PCI EXpress for DSP

DSPs with PCI Express Interface Expand Embedded Connectivity Options

Lower cost broadens usage of DSPs in real-time, range of other markets.


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While PCI Express (PCIe) has become wildly successful throughout virtually all market segments, a few were slower to adopt this powerful serial interconnect standard. Wireless base stations, for example, have yet to take advantage of PCIe’s speeds (now 8 Gbit/s in PCIe Gen 3).  In the case of the base stations, this is because the main processor used is a DSP, which vendors had not integrated with PCIe.

Instead, they offered their own proprietary interfaces or Serial RapidIO (SRIO) interface, meaning real-time/embedded-systems designers had limited choices when looking to connect several different endpoints. This in turn meant those designers had to depend on expensive FPGAs or ASICs to develop endpoints; because the ecosystem surrounding the proprietary and/or SRIO interface was very limited, ASICs and FPGAs essentially became the only choices for the wireless base stations. DSP vendors such as Texas Instruments, Freescale and LSI, however, have started to integrate PCIe natively into their product offerings, enabling designers over the past year to take advantage of the huge ecosystem behind PCIe.

PCI Express Blankets Market Segments

With the 700-plus members in the PCI Special Interest Group having deployed the standard into markets ranging from consumer and small-office electronics to enterprise and cloud-computing systems, the ecosystem for PCIe is both enormous and responsible for bringing down the cost of designs using the interconnect. That ecosystem has developed over the past decade because PCIe has successfully penetrated 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 and AMD; 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 “marriage” of PCIe and processors, and those devices’ use in high-volume products, has helped to drive the presence of PCIe into so wide a range of market segments and more importantly lower the costs for end-users, making it a win-win for all involved.

Now DSPs incorporate the PCIe interface, which offers designers more options for connectivity and at an even lower price. This is especially important given the penetration of PCIe into rapidly evolving DSP product categories, which include not only wireless base stations but other market segments such as video surveillance systems, video communications, medical and biological imaging, home A/V equipment, and digital video recorder/network video (DVR/NVR) boxes.

This new development, blending the two distinct yet compatible technologies, benefits DSP makers, system designers and end-users alike. Designers have numerous options in PCIe interconnect and are now able to maximize both the performance and power efficiency of DSP designs. Additionally, 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.

DSP Providers Recognizing Importance of PCI Express

DSP vendors have realized that the markets in which they’re active are demanding more connectivity options. Designers need a standard interface that is both inexpensive and widely used by millions, with a solid ecosystem that guarantees multiple sources for any device.  Additionally, DSP vendors no longer have to add multiple interfaces to satisfy their varied customer base; they can add one lane of PCIe 2.0 (Gen 2) at 5 Gbit/s 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. Also, since PCIe has been around since early in this decade, the cost structures are now at a point where it makes economic sense to utilize the interface. Furthermore, with the ecosystem so huge, connecting to PCIe endpoints is extremely easy. Last but not 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.

Since most DSPs with PCIe have only two lanes configured as one x2 PCIe port, switches optimized for DSPs ensure connectivity to the large number of endpoints. Let’s take a look at some of the usage models in applications using DSPs and low-lane-count PCIe switches.

DVR/NVR – Digital Video Recorder / Network Video Recorder

In the DVR/NVR usage model illustrated in Figure 1, the single x1 interface from the DSP is being fanned-out to connect to several endpoints, which are 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 difficult to implement on the FPGA or ASIC, not to mention in the video encoder. 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.

Figure 1
Digital video recorder/network video recorder.

Figure 2
Digital video recorder/network video recorder, with x2 upstream connection.

Since the connection between the DSP and the PCIe switch is now x2 wide, as shown in Figure 2, there isn’t any bottleneck in the system. In fact, the trend in the market is that most designers no longer need to do their own ASICs; most everything is moving toward an ASSP because the interface is now standardized on PCIe. Having a standard interface helps develop the ecosystem around DSPs. 

Figure 2
Digital video recorder/network video recorder, with x2 upstream connection.

Because DSPs support only one x2 PCIe Gen 2 interface, a PCIe switch plays an even more significant role in the design: It not only provides fan-out but also helps balance the speeds and feeds in the system, due to its flexibility to operate at both Gen 1 (2.5 Gbit/s) and Gen 2 speeds on each lane or port independently. This is tricky, however, because the market segments using DSPs are varied. Some, as in enterprise systems, need the PCIe switch to provide a large number of lanes and ports for scalability (Figure 3), while others must satisfy the “Three Ps” essential in consumer designs such as set-top boxes: package, power and price. Since the boards and systems in some segments are size-limited, it is crucial for the PCIe switch to come in the smallest possible package. Power is of extreme importance in these designs, as many feature no heat sinks and/or airflow. It is essential that these designs require no unnecessary components, which would drive the total system cost up while adding to their power draw.

Figure 3
A typical DSP Farm

DSP Farm 

In this usage model (Figure 3), 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 without incurring a high expense for implementing it. The PCIe switch and PCIe interface on the DSPs enable these designers to use a technology that is widely available and is extremely cost-competitive compared to other technologies. Figure 3 shows a perfectly load-balanced usage model—the x8 upstream from the PCIe switch provides up to 40 Gbit/s, and the four x2 ports from the DSPs provide 40 Gbit/s as well, resulting in no bandwidth bottleneck in this system.

Figure 3
A typical DSP Farm

The flexibility of PCIe switches is really critical for such applications. In Figure 3, the DSP farm has only four 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 a x16 upstream port, the PCIe switch vendor must support such configurations.  Obviously, the higher the number of DSPs connected in a DSP farm, the greater the processing power and the more powerful such a system will be.

Figure 3
A typical DSP Farm

Fan-Out Connections Using PCIe, DSPs

While not typically considered a real-time/embedded design, set-top boxes illustrate how designers working in several market segments can use DSPs to implement fan-out connections to a Wi-Fi radio and a USB 3.0 endpoint, as shown in Figure 4.

Figure 4
Fan-out in a 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 toward more and more devices incorporating this technology, thus driving up the need for PCIe connectivity. In Figure 4, again, there is no bandwidth bottleneck. The 1 x2 Gen 2 upstream port from the switch provides 10 Gbit/s and the 2 x1 Gen 2 downstream ports together provide 10 Gbit/s—perfectly load-balanced without any bottleneck!

Figure 4
Fan-out in a set-top box.

With PCIe technology having become as ubiquitous as it now is, along with DSP-based designs now adopting PCIe on a large scale, we can look forward to an even broader ecosystem in which DSPs and PCIe complement one another seamlessly. Still, DSPs likely will continue to be limited in the number of PCIe lanes and ports they can support, making versatile PCIe switches all the more important in DSP-based designs.  

To satisfy the requirement for DSP-friendly PCIe switches, vendors such as PLX Technology are bringing to market devices that are both power- and space-efficient while also flexible and high in performance. These switches presently are being designed into the full range usage models cited above. DSP and PCIe silicon vendors will continue to speed the evolution of the PCIe ecosystem, giving designers the necessary elements to further deploy the PCIe Gen 2 standard, along with the emerging PCIe Gen 3.

PLX Technology
Sunnyvale, CA. 
(408) 774-9060.