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Picking a RapidIO switch

Posted: 12 Mar 2009     Print Version  Bookmark and Share

Keywords:RapidIO interconnect  data plane  switch 

The RapidIO interconnect offers many advantages for embedded systems. It combines high bandwidth with low cost and low power, and it is suitable for both control plane and data plane applications. Successful RapidIO systems use switch-based topologies, so one of the key decisions system designers must make is which switch to use. This article outlines the decision factors that designers should consider, organized by project development phases: system design, implementation, system verification, and system evolution. A last section discusses support services, which impact all stages of project development.

System design
Designers have many different options for implementing a RapidIO interconnect. RapidIO supports all possible topologies(tree, ring, star, etc.), and different inter-processor communication paradigms (read/write and messaging).To choose the system, the designer must answer questions such as: Does my design work? Is this the best design? Where are the risks with this design? In addition to setting the system design, these questions can impact which switch product, and which switch supplier, is right for your application. Some basic criteria to consider when selecting a RapidIO switch are performance�including packet latency and throughput�and the amount of power the switch consumes. As a first step, the designer should perform a spreadsheet analysis to compare the latency and throughput against the design requirements. This quickly eliminates any switches that simply will not work in the design. If no switches pass this analysis, the designer must find a way to reduce the design requirements.

Often, systems have complex traffic patterns that defy simple spreadsheet analysis. To confirm that the chosen system design will work, and that it is the best one, two approaches may be used: Prototyping, and system modelling. Your switch vendor should be able to support both of these approaches.

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Figure 1: This evaluation board is an AMC width card that provides an FPGA and a multi-core DSP. It can be used for system prototyping.

If you choose to prototype, the switch vendor should be able to provide supporting hardware, either directly or though a partner. RapidIO is incorporated into a wide range of commercial off the-shelf boards. These boards are available with devices from all the popular DSP, microprocessor, and FPGA vendors. The boards are also available for the standard embedded system form factors, including AMCs, uTCA, ATCA, VITA 42 and VITA 43 (Figure 1). For prototyping on a smaller scale, designers cause a Serial RapidIO Development Platform. The switch vendor should also provide software tools that can configure the assembled prototype system quickly. The vendor should also provide evaluation tools to generate traffic flows representative of the expected system design. Look for evaluation tools that support comprehensive performance measurement and configurable congestion-based event detection mechanism. These tools allow system designers to rapidly understand the performance characteristics of various system designs. In some cases, prototyping is not an appropriate strategy. If the system is too large to be adequately prototyped, or if there simply isn't time or money to build a prototype, your switch vendor should be able to help by providing system +++OP modelling tools. There are two aspects of the system modelling environment that are key: The accuracy of the switch model, and the flexibility of the traffic generation facilities. The need for an accurate switch model is obvious, as it provides the designer with a credible' map' for the design of the switch. Flexible traffic generation facilities allow progressive refinement of the system model, allowing the system design to evolve as the understanding of system behaviours and risks grow. Every architect strives to deliver a system design that will exceed their customers' expectations. A good switch supplier offers robust technical support to assist system design activity far beyond delivering simple latency, throughput and power data. To enable fast, efficient prototyping, switch suppliers must have a strong presence in the RapidIO ecosystem. The switch supplier must ensure that its switches and design support tools are interoperable and software compatible with every end-point option available. A reputable RapidIO switch vendor must be able to provide system modelling tools to ensure fast reliable system design, ensuring optimal opportunity for success.

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Figure 2: Signal integrity is a key concern in RapidIO designs.

Implementation
Once the system design has been determined, implementation risk factors must be considered. Is there an easy way to learn how to design-in the switch? How do I design for signal integrity? What is the best implementation of my system design? Does the vendor provide sufficient tools and data to enable ease of design? As with any device, risks associated with using a switch are reduced the longer it has been in production, as switch suppliers are more likely to have comprehensive documentation for hardware and software design. This will offer the designer a better outline of the steps required to integrate the switch into their system. Ideally, the documentation is laid out in a step-by-step, easy to use format. It's best if the switch has been used in a wide variety of system designs. This reduces the likelihood that your system will present problems that the switch vendor has not yet encountered.

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