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.