Resistive touch screen structure and touch screen principle

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

A resistive touch screen is a sensor that converts the physical position of a touch point (X, Y) in a rectangular area into a voltage representing the X and Y coordinates. Many LCD modules use resistive touch screens, which can use four, five, seven or eight wires to generate the screen bias voltage and read back the voltage of the touch point.

In the past, in order to read the coordinates of the touch point on a resistive touch screen into a microcontroller, a dedicated touch screen controller chip was required, or a complex external switch network was used to connect to the microcontroller's on-chip analog-to-digital converter (ADC). Sharp's LH75400/01/10/11 series and LH7A404 microcontrollers have an on-chip ADC with touch screen bias circuitry, which uses a successive approximation register (SAR) type converter. These controllers allow for a direct interface between the touch screen sensor and the microcontroller, control of all touch screen bias voltages without CPU intervention, and recording of all measurements. This article will detail the structure and implementation of four-wire, five-wire, seven-wire, and eight-wire touch screens.

SAR Structure

There are many ways to implement SAR, but its basic structure is very simple, see Figure 1.

Figure 1 Basic structure of SAR

The structure stores the analog input voltage (VIN) in a track/hold, and the N-bit register is set to the intermediate value (i.e., 100..0, where the highest bit is set to 1) to perform a binary lookup algorithm. Therefore, the output (VDAC) of the digital-to-analog converter (DAC) is half of VREF, where VREF is the reference voltage of the ADC. After that, a comparison operation is performed to determine whether VIN is less than or greater than VDAC:

1. If VIN is less than VDAC, the comparator output is logic low, and the highest bit of the N-bit register is cleared to 0.
2. If VIN is greater than VDAC, the comparator output is logic high (or 1), and the highest bit of the N-bit register remains 1.

After that, the SAR control logic moves to the next bit, forces the bit to high, and performs the next comparison. The SAR control logic will repeat the above sequence of operations until the last bit. When the conversion is completed, an N-bit data word is obtained in the register.

Figure 2 shows an example of a 4-bit conversion process, where the Y axis and the thick line represent the output voltage of the DAC.

Figure 2 4-bit conversion process

In this example:
1. The first comparison shows that VIN is less than VDAC, so bit [3] is set to 0. The DAC is then set to 0b0100 and a second comparison is performed.
2. In the second comparison, VIN is greater than VDAC, so bit [2] remains at 1. The DAC is then set to 0b0110 and a third comparison is performed.
3. In the third comparison, bit [1] is set to 0. The DAC is then set to 0b0101 and a final comparison is performed.
4. In the final comparison, bit [0] remains at 1 because VIN is greater than VDAC.

Touchscreen Principle

A touchscreen consists of two transparent layers stacked one on top of the other. Four-wire and eight-wire touchscreens consist of two layers of transparent resistive material with the same surface resistance. Five-wire and seven-wire touchscreens consist of a resistive layer and a conductive layer, usually separated by an elastic material. When the pressure on the touchscreen surface (such as by a pen tip or finger) is large enough, contact is made between the top and bottom layers. All resistive touchscreens use a voltage divider principle 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 the positive reference voltage (VREF), and the lower resistor (R2) is grounded. 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 in a specific direction on a resistive touch screen, bias a resistive layer: connect one side to VREF and the other side to ground. At the same time, connect the unbiased layer 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 divided 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

Four-wire touch screens contain 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, see 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 touch.

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 either the left or 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 the bus set to 0V to the negative reference input of the ADC. Five-wire touch screen

A

five-wire touch screen uses a resistive layer and a conductive layer. The conductive layer has a contact, usually at the edge of one side. The resistive layer has a contact at each of the four corners. To measure in the X-axis direction, bias the upper left and lower left corners to VREF, and the upper right and lower right corners to ground. Since the left and right corners are at the same voltage, the effect is similar to connecting the left and right buses, similar to the method used in a four-wire touch screen.

To measure along the Y-axis direction, bias the upper left and upper right corners to VREF, and bias the lower left and lower right corners to 0V. Since the top and bottom corners are at the same voltage, the effect is roughly the same as connecting the bus to the top and bottom edges, similar to the method used in a four-wire touch screen. The advantage of this measurement algorithm is that it keeps the voltages at the top left and bottom right corners constant; 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 top left corner (biased at VREF) to the positive reference input of the ADC and the bottom left corner (biased at 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 top left and bottom right corners. When performing a screen measurement, connect one wire from the top left corner to VREF and another wire to the positive reference of the SAR ADC. At the same time, one wire from the bottom right corner is connected to 0V and another wire is connected to the negative reference of the SAR ADC. The conductive layer is still used to measure the voltage of the voltage divider.

Eight-wire touch screen

The implementation of an eight-wire touch screen is the same as a four-wire touch screen, except that one wire is added to each bus. For the VREF bus, one wire is connected to VREF and the other wire is used as the positive reference input of the SAR ADC's DAC. For the 0V bus, one wire is connected to 0V and the other wire is used as the negative reference input of the SAR ADC's DAC. Any of the four wires on the unbiased layer can be used to measure the voltage of the voltage divider.

Detecting the presence or absence of touch

All touch screens can detect whether a touch has occurred by pulling one layer up with a weak pull-up resistor and pulling 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 there is a touch. The problem with this method is that the touch screen is a large capacitor, and it may 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 gives 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), some problems will occur in the calculation, which can be effectively improved by using the weak pull-up method.

Figure 5 RTOUCHFigure

6 Touch screen resistance calculation method