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Four wire resistive touch screen control and single chip microcomputer interface

There are many kinds of interface types of human computer dialogue, such as display, LCD, LCD and LED, etc.. The latter is a new technology which has just developed in recent years. It is through the computer technology to deal with the sound, images, video, text, animation, etc., and to establish a certain logical relationship between these information, so that it can become a set of interactive information access and output system. In other words, it can be integrated with the intention of the release of information and the recipient of the information needs and acceptance habits, information collection, processing, integration and two-way communication. The touch screen system is simple, economical and efficient, and has the characteristics of good man-machine interaction, easy operation, fast input speed and so on. By combining it with the rapid development of computer network and multimedia technology, users simply touch the screen with a finger, we can carry out information retrieval, data analysis, we can even make immersive, lifelike effect; a keyboard input simple, intuitive, fast, consists of Fuduocai ability, than any previous media more affinity.

Touch screen and MCU interface, it can make a number of small system operation more convenient and quick. The most important problem is to determine the coordinates of the touch point, this paper takes the four wire resistive touch screen as an example, through the single chip microcomputer control to accurately sample the coordinates of the touch point, as the point to which the key is to turn to the corresponding processing software for processing.

一, four wire resistive touch screen

Resistive touch screen is a part of the screen with the display surface is very complex with multi-layer composite film, from a layer of glass or organic glass as a base, the surface coated with a layer of transparent conductive layer, above a layer of surface hardening treatment, smooth scratch resistant plastic layer, its internal surface is coated with a layer of transparent conductive layer, there are many small (less than 1/1000 inches) of the transparent insulation between the insulation, see figure 1.

When the fingers touch the screen, the two layers of insulation are in contact with the Vref axis direction, which makes the voltage from the Y axis to zero. The detection layer's voltage is zero. The controller detects the A/D axis coordinates, and the resulting voltage value can be compared with the Y, the same as the X axis. Resistance type touch screen is the key to the material, the resistance screen according to the number of lead wire, divided into four lines, five lines, six lines and other multi line resistance touch screen.

二、Four wire resistive touch screen control

The touch screen and the A/D microcontroller interface can be applied to the X, Y direction by 4 transistors or MOS tube respectively, and through the A/D channel to read X, Y direction of the voltage value to achieve (see Figure 3). The data reading control of the touch screen input A/D can be used to query the timing and interrupt mode.

The 20ms timing in query mode, touch screen touch state of MCU V1, V3, V4 as in V2 conduction, (MCU control port PE2 is "1"); once the touch action, touch screen via V5 PE5=0 (R5, R6 touch screen resistance relatively low resistance), MCU time to query "PE5=0", immediately to V3 conduction juxtaposition of PE6=0, form the current loop in the Y direction, start the MCUA/D conversion of -ADC1 channel, Y axis value can be read by X+ MCU, V3 V2; also closed, open the V1, V4, MCUA/D, ADC0 coordinates conversion start channel can be read out by Y+ X axis value X, Y axis; sampling after the PE6=1, V1, V3, V4, V2 conduction, read PE5, if PE5=0 is not lifted, button, continue sampling and digital filtering, if PE5=1, then the response button button up.

V1, V3, V4, X (), the PE2 (PE5), the control, V2 control (MCU); once the touch action, the touch screen through the V5, V3, MCU, direction, the direction, the direction, direction, direction, Y direction, direction, direction, starting MCUA/D conversion - channel, channel, channel, the can read the axis, the ADC1,,,,,,, V2,,,,,,,,,, the, X+,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,, V3,, Y,,,, MCU, PE6=0 X, Y, PE6=1 MCU,,, ADC0, V1 Y, V4, MCUA/D Y+, X,,,, to return from an interrupt service routine. In this way, when the coordinates of the Y and X axis are measured, the effect of the diode leakage current can be neglected.

When the query, regardless of whether there is a touch input, each 20ms will need to interrupt a time, need to take up CPU more time, in order to improve the utilization of CPU, you can use the interrupt mode, when there is a touch signal to generate interrupt request.

Touch screen and a built-in A / D MCU interface design is simple, low cost, but touch screen input can only complete some simple input control operation, such as menu selection, type selection button and so on. If the request for such as the Chinese character handwritten input, such as the requirements of linear, high precision, such as the touch screen input, you need to use a dedicated A/D interface.

In order to prevent the high energy pulse signal through the touch screen is coupled to the A / D converter, resulting in chip damage or deterioration in performance, in practical application, also need to clamp diode and a filter capacitor is added between the touch screen of the X, y + output wire and a / D input channel, can also protect the A / D in the input line penetrates into ferrite rings, as shown in Figure 4. This pulse signal in before entering the A / D converter to be filtered, prevent the pulse signal amplitude exceeds the power supply voltage and damaging the device, protection of the A / D is not damaged.

On a：Hui Huang: Four line / five wire resistive touch screen works

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