# Green IoT: A Review and Future Research Directions
## 7 Energy-Efficient Microcontroller Units and Integrated Circuits  
A microcontroller unit (MCU) is an energy-efficient and cost-effective solution for embedded computing applications. It has programmable input/output peripherals, memory, and one or more processors, all integrated into a single circuit. This makes it ideal for use in IoT technology, particularly in devices powered by batteries or solar energy [119]. To improve energy efficiency, the data processing component should only be active for brief periods during which it reads, processes, and transmits information. The rest of the time, it remains inactive, a process known as duty cycling. The microcontroller unit normally waits for a sensor event or internal timer to wake it up, while it is in a deep sleep state for the majority of the time. Upon waking, the device restores power and clock functions, retrieves new data, processes it, and then returns to sleep after saving its state [108]. The power consumption pattern of a microcontroller commonly used in IoT applications is illustrated in Figure 15. Figure 16 provides an overview of key features for several MCUs frequently employed in ultra-low-power applications. However, many commercially available low-power microcontrollers are not able to cater to the performance needs of certain applications within the limitations of small coin batteries and energy-harvesting devices [120]. The latest CMOS ICs have the capability to significantly enhance energy efficiency by delivering the required functionality while maintaining a compact size and extremely low voltage. This can result in a remarkable reduction in overall system size and a lifespan increased by as much as ten times [121; 122]. The strategy is to decrease the voltage supplied to the chips to a value just above the threshold voltage. Extensive research has been conducted on aggressive voltage scaling, including its limitations and drawbacks [123]. A major challenge with operating at low voltage levels is a decrease in performance, which can restrict the extent to which voltage scaling can be employed for a given processing need. Using application specific integrated circuits (ASICs) to implement hardwired functionality to get around the performance loss in ultra-low voltage devices is a popular way to solve this issue. With frequencies of tens to hundreds of KHz and power consumption ranging from a few to hundreds of microWatts, digital processing  
Figure 16: A summary of the key characteristics of MCUs commonly employed in ultra-low power applications. Figure 15: Demonstration of typical power consumption scenario of a microcontroller for generic IoT applications. systems are capable of meeting the performance requirements of applications even at extremely low operating voltage by using specialized circuits. These specialized systems, which are developed as system-in-package (SiP) or system-on-chip (SoC) devices, combine analog signal processing, analog front end, digital signal processing, and power supply circuits (batteries, harvester, or both), resulting in highly compact designs [124]. The traditional fields of ultra-low power device applications, such as wearable or implanted sensors for health monitoring, have made extensive use of this dedicated ASIC approach [125; 126; 127]. While these devices greatly reduce power consumption, they lack flexibility because their functionality is limited to the specific service. Additionally, their performance cannot be adjusted, as they are created with a particular use case in mind. However, these restrictions can be lessened by introducing some runtime reconfigurability to the integrated circuits and by extending the working range through voltage and frequency scaling when the intended algorithms are more general and can be employed for numerous applications. There are numerous examples of this type of device, including vision sensors, where fundamental operations executed with specialized processors or dedicated accelerators can be shared among various applications [128; 129].