Proving the Business Case for the Internet of Things

Optical heart rate sensor cuts complexity for wrist-based wearables

Steve Rogerson
January 19, 2016
Silicon Labs has introduced an optical heart rate sensor designed to reduce the cost and complexity of wrist-based heart rate monitors (HRMs).
The Si1144 HRM includes a low-power optical sensor module paired with an energy-friendly EFM32 Gecko microcontroller (MCU) running an HRM algorithm. The module integrates an optical sensor, green LED, LED drivers supporting up to two external LEDs, analogue-to-digital converter, control logic and an I2C digital interface.
According to Texas-based Silicon Labs’ market estimates, 100 million units of HRM-enabled devices will be sold per year by 2018, with the majority of those units being wrist-based wearables. This HRM product addresses this large and growing market across a wide range of wearables including activity-tracking fitness bands, pedometers and smart watches, in addition to providing HRM capabilities for gym fitness equipment, bathroom scales and geriatric monitoring devices.
Heart rate monitoring is one of the most sought-after biometric sensing technologies available today for people of all fitness levels, from serious athletes looking to improve athletic performance to people simply seeking a healthier, more active lifestyle. Accurate HRM enables precise calculation of expended calories, making it easier to maintain dieting regimens.
Traditionally, heart rate measurement has been limited to the use of chest straps linked to an external device, such as a specialised fitness watch or a smartphone. These HRM products pose unique problems: chest straps are often inconvenient and uncomfortable to wear while smartphones can be difficult to monitor when running or cycling.
Wrist-based HRM technology changes the biometric monitoring game by providing a more convenient, comfortable way to measure heart rate rivalling the accuracy of chest-strap-based designs. The measured results vary widely, and many of these HRMs are costly as well as power hungry, reducing battery life. Complex motion artefacts also combine to make a wrist-based HRM a complicated design challenge.
“Silicon Labs’ Si1144 HRM addresses the challenges of wrist-based heart rate measurement by providing an accurate, easy-to-implement and cost-efficient solution priced well below competitive offerings,” said Daniel Cooley, vice president of marketing for IoT products at Silicon Labs. “Additional benefits of Silicon Labs’ HRM include industry-leading power consumption and a very small footprint, making the Si1144 module a very good fit for power-sensitive, space-constrained wearables.”
Features include accurate sensing of weak blood flow signals on the wrist and a choice of two algorithms to support static HRM and optional dynamic, motion- compensated HRM using data from an external accelerometer .
It pairs an optical module with a Pearl Gecko MCU containing a DSP-enabled ARM Cortex-M4 core for low-power, high-performance designs or with a Cortex M3-based Jade Gecko MCU for simpler, cost-sensitive designs. It has less than 500nA standby current with 1.7 to 3.6V supply voltage.
The device comes in a ten-lead 4.9 by 2.85 by 1.2mm LGA module package with I2C serial communications at up to 3.4Mbit/s data rate. Samples and production quantities are available today.