Ultra-low power microcontrollers for compact wireless devices
This paper describes design strategies for ultra-low power microcontrollers for use in various existing and emerging wireless applications. Emphasis is placed on practical techniques to reduce both active and standby mode power. Design examples draw from experience with the ARM Cortex-M3 core.
In particular an 8.75mm3 sensor system is detailed that includes a low voltage ARM Cortex-M3 microcontroller, custom 3.3fW leakage per bit SRAM, integrated solar cells, a thin-film battery, and an integrated power management unit. The 7.7µW system consumes only 550pW in a functional sleep state between sensor measurements and harvests energy from the solar cells to enable nearly-perpetual operation.
A dramatic paradigm shift is underway in the chip industry. For decades, the industry has bee n chasing gigahertz; companies have continued to sell chips by consistently making them faster. However, the industry has very quickly shifted its focus away from speed and toward energy efficiency. The chip industry finds itself today on the verge of a new generation of compact wireless devices embedded in all everyday objects, from smart credit cards to smart clothing to smart homes and buildings. With tens or hundreds of these smart objects for each person, the costs of daily recharging (or even weekly, monthly, or yearly recharging) are prohibitively high. Energy efficiency has therefore become the chief concern.
Microcontrollers sit at the heart of this new generation of compact wireless devices. The key challenge for microcontroller users is achieving unprecedented energy efficiency while also meeting the functional and performance requirements of increasingly feature-rich products. The ARM Cortex-M architectures have offered a platform with an excellent blend of energy- efficiency and performance. However, the architecture is only one piece of a complex puzzle. In this paper, we look at a range of chip-level and system-level considerations for the users of energy-efficient microcontrollers.
We begin with a short summary of the sources of energy consumption in a typical microcontroller. We continue with a peek inside an extremely energy-efficient microcontroller. We use an implementation of the Cortex-M3 architecture called Archimedes to illustrate the energy saving techniques available in cutting-edge microcontrollers. This particular implementation is the world’s most energy-efficient commercial-grade microcontroller and uses energy efficiency techniques that will sit at the heart of next-generation microcontrollers.
An understanding of this microcontroller’s internal functionality is useful for readers trying to understand energy-efficiency trade-offs in a microcontroller. We then take a system-level view and discuss the integration of an energy-efficient microcontroller with other off-chip components. We again use the aforementioned Cortex-M3 implementation as an example of system-level integration issues.
Readers of this paper should take away the following key points:
- Sleep mode energy is extremely important in microcontrollers and can easily dominate a system’s energy budget without careful management.
- Next-generation aggressively voltage-scaled microcontrollers with will offer dramatic active mode energy reductions over today’s microcontrollers.
- The availability of functional sleep modes will allow microcontroller users to maximize energy efficiency.
- An energy-efficient microcontroller and energy-efficient code can easily be defeated by poor system or board-level design. Highly-integrated microcontrollers can help address this problem.
Energy components in a microcontroller
Before examining the Archimedes microcontroller, it is important to consider the sources of energy consumption in a typical device. A microcontroller’s energy budget can generally be broken into three components: 1) active mode energy, 2) sleep mode energy, and 3) wake/sleep transition energy (i.e., the energy consumed when moving between sleep and active modes), as shown in Figure 1.
Fig 1: Sources of energy consumption in a microcontroller.
In a typical wireless sensing application like the temperature monitoring of a pharmaceutical product, an interrupt (e.g., a timer expiration) wakes up the microcontroller. After any sleeping components (e.g., memories, regulators, clock generators, etc.) have been woken, the chip does some computations and sensor measurements, saves the sensor data in local memory, and occasionally communicates the data over a wireless interface. Once these tasks are complete, the microcontroller returns to sleep mode until the next interrupt arrives.
The importance of each component of energy consumption varies from application to application. Figure 2 shows the relative contribution of each component for a typical commercial low power microcontroller with active mode demands of 200µA/MHz and sleep mode current of 500nA with RAM/register retention and a low power oscillator running. It is assumed that the microcontroller cycles between a 100ms active mode with clock frequency of 1MHz and a sleep mode of variable length.
It is further assumed that the microcontroller takes 1ms to transition between sleep and active modes and that power ramps at a steady rate from the sleep mode value to the active mode value (an overly simplistic assumption that is used for illustrative purposes).
Active mode energy consumption is the dominant source of energy consumption for short sleep times. However, for sleep times of 50s or more, sleep mode energy consumption actually dominates the total energy budget, a key takeaway for this paper. Though instantaneous power is significantly lower in sleep mode than in active mode, considerably more time is spent in sleep mode, and the total energy consumed in sleep mode soon dominates the total.
Sleep times of 50s or more are quite common in many wireless sensing applications like temperature logging for pharmaceuticals, temperature/humidity monitoring in homes/buildings, and a range of other data logging applications. Note that wake/sleep transitions can be a considerable energy component in certain systems but were a negligible energy component in this system.
Addressing energy consumption in the Archimedes MCU
At the extremes of energy-efficient system design, it is vital to understand the inner workings of the microcontroller itself. Incorrect use of the microcontroller and its features can easily increase energy consumption by an order of magnitude. The last section showed that active mode power and sleep mode power are the two key components requiring attention in a typical system. In this section, we explore how these sources of energy consumption are being attacked today in microcontrollers and how they will be addressed in next-generation microcontrollers.
As an example, we use a cutting edge Cortex-M3 microcontroller that has demonstrated unprecedented energy efficiency for any core, including the most efficient 8-, 16-, and 32-bit architectures. A block diagram for this research prototype microcontroller, called the Archimedes Microcontroller, is shown in figure 3 [1]. It includes a Cortex-M3 core along with 3 kB of retentive SRAM, 2kB of non-retentive (i.e., power-gated) SRAM, a capacitance-to- digital converter, and a temperature sensor.
Fig 3: Archimedes microcontroller block diagram
Archimedes uses several well-known techniques to reduce active mode energy. The use of the Cortex-M3 architecture, in particular, helps to address active mode energy. The Cortex-M3 is ideally suited to low active mode energy as it is a lightweight core with very few gates. Furthermore, the Cortex-M3 uses Thumb-2 technology to deliver better code density than 8-bit and 16-bit processors and can complete many typical operations in fewer clock cycles (and with less energy) than 8-bit and 16-bit processors.
Archimedes also uses several new techniques to reduce active mode energy. Most notably, Archimedes operates digital circuits at a supply voltage as low as 0.4V using an internal voltage regulator. This aggressive voltage scaling reduces the amount of charge used to represent a “1” and reduces active energy quadratically (assuming ideal voltage down-conversion).
Figure 4 confirms the dramatic active mode energy savings achieved as a result of voltage scaling (neglecting the power overhead of voltage conversion). Such aggressive voltage scaling has not traditionally been used since it requires a complete redesign of the microcontroller system, particularly the SRAM structures. The Archimedes device includes custom SRAM arrays capable of robust operation below 0.4V. New technology advances, like these custom SRAM arrays, have only recently made possible the commercial roll-out of an aggressively voltage- scaled design. Next-generation microcontrollers using aggressive voltage scaling will enable dramatic active mode energy reductions.
Fig 4: Active energy consumption as a function of supply voltage in the Archimedes microcontroller.
In addition to a focus on active mode energy, Archimedes includes a wide range of techniques to address sleep mode energy, a particular challenge for today’s microcontrollers. The Cortex-M3 architecture enables sleep mode energy reductions since it supports sleep modes in which unused components can be power gated. The Archimedes device includes components that are entirely power gated, partially gated, or ungated in sleep mode, as shown in figure 3. Most commercial microcontrollers today support various sleep modes in the same way.
However, architectural support for sleep modes is insufficient alone. While microcontrollers can achieve energy efficiency by power gating all components in the deepest sleep modes, such a mode is not practically useful to the microcontroller user. These deep sleep modes can only be exited by pulsing the reset signal, which generally requires that some other chip at the board level be active.
We introduce the concept of a more useful functional sleep mode in which components like SRAM, timers, and brown-out detect circuits remain active. It is vital that these components be implemented with energy-efficient circuit designs. The availability of extremely energy-efficient functional sleep modes will be critical tool for microcontroller users looking to improve power consumption.
The Archimedes microcontroller includes a custom 3kB retentive SRAM array that consumes only 3.3fW per bit or approximately 80pW for the entire array at 0.4V. Along with a custom sleep timer with special low power design, wake-up controller, and custom voltage converter with special low power design, the entire chip consumes only 550pW in a functional sleep mode. These power numbers are approximately 1000 times better than those of the leading low power microcontrollers today. While Archimedes is a research prototype not ready for commercial roll-out, it clearly shows that next-generation microcontrollers have room for significant improvement over today’s devices.
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.
References
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, shanson@ambiqmicro.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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