## 2 Challenges, Trends and Constraints in WSAN Hardware Design
### Embedded Systems Development Technologies and Market Trends  
Embedded systems technology has strongly been influenced by the dramatic changes that have occurred in the mobile phone market. In the last decade, consumer demand for ever more powerful smart phones have driven the electronics industry to design and manufacture high processing and low power MCUs and microprocessors (MPUs). This evolution has helped the introduction of mobile computing devices, e.g., tablets, etc., which in turn has acted as an additional reason for the development of new semiconductors, processors, sensors, batteries, and communication modules. Due to economies of scale of such markets, the cost of embedded systems has significantly diminished, whilst the cost-to-performance ratio has increased notably. Consequently, the design of hardware WSAN solutions has vastly been affected by the aforementioned changes. In Figure 3, the most important changes in technologies and approaches associated with the sub-parts of the typical WSAN node system, in the last decade, is illustrated. Obviously, nowadays, experienced designers and developers have plenty of choices at their disposal, in order to build either end-to-end generic commercial solutions or optimized application-specific solutions. In particular, in the field of agriculture, there are stand technologies for energy, communications, processing, etc. that can positively help towards the development of reliable and vigorous outdoor WSAN systems. Of course, the lack of skills and knowledge make these efforts difficult. Figure 3: Technological trends and changes influencing the design of a WSAN hardware system. On the other hand, this technological breakthrough brings about hurdles race conditions, because the revolutionary technologies have to be quickly assimilated and used, new development tools have to be launched to support designers in the previous effort as well as to produce new technology in their turn, whilst at the same time, under the pressure of stakeholders for robust and standardized solutions, the new solutions have to be commercialized as soon as possible. As a response to this perpetual need, many significant developments have been made by the electronics industry and market, in order to provide new tools, methods and solutions that encapsulate new technology and allow fast prototyping (such as the Mbed [56] and Codebender [57], and i-Sense [58]). The real revolution came from a new area of systems development, the so-called open-source software and hardware. According to this concept, the various design artifacts, i.e., documentation, circuits, software code, hardware project implementations, application case studies, etc. are freely shared among big users' communities under the license scheme of Creative Commons Attribution Share-Alike, which allows for both personal and commercial derivative works as long as the credit to original creator is granted [59]. The most successful case of the open-source design approach is the Arduino platform [60]. The Arduino platform (Arduino SRL, Scarmagno, TO, Italy) is a MCU-based board using an Atmel AVR 8-bit MCU, which provides all of the microcontroller's pins to pass-through pin-headers. Through these headers, all the major functional peripherals of the MCU are available to users. Users, in their turn, can connect other personal or commercial hardware boards, called shields, to the main Arduino board, in order to build their own specific applications. In order to make easy the firmware development process to users without much experience in embedded systems, Arduino provides a ready-made library of APIs in its integrated development environment (IDE). Thus, developers dispose an open-source hardware and software platform that allows the expandability and reusability they are looking for. Despite the fact that Arduino was originally established for education and hobbyists [61], it soon became very popular in research and development of real-life applications, even in demanding areas, such as agriculture.