USB 3.x - Feeling the need for speed

Gordon Lunn, FTDI

May 15, 2017

Gordon Lunn, FTDIMay 15, 2017

In today’s increasingly busy world, knowledge is everything and the time in which you can expect to receive such knowledge is critical. As such, data transmission in engineering terms has continued to evolve, so that large quantities of information can be transferred at a faster pace and carried significantly further than ever before. From early smoke signals (not the frying PCB kind) to modern superfast fibre optic cabling, every application has looked for new and innovative ways to improve communication capabilities. It must be agreed that there is unlikely to ever be any real change in this fundamental dynamic.

One technology that has seen relentless progress in order for it to remain relevant in modern high-speed systems is universal serial bus (USB). Originally unveiled back in the mid 1990’s, USB 1.0 was a 1.5Mbit/s slow speed serial solution that was predominantly aimed at replacing traditional PC data interfaces, such as RS232 and PS/2. The technology was robust enough to allow reasonable transfer rates over distances of up to 5m. This meant that it was suitable for a wide variety of PC peripherals - especially mice, keyboards, printers, etc.

Further improvements came a few years later, with the introduction of USB 1.1. This enabled full speed data rates of 12Mbit/s to be achieved. It was at this point that USB really started to take off and before long it would effectively become ubiquitous within the computing domain. What really helped propel the success of USB at this time was the tight adherence it had to standards. Standard connectors, and standard cable lengths would be pivotal in ensuring that it was relatively easy to swap peripherals in and out - especially as, in addition to data, the standard cables also made provision for the supplying power to peripherals. Another key aspect to the standards was the development of particular device classes, thereby permitting standard drivers to be accessible to multiple USB devices from different vendors without the cost and inconvenience associated with developing and subsequently installing numerous bespoke drivers.

By the turn of the century USB was well established and looking to grow into new markets and so another upgrade was warranted. The advent of USB 2.0 pushed the supported speed up considerably compared to the previous generations, reaching 480Mbit/s. Over time the power delivery dimension came to have greater value. This resulted in USB’s proliferation into the portable consumer electronics sector - becoming the established route for charging in digital cameras, MP3 players and smartphones. Now every airport and an increasing number of hotels will have USB-based charging points. 

For secure, lossless data transmission at USB 1.x and USB 2.0 speeds, a simple bi-directional, differential signal over twisted pair wiring has proved to be more than sufficient to ensure low noise and crosstalk. As the next chapter in the USB story begins, upholding of signal integrity needs to be accordingly dealt with via more sophisticated mechanisms.

SuperSpeed USB 3.0 (also referred to as USB 3.1 Gen1) has been responsible for an even more significant leap in performance. It offers data rates that are an order of magnitude greater - hitting the 4.8Gbit/s mark. This has opened up a whole new array of applications that USB can now comfortably target, such as faster, larger memory devices and the transfer of high definition (HD) video or imaging data. Thanks to this new level of operational throughput, USB technology has been able to maintain its relevance (while other interface technologies have become obsolete). It is now even offering a viable alternative to Ethernet, in scenarios where cable distances are short but speed and power delivery are both of importance.


Figure 1: The Speeds Corresponding to Each USB Generation (Source: FTDI)

A key factor responsible for aiding the ongoing evolution of USB is that this technology has retained backward compatibility to older legacy systems. This means that the new technology can still be adopted by applications currently unable to take full advantage of the greater speeds that USB 3.x possesses, but will be adequately future proofed for when the need arises. This more cautious approach to design and development has many engineering advantages over a complete wholesale change to some form of technology that has previously been unproven in an application area. However, that does not necessarily mean that design for USB 3.x hasn’t got its own challenges to be taken into consideration. The higher data rates enforce far tighter rules on PCB layout. Traces should be shorter to avoid timing issues, traces should be contained on one PCB layer where possible and superior grounding is required.

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