A system-level view of the energy-efficient microcontroller
Today’s microcontrollers offer a range of low power features, and the last section showed that next-generation microcontrollers will significantly improve energy efficiency further. However, the microcontroller is only one component in a typical system. Integrating multiple components at the board level has the potential to increase energy significantly without proper attention.
The energy associated with toggling board-level traces and the associated I/O drivers within each component can be very large. More importantly, most board-level components are not designed with the same attention to energy-efficiency as today’s leading low power microcontrollers and can easily dominate an energy budget.
As an example, consider a microcontroller-based system that incorporates energy harvesting functionality at the board level, as shown in figure 5. A solar cell harvests energy from incident light and a boost converter component boosts the solar cell output to charge a battery. A leading commercial microcontroller may have a 500nA functional sleep mode with SRAM retention and a timer running. However, a typical low power boost converter component may draw 10µA or more, effectively dominating the energy budget.
Fig 5: Board-level energy harvesting system.
It will therefore be the job of low power microcontroller providers to incorporate key functionality into next-generation microcontrollers. Single-chip integration also has the important benefit of reducing the bill of materials in a system. Microcontroller vendors are beginning to incorporate wireless communication in their devices (and vice versa), and features like support for energy harvesting will follow shortly. We again use the example of the Archimedes Microcontroller to illustrate the benefit of integration.
As shown in figure 3, Archimedes includes on-board boost converter. By applying many of the low power techniques used elsewhere in the microcontroller, the boost converter consumes only a fraction of the 550pW functional sleep mode power budget. Figure 6 shows Archimedes integrated with a Cymbet 12µAh solid-state battery and an ultra-compact solar cell in a volume of only 8.75mm3
. Due to the extreme energy efficiency achieved by Archimedes and its integrated boost converter, a tiny battery and solar cell are capable of delivering power for nearly perpetual operation. This clearly shows that highly integrated microcontrollers have the potential to significantly improve system-level energy efficiency.
Fig 6: Energy harvesting system with Cymbet solid-state battery (bottom layer), Archimedes microcontroller with integrated boost converter (middle layer), and solar cell (top layer).
Energy efficiency is becoming a central concern for the chip industry as chip users seek to develop more compact wireless devices with longer operational lives. Microcontrollers, in particular, are at the heart of these compact wireless devices. In this paper, we reviewed the key sources of power consumption in microcontrollers and considered Archimedes, the world’s most energy-efficient commercial grade microcontroller. Studies of Archimedes revealed some of the techniques used to minimize energy consumption in today’s microcontrollers and provided a snapshot of some new features that will emerge in next-generation microcontrollers. It is this next generation of microcontrollers that will truly drive the growth of compact wireless devices emerging today.
1. G. Chen, M. Fojtik, D. Kim, D. Fick, J. Park, M. Seok, M.-T. Chen, Z. Foo, D. Sylvester, D. Blaauw, “Millimeter-Scale Nearly Perpetual Sensor System with Stacked Battery and Solar Cells,” International Solid-State Circuits Conference, pp. 288-289, 2010.
About the author:
Scott Hanson, email@example.com
, is CEO of Ambiq Micro Inc, Austin, Texas.
This paper was presented at ARM TechCon 2010 - more details of ARM TechCon 2011 in October are available at www.armtechcon.com
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