## 2 Challenges, Trends and Constraints in WSAN Hardware Design
### Open-Source-Hardware Architectures versus Open-Architecture Systems
#### 2.3.2 Power Management Constraints  
1. _Poor energy conversion efficiency_: Due to the fact tha, all the OSH expandable architecture solutions have originally been designed for pilots and proof of concepts in indoor test environment, they disregard the need for efficient power management. For WSAN applications in the field of agriculture where the hardware nodes have to be battery-operated, the existing OSH architectures entail problems, because these solutions are not energy optimized. Arduino-like as well as the SBC hardware solutions require external +5 Vdc power sources, which in most of the cases is coming from the USB port of a personal computer. Some of these solutions also accept external supply voltages above the +5 Vdc, usually ranging from +7 Vdc up to a maximum of +18 Vdc. To keep the manufacturing cost down, the shields designers' choice, for the external voltage management, is to use linear voltage regulators. These electronic components don't require much physical space on the boards, but, on the other hand, they suffer from very low energy efficiency. Also, the higher the external supply voltage value is from the base +5 Vdc value, the more energy is lost in form of heat at the regulator's package. Therefore, the use of this type of voltage regulators, in the multi-board architectures, downgrades the overall energy efficiency of the final system. 2. _Inability to ensure the power of the expansion shields_: After the regulation of the external voltage, the voltage supply signal of the MCU, e.g., +5 Vdc, is routed to the related BECs pins in order to power the expansion shields. Unfortunately, the amount of power that can be drawn from the expansion shields is limited by the particular voltage regulator of the main-board shield [88]. When the expansion shields require levels of power higher than that sourced from the main-board,then, some or all of the shields must have their own power source circuits, in order to be able to accept external power. 3. _Power cabling ataxia:_ The high power consuming shields have their own connection terminal blocks or headers separated from the common BECs. In this way, the total shields constructions are suffering, not only from poor energy inefficiency, but also from power cabling ataxia. The power cabling burdens the ESD and EMI performance and makes the system vulnerable to noise interferences. This problem is significantly escalated when, in the multi-board system, there is the need for secondary voltages, e.g., +3.3 Vdc for some shields with low power MCUs. 4. _No provision for voltage signals other than logic levels:_ Various difficulties appear when higher voltages than the basic +5 Vdc and +3.3 Vdc, e.g., +12 Vdc, are required to drive actuators' loads. In this circumstance, the use of particular external power supply units for the system is mandatory. From the physical layer perspective, none of the existing OSEP mechanisms of expansion is supporting the physical connection of multi-value voltage signals. 5. _Energizing unnecessary circuits:_ In the expandable architectures boards, it is very popular for the main-boards and their shields to have some extra circuitry for general-purpose use, e.g., LEDs, MEMS-based sensors, etc. Actually, this is a very common practice also in WSAN end-solution and COTS solutions [43]. Such extra circuitry may be useful for testing, during the development phase, but it is totally useless in the final in-situ application, because it wastes significant amounts of energy. For battery-operated WSAN systems in the agricultural environment, this testing circuitry degrades the valuable available energy. Unfortunately, the existing OSEP architectures do not have any provision for this constraint, so there is no mechanism for developers to disengage that extra circuitry, in order to build energy optimized systems.