If a product designer wants to add a capacitive touch screen to a device, he must meet many sometimes conflicting requirements. This kind of difficulty usually leads to a question: whether to improve the robustness of noise and sacrifice the display's response performance. Consumers hope to get real-time response when moving their fingers on the touch screen. A good touch controller can send a very fast update information stream to the operating system, so that the application always knows the current position of the finger and reacts in real time. Moreover, although it is very good to browse content with finger input, in terms of creating content, a pen or stylus provides many advantages, allowing users to write and draw. A good touch interface should support both input methods. System designers should choose components that are easy to integrate in their products. Touch controllers with pre-installed standard firmware can significantly shorten development and debugging cycles. Designers do not need to write complex touch sensing codes, but only need to select a set of parameter values ​​suitable for the specific requirements of their projects and apply these values ​​to standard 2-wire or 3-wire interfaces. For example, consumer demand for affordable portable products has driven manufacturers to produce touch screens with fewer material layers. To achieve this, the sensor no longer uses an indium tin oxide (ITO) "shield" layer, and the ITO layer is traditionally used to protect touch screen sensors from interference from the display device underneath. Figure 1: Compared with the next generation touch screen stack, the existing touch screen stack has fewer "touch-on-lens" stacks, which can reduce the thickness of the display screen and increase the brightness. However, due to the lack of ITO shielding layer, if the touch screen controller is not designed for this problem, the noise problem of the display screen will increase sharply. Without the shielding layer, the interference signal from the display below the sensor will be many times stronger than the signal from the finger above. Solving this problem has brought very serious challenges to the touch screen controller chip. Another challenge for portable touch systems comes from battery chargers, which may be a bit surprising. In-car chargers, wireless chargers, after-market chargers and other chargers usually use switching technology to control their output voltage, and noise when switching frequencies will affect the operation of the touch screen. The following figure shows the image displayed when the user draws a set of diagonal lines with his finger on the drawing application of the smartphone. The same smartphone is connected to different chargers to capture each image. Obviously, the touch screen controller used in this smartphone is not robust enough to cope with charger noise. Figure 2: Line drawing test results of touch screens of smartphones connected to different chargers. When some chargers are connected, the touch screen shows false touches, poor linearity and high jitter, which are all poor user experiences example. Although there are relatively simple ways to address these challenges individually, to solve all the problems at the same time requires the use of more complex systems. The working principle of the touch controller IC is to transmit the voltage signal to the sensor and measure the amount of charge of each capacitor in the array. Noise from the display and charger changes the effective signal voltage and disturbs the measurement results. However, if the signal voltage is increased, the interference becomes relatively small, which can bring more accurate measurement results. This technique is simple and effective, so it is widely used in touch controllers. For example, Atmel ’s maXTouch® S series devices are driven with high voltages up to 24V, which can improve the signal-to-noise ratio in noisy environments. The noise generated by LCD, OLED and other display components is not always the same. As the image is constantly refreshed, these noises will follow a repeating pattern of noisy and quiet periods: starting from the pixel horizontal line at the top of the image, updating the color value of each horizontal line until it reaches the bottom of the screen, and then returning to the top again, starting the next Frame operation. Electrical activity usually has short pauses between successive horizontal lines, and pauses between frames will be longer. Some touch controllers try to take advantage of these pauses called "blanking intervals" because the noise level at these times is usually much lower. However, this method has several limitations. First of all, there are very few display components that are specially designed to provide synchronization signals to other components such as the touch controller, so it is difficult for the touch controller to know when it will be quiet. Secondly, the blanking signal interval is not always quiet, and in some cases, the interval is too short to be used. Finally, and most importantly, synchronization with the display assembly means that the touch controller loses its freedom to choose its operating frequency. Therefore, although this technology successfully avoids the noise of the display screen, it becomes more difficult to avoid noise from other sources such as chargers. Therefore, in the actual system, synchronizing with the display screen component is not a practical method.
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