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Maintaining good touchscreen user experience

Posted: 31 Jul 2013 ?? ?Print Version ?Bookmark and Share

Keywords:Capacitive touchscreens? smartphones? Tablets? battery life? LCD?

The LCD is a big portion of the power draw from the overall system. Power usually scales with larger screens due to the increased LCD size. One way of maintaining battery life is to put a larger battery pack in the system. However, this increases the weight of the system and affects the user experience in terms of portability. Another alternative is to decrease performance by reducing refresh rate, reducing transmit voltage, disabling various digital filters, or using the lowest possible analogue and digital power supplies. Again, these solutions negatively impact the user experience so they are not ideal options.

As weight and performance are key factors to a good device, the best resolution for extending battery life is to optimise power draw for individual components in the system. From a touchscreen controller point of view, that means having flexible power management schemes for the device.

The overall power consumption depends on the state or usage of the device (figure 4). A smart and energy efficient touchscreen controller has multi-state power management in which each state has a unique scheme to lower power consumption, such as an active state, low power state, and deep sleep state. This is all managed by the touch controller's configuration parameters.

The active state provides the fastest touch response time because the touchscreen is actively scanned to determine the presence of a touch and identify the coordinates.

The low power state is entered when no touch is detected after a certain time during the active state. This state further reduces power with corresponding increase in the response time. Any touch detected will automatically switch the device into active state.

The deep sleep state has the lowest power consumption. No scanning is performed and no touches are reported. An interrupt is required to wake up the touch screen controller and put it into active state.

Figure 4: Power usage depends on LCD UI configuration state.

The various power states are determined by the system environment. For example, if the screen hasn't been touched in a while, the system will deactivate the user interface to save battery life. This is done by the host managing the components in the device, for example by turning off the LCD screen and placing the touch controller into a low-power state. When a touch is detected in the low-power state, the touchscreen controller will transition to active mode and continue scanning to determine the touch coordinates on the panel. If no touch is detected in the low-power mode, the host will drive the touch controller into deep sleep to conserve power. These dynamic power management states provide consumers flexibility between touch performance and power consumption for mobile devices on-the-go.

Maintaining satisfactory user experience as touchscreens grow takes a system wide approach. Touchscreens are limited by physics, and if capacitive touch is to remain the technology of choice in mobile consumer electronic devices, then ingenuity and integration are key. New touchscreen materials are being developed to increase panel speeds, and host processing architectures are being defined to offload some of the heavy number crunching. Hardware and software improvements are constantly being made to increase signal strength while filtering out noise. A system wide approach to power consumption is being used to increase battery life. Making this all more cost effective is the next big challenge for designers.

About the authors
Todd Severson is a Product Marketing Engineer for TrueTouch touchscreen solutions at Cypress Semiconductor Corp. He has a BS degree in Engineering Management with a concentration in Mechanical Engineering from the United States Military Academy.

Henry Wong is a Senior Product Marketing Manager for TrueTouch touchscreen solutions at Cypress. He has a BS degree in Computer and Systems Engineering from Rensselaer Polytechnic Institute. Henry has over 16 years of experience in engineering and marketing experience in the semiconductor and consumer electronics industry worldwide.

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