PCI Express (PCIe) has successfully penetrated business-focused market segments -- graphics, storage, servers, communications, and embedded systems – helped immensely by the ubiquity of PCIe interfaces on everything from high-end CPUs from Intel; GPUs from nVidia: embedded processors from Freescale, AMCC, and Cavium; and consumer products from Atheros, Marvell, Broadcom, Infineon, and Conexant.
Wi-Fi modules, set-top boxes, cable modems and home gateways now feature PCIe, and both the volume and number of product types are growing by leaps and bounds. This has helped to drive the presence of PCIe in all market segments ranging from graphics, servers, storage, communications and consumer products, to improve the ecosystem and to lower costs for customers, making it a win-win for all involved.
What has not been well known but has emerged recently is the presence of the PCIe interface on DSPs from Texas Instruments, Freescale, and LSI. In the past DSPs had either proprietary interfaces or Serial RapidIO (sRIO), but now DSPs have PCIe interface as well, offering designers more options for connectivity, and at lower price. This is especially important given the penetration of PCIe into rapidly evolving product categories, such as wireless base stations, video surveillance systems, video communications, medical and biological imaging, home A/V equipment, and digital video recorder/network video (DVR/NVR) boxes – all markets that continue to be bastions of DSPs.
This article will look at this development with examples of usage models and how it benefits the DSP makers, designers and end users - the entire ecosystem. This article will also look at the options designers have in PCIe interconnect and help them maximize both performance and power efficiency of DSP designs. Additionally, it will illustrate how that efficiency is achieved by taking advantage of PCIe switches’ flexible ports and lanes, small packages and unique ability to fan out to a number of endpoints.
From its debut of actual silicon in 2004, PCIe has penetrated every market segment, not the least of them consumer electronics, along with graphics cards, add-in cards and motherboards. Yet even after all that success, there has been one major portion of the market segment that to date hasn’t adopted PCIe – wireless base stations. This was primarily because the available DSPs used were either sRIO-based or the manufacturers used their own ASICs with proprietary buses.
All that’s changing now, primarily because the DSP vendors have realized that the market is demanding more connectivity options. Designers need a standard interface that is both inexpensive and widely adopted by thousands of customers, with a solid ecosystem that guarantees multiple sources for any device.
Also, DSP vendors now don’t have to add multiple interfaces to satisfy their varied customer base; they can now add one lane of PCIe 2.0 (Gen 2) at 5Gbps and save significant space by removing several pins of any proprietary interface on their packages, enabling them to offer more cost- and function-optimized devices.
Since PCIe has been around for more than seven years, the cost structures are now at a point where it makes economic sense to utilize this interface. Furthermore, the ecosystem is huge, which makes connecting to PCIe endpoints extremely easy. Last but not the least is the emergence of endpoints based on the latest complementary interconnect technologies such as USB 3.0, whose bandwidth requirements are supported by PCIe Gen 2 PCIe.
Most of the DSPs have only two PCIe lanes configured as 1 x2 PCIe port, a PCIe switch optimized for this market ensures connectivity to the large number of endpoints. Let’s take a look at some of the usage models in these applications driving the need for a PCIe switch.
In DVR/NVR usage model illustrated in Figure 1 (below), the single x1 interface from the DSP is being fanned-out to connect to several endpoints, which have PCIe native. If the PCIe interface did not exist on the DSP, then the designers would have to use proprietary interfaces to connect these endpoints, which would have been a headache to implement on the FPGA and ASIC, not to mention in the video encoder.
Fig 1: Digital video recorder/network video recorder.
Avoiding a bottleneck
For designers worried that the bandwidth between the processor and the PCIe switch could be a bottleneck, the PCIe switch offers a x2 configuration for the DSP that supports such a configuration, as shown in Figure 2.
Fig 2: Digital video recorder/network video recorder, with x2 upstream connection.
In this example, since the connection between the DSP and the PCIe switch is now x2 wide, there isn’t any bottleneck in the system.
Another important usage model is that of a DSP farm, as shown in Figure 3 (below).
Fig 3: A typical DSP farm.
In this usage model, several DSPs are connected to a central PCIe switch through which the processing power of the system is greatly enhanced. Such a usage model is extremely attractive for vendors who want to enhance the performance of their system but the expense of implementing it. The PCIe switch and PCIe interface on the DSPs enables these designers to use a technology that is widely available and is extremely cost-competitive compared to all other technologies. Figure 3 shows a perfectly load-balanced usage model – the x8 upstream from the PCIe switch provides up to 40Gbps and the four x2 ports from the DSPs provide 40Gbps as well – so there is no bandwidth bottleneck in this system.
The flexibility that a wide portfolio of PCIe switches offers is really critical for such applications. In Figure 3, the DSP farm has only 4 DSPs, but designers need the assurance that a PCIe switch will not be a limiting factor if they want to scale their designs. If the designer decides to scale up to 12 or 16 (or more) DSPs and wants an x16 upstream port, for example, (s)he wants the PCIe switch vendor to support such configurations. Obviously, the more number of DSPs connected in a DSP farm, the higher the processing power and the more powerful such a system will be.
In the set-top box market, designers use DSPs need to implement fan-out connections to a Wi-Fi radio and a USB 3.0 endpoint, as shown in Figure 4.
Fig 4: Fan-out in set top box.
In this usage model, the PCIe switch is providing pure fan-out connectivity to multiple endpoints. With USB 3.0 emerging as the successor to USB 2.0 (thanks to the 10x speed bump), the trend is pointing towards more and more consumer devices incorporating this technology, thus driving up the need for PCIe connectivity.
PCIe technology has become truly ubiquitous, and the last remaining markets – DSP-based designs -- are now adopting PCIe on a large scale. These DSPs support PCIe Gen 2 for the higher speeds. However, DSPs are always going to be limited in the number of PCIe lanes and ports they can support, which often won’t be sufficient for designers. In fact, most of DSPs support only one x2 PCIe Gen 2 interface. This is where the PCIe switch comes into play; it will not only provide fan-out but also help balance the speeds and feeds in the system, due to its flexibility to operate at both Gen 1 and Gen 2 speeds on each lane or port independently.
This is tricky, however, because the markets using DSPs are varied. Some market segments need the PCIe switch to provide a large number of lanes and ports for scalability, as shown in the DSP farm usage model (Figure 3) and also satisfy the “Three Ps” essential in the consumer market for designs such as set-top boxes: package, power and price.
Since the boards and systems in these markets are essentially size-limited, it is crucial for the PCIe switch come in the smallest-possible package. Power is of extreme importance in these markets – simply because these are consumer products with small board sizes and no heat-sinks and/or air-flow. It is essential that a design requires no extra components, which would drive the total system cost up, making low power all the more important.
Vendors such as PLX Technology satisfy these requirements with flexible, high-performance yet power-efficient PCIe switches, which are being designed into the full range usage models cited above. These vendors will continue to play a key role in the development of the PCIe ecosystem with the widely deployed PCIe Gen 2 standard, as well as the forthcoming PCIe Gen 3 and its 8Gbps transfer rates.
About the author:
Krishna Mallampati (firstname.lastname@example.org) is product marketing director for PCIe switches at PLX Technology, Sunnyvale, Calif.