Sensing Products Solving Business and Technology Issues

It is difficult to imagine, when designing such tiny platforms, that there is no limiting factor around battery life. Think how differently each design decision has to be made, in order to get the best battery life possible. Just like other devices within an embedded design, sensors have to be turned on in a smart way so that they don’t burn all the available power too quickly.

Being able to have always-on sensors for complete context awareness is what everyone is aiming for. Reading data in real time out of motion or environmental sensors or even microphones can considerably expand the system capabilities and enable endless possibilities.

What is “Always-On” Capability?

IoT solutions, such as fitness tracking, indoor navigation and gesture recognition, require the system to constantly collect and process data so none of the contextual information is lost. In an always on sensing application a motion and/or environmental sensor would be collecting data constantly. The sensor is connected to the interrupt pin of the processor, where upon receiving an event wakes the processor and starts to collect and process data. Always-on experience can be achieved using different ways, for example by:

1. Using an ultra-low power sensor to interrupt the processor and wake it from sleep

2. Using techniques which take advantage of low power mode or special configuration of the sensor

3. Buffering the data i.e. “store and forward” minimizing the amount of radio setup times and batching the data to be sent.

Of course, these examples can all be used in combinations to get the best out of the system. 

We Propose an ALWAYS-ON Sensor Approach

One of the motion sensors offered by STMicroelectronics that has the best capability in term of power and performance is the LSM6DS3 which combines a state of the art accelerometer and gyroscope. The LSM6DS3 can easily provide an always-on experience thanks to multiple features offered by this chip. The LSM6DS3 provides the following:

•   Ultra-low power accelerometer and gyroscope

•   Both elements, accelerometer and gyroscope, can consume as little as 425 uA, while both on at the same time

•   Smart FIFO, which can store up to 8 Kbyte of data

•   Sensor hub, which will help collect data from up to 4 auxiliary sensors that can be directly connected to the LSM6DS3

  using an I2C master bus 

0. With the low power accelerometer and gyroscope, embedded FIFO, significant motion detection and sensor hub capability can significantly lower the overall power consumption and MCU/CPU load. Here is an example of a system using the FIFO buffer to keep the MCU in standby/asleep as long as possible while at the same counting steps with the LSM6DS3 embedded pedometer function. 

Multi-Mode Microphone

In addition to motion, a very commonly used sensor is the microphone. STMicroelectronics recently introduced a new microphone called MP23DB01MM which enables an always-on experience thanks to very low power consumption figures:

•   Dimension: 2.65 x 3.5 x 0.98 mm3

•   Digital output interface

•   Acoustic overload point: 120dBSPL

•   SNR: 64.5 dB (Normal mode)

•   SNR: 64.5 dB (Low Power mode)

•   Sensitivity: -26dBFS

•   Supply voltage: 1.6V to 3.6V DC

•   Max Current consumption:

o 650 uA (Normal mode)

o 230 uA (Low power mode)
 PSR: <-80dB across whole audio band (both operative modes)

This digital microphone works in three different operation modes:

•   Normal mode: high performance, high current consumption

•   Low Power mode: reduced performance, low current consumption

•   Power Down mode: standby, minimum current consumption 

The operation mode is chosen detecting the clock frequency. With a low power mode consuming of 230 uA, IoT solutions are now enabled for an always-on experience.

MP23DB01MM is best suited for battery operated application such as wearable or portable devices, featuring voice activation and always-on capabilities. Combining Audio + Motion are 2 key sensing features used by Android as part of Contextual Awareness Hub.

Environmental Sensors

Environmental sensors can also be used in an always-on experience, with currents as low as 4 uA on pressure, humidity/temperature and UV index sensors. 

Putting a Whole Solution Together Using BLUEMICROSYSTEM1

BLUEMICROSYSTEM1 is an expansion software package for STM32Cube. The software runs on the STM32 and includes drivers that recognize the Bluetooth Low Energy (BlueNRG) and four sensor devices (HTS221, LPS25H, LSM6DSx, LIS3MDL) as well as auxiliary sensors which can be plug-in to the DIL-24 socket. The expansion is built on STM32Cube software technology to ease portability across different STM32 microcontrollers. The software comes with examples of implementation of the drivers running on the Bluetooth low energy expansion board (X-NUCLEO-IDB04A1) and the Motion MEMS and environmental sensor expansion board (X-NUCLEO-IKS01A1) when both are connected to a STM32 Nucleo development board (NUCLEO-F401RE). 

Success Story

A design was started from scratch using a BLUEMICROSYSTEM1 board where all the elements were put together and all example codes were downloaded from STMicroelectronics website. After working on the firmware to reach the desired solution, the design was miniaturized to obtain the final form factor. The firmware was then ported to the final board. BLUEMICROSYSTEM1 allowed a very fast design turnaround time.