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Differential Signaling Challenges: Is Serialization a Paradigm that should be Broken?

By Mike Fowler, principal strategic marketing engineer with Fairchild Semiconductor's Interface Products Group in South Portland, Maine.

Next year and following will see major growth in serial data streaming techniques in the interface microelectronics arena, as manufacturers continue to adopt the technology for intra-system connections. Advances in serialization technology over the last 20 years, combined with the increasingly obvious limitations of parallel signaling, are speeding this transition.

As electronic technology developed in sophistication, the inability of parallel transmission to accommodate higher speeds and wider words became more apparent. As it turns out, designing systems with wide parallel word paths is too cumbersome and presents serious technical challenges in the areas of noise, power, speed and cost. Alternatively, low voltage differential signaling (LVDS) combines extremely low power consumption and exceptionally low electrical and radiated noise, with data transmission speeds hundreds of times faster — up to tens of Gigahertz — versus parallel single-ended signaling.

As the name implies, differential signaling operates at the receiver by comparing the difference between two signals. The constant current used in most forms of differential signaling reduces the amount of noise induced into the electrical system. The opposing currents of the two signals that comprise the differential signal cancel out a large portion of the magnetic field of the other signal, thus reducing radiated emissions. In this way, noise induced into one signal is conversely induced into the other signal, so that the difference between the two signals is not affected. This phenomenon allows the differential signal to operate at reduced signaling levels compared to single-ended signals, thus reducing emissions even further.


Basic Intra-System Data Interface Employing a Serializer/Deserializer (SerDes) for Low-Power, High-Speed Serial Data Streaming

Improvements in differential signaling and serialization technology enabled one of the first viable applications of intra-system serialization — notebook computers. Since their inception, notebooks have employed standard parallel techniques to transmit image data from the graphics processor to the display, through the hinge that connects the base unit to the screen. Display resolution and color palette size improvements drove the need for faster speeds and more bits per pixel. The greatest drawback to using parallel single-ended techniques, however, proved not to be the issue of speed, but the substantial increase in radiated noise from the greater number of parallel signals needed to provide the additional bits. A more attractive alternative, serialization via differential signaling techniques, not only reduces emissions to meet stringent government mandates but also limits the number of wires running through the small-diameter hinge connecting the base unit to the display — further increasing the mechanical integrity and reliability of the connection.

Once crosspoint switching became available at the silicon level, serialization of the communications transmission network moved from the inter-system to the intra-system level. Transmitting the serialized data through the system in the same form it was sent over the network was the natural choice. The input signals must be synthesized to operate at faster speeds to meet the high-speed requirements of today's communications world. Both optical and high-speed differential signaling are being investigated and used to achieve these goals.

Improved serialization techniques have opened the door to new applications, particularly in the ultra-portable realm of cell phones and battery-powered devices. Cell phones, for example, are encountering many of the same challenges faced by notebook PCs almost ten years ago, including higher-resolution displays with more colors. Cameras and other convergent functions now appearing in cell phones add complexity to the challenge of sending signals across the hinge in clamshell, or flip, designs. The bi-directional microcontroller interface occasionally used to lower power between the base and the display poses a new challenge of how to efficiently provide serialization in opposite directions.

Bi-directional serialization provided the solution, but the microcontroller interface posed its own challenge for the serializer. In the past, all serializers were provided with a constant clock at the parallel data rate that was used to develop the high-speed serial clock used for the serial transmission of the data. This parallel clock was not something that was normally available with a microcontroller interface. With the latest serializers and deserializers, it is still necessary to provide a constant clock, but not necessarily at the same frequency as the interface.

Three additional considerations — power, size and cost — grew in importance with the use of serializers in ultra-portable consumer applications. Battery life, an important consideration in all mobile applications, becomes even more critical when the application itself limits the size of the battery, and the primary location of the device is in the pocket rather than on the desktop. Innovative technology leaders like Fairchild are now developing techniques that will continue to lower the active power and component size of the serializer and deserializer, while at the same time increasing the solution's cost-effectiveness.

For the last ten years at least, the industry has struggled with how to best integrate serialization. Some of these key integration issues center around the twofold need for high-speed clocking and highly tailored I/O. The integration of high-speed signaling has proved an extremely difficult challenge due to signaling power requirements and the noise generated by the rest of the device. A significant breakthrough for VLSI, LVDS is a fairly noise-free and immune technology that offers the double advantage of very low power consumption and excellent noise-rejection characteristics. The power and electromagnetic interference (EMI) requirement of mobile applications has provided a further challenge in this area. Fairchild is meeting these challenges with innovative new techniques that promise extremely low power and EMI.

Given the move from parallel to serial data streaming in faster, smaller applications, new questions arise. In the past, for example, the need to run Ethernet at precisely 10 Mbits/s, 100 Mbits/s, and 1 Gbits/s was largely unquestioned. But are there advantages now to varying serialization frequencies based on other needs? Is serialization a paradigm that should be broken, or does the industry just think in terms of serialization due to the fact that parallel used to be the primary method of transmission within a system? Should the serial techniques be moved so far back into the system that serialization of a parallel path never needs to happen again?

One thing is certain. Industry leaders will continue to drive the market with innovative new solutions that will challenge the conventional wisdom by “thinking outside the box.” The inherent drawbacks of single-ended parallel techniques, combined with the advantages of differential serial data streaming and serializer/deserializer advances, are sure to accelerate the proliferation of serialized intra-system data interfaces in all application areas.

Mike Fowler can be reached via email at Michael.fowler@fairchildsemi.com.

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