# Edge Computing: A Survey On the Hardware Requirements in the Internet of Things World
## 3 Ultra-Low Power MCU Architectures
### Near-Threshold MCU Architectures  
Generally, MCUs work with a power supply voltage well above the threshold voltage of transistors. Nevertheless, the power supply voltage can be scaled during deep sleep mode in order to reach sub-threshold condition, as shown in Figure 7. Indeed, smart voltage scaling to make transistors work in the near-threshold region leads to a new low-power era. However, making a circuit working in the near-threshold region is a complex task as reliability problems and performance degradation can arise due to several factors, including fabrication process variations.  
Figure 5: Clock gating: instead of using an enable signal, the clock is directly disabled.  
Figure 6: Power gating: regions of the circuit are decoupled by transistors from the true supply rails.  
Indeed, the behavior of a circuit can degrade due to sensitivity to process variation (i.e., channel length, doping concentration, etc.), voltage and temperature (PVT). PVT compensation requires special circuits to work correctly.  
It is worth noting that special circuits are embedded within the main circuit, so they are exposed to the same conditions of the main circuit, i.e., aging, high temperature and current. In order to be effective, these special circuits must probe the current and provide feedback, by adjusting the power supply to prevent unpleasant problems such as meta-stabilities. Common implementations of probing subsystems are canary circuits and razor flip-flops.  
Canary circuit is a replica of the critical path that is monitored in order to adjust the power supply. Though being a simple approach, it only provides information about global process variations, while no local information can be evinced as well as local PVT conditions, this is due to the fact that the real critical path is placed somewhere else.  
Razor flip-flop approach relies on lowering the supply voltage until a critical point. Working so close to the limit, errors can occur due to time constraints violation. However, no error is propagated thanks to shadow flip-flops that can restore the correct value. Shadow flip-flops are scanned using a delayed clock signal that preserves their integrity and errors are detected by comparing these values to the one in the real critical path. Global and local variations are both considered in this solution since the device works in borderline conditions. In order to correctly apply razor flip-flops feedback, the designer must have access to the low-level circuit and this is not always possible, especially when dealing with externally engineered cores with no additional information than the top level interface protocol.  
In contrast to voltage supply control, transistor body biasing represents an alternative to PVT compensation. The main advantage of this solution is that adjusting transistors threshold only modifies the leakage component, whereas the above approaches impact both on leakage and dynamic power. Moreover, body biasing is very effective when working near-threshold, as it features a more efficient and simpler circuit for polarization of p-well and n-well regions than DC-DC (direct current) regulators for the supply voltage.