Embedded Systems Design Europe - March 2008 - (Page 33)

communications tions for encoding the data. There has been some debate over which scheme is more advantageous. The Ethernet and IEEE standards describe Option B as the method for encoding data, while the original Manchester-encoded speciﬁcation described Option A. Confusion may occur when looking at the voltage signals on a Manchester-encoded line. Most line drivers will invert their signals, which have led some to believe that Option B is being implemented. In the end, when developing your own custom-designed embedded system, the choice as to which option is best may be purely academic. However, once a choice has been made, consistent implementation is a necessity. With this information, let’s construct a Manchester-encoded bit stream using Option A. We will use the binary bit pattern shown in Figure 1. The ﬁrst thing to do is establish what are called bit boundaries. These are the points in time where a level transition occurs. Bit boundaries are analogous to one clock period in an NRZ scheme. It’s these bit boundaries that deﬁne the points where Manchester encoding of the individual bits will occur. Figure 4 illustrates the bit stream constructed using Manchester encoding. Each point where you see an arrow is deﬁned as the bit boundary. The arrow indicates the direction of the transition. So, using Option A from Figure 3, the ﬁrst binary “1” bit is translated by a transition from a high to a low level. One clock period later, another transition occurs. This time a binary “0” bit has been encoded by a low- to highlevel transition. Further down the bit stream, you will notice a transition that looks out of place. It occurs at a point halfway between two clock periods. This is called the setup point. The reason for the setup point is to ensure the signal is at the correct level prior to the next bit boundary. CONSTRUCTION OF MANCHESTERENCODED DATA Manchester encoding is very easy to construct. You simply combine the sewww.embedded.com/europe | embedded systems design europe | MARCH 2008 chronization between the transmitter and receiver even more sensitive to bit encoding errors. Since the line is in one state for a relatively long period of time, there are no transitions. Without transitions on the data line, it becomes impossible to see where a bit boundary is located. This can result in erroneous data encoding. The next section describes an encoding scheme that addresses these issues and provides a unique alternative to the traditional method of data transmission. MANCHESTER ENCODING Manchester encoding offers distinct advantages over other digital encoding schemes. It has become a popular standard for low-cost radio frequency communication of digital data. Even Ethernet employs Manchester encoding that was used to deliver this article to your computer (if you’re reading this online). So what exactly is Manchester encoding and how can it be used effectively in a low-cost embedded systems design? Manchester encoding was ﬁrst developed back in the late 1940s at the University of Manchester in Manchester, England. Given the time period and location, one with a proclivity for history might be inclined to believe its development was perhaps a by-product of research done by an obscure World War II code-breaker at England’s infamous Bletchley Park. In reality, Manchester encoding was the result of research done at the University of Manchester into phase modulation techniques used for reading and writing digital data onto a magnetic storage device. Since that time, Manchester encoding has gained wide acceptance as the modulation scheme for low-cost radio-frequency transmission of digital data. One of the most signiﬁcant characteristics of Manchester encoding is its unique way of representing digital data. Rather than representing data based on a particular level, Manchester encoding uses transitions (see Figure 3) to identify a binary one or zero. In more traditional encoding schemes, a separate clock signal determines when to sample the data line. Manchester encoding uses one signal to identify the data. Figure 3 shows two different op- 33 031-032-033-034-035-ESDE.indd 33 5/03/08 17:07:38 http://www.embedded.com/europe

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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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