High Speed Interconnects

Thunderbolt: A Potential High-Speed, Multiprotocol Serial Interconnect

With video and graphics content growing in all application areas including embedded, is there a role for another high-speed interconnect that can transfer a variety of other protocols at 10 Gbit/s?


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Once upon a time there was LightPeak. LightPeak was/is Intel’s bid to bring optical interconnect technology into the personal computer realm. Introduced in 2009, LightPeak was technically successful. It initially operated at 10 Gbit/s with the solid plans to go to 100 Gbit/s, leapfrogging the speed of copper wire connections. A 12 mm x 12 mm module that included an optical module and an optical IC connected the lasers, the photo detectors and the optical cables using off-the-shelf parts. Intel was also hard at work doing research to integrate these onto a single silicon chip. The idea of LightPeak was also to be able to encapsulate and carry multiple different protocols over the same physical link.

The market, however much it liked the capabilities of the technology, was not comfortable with the price point and so LightPeak was never widely deployed. Intel continued to look for ways to bring the price point down and decided to work on an electrical solution. In collaboration with Apple, Intel brought the technology to market with an electrical version that is—at 10 Gbit/s—the functional equivalent of LightPeak. It is now deployed in almost all Apple computers and is named Thunderbolt.

The underlying protocol, the capabilities and the speed are identical to the original LightPeak specs, but the connection technology is now a much less expensive electrical technology in which there are no optical transceivers. This was partially accomplished by moving some of the electronics, and hence some of the cost, into the cable. This makes it more palatable to vendors and OEMs because they can build it in and have it as a part of their machine’s features, and then if someone really wants to use it, they can pay a little more for the cable. This electrical version, however, will definitely stay in the 10 Gbit/s range and will not go to the world of 100 Gbit/s envisioned for LightPeak.

The Thunderbolt cable carries two full-duplex channels at 10 Gbit/s in each direction. The input and output protocols of the Thunderbolt host and device controllers are PCI Express and DisplayPort. The cables are electrically active with circuitry at each end so that each direction in each channel can be data and/or display. For this reason, the incoming PCIe or DisplayPort protocols are mapped to the Thunderbolt transport protocol, which uses a 4-byte header and 64/66 encoding as opposed to the 8b/10b encoding of the inputs. This transport protocol is carried between the two—host and device—controllers, where it is translated back into the underlying protocol (Figure 1).

Figure 1
The underlying PCIe or DisplayPort protocols are mapped by the Thunderbolt controller into the Thunderbolt transport protocol and then decoded by the destination controller.

Rather than try to build Thunderbolt controllers that could translate a whole bunch of different protocols back and forth between the Thunderbolt transport protocol, Intel chose to use one data and one display protocol. They selected PCIe and DisplayPort as the gateways to the outside and they can use standard PCIe and DisplayPort drivers. There is also a wide variety of low-cost controllers on the market that can translate between these two protocols and most of whatever else would be needed in a system.

For example, DisplayPort can easily be translated to HDMI, and PCIe can interface to such things as FireWire, USB 2.0 and 3.0, Ethernet, SATA, RS-232 and more. This relieves the burden of complexity from the Thunderbolt controllers and gives OEMs and users options to select only the interconnects they need. The Thunderbolt controller in the computer supports PCIe Gen 2 with x4. At 5 Gbit/s per lane, that translates into 20 Gbit/s, which would seem to take up the entire capacity of the Thunderbolt cable (2 x 10 Gbit/s each direction).

An example of such a controller is shown in Figure 2. The Thunderbolt device controller has interfaces for DisplayPort output and four lanes of PCIe I/O. Those PCIe lanes, however, can connect to very widely available, low-cost interface controllers for a variety of interconnect protocols that connect to the outside by way of their commonly used connector ports.

Figure 2
The Thunderbolt controller’s external signals are PCIe and DisplayPort. These can be translated by interface controllers in devices or hubs to accommodate the interconnect needs of a wide variety of devices and interconnects.

Intel has not given up entirely on the optical technology. The current electrical Thunderbolt cable can extend up to three meters and, connected to a single Thunderbolt connector on a PC, can daisy chain up to six Thunderbolt devices, the last of which can be a native DisplayPort device. By incorporating optical transceivers into the connectors at the end of an optical cable, it would be possible to plug such a cable into the electrical ports of any device and extend the reach of a cable to 50 meters. That will not, however, translate into a faster data rate, which will remain at the 10 Gbit/s capacity of the electrical implementation. According to Jason Ziller of Intel’s Optical I/O Project Office, media creators are looking for a lighter and less expensive high-speed cable.

While it is currently too early to tell, it will be interesting to see if this will constitute a competition to current implementations of PCIe over cable that are being used in such areas as data centers. It is still to be seen whether Thunderbolt will break out into the general market or whether it will remain an Apple-specific interconnect. There have been some ominous rumblings from CEOs in this arena that it is difficult if not impossible to purchase chips from Intel for Thunderbolt without Apple’s approval. If that is indeed the case, it would be a serious impediment to the spread of Thunderbolt to PCs and other system designs. Since a Thunderbolt port is also an option in the new Ultrabook specification being promoted by Intel, this seems like an uncertain situation.

It turns out that there is a misunderstanding that has appeared elsewhere in the industry press as well. According to Intel’s Jason Ziller, “We are currently in a situation where we cannot support every request that has come in, since we have had tremendous response to Thunderbolt from the industry.  If we sell Thunderbolt chips to a customer, we must be able to provide technical support to them in order for them to develop and ship a certified product.  We are currently taking steps to expand our enabling capability next year so that many more customers can develop products.  But all of this does not require Apple’s approval. “

This also brings up the question, what does something like Thunderbolt have to do with embedded systems? The answer to that is not yet clear save for the fact that developments in the PC realm, once they become high volume and low cost, tend to migrate to areas of embedded development where they are needed. Such has certainly been the case with PCIe and USB to name a few. That being said, an interconnect like Thunderbolt would seem to appeal to a narrower class of applications that demand very high performance yet, although functionally quite specialized, nonetheless have a large user base. Among these are media creators. Again, this will depend on the technology itself being freely available.

However, media creators seem very attracted to the light weight and low cost associated with Thunderbolt. As device controllers get built into cameras and other video equipment, it becomes possible to bring video onto a laptop in the field in order to do things that otherwise would require bringing the video into a studio and using a workstation. Some manufacturers are reportedly making devices that perform certain specialized functions that were previously implemented on cards plugged into workstations so that even such targeted functionality becomes available at remote locations, speeding production and lowering costs.

Currently the main applications for Thunderbolt fall into three areas. The first is high-performance storage with high-speed, high-capacity SSDs gaining significant ground. The second is media creation as indicated above and the third is expansion to multifunction devices such as multifunction monitors, I/O expanders such as cards and hubs with different interface connections. With the growing amount of video content and performance beginning to appear in the embedded space, there are an number of high speed connectivity options. It remains to be seen if Thunderbolt will be a part of that.