Embedded Systems Design Europe - March 2008 - (Page 32)

communications a proprietary one-wire network. This novel design provides power and communications all on a single wire. These devices work well for low-cost designs where the power required from the single wire is not signiﬁcant. In wireless network applications, numerous solutions based on the use of the ZigBee protocol atop the IEEE 802.15.4 standard for physical layer and medium access control in low-rate wireless personal area networks are available off-the-shelf from companies as diverse as Amber, Freescale, Microchip, and Texas Instruments, among others. In addition, Microchip has an even lower bit-rate solution, combining its more sparse proprietary MiWi protocol atop the 802.15.4 physical layer. Freescale, in addition to its full ZigBee products, offers developers alternatives that use lower bit-rate protocols atop the 802.15.4 physical layer. WHEN OFF-THE-SHELF IS NOT ENOUGH But even with so many options, both wired and wireless, available off the shelf, there are a signiﬁcant number of networked control designs where cost and physical size continue to place a heavy burden on our designs. In those cases, the usual list of suspects may not be viable after all. Embedded developers are then challenged to dig deep and ﬁnd solutions that satisfy their performance requirements, while maintaining a small physical size and a lower product cost, all without sacriﬁcing data integrity. For those systems requiring more power than is capable from one-wire solutions, standard off-the-shelf components may not be feasible. The solution may require a custom design that strives to balance functional requirements with a number of signiﬁcant constraints. When cost constraints are tight and performance requirements less stringent, it might be worth your while to use a “back to the future” strategy and consider the use of a well-known and welltested serial communications protocols 32 such as Non-Return to Zero (NRZ) or Manchester encoding as a possible solution to your embedded design. All digital serial communications share one thing in common; they all send a series of ones and zeros. From this point, the differentiating factor becomes the level of the signal, the organization of the data, how the data is encoded, and how the data is synchronized. In a design that is sensitive to component availability and overall cost, it’s highly desirable that serial data is transferred with a minimum number of wires and is built from readily available components. NON-RETURN TO ZERO (NRZ) One of the simplest ways to transmit digital data is by having a separate clock and data line. In this approach, a clock signal of constant frequency is synchronized with its corresponding data. Depending upon the preference of the designer, the data is either latched on the rising or falling edge of the clock. Figure 1 illustrates a typical NRZ implementation. In this example the binary string “1010011” is being transmitted. Each bit is identiﬁed on the rising edge of the clock signal. As simple as this process is, you need to consider at least three fundamental drawbacks: 1. An additional line must support the clock signal. This is required to latch the data accurately. Depending upon the quality and length of the transmission line, additional circuitry may be required to provide the proper drive capability. 2. The second problem occurs if you decide to eliminate the clock signal. In this scenario the receiver will require an internal clock that is in near-perfect synchronization with the transmitter. Any phase shift between the transmitter and the receiver can cause bit errors to occur. This may seem a trivial matter. However, when the clock frequency becomes high enough, the sensitivity to phase differences between the transmitter and receiver becomes more signiﬁcant. 3. A third limitation occurs when an NRZ transmission contains a long string of ones or zeros, as shown in Figure 2. This can make the syn- MARCH 2008 | embedded systems design europe | www.embedded.com/europe 031-032-033-034-035-ESDE.indd 32 5/03/08 17:07:23 http://www.embedded.com/europe

Table of Contents Feed for the Digital Edition of Embedded Systems Design Europe - March 2008

Embedded Systems Design Europe - March 2008 Distributors to Increase Embedded Focus Kontron and Quanta to Join Forces Coverity Raises $22m as European Business Booms Help is at Hand for Europe's Industrial Control Developers Milestones in Embedded Systems Microsoft is Recruiting for Embedded Center in Aachen European Designers to Win Cash for Green Designs Duo Work on Smaller Form Factor Europe Invests in Real-Time Java for Multicore Systems Curtiss-Wright Buys Pentland Systems Designing DSP-Based Motor Control Using Fuzzy Logic Lower the Cost of Intelligent Power Control with FPGAs Virtualizing Embedded Linux Back to the Future: Manchester Encoding Is Multicore Hype or Reality New Products Advertising Contacts

Embedded Systems Design Europe - March 2008

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