Low‐power MCU benchmarking: what datasheets don’t tell youThe discussion of generating low power products started decades ago. Since then, the industrial engineering and scientific community have developed new energy sources, architectures, and integration technologies. Battery capacity and technology have been extended to deliver better components for ultra‐low power products. Semiconductor companies have developed energy aware microcontrollers, system architectures, and low‐power, low‐leakage process technologies. Low‐power integrated oscillators, sophisticated power management systems, and higher performing analog circuits enable more sophisticated solutions.
These advances and new software methods have enabled operating code with lower energy demand and/or more functionality at the same or reduced energy levels. In this article we discuss the advantages and traps of different implementations of ultra-low power. We discuss parameters of components and the relevance of proposed solutions, and conclude with an outlook on the future and trends of ultra‐low power advancement.
Introduction to ultra‐low power
Many articles and publications address low power consumption, but there are no agreed upon criteria defining low‐power systems and components such as software, embedded processors, single devices or device families. Also, there is no widely established method supporting low‐power or ultra‐low power benchmarking.
Presently, low‐power programmable processors are expected to operate in high and medium performance applications. Low‐power processors are in smart phone and tablet computers, deal with internet connectivity and AV‐streams, and are used in challenging industrial and automotive control. Low‐power programmable applications are dealing well with energy at high performance levels.
Compared to low‐power systems, ultra‐low power and MCU devices operate with restricted energy budgets and long operating times. The charge budget present for end‐user products is often much smaller than 1000mAh and the expected operating time is many years. Ultra‐low power applications optimize the total system energy performance and may manage the energy source, MCU device, life‐time environment and software, and system hardware. Energy efficiency involves much more than CPU architecture, the number of operating modes, and current parameters at room temperature.
Circuit aspects to get to ultra‐low power
Ultra‐low power MCUs have several operating modes in addition to individual enable and power‐on control capabilities. The digital portion of the microcontrollers use clock and power gating to reduce energy consumption. The power gating minimizes the leakage current while clock gating optimizes dynamic current consumption.
Datasheets and user guides reflect the individual implementations, which vary from vendor to vendor. They reflect current parameters for the different operating modes, and the user guides provide the system wake‐up conditions when returning from low‐power modes. This information also enables the user to identify all data and parameters to be saved and restored. Many sources discuss background operation; no software is executed but the MCU still executes functions and tasks required by the application.
Ultra‐low power applications and MCUs are dealing with a restricted amount of energy. For further exploration, a CR2032 coin cell (Figure 1) is used with 230mAh @3V. A part of the coin cell’s energy is exhausted through self‐discharge; another part is not viable due to the increased source impedance near end‐of‐life. Both are assumed to take 20% of the capacity away.
Figure 1. At the heart of many ultra-low-power microcontroller designs is the familiar CR2032 battery cell used in many consumer and mobile electronics and wireless embedded devices.
The source impedance leads to increased voltage drop and prevents the full discharge, avoiding reset situations. Thus, the entire system has 184mAh for all hardware, code execution, and low‐power modes. 40% is used for the system hardware; the other 60% of energy is used for code execution and the real‐time clock mode.
The energy budget for executing code depends on the device and process technology. Calculating the available average current for executing code confirms that the temperature of the system or product has a significant influence on the ultra‐low power performance. One conclusion is that the architecture is important but the device specifications are more so – at least in ultra‐low power applications.
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