Meeting the Need for SuperSpeed USB

By Gervais Fong, Product Marketing Manager
Synopsys DesignWare USB PHY IP

Since the introduction of the original USB standard in 1995, the USB interface has become the one of the most successful connectivity standards. In today’s highly connected world, USB connections are found in the computing, consumer, mobile, industrial, and automotive segments. Products are far ranging - from PC’s to portable audio/video players to cell phones and digital TVs. With the trend of increasing data storage requirements driven by applications such as high definition video combined with the desire to move this data quickly between host, storage, and portable devices, it was only a matter a time before there was a need to make this well know standard even faster. This heralds the third generation of this ubiquitous standard - the arrival of USB 3.0.

Super fast! USB 3.0, also known as SuperSpeed USB, is defined as a 5Gbps connection - 10x faster than USB 2.0. Attractive for portable and non-portable applications, USB 3.0 is designed to provide high bandwidth data transfers within an efficient power profile. Supporting low speed (1.5Mbps), full speed (12 Mbps), high speed (480Mbps), and the new SuperSpeed (5Gbps) data rate, USB 3.0 is fully backwards compatible with legacy USB 2.0 and 1.1 products.

The table below illustrates the speed advantage of SuperSpeed USB. For example, a high definition video with a file size of 27 GBytes would take approximately 14 minutes to transfer using USB 2.0. With SuperSpeed USB, the same file would take about one minute! This extremely fast data transfer capability makes it very attractive for a wide range of applications.

The much anticipated version 1.0 of the USB 3.0 specification will be a key catalyst for driving the development of SuperSpeed USB-enabled products. Applications expected to take early advantage of SuperSpeed USB include media storage, media creator, and media player products. Today, media storage devices such as hard disk drives are over one terabyte in size and expected to increase to many terabytes within the next five years. Media creator products such as digital camcorders and digital cameras are quickly becoming capable of shooting high definition video driving up file sizes. And lastly, media players, such as video and music players, continue to increase their storage requirements and thus, drive the need for faster data sync -and- go functionality using SuperSpeed USB.

The creators of the USB 3.0 standard, the USB 3.0 Promoter Group, recognized the huge benefit of retaining backwards compatibility with legacy USB 2.0 and USB 1.0 products. As a result, they maintained the software compatibility at the driver and application level. The existing USB connector receptacle form factors were maintained thereby ensuring that USB 2.0 products could be plugged into USB 3.0 products and still operate. For example, with the new USB 3.0 cable supporting the standard A connector form factor, extra SuperSpeed pins were added to the existing Hi-Speed USB pins. Similar changes were also applied to the USB 2.0 B and micro-B connectors. With these changes to the connector design and the USB negotiation protocol, two USB-enabled products are able to auto-negotiate their maximum USB operating speed regardless of what USB standard they support.

There are multiple changes within the USB 3.0 specification. Here’s a sample of some of the key changes affecting different aspects of the system implementation:

Bus Interface:

• Existing USB 2.0 interface does not change (VBUS, D+, D-, GND).
• Add a pair of uni-directional data paths – one for SuperSpeed Receive (SSRX+, SSRX-) and another for SuperSpeed Transmit (SSTX+, SSTX-). With an additional ground pin, five additional pins are added to the existing four pins for USB 2.0.
• Maximum cable length – 3 meters.

• Only one speed path can be operational at a time. For example, if the SuperSpeed path is running, the Hi-Speed path is not allowed to operate.
• The only exception is internal to hub implementations only - USB 2.0 and USB 3.0 datapaths are allowed to operate independently.

Architecturally, the datapaths for traditional Hi-Speed USB and SuperSpeed USB are separate from one another. The following diagram pictorially depicts an example USB 3.0 implementation connection from an AHB, AXI, or OCP System on Chip bus that divides into separate USB 2.0 and USB 3.0 datapaths. The USB 2.0 datapath remains unchanged. It routes through the MAC and into the USB 2.0 PHY utilizing the traditional UTMI+ or ULPI interfaces. The new USB 3.0 datapath routes through the USB 3.0 MAC and PIPE3 interface into the PHY. The PIPE3 interface can be configured for 8, 16, or 32 bit operation.

Another key objective of the SuperSpeed USB standard was to provide high data transfer performance while maintaining an efficient low power profile. Unlike the older USB 2.0 standard, there is no broadcast or host polling in SuperSpeed USB. Only when the bus is active is power consumed. This dramatically decreases the amount of power consumed; especially, when the link is idle.

The power management features defined in SuperSpeed USB will help maximize power savings. Four different power management states are available:

• U0: Fully operational
• U1: Link idle w/ fast exit
• U2: Link idle w/ slow exit
• U3: Suspend

However, moving from USB 2.0 to USB 3.0 is akin to moving from driving on the city street to cruising on the autobahn. There will be design challenges associated with the implementation of a 5Gbps link. For example, most future USB 3.0-enabled products will be cost driven. Using lower cost materials and overcoming signal integrity issues while maintaining excellent 5 Gbps performance will be demanding. And technical issues notwithstanding, achieving this while meeting ever shrinking time to market window requirements will also be very challenging.

One solution to these challenges is to consider commercially available IP. It can lower risk, provide access to USB and SERDES technical expertise, decrease cost, and shorten development schedules.

With all the advantages of SuperSpeed USB and the broad market interest, it is clear that the USB standard will continue to be one of the most, if not the most successful connectivity interface standard in history.

For more information about Synopsys’ complete DesignWare SuperSpeed USB IP Solution including device controller, PHY, verification IP, virtual platform and drivers, please go to www.synopsys.com/usb.


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About the Author

Gervais Fong is a Senior Product Manager for Mixed-Signal PHY IP at Synopsys. Gervais has over 15 years of experience holding product marketing and product management positions covering ASIC, FPGA, EDA, and IP products. Gervais holds a Bachelor of Science degree in Electrical Engineering and Computer Science from the University of California, Berkeley.