Power management is an often overlooked yet critical area for both consumer and industrial IoT devices. In a battery powered device, optimizing dynamic as well as static power is imperative; power optimization can be addressed in three ways:
- Power management control
- IP implemented for low power
- Power aware software
Power management control should address the inclusion of voltage and frequency scaling. For this to be optimally implemented inside an IoT device, the system designer needs to identify known power states for each of the major functional blocks within the device. The table below provides an example of valid power states for multiple blocks within an SoC.
IP blocks for IoT should be designed to include power control wrappers for power and frequency scaling as shown below. Imagination already provides power control wrappers that will enable a functional IP block to be set to a valid power state within the device.
For IP to be implemented for low power, the system designer must first identify the power management objectives. In the case of an IoT device that is turned off for significantly longer time periods of time, leakage power will dominate the power consumption of the device. In the example of leakage power domination, process selection (i.e. choosing a process technology with low leakage) is an imperative. In addition, implementing the chip with high Vt values and using power gating where possible will further reduce leakage.
If the device is turned on for the majority of time (e.g. sensor hubs), dynamic power will dominate. To reduce the dynamic power, voltage and frequency scaling should be implemented as a part of the power management function. In addition, choosing processes and memory IP that can operate over low voltages, such as operating in the 0.7v to 0.8v regime in a 40nm process, is highly desirable.
A useful scheme to reduce power for a CPU is close timing based on reduced values of supply voltage; this is commonly referred to as voltage scaling. For example, operating a MIPS M-class processor such as the M5150 CPU through voltage scaling at 0.95v will result in a power reduction of 23% (vs. the value when operating at the 1.08v minimum voltage).
However, a lot of the dynamic power savings would be lost if the wireless communications systems operate inefficiently. Bluetooth Smart (previously known as Bluetooth Low Energy) is positioned for very low power wireless communications, but the power reduction comes at the cost of reduced range point-to-point communications, and low data rates. For applications requiring higher data rates, Wi-Fi would be a suitable solution.
To address these requirements, Imagination has developed Ensigma Whisper, a low-power Wi-Fi offering that includes baseband processing. Energy-aware Wi-Fi is possible in Whisper by exploiting the low-power aspects within the 802.11 specification. For example, Whisper can operate 802.11n over a single 2.4GHz band radio at power consumption levels comparable to Bluetooth.
Power management is a significant issue for IoT devices that are mobile or are required to be powered by small batteries for extended periods of time. The IP used in such devices must be designed for low power and be easily integrated into SoC level power management schemes.