RTC Magazine

USB and PCI Express are two interconnect technologies that have created industry-wide appeal and a strong presence in computer and consumer electronics over the last two decades. During that time these technologies evolved in speed and functions to meet the needs of the market, while maintaining backward compatibility with the previous generations in hardware and software. The evolving robustness of these technologies is attracting other industry segments, such as embedded systems, to utilize them and take advantage of the their broad availability and lower cost due to economies of scale.  

USB 3.0 – SuperSpeed USB

USB technology has made significant progress since its introduction back in 1995 as a simple plug-and-play mouse and keyboard interface. Commonly known as USB 1.0, it runs at 1.2-12 Mbit/s. In early 2000, USB 2.0 was defined to support higher speeds of 480 Mbit/s that enabled a plethora of applications for fast data transfer and storage. Today, you see USB 2.0 camcorders, external disk drives, SSD thumb drives, digital cameras, printers, network adapters, and a wide range of other consumer-focused applications. USB is a key enabler of the digital revolution in consumer electronics.

As new bandwidth-demanding applications such as video players and high-speed disk drives started using USB 2.0, I/O became a bottleneck, so a new revision, USB 3.0, was developed. Also known as SuperSpeed USB and introduced in 2008, it enables a whopping tenfold improvement over USB 2.0, while running at 5 Gbit/s. In addition to the 10x speed over the previous version, USB 3.0 offers additional capabilities such as increased bus power, power management and explicit packet routing. Table 1 highlights the key differences.

Delivering on its plug-and-play and ease-of-use promises, the connectors for USB 3.0 are backward compatible with USB 2.0. Although the connectors are compatible, in actuality they differ, as USB 3.0 requires additional signals to support duplex high-speed communication and power. There are some limitations on USB 3.0 cable length compared to previous versions, as signaling rates are much higher. 

Although x86 CPU makers have yet to introduce chips that support USB 3.0, several other vendors have introduced products and are shipping them in large volumes. With its speed, new features and availability of silicon, USB 3.0 will deliver unprecedented I/O speed to applications such as embedded systems, hard drives, high-definition video, high-resolution cameras and multi-channel audio. Additionally, advanced storage applications such as DAS and NAS (Figures 1 and 2) are emerging that maximize USB 3.0’s performance.

PCI Express

PCI Express (PCIe) was introduced when the performance and capabilities of the parallel PCI bus peaked at 64-bit, 133 MHz. PCIe represents a dramatic extension of the PCI bus. It is a serial, point-to-point interconnect technology. PCIe has gone through its evolution from 2.5 Gbit/s to 8 Gbit/s signaling rates and to advanced features to meet the needs of embedded, graphics, storage and communication applications. While evolving in speeds and features as USB has, PCIe also maintained backward compatibility with earlier generations in three areas key to embedded designs: software stack, form factor and protocol. Table 2 highlights the differences between PCIe 2.0 and 3.0. Devices based on PCIe 3.0, at 8 Gbit/s, are expected to release later this year.

Like USB, PCIe is addressing the needs of an ever-increasing number of industry segments—some highly anticipated, others less so. In these applications, it provides scalable bandwidth between the CPU and I/Os in servers; matches the SAS/SATA and Fibre Channel (FC) data rates in storage; provides high-speed links to control or packet processors in communication; links real-time audio/video processors in consumer applications; and facilitates high-bandwidth serial links in many embedded applications. 

Rackmount and blade servers: These high-end applications would fall in one of two classes—rackmount server or blade server.

Graphics: This application is the key driver of PCIe technology, as it has brought economies of scale to reduce the cost of components. Video gaming, for one, has been growing by leaps and bounds, and graphics chip vendors are providing cutting-edge performance through high-resolution graphics-processing units (GPUs).  

Video distribution: Multiple-monitor computing enabled by PCIe is a major trend that increases productivity and enhances capability in desktop publishing, CAD, CAM, CAID, financial analysis, stock trading, software development, simulation and animation. 

Storage systems: PCIe provides the connectivity between the storage interfaces, such as FC, SCSI and SATA, and the processors that control or manage the storage system.

Industrial/embedded: PCIe technology is being adopted by many standards bodies in the industrial and embedded spaces, such as the advanced telecom, MicroTCA systems and AMC specifications. PCIe switches from companies such as PLX Technology are being used to connect embedded and network processors, as most of them are designed with PCIe interfaces integrated. 

Security systems: With numerous high-resolution cameras comes an extensive need for increased bandwidth and throughput in the systems. For example, a frame-grabber board takes video images from the cameras, processes them locally and feeds them info to the host for analysis and action (Figure 3).  

DVR cards and TV tuners: Digital video recorder (DVR) cards ship with a conventional PCI interface. However, because PCI slots are being eliminated and replaced with PCIe, next-generation DVR cards will be available with the PCIe interface.

How Do They Compare?

A quick comparison of PCIe 2.0 and USB 3.0 shows an array of similarities that impact embedded systems and other applications.

Peer-to-peer communication: USB hubs will support multiple end-devices, but these end-devices will not be able to send traffic directly to each other. PCIe switches support peer-to-peer communication. Once configured by the host, end-devices can directly communicate with each other. However, one must be careful when choosing the switch vendor, as some vendors require CPU support in peer-to-peer communication.

Cable: Cables and connectors for USB have provided an excellent migration path from one generation to the other. USB’s high volume due to the heavy use in consumer electronics has yielded very low cost for the connectors and cables for all USB generations. In the case of PCIe, cable and connector development has been slow due to various non-technology reasons. As a result, the PCIe Gen 1 cable spec took too long to develop and the spec is so complex that after several years the cost of the cable and connectors is prohibitively high. Although PCI Gen 2 has been around since early 2007, the cable specification for this generation is not complete yet. This has caused vendors to either stay away from PCIe or design proprietary connectors and cables to serve their needs.

Bandwidth: USB 3.0 runs at 5 Gbit/s and provides 4.8 Gbit/s data bandwidth simultaneously in both directions, which makes it very attractive for a lot of consumer and embedded applications. PCIe Gen 3 runs at 8 Gbit/s effectively providing 7.9 Gbit/s after encoding, and PCIe Gen 2 runs at 5 Gbit/s, providing 4 Gbit/s data bandwidth after encoding overhead.  PCIe not only has raw bandwidth advantage over USB, but it also allows scaling of this bandwidth up to 16-fold by combining sixteen PCIe lanes into one data pipe (port). These increments could be x2, x4, x8 and x16.

Quality of service: USB does not offer any quality of service and it is not needed in most applications that it serves. PCIe supports quality of service through multiple virtual channels, port arbitration and traffic classes. Embedded applications can take advantage of this feature. 

Error recovery: USB does not offer any error recovery mechanism from link failures. PCIe supports positive acknowledgement of data packets moving between devices and retransmits in case of errors.  PCIe also supports reducing the port width and/or speed of the link if too many line errors are observed.

Power management: USB supports some level of power management. PCIe supports exhaustive device, link and system-level power management features.  

Protocol enhancements: Some protocol enhancements have been made in USB 3.0 to support address-based routing of packets instead of broadcasting through USB hubs.

The PCI-SIG has developed an exhaustive list of protocol enhancements (as discussed above) to enable system designers to enhance performance and manage power consumption.

Both PCIe and USB are highly valuable technologies for embedded-system developers. USB is very useful in connecting to peripherals over a low-cost cable and connector combo and is available everywhere. Unfortunately, it does not scale beyond a 4.8 Gbit/s data pipe. PCIe offers a low-cost connectivity alternative in many embedded applications. PCIe scales well to support high-bandwidth applications and offers various enhancements compared to a simple data pipe like Ethernet.  

PLX Technology
Sunnyvale, CA.
(408) 774-9060.
[www.plxtech.com].