## 2 Challenges, Trends and Constraints in WSAN Hardware Design
### Embedded Systems Development Technologies and Market Trends
In recent years, the electronics industry realized the advantages of the open-source design approach and Arduino concept and it is foreseen as a new prosperous market. The idea of expandable modular hardware development tools and the application-centric programming concepts, of course, cannot be attributed to Arduino or to its successors. Many implementations, such as the Basic Stamp for MCU programming in Basic language at early 1990s [62; 63], and the e-Blocks modular hardware tools [64], targeted providing easy-to-build hardware embedded systems. The reason that these efforts didn't attract the popularity of Arduino concept is probably associated with the fact that they were single-source commercial solutions with negative cost and openness implications. The free support from a vast community of designers and developers, in the case of open-source platforms, has made the big difference, and it seems to be the solution to the way out of the demanding conditions for integration and use of new technology, especially in cases such as WSANs. Today, there are two popular competitive open-source platforms, namely the Arduino [60] (Figure 4a) and the Lauchpad (Figure 4b) from Texas Instruments (Dallas, TX, USA) [65]. For each platform, there are several add-on boards aiming to provide application-specific functionality, produced by the original creators or by third parties such as companies or individuals from various users' communities. Both Arduino and Launchpad provide several alternatives, in terms of processing power, number of input/output pins and peripherals. Figure 4: Expandable open-source platforms: (**a**) Arduino Uno; (**b**) Launchpad MSP430. In the meantime, all the key-player semiconductor manufacturers decided to launch Arduino-like, or Arduino-compatible, platforms, in order to promote their own new MCUs and microelectronics portfolios. Among these platforms, they are the Nucleo from ST-Microelectronics (Geneva, Switzerland) [66], the FRDM from Freescale/NXP (Eindhoven, The Netherlands) [67], the XPresso from NXP [68], and Blackfin DSP platform from Analog Devices (Analog Devices Inc., Norwood, MA, USA) [69]. Others, such as Infineon (Infineon Technologies AG, Neubiberg, Germany), have launched application-specific Arduino shields [70]. As the acceptance of the open-source expandable platforms (OSEP) increased, the introduction of the single-board-computers (SBCs) extended the capabilities of such platforms, regarding the processing and computational power, the use of open-source operating systems such as Linux, interface of low-cost USB communication modules (WiFi, ZigBee, Bluetooth, GSM modules etc. ), and the connectivity with cameras and screen displays, interfacing with audio sources and outputs, etc.. The most of these SBCs are miniature in their physical dimensions (i.e., credit-card sized), low-power and low-cost compared to mini computers. The expandable SBCs allow very easily the development of web-based applications that is very useful for WSAN applications in remote areas (common in agriculture). Some of the SBCs provide hosting of Arduino shields, in order to ensure compatibility with all the already existing application shields. Thus, the result of this compatibility is the reusability of hardware implementations. Among the most popular SBCs are the BeagleBone (BeagleBoard.org Foundation, Oakland Twp, MI, USA) [71], the Raspberry Pi (Raspberry Pi Foundation, Caldecote, Cambridgeshire, UK) [72], and the Galileo from Intel (Santa Clara, CA, USA) [73]. The most recent of these derivatives, namely the Raspberry Pi 2, the BeagleBone Black, and the Galileo Gen2 are illustrated in Figure 5. Following the introduction of these SBCs, other movements to the same direction took place either from Arduino (e.g., Arduino Tre, Leonardo, and Due) [74], or from well-known semiconductors industries such as Freescale/NXP (FRDM Kinetis KL64) [75]. A well-documented presentation and comparison of all the existing SBCs is given in [76].