Touch screen design challenges upgrade engineers how to improve the user experience?

Author: Cypress Semiconductor Corporation Todd Severson, Henry Wong

Touch-enabled consumer electronics continue to increase screen size every year. Touch screens are widely used in smartphones and have rapidly evolved into tablets. With the release of Windows 8, touch functionality is evolving to Ultrabooks, notebooks, and all-in-one PCs. As screen sizes continue to increase, the main challenge for capacitive touch is to maintain the higher handset performance that users expect on larger screen sizes. This means that more intersections on a larger surface area need to be scanned in the same amount of time. In addition, the processor must be able to operate with less signal and more noisy conditions while still trying to maintain its speed, accuracy, and responsiveness for an ideal user interface experience.

In 2007, Apple introduced the iPhone to open the application of capacitive touch screens in consumer electronics. This 3.5-inch screen-sized device introduces a multi-touch user experience that changes the way users interact with electronic devices. Touch screens are now standard on consumer electronics such as digital still cameras (DSCs), portable navigation devices (PNDs), e-readers, tablets, ultrabooks, and all-in-one (AIO) computers. As we have seen, one of the major trends in all of these devices is the shift to larger screen sizes. Capacitive touch screens are constantly evolving their existing product segmentation types as they enter new segments such as ultrabooks or notebooks. OEMs of top smartphones have switched from smart phones to super phones to differentiate their products by offering customers a larger screen size.

The main product segmentation types of today's consumer electronics are as follows: 3-5 inch screen size smartphone; 5-8 inch super phone or tablet phone; 8-11.6 inch tablet; 11.6-15.6 inch ultrabook And laptops up to 17 inches in size. In just five years of product history, tablets are considered one of the fastest-growing mobile devices, and sales are expected to surpass PCs in 2015. As a result, PC vendors are shifting their focus to touch-friendly designs, such as reversible laptops that can be used as laptops or tablets.

Figure 1 Forecast of global tablet and PC

Figure 1 Forecast of global tablet and PC

Users expect large screen devices to achieve similar performance and touch experiences as smartphones. The use cases that large screen devices need to handle are usually different from what we see on smaller phones. Laptops or PCs are more often used when plugged in. They have a larger surface area, so you can place your palm or other large objects on the screen when typing, and users usually place these larger devices on the desktop. Or use it on your knees instead of in your hand. All of these actions change the electrical performance of the device. The user performance with robust performance and fast response mainly includes: high sensitivity, ability to track multiple moving touch objects, recognition and tracking of fingers in various noisy environments, recognition and tracking of fingers under various environmental conditions, and maintenance Acceptable power consumption for ideal battery life. In other words, the essence of the user experience is the response that the system makes when you touch the screen under various conditions.

Capacitive touch screens work by generating a signal charge by driving the emission voltage to a sensor panel on the device. The touch screen controller then receives a signal that can determine the sensor capacitance by measuring the change in sensor charge. The current received by the chip is equal to the product of the panel capacitance and the voltage of the transmit driver (Q1 = C * VTX). The baseline circuit removes the nominal non-touch sensor charge, allowing the system to focus on measuring changes in sensor charge due to finger touch. This helps to improve the measurement, resolution and sensitivity of the touch.

With the development of capacitive touch screens, we are facing more and more technical challenges. The main problem with larger screens is that the emission voltage needs to cover a larger surface area, and the resistance and capacitance of the sensor also increase. The touch panel is limited by higher parasitic capacitance and resistance, which affects the time constant of the resistor-capacitor (RC), which causes the emission frequency to slow down. The transmit operating frequency affects signal setup, refresh rate, and power consumption. Our goal is to determine the highest transmit operating frequency required to achieve consistent touch response across the panel and minimize scan time and power.

Refresh rate

The refresh rate is the number of times the touch screen controller measures the on-screen touch measured in one second and reports it back to the host processor. The higher the refresh rate, the more x/y data coordinates the device will collect in a shorter amount of time, resulting in a responsive user experience. Most consumer electronics require a touch controller with a refresh rate greater than 100 Hz or approximately 10 ms. Specific applications such as digital tablet or point-of-sale (POS) terminals require even higher refresh rates to capture and recognize signatures and strokes that slide quickly.

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 intersections, and then process the data. The refresh rate is mainly affected by two major factors: the scanning speed of the screen and the processing speed of the scanned data. With the same sensor characteristics (3108 vs. 275), the 17-inch screen has 11 times more intersections than the 5-inch screen. In order to maintain the user experience of the 5-inch screen, the 17-inch screen requires more powerful scanning and processing functions.

One way to solve the scanning problem is to make sure that the touch controller has enough receive channels to scan the entire screen in a single cycle. Most touch screen overlays consist of a sensor pattern located under the glass of the housing, which contains a large number of "unit cells" arranged in the x and y directions, with the x direction for emission and the y direction. Used for receiving, or vice versa. The receive channel collects data and uses an analog-to-digital converter (ADC) to convert the mutual capacitance changes in each unit cell into digital data for the host to resolve the coordinates of the finger touch point. If the number of receive channels or ADCs is insufficient, multiple scans and longer time are required to scan the entire panel. This can result in fewer samples being taken in a given time period, resulting in a poor user experience.

There is a way to solve the processing problem by adding a larger processor to the touch controller or offloading some of the computations to the system's main processing unit. This means sending capacitive data to the host and running the algorithm on the application or graphics processor. One implementation is to use a touch screen controller to scan the sensor, 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 ID. Parallel processing enables a large number of digital operations on multi-gigahertz multi-core processors that act as touch screen and display host.

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