Some Discussions on Resistive Touch Screen Technology

    Touchscreen technology is beginning to penetrate beyond the consumer market into the medical, industrial, and automotive markets for reasons of aesthetics, maintenance, cost, and hygiene. With the advent of touchscreens, a number of touch technologies have emerged, such as capacitive, resistive, inductive, surface acoustic wave, and infrared touch technologies. Each design technology has its own advantages and disadvantages. Capacitive touchscreens are based on electrodes on a printed circuit board and are popular with users for touch keys, sliders, and scrolling, which add a lot of value to the user experience with easy touch. Surface acoustic wave touch technology is based on sound waves and exists in designs where transparent displays are required, such as amusement parks and indoor environments with a lot of traffic. Infrared touch technology is based on a light interruption method and is mainly used for low-resolution, very large screens. Inductive touchscreen technology is mainly used for panels made of plastic, aluminum, or stainless steel, or panels that will be exposed to liquids. Of these, resistive touchscreen technology is the most cost-competitive and can be easily integrated into embedded designs. This technology is mainly used to design touchscreens with panel sizes up to 19 inches. Support for finger touch detection and stylus detection has expanded the application range of resistive touch technology in consumer electronics (see Figure 1).

Figure 1: Finger and stylus detection makes resistive touch screens better.

　　This article will mainly discuss the characteristics of resistive touch screen technology, issues that should be paid attention to during the design process, and potential application areas.

　　Understand the design and controller selection requirements for electronic touch sensors

　　Because resistive touch screens are now readily available and their prices are decreasing over time, the technology is finding more and more applications. To choose the best touch screen technology, application designers must consider the application requirements in depth. Resistive touch screen technology requires only a simple PCB design, unlike capacitive and inductive touch screen technologies, which require electrodes or coils to be etched on the PCB. Because the touch screen is directly overlaid on the display, it saves PCB space required for mechanical switches or capacitive touch key electrodes. It is not recommended to use resistive touch screens in harsh environments, such as mines or construction sites with frequent explosions or excessive dust. Even minor damage to a resistive touch screen can affect touch accuracy and linearity.

　　How resistive touch screens work

　　1. A resistive touch screen is a transparent glass panel covered with a touch-responsive film.

　　2. Resistive touch screen panels consist of two resistive layers (indium tin oxide) with a thin separator layer in between.

　　3. The two film layers of a resistive touch screen form a resistor network that acts as a voltage divider circuit for the touch position detection function.

　　4. The touch screen causes a voltage change on the voltage divider composed of a resistor network. This voltage is used to determine the location of the contact point that touches the screen.

　　5. The touch screen controller (TSC) converts the captured analog voltage signal into a digital touch coordinate signal. It has a built-in analog-to-digital conversion channel and acts as a voltmeter to measure analog voltage.

　　6. After the screen is touched, the touch controller, which acts as a voltmeter, first applies a voltage gradient VDD at point X+ and a ground voltage GND at point X-. Then, the analog voltage on the Y-axis resistor is detected and converted into a numerical value to calculate the X coordinate using an analog-to-digital converter (Figure 2). In this case, the Y-axis becomes the sensing line. Similarly, applying voltage gradients at points Y+ and Y- can measure the Y coordinate.

　　7. Some touch controllers also support touch pressure measurement, i.e. Z-axis measurement. When measuring the Z-axis coordinate, the voltage gradient is applied to the Y+ axis and the X-axis. [page]

Figure 2: Resistive touch screen: X coordinate measurement:

　　Resistive touch sensing comes in two main forms: software touch sensing solutions and dedicated touchscreen controller chips.

　　In software haptic solutions, the microcontroller must take on all touch detection and coordinate calculation tasks. The microcontroller-based software algorithm uses the internal microcontroller to measure the touch position voltage, perform touch detection functions and coordinate processing functions.

　　In a dedicated touch screen controller, the controller initiates an interrupt request to the system host (microcontroller) to detect a touch event and outputs digital data representing the touch coordinates. The main processor (MCU) then reads the digital data and executes the operation command expected by the customer.

　　The design method based on MCU calculation parameters requires the main processor to be very fast to manage frequent touch operations. This is not a very reliable design for fast touch detection applications. Because there is no data averaging and touch detection delay function, the detection accuracy of this type of design is relatively low. Dedicated touch screen controller chips with data sampling, measurement value averaging, touch detection delay configuration and digital touch coordinate calculation functions are real touch screen controllers. These chips are easy to integrate into product designs and have higher performance.

　　Resistive touch screen classification

　　According to the number of sensing lines on the touch screen, resistive touch screens can be further divided into three categories: 4-wire, 5-wire and 8-wire. The strip electrodes of the 4-wire touch screen are installed on two different resistive layers (X+, X- on the same layer, Y+, Y- on another resistive layer). The 5-wire touch screen has circular electrodes (X+, X-, Y+ and Y-) only on the bottom layer. The top layer is used to measure the voltage during the touch process, and the voltage gradient is only applied to the bottom layer.

　　The working principle of 8-wire touch screen is similar to that of 4-wire touch screen. It just adds a reference voltage line to each line, so the number of lines reaches 8. The newly added 4 lines are used to provide reference voltage to the original 4 lines. 8-wire touch screen adopts the measurement principle of ratiometric analog-to-digital converter.

　　Because of low cost and simple touch sensing algorithm, 4-wire touch screens are widely used in low-end consumer electronics. 5-wire and 8-wire touch screens are mainly used in expensive high-end medical equipment and important industrial controllers.

　　The main components of a touch screen solution include a touch screen panel, a touch screen controller (TSC), a display panel, and a main processor, as shown in Figure 3. The main processor can be a low-end microcontroller. The main processor manages the initialization of the touch screen controller using a one-wire or two-wire interface protocol (I2C/SPI) and reads digital coordinate data. The main processor is also responsible for converting user touches into required operations, such as volume adjustment, picture replacement, or writing display. Most consumer electronic products have a display panel, and human-computer interaction icons can be displayed on the same display panel. [page]

Figure 3: Resistive touch solution block diagram

　　Designing an application system with a touch user interface depends on the design complexity of the touch screen resolution requirements. The touch screen resolution also depends on the resolution of the analog-to-digital converter of the touch screen controller. Another important factor is the power consumption of the touch screen controller. It is recommended to use a controller with interrupt function and low-power standby mode. When there is no touch operation, the controller enters a low-power standby state to save power; when a touch event is detected, the controller will wake up and perform the touch voltage decoding function. This function has become a basic requirement for portable devices because every coulomb of power in the battery of portable devices is very precious.

　　Choosing a touch screen controller with built-in buffer is very beneficial for frequent touch detection applications. For example, writing is a continuous touch operation. If the touch screen controller includes a FIFO buffer, data processing can be performed after the FIFO buffer is full, which can reduce the processing overhead of the main processor. When the screen is large (>6 inches), the noise picked up by the touch screen conductive plate will affect the accuracy of the touch screen. Adding capacitors on the touch screen (on the X+/X-, Y+/Y- axis) can reduce high-frequency noise.

　　A resistive touch screen example

　　To understand the principle of resistive touch solutions, we analyze an off-the-shelf low-cost tablet solution (Figure 4). In this example, the resistive touch screen controller uses ST's advanced STMPE811 controller, and the main processor uses ST's STM32 high-density 32-bit microcontroller.

Figure 4: Handwriting tablet solution

　　This solution allows users to experience the beauty of real-time handwriting on a TFT-LCD panel. On a 4-wire resistive touch screen, the X and Y coordinates of the stylus are mapped to a line drawing within the TFT-LCD panel. In the existing handwriting tablet design, a 2.4-inch touch screen is mounted on a 2.4-inch (QVGA resolution) TFT-LCD panel. Most mobile phones and PDAs use low-resolution displays. To ensure that the touch detection coordinates are accurately mapped to the display, special attention should be paid to the resolution of the touch screen and display panel. The change in touch coordinates along the touch screen's resistive axis (X/Y) is another important consideration, which is related to the brand of the touch screen. On some touch screens, the coordinate values ​​obtained from the touch screen controller gradually decrease from top to bottom along the axis of the touch screen, and vice versa.

　　In this example, the touch screen controller is connected to a 32-bit microcontroller via an I2C protocol interface. The TFT-LCD panel is connected to the microcontroller via the flexible interface (FSMC) of the microcontroller, and various parameters of the touch screen controller, such as the sampling speed and average value of the analog-to-digital converter, are configured by the microcontroller. In addition to the I2C protocol interface, the touch screen controller provides an output interrupt pin for initiating a touch detection interrupt request to the main processor. This interrupt pin is connected to the external interrupt port pin of the microcontroller. The touch screen controller used in this solution includes a 12-bit analog-to-digital converter and a FIFO buffer that can temporarily store 128 touch data sets. When the touch screen controller detects a touch event, the microcontroller receives an interrupt request from the external interrupt port pin and then reads the data in the touch screen controller FIFO buffer via the I2C protocol. Each X-axis and Y-axis coordinate data uses a 12-bit value. The software mapping from the touch screen coordinates to the pixel display coordinates is calculated based on the resolution of the display panel and the resolution of the touch screen. The microcontroller processes the coordinates of the TFT-LCD pixel display and then displays the corresponding TFT-LCD pixels. The resolution of the 12-bit analog-to-digital converter is more than enough. As a result, a very precise touch point can be achieved, which can cause consecutive pixels to light up, giving the user a real-time line drawing feeling (Figure 5).

Figure 5: Handwriting tablet implementation process

　　Because the touchscreen controller has a built-in FIFO buffer, it is easy to manage the microcontroller processing overhead. In addition, a color table can be displayed on the side of the display panel. The color of the text can be selected, and by clicking a color in the table, the next line drawing will be the selected color. The solution also provides an icon for a clear button, which can be touched to clear the screen when the screen is full of content. In this way, designers can easily implement a brush function. This application may be the basis of a child's drawing toolbox. (Menka Tangri, STMicroelectronics)