What’s All the Buzz About Thunderbolt?

By Dan Harmon, Texas Instruments

The new Intel wired external interface, Thunderbolt™ technology, with technical collaboration from Apple, is all the buzz these days. Developers are already wondering whether Thunderbolt technology is going to replace USB 3.0 as the main I/O of choice on PCs. But before diving into the USB 3.0 versus Thunderbolt discussion, let’s take a quick review of what Thunderbolt really is.

According to the Intel Thunderbolt technology product brief, the basic concept is to take the DisplayPort and PCI Express (PCIe) interfaces, multiplex them, and then send out a combined transmission from the host system. The peripheral system will then take the Thunderbolt signal in, de-multiplex the PCIe and DisplayPort content and provide those for use by the peripheral system.

One implication of this has been lost in the buzz: The source of the DisplayPort and PCIe data is not changing and therefore the performance remains identical for those interfaces. In other words, if the source is DisplayPort 1.1, then the throughput across is still at DisplayPort 1.1 rates (2.7 Gbps).If the source is DisplayPort 1.2 capable, then the performance for the DisplayPort throughput bumps up to the 5.4 Gbps of DisplayPort 1.2. For PCIe, the assumption is that it will be an x1, Gen 2 link,which drives 5 Gbps on both Tx and Rx lines. If you add the bandwidth of PCIe x1 Gen2 to that of DisplayPort 1.2, you get approximately 10.4 Gbps versus the Thunderbolt port transmission speed of 10.3 Gbps. This means that the mux chip must be doing more than simply time-slicing the two signals together. It must be eliminating some of the protocol overhead and replacing it with Thunderbolt protocol-specific transmission overhead.

So if you really are not getting any better DisplayPort or PCIe performance than what an existing system is capable of, what is the value of Thunderbolt technology? What is likely is form factor and ergonomics.

Thunderbolt technology (originally codenamed Lightpeak) can enable the elimination of various connectors on a system to be replaced by a single Thunderbolt connector. For example, removing the DisplayPort receptacle is obvious. Less obvious, could be the elimination of the eSATA, FireWire, HDMI, and even USB receptacles depending on the needs of the system. This opens up the possibility of some very slim form factors while still enabling multiple high-bandwidth I/O. This prompts several follow-on questions: What will this cost to the system implementers? How much more will the additional chip and the new receptacle cost than the existing connectors? Are end users willing to pay the premium for sleeker form factors, but no additional performance?

Getting back to the initial question of Thunderbolt technology versus USB 3.0,these technologies appear to be complementary rather than conflicting. USB will not go away as a key interface for PC systems – it has become too ubiquitous. One way to think of this is to consider that for most portable PCs, ultimately there is a docking solution for fixed-usage scenarios such as in the office. Thunderbolt technology would be an ideal interface for a new docking paradigm – a cabled dock. A single cable passes both the monitor interface (DisplayPort) as well as the data interface (PCIe).Then, inside the dock, the PCIe interface can be used to add any other data I/O that are typically available in docks today, such as USB 3.0, Firewire IEEE-1394 or eSATA via the use of PCIe packet switch and various PCIe-based host controllers, such as the Texas Instruments TUSB7340 for USB 3.0 or the XIO2213B for 1394. These docks also can offer the full user I/O experience via additional downstream devices such as flash media readers or stereo audio, as well as ports for thumb drives. The DisplayPort signal can be mapped directly to a DisplayPort receptacle or, if the dock wants to support HDMI in addition to DisplayPort, then a simple 1:2 switch with level-shifting capabilities, such as the Texas Instruments SN75DP122A (DisplayPort 1.1++) or soon-to-be-available DisplayPort 1.2++ can be used.

Finally, there are legal regulations that mandate that USB continues to be available for the foreseeable future. The Chinese government and the European Union Commission both have mandated that all mobile phones use the micro-B USB receptacle for charging and cannot have any other type of dedicated power receptacle. While this does not necessarily mean that a PC must have a USB connector as a dedicated wall charger, USB in the host system offers a simple means to not only charge a mobile phone, but to exchange data with the PC. With the passing of these governmental rules, most other portable consumer products (personal navigation devices, portable music players, portable media players, tablets, eBooks, etc.) have chosen to use the same micro-B receptacle as their only charging port as well.

At first glance, it may appear that Thunderbolt protocol is a new I/O standard that could replace SuperSpeed USB; however, in actuality, both technologies will reside on the same system and work together to deliver high-speed capabilities to the end user. Thunderbolt technology will continue to rely on underlying protocols such as DisplayPort, PCI Express or USB for the real data transfer between PCs and peripherals.


Dan Harmon is product marketing manager for consumer and computing interfaces at Texas Instruments. He also serves as TI’s USB-IF representative and chair for TI’s USB 3.0 Promoter’s Group. He earned a BSEE from the University of Dayton and a MSEE from the University of Texas at Arlington. You can reach Dan at ti_danharmon@ti.com.