not specify controller I/O signal or timing requirements, so different controllers will have unique behaviors that have to be taken into account during analysis. For example, controllers might perform deskewing on an interface, byte, nibble or individual bit basis - or not at all.

Ensuring that an interface will work requires ensuring that signal quality and timing requirements are met for all signals and inter-group relationships, including controller-specific behaviors. This requires simulating all signals and post-processing waveform data to extract eye measurements and interconnect flight times for use during timing calculations. Performing this analysis for a complete DDR interface is difficult, since there are dozens of signals involved. Ideally, this analysis should be fully automated, because of the complexity and number of analysis steps involved.

Once a suitable routing strategy has been defined, constraints can be captured graphically and automatically driven into layout.

Pre-layout analysis defines a set of layout guidelines that should allow a system to work properly, if the pre-layout exploration was comprehensive and the layout rules were completely followed. Post-layout verification analyzes the behavior of the design as it was actually laid out, catching cases where the guidelines weren't followed correctly or simply weren't comprehensive enough.

Both forms of analysis are important. Pre-layout exploration helps optimize layout efforts and avoid excessive rework. Post-layout verification helps ensure the design is ready for prototype verification and doesn't contain problems that will make it fail in the lab, where debugging, updating and refabricating are time-consuming and expensive.

Pre-layout exploration establishes expectations for how the design will work, and what the operating margins will be. Post-layout verification needs to perform the same analytical process and report results the same way as pre-layout exploration, so the two sets of results can be readily compared. Ideally, the analysis process should be fully automated, due to the complexity and number of steps in the process. That's exactly what HyperLynx DDR analysis does - use the same automated analysis flow that reports the same results in the same format - so that any issues that arose during layout can be quickly isolated and resolved.

Full-interface DDR analysis is a complex, protocol and device-specific process. The exact analytical process, waveform measurements and timing calculation differ based on the DRAM technology and controller being used. HyperLynx understands the protocol requirements for DDR-2,3,4,5 and LPDDR-2,3,4,5 technologies, including buffered (registered) DDR5 memories. HyperLynx uses a combination of timing models and analysis wizard setup options to establish the controller's capabilities and how to configure the analysis. Controller capabilities specified through the analysis wizard include 1T/2T address timing, read and write leveling, dynamic termination setup, DQ/DQS deskewing capabilities, and more.

As data rates increase, interactions between signals and the Power Delivery Network (PDN) become more important and can consume a significant portion of the design's available operating margin. Modeling these effects requires an accurate simulation model for the combined signal/power delivery network. HyperLynx DDR analysis is seamlessly integrated with the HyperLynx Advanced Solvers hybrid solver to generate these simulation models. With Power-Aware analysis, the effects of non-ideal signal return paths, return path current sharing and simultaneous switching noise can be selectively included or excluded from the analysis, allowing the magnitude of their impact on operating margins to be quantified.

DDR5 memory represents an entirely new chapter in DDR modeling and simulation, due to the inclusion of equalization circuitry in device receivers. This requires a new generation of DDR5 (IBIS-AMI) simulation models and simulation techniques. Additionally, DDR5 mandates calculating eye margins at 1e-16 probabilities, which is not possible with conventional DDR simulation techniques. HyperLynx fully supports DDR5 IBIS-AMI simulation models with the latest features and supports multiple simulation methods to provide different tradeoffs between simulation speed and accuracy. HyperLynx also allows IBIS-AMI models to be used with single-ended analog drivers that have varying rise/fall impedances and edge rates -something not natively part of the IBIS-AMI spec itself.