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Creating winning designs for wearables

Posted: 30 Sep 2014 ?? ?Print Version ?Bookmark and Share

Keywords:smartphones? Internet of Things? IoT? microcontroller? MCUs?

One of the primary ways to optimise a low-power embedded design is to find the lowest sleep mode that still provides adequate response to real-time events. Most MCUs using the Cortex-M processing core support multiple sleep modes.

Silicon Labs' EFM32 Gecko family, for example, uses standard 32bit ARM Cortex-M cores combined with an energy-optimised peripheral set and clocking architecture. Designed specifically for energy-sensitive applications, this architecture features a range of power modes enabling developers to achieve the optimal energy efficiency required by wearables. Typical low-power modes used in many MCU architectures include:

Sleep/Standby: Enables quick return to active mode (usually via interrupt) at the expense of slightly higher power consumption.

Deep Sleep: Leaves the MCU's critical elements active while disabling high-frequency system clocks and other non-essential loads.

Stop: A deeper version of Deep Sleep mode that enables further power savings while retaining limited autonomous peripheral activity and fast wakeup.

Off: This 'near-death' state preserves the minimum compliment of functionality needed to trigger wakeup from an external stimulus.

Smart peripherals, equally smart designs
Many MCUs are equipped with at least a few peripherals that perform routine timekeeping, I/O, and housekeeping tasks while the CPU remains in one of its low-power sleep modes. Some MCUs are also equipped with autonomous peripherals that perform multiple functions (e.g., counters/timers, ADCs, DACs, GPIOs, serial transceivers, etc.) without CPU intervention. For example, all of the on-chip peripherals supported by EFM32 Gecko MCUs can function autonomously and remain active in one or more of the device's sleep modes.

In addition to the counter/timer, ADCs, DACs, GPIOs, and serial communication elements, many ultra-low-power, ARM-based MCUs offer the following peripherals:
???A capacitive sense controller that senses touchpad contact and coordinates within an n-by-n grid with minimal or no CPU intervention.
???An LCD driver that can drive numeric LCD or TFT display via DMAs from memory without CPU intervention.
???Analogue comparators that enable monitoring of threshold voltages for alert/alarm conditions without CPU intervention.

In some ultra-low-power MCU architectures, the activities of peripheral functions, including serial communications, counters/timers, analogue and digital comparators, and higher-level I/Os, can be coordinated by a separate low-power bus that enables events and signals from one peripheral to be used as input signals or triggers by other peripherals. This bus architecture can ensure timing-critical operation with minimal CPU overhead as well as reduced software overhead, resulting in extremely low-energy wearable designs and long battery life.

Winning the wrist-top revolution
Designing winning products for the wrist-top revolution requires a deep understanding of the new realities of wearable application requirements and a fresh approach to integrating complex technologies and high-performance components into space-and power-constrained designs. Smart watches, portable fitness trackers, smart glasses, and other wearable computing devices are changing everything we know about designing portable electronics.

Wearables are rewriting the rules for design engineers who must seamlessly integrate sophisticated sensing, computing, display, and wireless technologies into affordable, compelling, ultra-compact designs that can operate for months on a single user-replaceable battery or other limited energy sources. New wearable computing products are appearing on the market at an ever-increasing pace, resetting our expectations for the end user experience with each design innovation. And this Wrist-top Revolution has only begun.

About the author
Raman Sharma, Director Field Marketing, Americas, joined Silicon Labs through the acquisition of Energy Micro, where he served as the VP of Sales, Americas. Raman now leads the field marketing efforts for Silicon Labs' 32bit MCUs and wireless products. Raman was formerly the Global Medical Segment Manager with Freescale Semiconductor and has fifteen years of experience in product design and technical sales with companies including OKI Semiconductor, Xilinx, and Compaq Computer.

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