# Green IoT: A Review and Future Research Directions
## 7 Energy-Efficient Microcontroller Units and Integrated Circuits
However, it is crucial to note that sequential instruction execution and low operating frequencies may result in performance that is insufficient for the requirements of applications when operating at low voltage. The utilization of software parallelism is a key component to enhance the performance of ultra-low power processors while preserving a high level of flexibility. As it has been demonstrated that under some circumstances parallel processing at low voltage can be more energy-efficient than sequential computation at a higher voltage, using multicore platforms can benefit high application workloads [130]. High-end embedded applications, where multi-core architecture has become the standard, have made full use of this concept. Table 7 summarizes recent processors that are energy-efficient in terms of different features. Additionally, Figure 17 compares the power efficiency of multi-core processors vs. single-core processors at various operating points. Each operating point is dependent on the workload as well as the frequency and voltage required to achieve the performance shown. At low operating points (workload), single-core processors (blue line) are more power-efficient than multi-core processors (red line). This is because single-core processors can enter low-power states more easily than multi-core processors. When idle, single-core processors can enter sleep states, which significantly reduces power consumption, whereas multi-core processors often have to keep at least one core activity to manage system tasks [131]. However, as the operating point increases, multi-core processors become more power-efficient. This is because the clock frequency of single-core processors increases linearly with the operating point, while the power consumption increases quadratically. In contrast, the clock frequency of multi-core processors increases less steeply with the operating point, and power consumption increases at a slower rate [132]. In conclusion, the power efficiency of multi-core processors vs. single-core processors at various operating points is a complex topic that depends on several factors. At low operating points, single-core processors are more power-efficient, but as the operating point increases, multi-core processors become more power-efficient. The number of cores, cache size, workload, and power management techniques all play significant roles in the power efficiency of multi-core processors. \begin{table}
\begin{tabular}{c c c c c c c} \hline
**FeatureProcessor** & **MSP430** & **ReiSC** & **TMS320C64x** & **FRISBEE** & **ARM CortexM3** & **OpenRISC** \\ \hline Number of cores & 1 & 1 & 1 & 1 & 64 & 4 \\ Data format & 16-bit & 32-bit & 32-bit VILW & 32-bit VILW & 32-bit & 32-bit \\ Technology & CMOS & CMOS & CMOS & FD-SOI & CMOS & FD-SOI \\ VDD range (V) & 0.4 (1.0) & (0.4-1.2) & 0.6–1.0 & 0.4-1.3 & 0.65–1.15 & 0.32–1.2 \\ Max freq. (MHz) & 25 & 82.5 & 331 & 2600 & 80 & 825 \\ Power cores. (\(\mu\)W/MHz) & 7.7 & 10.2 & 409 & 62 & 317 & 20.7 \\ Best Perf. (MOPS) & 25 & 57.5 & 662 & 2600 & 1600 & 3300 \\ Energy eff. (MOPS/ mW) & 64.5 & 68.6 & 4.5 & 16 & 3.9 & 193 \\ \hline \end{tabular}
\end{table} 
Table 7: Overview of recent processors that are both energy-efficient and ultra-low-power.