# Microcontroller Unit-Based Wireless Sensor Network Nodes: A Review
## 1 Introduction  
Of late, there have been numerous research efforts in the fast-growing field of wireless sensor devices. This is being driven by the advent of many Internet-of-Things (IoT) applications and scenarios that utilize wireless sensor networks (WSNs) for providing an efficient sensing and communication infrastructure. A wireless sensor network consists of distributed wireless sensor nodes or motes that can be used for detecting various physical and environmental phenomena. The nodes collaboratively send the sensed data to a base station or sink for further processing and decision-making. Examples of sensed parameters include temperature, pressure, moisture, motion, liquid level, etc. Wireless sensor networks are very useful in a wide range of applications that include environmental monitoring, precision agriculture, IoT, water quality monitoring, animal tracking and so forth [1, 2, 3, 4]. Overas the years, several practical wireless sensor nodes or motes have been developedand introduced in the market to facilitate the implementation of various wireless sensor network scenarios. Advancements in wireless sensor node technologies [5] facilitate the cutting-edge research and real-world implementation of WSNs in countless applications around the globe. A sensor node typically consists of the sensing, computation, communication and power supply sub-systems. The wireless sensor node platforms are resource-constrained when it comes to the power supply, computation and storage capabilities, etc. [6]. Some of the objectives of designing and developing a wireless sensor node include ultra-low-power operation, low cost per node, small size and reconfigurable software and hardware. The designing of increasingly smaller and more affordable wireless sensor node platforms of ultra-low power consumption has become a hot research topic for the research community [7]. The lifetime of a WSN strongly depends on the supply of energy for powering the sensor motes. Traditionally, sensor nodes are powered by small batteries of limited capacities. In numerous application scenarios, many deployed sensor motes are required to last unattended for an unlimited lifetime in remote and harsh fields. In such deployment scenarios, battery replacement becomes a big challenge. The energy consumption of the motes directly impacts the lifetime of the WSNs. Therefore, ultra-low power approaches together with energy-harvesting techniques are critical towards the ubiquitous and perpetual operation of WSNs [8]. Due to the different requirements of various WSN applications, different wireless sensor node platforms are being developed. For instance, some tracking and monitoring application scenarios of WSNs require the use of wearables. Over the years, several wireless sensor nodes based on the MCU, field programmable gate arrays (FPGAs), system-on-chip (SoC), application-specific integrated circuit (ASIC) and other platforms have been designed and developed [9]. In the literature, there is a limited number of review articles on wireless sensor node platforms. In 2009, [10] presented a comparative review of only six selected sensor motes. Authors in [2] provided a survey on the protocols, platforms and simulation tools of selected wireless sensor platforms. A comparison of the performance of two commercial motes in practical situations was performed by [11]. A review of sensor motes targeted only for IoT applications was presented in 2018 by [12]. However, in this proposed study, the selected wireless sensor nodes are used for a wide range of applications. Furthermore, the authors of [9] provided a detailed review on wireless sensor nodes spanning from simple MCU-based to complex programmable logic devices. In the literature, there are several other published articles that list and compare different motes that can be used for different IoT applications [13; 14; 15; 16; 17]. However, these articles do not include the motes that have been recently proposed and published in the literature.