Back to the future: Manchester encoding - Part 1
When commercial options fail, try using Manchester encoding and other time-tested protocols in low-cost, low bit-rate serial communications.
When developing an embedded system that requires basic serial network communications, the list of design options are plentiful. But when all of the off-the-shelf alternatives fail to meet your requirements, it's worth remembering that traditional and well-tested protocols, such as Manchester encoding, offer some unique alternatives to the current commercial methods of wired and wireless control-data transmission.
The off-the-shelf choices are numerous. Many of today's microcontrollers offer such a wide variety of network peripherals that the possibility of not finding the right one for your application seems highly remote. Whether your system architecture requires a peer-to-peer or multi-drop topology, standards such as RS-232 and RS-485, to name a few, have loyally served the embedded development community for a long time. In addition, today's technology has expanded to include CAN bus and Ethernet as possible embedded network solutions.
For applications that are cost-sensitive, have a low data rate, and are physically constrained, some component manufacturers offer a variety of low-cost devices that interface to a simple network scheme. Dallas Semiconductor has taken it one step further and designed a family of components that communicates via 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 well-tested serial communications protocols such as Non-Return to Zero (NRZ) or Manchester encoding as a possible solution to your embedded design.