Low Power Wearable Device

Technology

Wearable technology is creating a big buzz in the market. The technology is on the rise in both personal and business use. There are variety of amazing products available in the market with unique functionality and great features. From glasses to shoes and watches to gloves, everything which is smart and can be worn comes under wearable Technology.

 

All of us are familiar with the fact that watches have always been a timeless piece, and as the big brands like Apple watch and other smart watches have entered the market, changes or advancements have become the need of the hour and will enable you to survive in the competition.

 

To give a tough competition to our competitors, we always came up with something unique and innovative devices which helped us to stay ahead or at par with our competitors. This time also we challenged ourselves to develop “World’s First Mechanical hybrid Smart Watch.

 

Idea

It started when we got a client’s requirement to develop a smart watch with all the features in it and which maintain its analog look in intact form. But this was not like any other ordinary watch, it should have Dual mode Bluetooth (BLE and Classic), Transparent organic light-emitting diode (TOLED), Capacitive touch panel for good user experience and human activity tracker. Every now and then we generally hear people saying this- “It’s just another device to charge every day” and “too cumbersome and too limited in use” are the common refrains. So, this client wanted a watch that runs on low power consumption and lasts for a long time with all above mentioned features.

 

We knew that it wouldn’t be a cakewalk to compete in this dynamic market of technology and innovation with any ordinary wearable device. But, we really felt the essence of these words when we started working on it.

 

Having been involved in the green-field design of various wearable concepts, we understand the right combination of hardware and firmware that goes into a turnkey product.

 

Hardware

Most of the wearable devices available in the market run on  battery supply. Now, the main challenge was to make a smart watch that should last for many days on a single charge with all provided functions.

 

Taking into consideration all these things, we tried to make the best out of the available hardware and firmware options. The hardware in it comprises a microcontroller, TOLED with bonded capacitive touch panel having a small resolution, Bluetooth dual mode supported chip (Bluetooth classic and Bluetooth low energy), 9-Axis sensor, vibration motor, microphone and an external storage flash.

 

Talking about the firmware architecture, it contains RTOS, graphical lib, Bluetooth stack and human steps tracking lib.

 

Knowing the fact that system firmware architecture is responsible for low power application, a lot of time and energy was being spent on research and development.

 

Firmware

The firmware architecture used in the watch is designed in such a way that for a large portion of time, it keeps the main microcontroller and other peripherals of the watch in either shutdown or sleep mode which ultimately saves power. To suffice this, an entire firmware is divided into granular tasks and each task follow the scheduling algorithm and it executes when it is needed based on time or interrupt. There is no dead loop in the task which hangs the system for a long time to serve a particular task.

 

RTOS is used in firmware as a middle layer to help in scheduling. In this RTOS layer, we used “Tickless idle mode”, which optimizes the power consumption by keeping the system in sleep mode when no pending task or interrupt is observed.

 

Another challenge was to work on the system frequency to make it highly efficient.

 

System frequency

The system consumes more power when the operating frequency is higher and vice versa. So, taking this into consideration when system is ready to go in deep sleep mode, its main frequency is lowered up to a considerable level to save power and then the system goes into deep sleep mode. It rolls back to original frequency when any known interrupt is observed. In active mode, the main system frequency is reduced as much as possible without compromising its overall performance.

 

For communication, UART interface is used between Bluetooth chip and Microcontroller. To keep the system in sleep mode for a  longer time, the flow control pins RTS and CTS are used which wakes up the microcontroller when any data available over Bluetooth.

 

We used internal sleep feature of Bluetooth chip that keeps the Bluetooth chip in shutdown mode when no activity is observed over it. In sleep mode, a UART peripheral is de-initialized, its supply clock is stopped, and pins (RX and TX) are configured as an input, also CTS pin of the controller is configured as an interrupt pin which is tied to RTS pin of Bluetooth chip. So, when Bluetooth chip has any data, microcontroller gets initialized through interrupt and prepare system  to handle data from Bluetooth chip.

 

In deep sleep mode, almost all other peripherals are de-initialized and pins are configured in such a way that it consumes less power.

 

Bluetooth connection parameter

Bluetooth feature is that which allows other devices like mobile, laptop, etc. to connect with it. To reduce the power consumption in BLE mode, Bluetooth provides parameters which need to be tuned accordingly. But, tuning the parameter somehow affects the performance. So this parameter is tuned on the basis of the requirement of application.

 

The following parameters which are being tuned are mentioned below:

 

  • Connection Interval
    Through this parameter, Central device identifies that how often it ask the data from the peripheral. It allows the range between 7.5 ms and 4s.
  • Slave latency
    Based on this parameter, peripheral doesn’t respond when central unit asks for data up to the number of this parameter’s time. This enables the peripheral to stay for a long time in sleep mode.
  • Connection supervision timeout
    This parameter determines the timeout ranging from the last communication happened till the time link is considered lost.
  • Advertising Interval
    Based on this parameter, Bluetooth peripheral device goes in advertisement mode and transmits advertising information and may respond with more information upon request from other devices. It must be between 20ms to 10.24 seconds. It could be in step of 0.625ms.
     
    This parameter needs more attention to configure as it has an effect more on discovery and connection performance. As per our experience, this could be the reason to consume more power if the interval is set very low. So, we considered the apple Bluetooth design guideline and then set this parameter.
     
    In our case, Minimum interval time is 546.25 ms and Maximum interval time is 558.75 ms. (Click here to download pdf file) 
  • Deep sleep mode
    This is the mode when system consumes a very small amount of power to operate itself. So mainly when there is no activity, the system prepares itself to get into this mode. Before getting into this mode, the system stops all peripherals, configured related pins to an adequate status (input pull up or pull down, open drain) which lowers down its leakage current, system main clock frequency is reduced as much as possible, use “Tickless idle mode” of RTOS, configured some of the pins as interrupt pin to awake microcontroller. In this mode, the display remains turned off and capacitive touch panel is configured in sleep mode.

 

Battery charge analysis

For optimal use of battery charge, it is required to analyze  battery charge / discharge vs time curve. Here we have practically analyzed this curve and defined the lower threshold of charge on which the watch performs well. From this analysis report, we have implemented battery life algorithm and achieved more battery life further.

 

Achievement

All these things succeeded us in designing this smartwatch and helped us to come up with “First mechanical hybrid smart watch”.

 

What makes our hybrid smart watch stand out was that we were the first movers in this category to launch such device. Finally, we were able to ship 3000 pieces in a targeted time frame and it leads to 40% reduction in development cost for our client.

 

After this great achievement, large numbers of clients approached us with requirements of smart wearables like smart earpieces, clothes and shoes in wearable category. This was just a small step towards success, there are still a lot of miles to go.

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