Matthew Vildzius
ECE4007 L03 EM1
Advisor: Eric Maxwell
Group TB-Eye
Ultra Low Power Microcontrollers
Introduction
Battery powered devices are more prevalent and more powerful than ever before. Advances in
battery technology and low power electronics have enabled new applications such as wireless
health monitors. A key component in many current battery powered devices is a low power
microcontroller (MCU) that controls the device. Recent advances in microcontroller technology
enable devices that have up to twenty years of battery life but may perform millions of
instructions per second when active [1]. This report looks at leading ultra low power
microcontrollers on the market today.
Commercial Applications
Low power microcontrollers are often used in battery powered devices such as remote controls,
wireless sensors and data collection, and consumer devices. Power drawn by the MCU is
minimized by keeping the device in sleep mode unless it is activated by another component of the
circuit. Low power MCUs are used in wireless health monitors such as heart rate bands or the
Nike+ sensor for running shoes. These devices must record and wirelessly transmit data to a
display unit (typically a watch or smartphone), and are expected to have long battery life. The
Nike+ sensor has a non-replaceable battery rated to last 1000 hours of normal use, or several
years if used for one hour per day [2]. The Nike+ sensor uses one of Microchip’s Extreme Low
Power (XLP) microcontrollers, the Microchip PIC16F688, in addition to a low radio module and
an accelerometer [3]. The PIC16F688 device consumes 1uA in sleep mode with the Watchdog
Timer running, and as low as 1.1mA in run mode at 1MHz [4]. The PIC16F688 is available for
$1.33 each [5].
Another series of ultra low power microcontrollers is the TI MSP430. The MSP430F20xx, for
example, uses 0.6uA in sleep mode with the Watchdog Timer active, and just 300uA running at
1MHz [6]. The MSP430 has several unique features, such as brown-out protection that requires
no additional power but is still able to reset the device if the supply voltage drops, and the ability
to automatically disable unused peripherals like the analog to digital converter and serial
communication circuitry [7]. A MSP430F2013 sells for $3.33 each [8].
Underlying Technology
There are several techniques microcontroller manufacturers can use to reduce power
consumption. Typically a low power MCU spends most of its time in a sleep state where most of
the device is inactive. In different levels of sleep, on-chip peripherals such as timers and parts of
the CPU may be shut down. To reduce power farther, the clock may be slowed down or turned
off. In the lowest power state the only parts of the MCU that remain active are the RAM and a
circuit to handle external interrupts to wake the device [9]. An important feature is the ability to
quickly stabilize the high speed clock when the device wakes from sleep so that instructions can
be executed and the device can go back to sleep mode without consuming too much energy. The
TI MSP430 can stabilize an 8MHz clock in just 292ns, compared to up to 1ms for some
competing devices to reach full speed [9]. Another specification to consider is low IO pin
leakage. At low current scales, leakage current on the IO pins becomes an issue especially on
high pin count devices. The TI MSP430 also uses a new type of random access memory (RAM)
called Ferroelectric RAM, or FRAM. Unlike Dynamic RAM (DRAM), FRAM is non-volatile, so
no power is required to preserve data. FRAM also has an advantage over traditional EEPROM
that is commonly used for non-volatile storage in that it can withstand trillions of write cycles,
not just thousands [10].
Building Blocks for Implementation of Low Power Microcontrollers
An important part of a low power microcontroller device is software that minimizes power
consumption by turning off unnecessary peripherals, using the microcontroller efficiently, and
staying in sleep mode as much as possible. TI provides a recommended program flow as well as
information to write programs that minimize power consumption. The code running on the device
must be configured in a specific way to take advantage of features such as the low power
watchdog timer and varying clock frequency [11]. In addition to software, the hardware
connected to the microcontroller must be designed with low power in mind. It is important to
keep the MCU in sleep mode as much as possible and wake it up with external interrupts from
another part of the circuit when necessary. Continuously sampling a sensor, for example, would
use prohibitive amounts of power so instead another solution must be found, such as triggering
the microcontroller when the sensor detects a signal of interest [3]. By combining a carefully
designed circuit, efficient code, and a low power microcontroller, it is possible to make a device
that has years of battery life without sacrificing functionality or user experience.
References
[1]
Microchip Technology, “nanoWatt XLP eXtreme Low Power PIC®MCUs,” Apr. 2010
[Online]. Available: http://ww1.microchip.com/downloads/en/DeviceDoc/39941d.pdf
[Accessed 5 Sept. 2011].
[2]
Nike, “Nike+ SportBand User’s Guide,” [Online]. Available:
http://nikeplus.nike.com/nikeplus/utility/v1/en_US/help.html [Accessed 6 Sept. 2011].
[3]
D. Carey, “Runners get iPod virtual trainer,” EETimes, Aug. 2006. [Online]. Available:
http://www.eetimes.com/design/other/4004584/Runners-get-iPod-virtual-trainer
[Accessed 6 Sept. 2011].
[4]
Microchip Technology, “14-Pin Flash-Based, 8-Bit CMOS Microcontrollers with
nanoWatt Technology,” PIC16F688 Datasheet, 2007.
[5]
Digikey, “Digi-Key - PIC16F688-E/SL-ND,” digikey.com, Sep. 2011. [Online].
Available:
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=PIC16F688-E/SLND [Accessed 5 Sept. 2011].
[6]
Texas Instruments, “MSP430F20x3, MSP430F20x2, MSP430F20x Mixed Signal
Microcontroller,” MSP430F20x3, MSP430F20x2, MSP430F20x Datasheet, Aug. 2005.
[Revised Aug. 2011].
[7]
Texas Instruments, “MSP430F21x1 Architecture Summary,” Application Report
SLAA217, Oct. 2004.
[8]
Digikey, “Digi-Key - 296-19705-1-ND,” digikey.com, Sep. 2011. [Online]. Available:
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=296-19705-1-ND
[Accessed 5 Sept. 2011].
[9]
Texas Instruments, “Choosing An Ultralow-Power MCU,” Application Report
SLAA207, Jun. 2004.
[10]
Texas Instruments, “Low-Power FRAM Microcontrollers and Their Applications,” White
Paper SLAA502, Jun. 2011.
[11]
Texas Instruments, “MSP430 Software Coding Techniques,” Application Report
SLAA294A, Mar. 2006 [Revised Aug. 2006].