Detailed analysis of the principle of resistive touch screen

Many LCD modules use resistive touch screens, which are equivalent to sensors that convert physical positions into voltage values ​​representing X and Y coordinates. Usually, there are 4-wire, 5-wire, 7-wire and 8-wire touch screens to achieve this. This article introduces the SAR structure, the principles and structures of the four touch screens, and the methods of detecting touch in detail.

Touch screen principle

Touch screens consist of two transparent layers stacked one on top of the other. Four-wire and eight-wire touch screens consist of two layers of transparent resistive material with equal surface resistance. Five-wire and seven-wire touch screens consist of a resistive layer and a conductive layer, usually separated by an elastic material. When the pressure on the touch screen surface (such as by a pen tip or finger) is large enough, contact is made between the top and bottom layers. All resistive touch screens use the principle of a voltage divider to generate voltages representing the X and Y coordinates. As shown in Figure 3, the voltage divider is implemented by connecting two resistors in series. The upper resistor (R1) is connected to a positive reference voltage (VREF), and the lower resistor (R2) is connected to ground. The voltage measured at the junction of the two resistors is proportional to the resistance of the lower resistor.

Figure 3: The voltage divider is realized by connecting two resistors in series.

To measure a coordinate at a specific direction on a resistive touch screen, a resistive layer is biased: one side of it is connected to VREF and the other side is grounded. At the same time, the unbiased layer is connected to the high impedance input of an ADC. When the pressure on the touch screen is large enough to make contact between the two layers, the resistive surface is separated into two resistors. Their resistance is proportional to the distance from the touch point to the biased edge. The resistance between the touch point and the grounded edge is equivalent to the lower resistor in the voltage divider. Therefore, the voltage measured on the unbiased layer is proportional to the distance from the touch point to the grounded edge.

Four-wire touch screen

A four-wire touch screen consists of two resistive layers. One layer has a vertical bus at the left and right edges of the screen, and the other layer has a horizontal bus at the bottom and top of the screen, as shown in Figure 4. To measure in the X-axis direction, bias the left bus to 0V and the right bus to VREF. Connect the top or bottom bus to the ADC and make a measurement when the top and bottom layers are in contact.

Figure 4 Two resistive layers of a four-wire touch screen

To measure in the Y-axis direction, bias the top bus to VREF and the bottom bus to 0V. Connect the ADC input to the left bus or the right bus, and measure the voltage when the top and bottom layers are in contact. Figure 5 shows a simplified model of a four-wire touch screen when the two layers are in contact. For a four-wire touch screen, the ideal connection method is to connect the bus biased to VREF to the positive reference input of the ADC, and connect the bus set to 0V to the negative reference input of the ADC.

Five-wire touch screen

A five-wire touchscreen uses a resistive layer and a conductive layer. The conductive layer has a contact, usually at one edge of the side. The resistive layer has a contact at each of the four corners. To measure in the X-axis direction, the top left and bottom left corners are biased to VREF, and the top right and bottom right corners are grounded. Since the left and right corners are at the same voltage, the effect is similar to connecting the left and right sides to a bus, similar to the method used in a four-wire touchscreen.

To measure along the Y axis, the upper left and upper right corners are biased to VREF, and the lower left and lower right corners are biased to 0V. Since the upper and lower corners are at the same voltage, the effect is roughly the same as connecting the top and bottom edges to a bus, similar to the method used in a four-wire touch screen. The advantage of this measurement algorithm is that it keeps the voltage at the upper left and lower right corners unchanged; however, if grid coordinates are used, the X and Y axes need to be reversed. For a five-wire touch screen, the best connection method is to connect the upper left corner (biased to VREF) to the positive reference input of the ADC and the lower left corner (biased to 0V) to the negative reference input of the ADC.

Seven-wire touch screen

The implementation of a seven-wire touch screen is the same as a five-wire touch screen, except that one wire is added to the upper left corner and one wire is added to the lower right corner. When performing screen measurements, connect one wire in the upper left corner to VREF and another wire to the positive reference of the SAR ADC. At the same time, connect one wire in the lower right corner to 0V and another wire to the negative reference of the SAR ADC. The conductive layer is still used to measure the voltage of the voltage divider.

Eight-line touch screen

The eight-wire touch screen is implemented the same way as the four-wire touch screen, except that one line is added to each bus. For the VREF bus, one line is used to connect VREF and the other line is used as the positive reference input of the SAR ADC's digital-to-analog converter. For the 0V bus, one line is used to connect 0V and the other line is used as the negative reference input of the SAR ADC's digital-to-analog converter. Any of the four lines on the unbiased layer can be used to measure the voltage of the voltage divider.

Detecting contact

All touch screens can detect whether a touch has occurred by pulling one layer up with a weak pull-up resistor and the other layer down with a strong pull-down resistor. If the measured voltage of the pull-up layer is greater than a certain logic threshold, it indicates that there is no touch, otherwise it is a touch. The problem with this method is that the touch screen is a large capacitor, and it may also be necessary to increase the capacitance of the touch screen leads to filter out the noise introduced by the LCD. Weak pull-up resistors connected to large capacitors will increase the rise time and may cause false touches to be detected.

Four-wire and eight-wire touch screens can measure the contact resistance, which is RTOUCH in Figure 5. RTOUCH is approximately proportional to the touch pressure. To measure the touch pressure, you need to know the resistance of one or two layers in the touch screen. The formula in Figure 6 shows the calculation method. It should be noted that if the measured value of Z1 is close to or equal to 0 (when the touch point is close to the grounded X bus during the measurement process), there will be some problems in the calculation, which can be effectively improved by using a weak pull-up method.

Figure 5 RTOUCH

Figure 6 Touch screen resistance calculation method