How to maintain the user experience during the touch screen

Screen sizes of touch-screen consumer electronic devices are increasing year by year. The touchscreen has become popular through smartphones and has quickly seized the tablet sector. With the release of Windows 8, the touchscreen is developing in the fields of hyper-, notebook and All-in-one (all PCs). As the screen size continues to grow, the main challenge facing capacitive touch is how to meet the user's expectations and make the larger size screen have the same high performance as the mobile screen. This means that more nodes on the larger surface need to be scanned at the same time. In addition, to achieve an ideal user interface experience, the processor must be able to work with less signal and more noise, while striving to maintain its speed, accuracy, and responsiveness.

Apple launched the iphone in 2007 to soar the use of capacitive touch screens in consumer electronics. The 3.5-inch screen device introduces a multi-touch user experience that changes the way users interact with electronic devices. Now, the touchscreen has become a standard configuration for consumer electronics such as digital cameras (DSC), portable navigation devices (PND), e-readers, tablets, hyper-computers, and All-in-one machines (AIO). As we can see, one of the main trends in the development of these devices is to challenge larger screen sizes. Capacitive touch screen in the development of ultra-polar or notebook computers and other new market segments, but also continue to develop their existing market areas. OEM manufacturers of top smartphones have moved from smartphones to super phones to differentiate their products by providing customers with larger screen sizes.

The main product breakdown types of current consumer electronics products are as follows: Smartphones with a 3-5-inch screen, 5-8-inch super cell phones or tablets, 8-11 6-inch tablets, 11.6-15.6-inch ultra-polar, and laptops up to 17 inches. In its 5-year product history, tablets were considered the fastest-growing mobile device, predicting that sales would overtake PCs by 2015. As a result, PC vendors are beginning to shift their focus to user-friendly touch design, such as flip laptops with the same functionality as laptops or tablets.

Users expect large-screen devices to have the same performance and touch experience as smartphones. The use cases that large-screen devices need to deal with are usually different from what we see on smaller handsets. Laptops or PCs are more often plugged into the power supply, and they have a larger interface, so you can place the palm or other large objects on the type when typing, and users will usually put these larger devices on the desktop or on the knee rather than in the hands. All of these behaviors change the electrical properties of the equipment. Robust and responsive user experience mainly includes: high sensitivity, ability to track multiple mobile touches, identify and track fingers in various noise environments, identify and track fingers under various environmental conditions, and maintain acceptable power consumption to achieve ideal battery life. In other words, the nature of the user experience is the response that the system makes when touching the screen under various conditions.

The Capacitive touch screen works by transmitting the voltage to the sensor panel on the device to generate a signal charge. The touch-screen controller then receives the signal, which determines the sensor capacitance by measuring the change in the sensor's charge. The current that the chip receives is equal to the product of the panel capacitance and the voltage of the transmitting drive (Q1 = C * VTX). The underlying circuit can remove the rated non touch sensor charge, which allows the system to focus on measuring the change in the charge of the sensor as a result of finger touch. This helps to improve touch measurement, resolution, and sensitivity.

With the development of capacitive touch screen, we are faced with more and more technical challenges. The main problem with larger screens is that the emission voltage needs to cover a larger surface area, as well as an increase in the resistance and capacitance of the sensor. The touch panel is subject to a higher parasitic capacitance and resistance, which affects the time constant of the resistor capacitor (RC), which results in slower emission frequencies. The frequency of launch will affect the signal establishment, refresh rate and power consumption. Our goal is to determine the maximum firing frequency required to achieve a consistent touch response on each panel while minimizing scan time and power consumption.

Refresh Rate

The refresh rate is the number of times a touch-screen controller has measured the touch and reports it back to the host processor in a second. The higher the refresh rate, the more y data coordinates the device collects over the shorter time, providing a responsive user experience. Most consumer electronics require a touch controller refresh rate greater than 100Hz or about 10ms. Specific applications such as digital drawing boards or point-of-sale (POS) terminals require even higher refresh rates to capture and recognize signatures and fast-slipping strokes.

For large screens, maintaining a fast refresh rate is challenging because the touch controller needs to scan a larger surface area, collect data from all nodes, and then process the data. The refresh rate is mainly influenced by two factors: the scanning speed of the screen and the processing speed of the scanned data. Under the same sensor characteristics (3108 to 275), the 17-inch screen has a node that exceeds 11 times times the 5-inch screen. To maintain a 5-inch screen user experience, a 17-inch screen requires more powerful scanning and processing power.

One way to solve the scanning problem is to make sure that the touch controller has enough receive channels to scan the entire screen at once. Most touch-screen stacks are made up of sensor patterns located under the Shield glass, which contains a large number of "unit cells", which are arranged in X and y directions, where the x direction is used for launch, the y direction is used for receiving, or vice versa. The receiving channel collects data and converts the mutual capacitance in each unit cell to digital data using a Analog-to-digital converter (ADC) to allow the host to parse the coordinates of the finger touch points. If the number of receive channels or ADC is insufficient, multiple scans and longer scans of the entire panel are required. This leads to a bad user experience, resulting in fewer samples available within a given time.

One way to help solve the problem is to have a larger processor for the touch controller, or to unload part of the operational tasks into the system's main processing unit. This means that the capacitance data is sent to the host side and the algorithm is run on the application or graphics processor. One implementation is to scan the sensor with a touchscreen controller, search for the first touch, and then transfer the image to the host processor. The host then processes the entire array, filters the noise, finds the touch coordinates, and tracks the finger IDs. Parallel processing allows a large number of digital operations to be performed on a thousands of MHz multi-core processor as a touchscreen and display host.

Signal-to-noise ratio (SNR)

Snr refers to the ratio of signal power to noise power, in other words, the ratio of useful information to error or irrelevant data. The sensor on the touchscreen panel is equivalent to a large antenna that can receive system and ambient noise such as fluorescent lights, LCD screens or chargers.

The larger the screen, the larger the antenna receiving range, the more easily receive the noise and the receiving channel to reach saturation. This can greatly affect touch performance, resulting in false touch, touch interruption, or touch screen "locked" so that data cannot be reported at all. In order to eliminate these disturbances, the touch screen controller is required to enhance the signal or reduce the noise. Some of the main ways to improve the signal-to-noise ratio include increasing the transmitting voltage to enhance the signal, using hardware and digital filter to reduce the noise, or using frequency hopping away from the noise frequencies.

The increase of signal-to-noise ratio is proportional to the emission voltage. The transmitting voltage can be supplied by charge pump or Vdda drive. In most consumer electronic devices, charge pumps can be provided with a 2.7-3v power supply and can further elevate the power supply voltage to high voltage. The problem with the big screen is that the charge pump is limited in its ability to drive high capacitance panels. This means that an external pump or power supply must be added, which increases cost and power.

In the absence of sufficient signal, another method can minimize the noise. The first line of defense is to create a cleaner capacitor environment with a filter. If this method is ineffective, the second line of defense is usually to search for less intrusive frequencies using FH. As mentioned above, large-size panels have higher parasitic capacitance and resistance, which can affect resistance capacitance (RC) time constants, resulting in slower emission frequencies. Slow frequency means it is difficult to scan the entire panel outside the noise range. Higher emission frequencies provide a greater space for the touch controller to stay away from the noise source. The ideal maximum launch frequency of 350kHz or higher, but according to the customer's specific objectives, constantly in the signal-to-noise ratio, refresh rate and power balance between, to optimize each device. Stand-alone games on desktops are more responsive than power, while portable devices need to consider power consumption to prolong battery life.

Power

As mobility plays an increasingly important role in our lives, power consumption has become an important consideration for consumers in choosing portable electronic devices. Market research shows that most users believe that battery life is one of the most important features to consider when buying new portable devices.

Because of the size of the LCD screen, power consumption is usually proportional to the screen size. The power consumption of liquid crystal display screen occupies a large proportion of the whole system power consumption. One way to prolong battery life is to use a larger battery pack in the system. But this increases the weight of the system, which affects the user's portability experience. An alternative approach is to reduce device performance by reducing the refresh rate, lowering the transmit voltage, disabling multiple digital filters, or using the lowest possible analog/digital power supply. Again, these solutions can have a negative impact on the user experience and are therefore not ideal.

For good devices, weight and performance are key factors, and the best solution to extend battery life is to optimize the power consumption of individual components in the system. For a touchscreen controller, this means developing a flexible power management solution for the device.

Total power consumption depends on the status or use of the device. An intelligent energy-efficient touch-screen controller should have a multi state power management, such as active state, low power state and deep sleep state, each of which has its own unique program of reducing consumption. These are managed by the configuration parameters of the touch controller.

In an active state, the touch screen has the fastest touch response time, because the device scans the touch screen aggressively to determine the presence of the touch and identify the touch coordinates.

In an active state, if a touch event is not detected within a certain amount of time, the device enters a low-power state. This further reduces power consumption and increases response time accordingly. If the device detects any touch events, it automatically switches from a low-power state to an active state.

Low power consumption in deep sleep state. In this state the device does not perform any scanning and does not report any touch. You need to interrupt to wake the touchscreen controller and switch it to active state.

Different power consumption states are determined by the system environment. For example, if the screen is not touched for a period of time, the system will allow the user interface to stop moving to prolong battery life. This is accomplished by hosting various components on the device, such as shutting down the LCD screen and placing the touch controller in a low-power state. In a low-power state, once a touch event is detected, the touchscreen controller switches to active mode and continues scanning to determine the touch coordinates on the panel. If a touch event is not detected in a low-power state, the host drives the touch controller into a deep sleep state to conserve power. These dynamic power management states allow consumers to flexibly manage touch performance and power consumption when using portable mobile devices.

With the development of touch screen, the system-level method should be adopted in order to ensure the user experience. Touch screen is limited by physical phenomena, so originality and integration are the key to make capacitive touch technology continue to be the choice of mobile consumer electronic products. New touch-screen materials are being developed to improve the speed of the panel, while also defining the host processing architecture to unload some of the heavy digital operations. Hardware and software are also being improved to remove noise while increasing signal strength, while people are using system-level power consumption to prolong battery life. The next big challenge for designers is how to make it all more cost-effective.

Author: Todd Severson and Henry Wong

Cypress Semiconductor Co., Ltd.