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Optimise UI power using wake-on-approach (Part 1)

Posted: 07 Jan 2014     Print Version  Bookmark and Share

Keywords:user interface  UI  wake-on-approach  proximity sensors  capacitive sensing 

Portable electronics that run on batteries such as mobile phones and tablets have stringent power requirements. Designers must leave no stone unturned to optimise the power consumption of these devices and enhance battery life. In such applications, and other applications as well, optimisation is achieved by making the device consume as little power as possible. Thus, it would make a significant difference if most of the blocks do not run when the device is not in use and the blocks turn ON and start running only when the user starts using the device.

This means that the device must have the intelligence to detect the start of the usage of the device and starts all of the required blocks. One simple way is to run just the user interface (UI) processing until the UI senses the user's input. When the UI detects a user's input such as a button touch or a press, the entire device wakes up and starts running all the blocks that are necessary.

Furthermore, if the UI is capable of detecting the approach of a user's hand, then all the blocks of the device, including those of the UI, can be turned OFF except for scanning the sensor that detects the hand approach. These proximity sensors are capable of detecting human hands or any conducting object at a distance without the need of any contact between the object and the UI. A device with proximity sensors operates in low power mode until a hand approaches. In the low power mode, only proximity sensors are scanned and pretty much no other activity is performed. Upon detection of a human hand, the proximity sensor wakes the device to active mode in which all the necessary blocks of the device get turned ON.

The proximity sensors that wake up the device from low power mode to active mode are called wake –on- approach proximity sensors.

Capacitive, inductive, and infrared are commonly known proximity sensing techniques. In applications like mobile handsets, laptops, white goods, and home appliances where the user interfaces are mostly touch panels, capacitive proximity sensing is being widely adopted because of greater reliability and aesthetics.

This article series explains how a capacitive proximity sensor can be used as a wake-on-approach sensor, and different applications where proximity sensors are used as wake-on-approach sensors are described. It also discusses the power consumption optimisation wake-on-approach proximity sensors bring and how to implement a wake-on-approach proximity sensor, hardware and software aspects of it, and design considerations.

Applications such as a wireless mouse, mobile phones, tablet PCs, remote-control backlighting, and laptop keyboard backlighting adopt such techniques that wake the device when the user starts using the device. These applications use proximity sensors to switch from low power mode to fully functional active mode.

Let us consider a capacitive-touch sensing device in low power mode when only the proximity sensor is scanned. Scanning only the proximity sensor reduces the total scan time, thereby reducing the average power consumption. When the user's hand approaches the user interface panel, the proximity sensor detects the presence of the hand and wakes up the capacitive device. Once woken up from its low power mode, the capacitive device moves to active mode and scans all the button sensors to detect the touches.

You might find it hard to indentify buttons on a TV front panel or tablet PC panel when you want to operate them as most of the user interfaces of today's UIs are touch panels and the buttons are hardly visible and indistinguishable from each other without backlighting. However, backlight LEDs for buttons, especially in portable devices like mobiles, tablets, PCs, etc., reduces battery life. A typical usage of wake-on-approach proximity sensors is to control the backlight on user interface panels using the proximity sensor. Whenever the device is in low power mode, the backlight is turned OFF to indicate the inactive mode of the equipment. Once a user's hand approaches the panel and the proximity sensor detects the same, the backlight is turned ON aiding the user in touching the correct buttons.

Another instance of backlight control is that the backlight LEDs have special LED effects like LED fading, breathing effects. But LED effects consume more power and hence the battery operated devices cannot afford to always drive effects on LEDs. With proximity sensing, only when the user starts using the device do the LEDs light up with special effects.

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