Embedded Systems Design Europe - March 2008 - (Page 35)

communications PC/104-CPU Board 500MHz, fanless, high functionality, inexpensive rugged design setup, the circuit will reliably decode Manchester-encoded signals. You will also notice that this circuit will work for both Option A and Option B. CLOCK SYNCHRONIZATION Another intrinsic value to Manchester encoding is the fact that the synchronizing clock is embedded within the signal. This fact is exploited in Ethernet, which uses on-board circuitry to maintain clock synchronization. A Digital Phase Locked Loop (DPLL) circuit monitors the incoming Manchester-encoded signal and makes adjustments to its internal oscillator to keep it in constant synchronization with the transmitter’s clock frequency. The DPLL functions by sampling the incoming Manchester-encoded data with its own local clock. A simple shift register, driven by the local clock, accumulates all the shifted bits. If the local oscillator is in synchronization with the transmitter’s clock, there will be an equal number of binary 1’s and 0’s across the shift register. If an imbalance occurs between binary 1’s and 0’s, the local clock is adjusted based on the number of binary bits off center. This is why you will ﬁnd a preamble at the beginning of each packet transmitted via Ethernet. Each Ethernet packet starts with an 8-byte (64 bit) preamble, which is used by the DPLL to “lock” into the correct frequency. Since the preamble doesn’t contain useful data, no data is lost. However, it does add more overhead to the data stream. A more esoteric version of Manchester encoding is a scheme called Differential Manchester encoding (DME). Think of it as Manchester encoding on steroids. DME is a more efﬁcient encoding scheme because it requires less bandwidth than standard Manchester encoding. The overhead of transmitting a data stream using DME is less because it doesn’t require a preamble, which is used by the DPLL to lock onto the clock frequency. Because of this, DME can be found in networks, such as fast Ethernet over copper twisted-pair wiring. DME differs from standard Manchester encoding in one simple way: Manchester encoding represents binary data based on a positive or negative edge transition at each bit boundary. DME represents data by the presence or absence of a transition between two bit boundaries. Simply stated, if a transition occurs between a bit boundary, it’s represented as a binary 0. An absence of a transition signiﬁes a binary 1. As a complement to this reintroduction to the basics of Manchester encoding for low-bit serial network applications, a second article is available online at Embedded.com. The article will leverage from the theory presented here and offer a practical, real-world example that illustrates the simplicity of implementing Manchester encoding into a real embedded design. Robert Guastella (robert.guastella@tennantco) is a senior controls engineer for Tennant Company in Minneapolis, Minnesota. He has over 22 years of experience in hardware and software design on products ranging from industrial controls, to digital servo drives, to automotive electronics. MICROSPACE MSM800SEL _ AMD Geode™ LX800 Processor with 500MHz _ up to 1 GByte DDR-DRAM SODIMM _ 4x USB V2.0, 2x COM, LPT1, FD, E-IDE, LPT1, KB, MS _ LAN Ethernet 10/100Base-T _ integrated 2D graphic controller CRT bis 1920 x 1440, 85Hz 24Bit Digital Video Interface (TFT) _ Operating Temperature 0°C up to +60°C _ Option: CompactFlash DVI- and LVDS-Converter Cablekit DIGITAL-LOGIC offers a large variety of Embedded Computers in PC/104, EPIC, EBX, 3.5", smartModule and other form factors. Further information: www.digitallogic.com www.embedded.com/europe | embedded systems design europe | MARCH 2008 35 031-032-033-034-035-ESDE.indd 35 5/03/08 17:08:14 http://www.digitallogic.com http://www.digitallogic.com 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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