# Energy-Aware System Design for Autonomous Wireless Sensor Nodes: A Comprehensive Review
## 4 Energy Management
### Voltage Supervisor for Cold Start  
Energy fluctuations and extreme low availability of ambient energy may lead to a total switch-off of a wireless sensor node. Most current wireless sensor nodes are not able to recover automatically by themselves from a depleted state in a so-called cold start. This is because the energy management requires a minimal energy level to drive a microcontroller-based system, which is not always available once the energy supply reaches a defined level of voltage. This leads to an oscillating behavior of the voltage supply, where the system tries to start, but does not succeed, as the needed energy for starting can be much more than the operation energy needed later.  
To ensure the wireless sensor node recovery in these cases, so-called voltage supervisors, voltage detectors or reset integrated circuits can be implemented. Other terms are used in the literature such as buffer circuit [86], control circuit [87], start-up circuit [88; 89; 90; 91], switching circuit [92], trigger circuit [93; 94; 95] and under-voltage lockout circuit [96]. Commercial voltage supervisors, such as TS831 [97], trigger the reset pin of the microcontroller, as it consumes much more power in the reset state than the active mode. For example, the average consumption of a benchmarking program on a microcontroller (MSP-EXP430 FR6989) at 3 V is approximately 100 \(\upmu\)A, but, during a reset with the same voltage input, it consumes more than 19.8 mA, which is 198 times more and is therefore critical for the energy budget [98]. Several voltage supervisors guarantee the operation only beginning from a certain voltage threshold, which is often around 1 V or more. At lower voltages, the state is undefined, leading to half opened MOSFETs, active loads or oscillating behavior and unwanted power dissipation. Such voltage supervisors are not suitable for typical cold start problems, at low voltage levels near 0 V, which could be a necessity for distributed wireless sensors supplied by energy harvesting from ambient, especially for non-critical applications.  
In [89], a hybrid harvester setup is proposed, where the secondary harvester and a passive extraction circuit provide the start-up energy for the control of the main extraction stage. The circuit is relatively complex and the energy for start-up is extracted from a piezoelectric transducer at low efficiency. This setup provides cold-start ability, but it does not treat the power consumption of the load. Complex voltage supervisors are interesting but also introduce a high-power consumption. The direct isolation of the microcontroller from the power supply is a better approach. The use of a voltage supervisor in conjunction with a MOSFET solves the problem of the high-power consumption in the reset state, which is the case in [98], where the proposed solution uses a semi-active circuit supervising the voltage and allowing the microcontroller input to stay enabled. This has the advantage that the microcontroller can switch itself off, after performing the necessary tasks. The circuit is able to perform a cold start from completely depleted storages, consumes much less power and prevents unnecessary energy usage. This solution is suitable for sensor nodes incorporating a microcontroller with a high off resistance.  
Voltage supervisors will gain more importance in the IoT context, where a massive use of wireless sensors is intended, as the energy aspect becomes a big issue for usability as well as in applications where the energy availability is subject to large fluctuations.