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Smartwatch doubles as IoT device with Bluetooth

Posted: 18 Sep 2014 ?? ?Print Version ?Bookmark and Share

Keywords:wearable? smartwatch? IoT? low power? Bluetooth?

The Cortex-M series has been implemented in an extremely wide range of general-purpose microcontrollers from many of the world's leading semiconductor companies. However, a lot of existing wearable products have used the highly flexible ARM Cortex-M3, as it consumes a small amount of power while delivering optimal performance and code density.

In fact, the high availability and sheer cost competitiveness of Cortex-M3 based MCU solutions from so many chipmakers has meant it has become ubiquitous and highly attractive to developers. An example of a Cortex-M3 based MCU is ST's STM32 MCU family, which has already seen wide deployment in many wearable products such as the Fitbit Flex activity tracker and the Pebble smartwatch.

MCUs based on the Cortex-M0 and Cortex-M0+ are not only enabling ultra-low-power coupled with performance but also provide advantages for developers in terms of size and integration. One example is the Freescale Kinetis KL03 MCU, which is claimed to be the world's smallest ARM-powered MCU.

Based on a Cortex-M0+ processor running at 48MHz, the device is available in an ultra-small 1.6 x 2.0mm2 wafer-level chip-scale package (CSP). According to Freescale, the KL03 consumes 35 per cent less PCB area, yet delivers 60 per cent more GPIO (general-purpose input/output) than the nearest competing MCU.

One difference between the Cortex-M processors is instruction set support. The Cortex-M0, Cortex-M0+ has a reduced set of instructions which reduces the complexity (and size) of the core. The richer instruction set of the Cortex-M3 and -M4 is better suited to more complex data processing. The Cortex-M4 also offers DSP instructions and an optional single-precision Floating Point Unit (FPU). Minimising power consumption is absolutely critical in wearable products. The Cortex-M processors feature two, architecture-defined sleep modes which deliver static power figures of less than 0.7W/MHz for the Cortex-M3 and Cortex-M4.

ARM Cortex-M

Figure 1: The ARM Cortex-M series of processor cores.

MCU and Bluetooth combination

While Cortex-M based MCUs can clearly be combined with a low-power single- (Bluetooth LE) or dual-mode Bluetooth module in a tiny, yet powerful, wearable product, an alternative approach that delivers greater integration and targets more complex designs is perhaps the use of a System-on-Chip (SoC) device that combines both a Cortex-M processor core and Bluetooth LE transceiver functionality.

One example is the nRF51822 from Nordic Semiconductor, which is a highly flexible multi-protocol SoC that is ideally suited for Bluetooth LE and 2.4GHz ultra-low-power wireless applications. The nRF51822 is built around a 32bit Cortex-M0 core with 256kB Flash and 16kB RAM. The embedded 2.4GHz transceiver supports Bluetooth LE version 4, in addition to proprietary 2.4GHz operation.

Nordic Semiconductor nRF51822

Figure 2: Block diagram of the Nordic Semiconductor nRF51822.

A second example is Dialog Semiconductor's SmartBond DA14580, which is a single-mode Bluetooth LE version 4.0/4.1 SoC that integrates the Cortex-M0 processor. The DA14580 draws only 4.9mA during transmission and reception and less than 600nA at 3V in deep sleep mode.

The device can also run from voltages as low as 0.9V and is also ideal for energy harvesting in completely autonomous systems. Available in a wafer-level CSP, with dimensions of just 2.5x2.5x0.5mm, the DA14580 requires only five external components and can operate from a single coin cell, enabling use in the smallest of wearables.

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