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How to easily face the design challenges of multi-point capacitive touch screen

Multi-touch capacitive touch screens have and are continuing to change the way people interact with handheld devices and bring people many new operating experiences. From mobile phones to e-books, Electronic writing tablets, navigators, electronic game consoles and notebook computers, all have abandoned the original touch buttons, and are competing to choose multi-point capacitive touch screens for human-computer interaction. In particular, the emergence of I-Phone and I-Pad has made the multi-point capacitive touch screen deeply rooted in the hearts of the people. However, the design of a multi-point capacitive touch screen is not easy and straightforward. Strictly speaking, the multi-point capacitive touch screen technology is not yet a fully mature technology, it is still a technology in the development stage and is constantly developing and improving. For a multi-point capacitive touch screen designer, there are still many design challenges in front of it. This article describes the design challenges of multi-point capacitive touch screen design and how to use the TTSP scheme to help designers face these challenges and make multi-point capacitive touch screen design easier than ever. challenge

design challenge one

The first challenge from the multi-point capacitive touch screen design is how to convert the tiny mutual capacitance changes due to finger touch into a digital signal with sufficient resolution. We know that, in general, multi-touch is based on the principle of mutual capacitance sensing, and mutual capacitance is the parasitic capacitance at the intersection of the transmitting sensing strip and the receiving sensing strip. This capacitance is very small, usually 0.2~4pF, and The change in mutual capacitance caused by finger touch is even smaller. The detection of this tiny mutual capacitance change requires not only a hardware detection circuit that is highly sensitive to capacitance changes to convert weak analog power to digital signals, but also requires corresponding software for control and coordination to ensure that the entire touch screen is displayed. Each point has a high enough sensitivity to the finger touch signal.

Design Challenge II

How to obtain fast enough scan time is the second challenge of multi-point capacitive touch screen design. For a single-point touch screen with M rows and N columns of sensing bars, using self-capacitance scanning, it only needs to scan M rows and N columns, respectively, and can calculate according to the signals of each row and each column, and locate the coordinates of the finger on the touch screen. . The number of times it scans the sensor bar is M+N times. When you use a multi-contact mutual capacitance scan, since it must be a row and column intersection scan, its scan count is MXN times the number of scan intersections. For a 3.2-inch screen with 10 rows and 20 columns, the self-capacitance scanning only needs 10+20=30 times, while the mutual-capacitance scanning needs 10X20=200 times. As the size of the touch screen gets larger, the number of scans increases faster and faster. In order for the user to have a better touch experience, it needs to scan the screen at least 50 times per second. This means that the scanning and data processing time of each point must be less than 100us, so as to ensure a fast enough response time. The larger the size of the touch screen, the more rows and columns, and the shorter the time.

Design Challenge Three

Touch buttons, touch sliders and touch pads all use copper foil as touch sensors, but touch screens basically use ITO (Indium Tin Oxides) material as the touch sensing layer. The resistivity of copper foil is so small that its resistance is almost negligible. ITO is transparent and conductive, but ITO has a relatively high resistivity. Usually, the resistivity of ITO on the touch screen is expressed by square resistance, that is, what is the resistance of one unit square. Generally, the sheet resistance of ITO ranges from 45 to 350 ohms, depending on the coating process of the touch screen manufacturer. Due to the existence of the ITO resistance, there will be a resistance of 3K~30K ohms at the near and far ends of each sensing strip on the touch screen. This resistance combined with the RC delay generated by the self-capacitance on each sensing strip makes the sensing The near and far ends of the strip will have different response times or charge and discharge times to the transmitted signal, resulting in different magnitudes of the finger touch signals at the near and far ends. In severe cases, this difference can reach more than 50%. How to eliminate or reduce this difference is the third challenge of multi-point capacitive touch screen design. Although choosing an ITO coating with lower square resistance is the most straightforward way to reduce this difference, usually the thickness of the lower square resistance ITO coating will be thicker, resulting in a decrease in transparency and an increase in cost. Unacceptable for many end customers.

Design Challenge Four

Signal-to-noise ratio (SNR) is one of the most important metrics in the design of multi-point capacitive touchscreens. For a touch screen, a large enough finger signal is far from enough. In fact, the touch screen is not placed on an ivory tower, and there are many sources of noise around it. For example, the LCD next to it is a noise source. Different LCDs and even different Display images have different noise levels and frequency spectra. Especially for some AC Vcomm type LCDs, it can generate current noise up to 15nA/mm2 and voltage noise above 1V on the surface of the LCD. Although an ITO shielding layer is placed under the touch screen, some designers have adopted the scheme, but the increase of the shielding layer leads to an increase in the thickness and cost of the touch screen, which also affects the visibility to a certain extent. Not all end customers are acceptable. The radio frequency signal of the mobile phone itself and the external electromagnetic waves will also interfere with it.When the terminal using the touch screen is powered by external mains, theadapterCan generate large common mode noise. There is also the annoying charger noise, the noise generated by the touch screen and the system itself such as AD conversion noise, switching noise, power supply noise and 8kV ESD noise used in ESD testing. In such a noisy environment, how to make the touch screen system have good noise immunity to the noise of various noise sources and obtain a sufficiently high signal-to-noise ratio is the fourth challenge of multi-point capacitive touch screen design.

Design Challenge Five

Finger positioning accuracy is the fifth challenge in multi-point capacitive touch screen design. Today’s end customers have higher and higher requirements on the positioning accuracy of fingers on the touch screen, especially the positioning accuracy on the edge of the touch screen. We know that the centroid algorithm is usually used to perform finger positioning calculations. However, due to the incompleteness of the sensing unit on the edge of the capacitive touch screen and the inherent lack of half of the weight signal on the edge of the finger, still using the centroid algorithm on the edge of the touch screen will bring large errors. Therefore, improving the finger positioning algorithm is not only applicable to the middle area of ​​the touch screen, but also to the edge area of ​​the touch screen to make the positioning of the finger touch more accurate, which is a challenge that must be faced in the design of the multi-point capacitive touch screen.

Design Challenge Six

Multi-touch gesture recognition and tracking. Multi-touch capacitive touch screens are designed for multi-touch and gesture recognition. Generally, touches of up to ten fingers can be recognized. The most common gestures are one- or two-finger gestures. It must not only recognize fourteen single-touch gestures (up, down, left, right, upper left, lower left, upper right, lower right, left rotation, right rotation, click, double click, tap and hold, and lift), but also Twenty-seven gestures capable of recognizing double touches (double touch up, double touch down, double touch left, double touch right, double left upward, double left downward, double touch Click to move up right, move down two touches, zoom out two touches, zoom in two touches, click two touches, move up one touch, move one touch down, move left one touch, move right one touch, One touch-up left, one touch-left down, one touch-up right, one-touch down-right, one-touch-down-left, one-touch-down-right, one-touch-down-right, one-touch-down-left, one-touch One-up and right-turn, one-touch-Z-shape move, one-touch-triangle move, one-touch-square move and one-touch-one-circle). Furthermore, in order to be able to track the movements of these fingers in real time when more than two fingers are touching, the temporary identification code assigned to each touching finger cannot be mistaken. It is a real challenge to the design of the gesture recognition algorithm and the computing speed of the chip.

Design Challenge Seven

Low power consumption.any useBatteryPowered mobile devices will have very strict power consumption requirements for each functional unit design, especially in the current low-carbon era. Of course, the multi-point capacitive touch screen as a functional unit in a mobile device is no exception. It is not easy to make a multi-point capacitive touch screen consume less than 35mW when fully activated and less than 100uW when in standby. If the design of a multi-point capacitive touch screen cannot meet this requirement, it will be in a very unfavorable position in the fierce market competition.

Design Challenge Eight

Waterproof performance is a landmark indicator to measure the design performance of multi-point capacitive touch screen. It appears that multi-point capacitive touchscreens using mutual capacitive scanning are inherently water resistant, which does not pose a design challenge. why would you said this? Because the touch screen using self-capacitance scanning, the direction of the signal change generated by the water droplet and the finger touch is the same, it is quite difficult to distinguish the water droplet from the finger touch. On the other hand, the direction of the change of the signal generated by the touch screen of the mutual capacitance scanning and the finger touch is exactly opposite, because the finger touch reduces the mutual capacitance, but the water drop increases the mutual capacitance. This gives the impression that a multi-point capacitive touch screen using mutual capacitance scanning has a natural waterproof capability and does not require special measures for waterproofing. The real situation is not so simple. When the water droplets drip onto the mutual capacitance screen, there will be no false triggering, but when the water droplets are wiped off and then touch the original place with a finger, it will not work. If you are lucky, you can return to the original finger touch sensitivity after a period of time. We know that a qualified product will not allow such a situation, let alone rely on good luck. Therefore, how to solve the problem of finger touch failure caused by water is another challenge in the design of multi-point capacitive touch screen. In fact, the problem of touch failure caused by water does not only refer to water droplets, it also includes water films and large pieces of water.

Design Challenge Nine

How to overcome the noise from the low-end charger is the ninth challenge of multi-point capacitive touch screen design. Especially in the Chinese market, a large number of low-end chargers are chosen by users. There are two special differences between the noise generated by this type of charger and other noises: the first is that its noise does not appear when there is no finger touch, it only appears when it is touched and is very strong, making an effective The touch becomes very unstable and becomes ineffective; secondly, this noise is a common mode noise from the charger and transmitted to the touch screen system through the ground wire. It is difficult to filter out by ordinary hardware filtering, commonly used digital filtering The filtering effect on it is also not ideal. So there must be an advanced filtering method to deal with the noise of this low-end charger.

Design Challenge Ten

Signal Consistency (SD). Many designers of multi-point capacitive touch screen will encounter such a problem, when their design is completed, the sample test finger touch signal strength meets the requirements. When they assemble the touch screen into the whole machine, or even prepare for mass production, a small problem will suddenly appear in front of them: a handheld device using a multi-point capacitive touch screen On a desk, touch doesn’t work. This is the problem of signal consistency, or what we call Signal Disparity, or SD for short. It is caused by the inconsistency between the amplitude of the finger signal when the touch screen is tested or held in the hand and the amplitude of the finger touch signal placed on the table. The amplitude of the finger touch signal placed on the table will be smaller than the amplitude of the finger signal when tested or held in the hand. When the amplitude difference between the two is large enough, the amplitude of the finger touch signal on the table cannot reach or exceed the finger signal threshold from time to time, and a valid touch cannot be captured. This signal inconsistency can become very severe with multiple and large fingers. How to solve the problem of signal inconsistency is the tenth challenge of multi-point capacitive touch screen design.

Although the ten challenges in the design of multi-point capacitive touch screen are listed above, in fact, to meet the increasingly high requirements of customers, the design of multi-point capacitive touch screen is not limited to these ten challenges. For example, in order to obtain a thinner touch screen, a lamination technology (referred to as Sensor On Lens) in which the ITO sensing layer is directly coated on the top glass has been and is being implemented. This screen is closely attached to the LCD screen, making the LCD On-screen noise effects are greatest on touchscreens. This makes the design of multi-point capacitive touch screens face more severe challenges. In addition, the inability to use a stylus on a capacitive touch screen for a long time has always been a pity in the design of the capacitive touch screen, and it has also been haunted by the vast number of capacitive touch screen users. Because the tip of the stylus is too small, it is difficult to generate a large enough coupling capacitance on the capacitive screen like the coupling capacitance generated when the finger touches, which becomes the biggest inherent deficiency compared with the resistive screen. Is it really impossible to use a stylus on a capacitive touchscreen? Are the designers of multi-point capacitive touch screens really helpless and helpless? It not only challenges the technical level of the multi-point capacitive touch screen designers, but also challenges their courage and wisdom! In addition, the design of the multi-point capacitive touch screen also needs to face the details problems that may occur during the use of the touch screen, such as the doughnut effect of big fingers; the proximity and proximity detection of faces when making calls on the touch screen on the mobile phone. Of course, single-chip, small size, and minimal peripheral components are also the performance that must be pursued in the design of multi-point capacitive touch screen. With the development of multi-point capacitive touch screen towards large-size screen, the design of multi-point capacitive touch screen will face more new challenges…

two. TTSP scheme easily realizes the design of multi-point capacitive touch screen

TTSP is the abbreviation of TrueTouch Standard Product. It is a standard product developed by Cypress for capacitive touch screen applications. TTSP is based on PSoC and embeds TTUM module specially designed for multi-point capacitive touch screen in it. Just like PSoC, it is also a true digital-analog mixed-signal processing chip. TTSP not only includes hardware circuit modules for detecting mutual capacitance and self-capacitance, but also includes very rich software; in its software, it not only includes programs for controlling and coordinating the work of hardware circuits, but also includes a variety of Signal processing and various algorithm programs, as well as the communication program with the main control chip and the Bootloader program. It is a true single-chip solution for a multi-point capacitive touch screen design.

1. Easy to use

Ease of use is the first feature of the TTSP scheme. In the TTSP scheme it does not require the user to write a single line of code. It only requires the user to define and set parameters or select parameters through pins to get all the required codes. It can be roughly divided into three steps to complete these settings. The first step is to set the number of sensor rows and columns on the touch screen through the Wizard Form of the TTUM module on the PSoC Designer development platform, and define each sensor on the row and column to the TTSP chip that can be used as sensors to transmit or receive. on the pins. This definition is done by clicking the sensor’s serial number in the row and column and dragging the mouse to the square representing the chip pin name. see picture 1.

Figure 1: Defining the sensor to the chip pins

The maximum resolution in the X and Y directions is also set here. In the other options folder of Wizard Form, you can also select some initial settings closely related to scanning, such as the frequency used for scanning, the number of neutron conversions in one conversion, and the number of cycles of the scanning signal used in one subconversion, etc. . The second step is to set parameters in the parameter table of TTUM. The parameter table of TTUM includes various parameter options such as finger signal threshold, noise threshold, the maximum number of fingers that can be given, a variety of digital filter choices, and so on. The third step is to select the communication protocol and protocol parameters. Communication protocols include I2C, SPI and UART. Protocol parameters include port and port number definition, communication rate, and so on. After all parameters are set, all the code is generated by clicking Generate/Building Project. The hexadecimal code is programmed into the TTSP chip, and the touch screen system can be debugged through the USB-I2C bridge tool and the debugging software TUNER equipped with the TTUM module. Figure 2 is an interface for debugging using TUNER.

Figure 2: An interface for debugging with TUNER

2. Fully functional

The TTSP scheme can provide touch detection of up to ten fingers, continuous tracking of four fingers, and can recognize up to fourteen gestures with a single touch and twenty-seven gestures with two fingers. It can implement not only mutual capacitance scanning, but also self-capacitance scanning. In fact, it can also implement the alternate scanning of mutual capacitance and self-capacitance. It is the use of this alternate scanning that improves the performance of the multi-point capacitive touch screen. The waterproof function design of the multi-point capacitive touch screen and the realization of the stylus make use of the alternate scanning and selective scanning of mutual capacitance and self-capacitance. TTSP allows users to use touch keys simultaneously in the same touch screen item, which is very beneficial to some mobile phone users who wish to use touch keys separate from the touch screen. The debugging function provided by TTSP also makes the development of multi-point capacitive touch screen more intuitive and easy. The TTSP scheme supports stylus and proximity detection to make it more complete.

3. Good performance

The TTSP scheme has not only high enough sensitivity to detect finger touches, but also high enough sensitivity to detect stylus “touches” and swipes. The TTSP scheme has strong noise immunity. It not only reduces input noise through reasonable hardware design, but also develops a variety of special filtering software for various noises to eliminate the influence of noise. Especially for low-end charger noise, it uses a specially developed filtering method now named “armor”, which effectively suppresses its noise. The selective use of these filters allows TTSP to have a sufficiently high signal-to-noise ratio. The positioning accuracy of the TTSP solution in the middle area of ​​the touch screen can reach 0.5 mm, and the positioning accuracy at the edge of the touch screen can be less than 1.5 mm. The TTSP scheme is also low-power, with power consumption of less than 35mW when fully activated and less than 100uW in standby state. The TTSP solution is a single-chip, small-volume and high-efficiency solution with only 4 to 5 small capacitors for peripheral components, which is convenient for FPC wiring.

three.concluding remarks

Although multi-point capacitive touch screen design has many design challenges, using the TTSP scheme can help designers easily face these challenges, making multi-point capacitive touch screen design easier and faster than ever.