The working principle and application of touch screen

1 The basic principle of the touch screen

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The working part of a typical touch screen generally consists of three parts, as shown in Figure 1: two transparent resistive conductor layers, an isolation layer between the two conductors, and an electrode. The resistive conductor layer is made of a resistive material such as indium tin oxide (ITO) coated on the substrate, the upper substrate is made of plastic, and the lower layer is made of glass. The barrier layer is a viscous insulating liquid material such as a mylar film. The electrode is made of a material with excellent electrical conductivity (such as silver powder ink), and its electrical conductivity is about 1000 times that of ITO.

The working principle and application of touch screen

The working principle and application of touch screen

  

When the touch screen is working, the upper and lower conductor layers are equivalent to the resistor network, as shown in Figure 2. When a layer of electrodes is applied with a voltage, a voltage gradient is formed across the network. If there is an external force that causes the upper and lower layers to contact at a certain point, the voltage at the contact point can be measured at another layer where the electrode is not applied with voltage, thereby knowing the coordinates at the contact point. For example, when a voltage is applied to the top electrode (X+, X-), a voltage gradient is formed on the top conductor layer. When an external force is applied to make the upper and lower layers contact at a certain point, the voltage at the contact point can be measured at the bottom layer. Then, based on the distance relationship between the voltage and the electrode (X+), the X coordinate of the place is known. Then, the voltage is switched to the bottom electrode (Y+, Y-), and the voltage at the contact point is measured at the top layer to know the Y coordinate.

2 touch screen control implementation

In many PDA applications, the touch screen is used as an input device, and the control of the touch screen also has a special chip. Obviously, the touch screen control chip has to do two things: one is to complete the switching of the electrode voltage; the other is to collect the voltage value at the contact point (ie A/D). This article takes the chip ADS7843 produced by BB (Burr-Brown) as an example to introduce the realization of touch screen control.

2.1 Basic Features and Typical Applications of ADS7843

The ADS7843 is a serial interface chip with a built-in 12-bit analog-to-digital conversion and low on-resistance analog switch. The supply voltage is 2.7~5 V, the reference voltage VREF is 1 V~+VCC, the input range of the conversion voltage is 0~VREF, and the maximum conversion rate is 125 kHz. The pin configuration of the ADS7843 is shown in Figure 3. Table 1 shows the pin function description, and Figure 4 shows the typical application.

The working principle and application of touch screen

The working principle and application of touch screen

The working principle and application of touch screen
2.2 ADS7843 internal structure and reference voltage mode selection

The ADS7843 is able to control the touch screen because its internal structure makes it easy to switch the electrode voltage and enable fast A/D conversion. Figure 5 shows its internal structure. A2~A0 and SER/ are the control bits in the control register for switching and reference voltage selection.

The working principle and application of touch screen

The ADS7843 supports two reference voltage input modes: one with a fixed reference voltage of VREF and the other with a differential mode with a reference voltage from the drive electrode. These two modes are shown in Figures 6(a) and (b), respectively. Using the differential mode of Figure 6(b) eliminates the effects of the switch-on voltage drop. Table 2 and Table 3 show the internal switch conditions corresponding to the two reference voltage input modes.

The working principle and application of touch screen

The working principle and application of touch screen

  

The working principle and application of touch screen

  

2.3 ADS7843 control word and data transmission format

The control words of the ADS7843 are listed in Table 4, where S is the data transmission start flag and this bit must be "1". A2~A0 for channel selection (see Tables 2 and 3).

MODE is used to select the accuracy of A/D conversion. "1" selects 8 bits and "0" selects 12 bits.

Input mode for SER/select reference voltage (see Tables 2 and 3). PD1 and PD0 select the power saving mode:

The "00" power saving mode allows power to be lost between two A/D conversions, and the interrupt is allowed;

"01" is the same as "00", but it is not allowed to interrupt;

"10" reserved;

"11" prohibits the power saving mode.

In order to complete the electrode voltage switching and A/D conversion, the control word needs to be sent to the ADS7843 through the serial port. After the conversion is completed, the voltage conversion value is read through the serial port. A standard one conversion takes 24 clock cycles, as shown in Figure 7. Since the serial port supports bidirectional simultaneous transmission and can overlap between one reading and the next transmission of the control word, the slew rate can be increased to 16 clock cycles each time, as shown in FIG. If the CPU can generate 15 CLKs (such as FPGAs and ASICs), the conversion rate can be increased to 15 clock cycles each time, as shown in Figure 9.

The working principle and application of touch screen

  

The working principle and application of touch screen

The working principle and application of touch screen 2.4 A/D conversion timing programming

A typical application of the ADS7843 is shown in Figure 4. Assume that the μP interface is connected to P1.3~P1.7 of 51 MCU. Now, taking 24 clock cycles for one conversion as an example, the program design of A/D conversion timing is introduced.

; A / D interface control line

DCLK BIT P1.3

CS BIT P1.4

DIN BIT P1.5

BUSY BIT P1.6

DOUT BIT P1.7

; A/D channel select command word and working register

CHX EQU 094H; channel X+ selection control word

CHY EQU 0D4H; channel Y+ selection control word

CH3 EQU 0A4H

CH4 EQU 0E4H

AD_CH EQU 35H ; channel select register

AD_RESULTH EQU 36H ; store 12 bit A/D value

AD_RESULTL EQU 37H

; Store the A/D value of channel CHX+

CHXAdResultH EQU 38H

CHXAdResultL EQU 39H

; Store the A/D value of channel CHY+

CHYAdResultH EQU 3AH

CHYAdResultL EQU 3BH

; Acquisition channel CHX+ block (CHXAD)

CHXAD: MOV AD_CH, #CHX

LCALL AD_RUN

MOV CHXAdResultH, AD_RESULTH

MOV CHXAdResultL, AD_RESULTL

RET

; Acquisition channel CHY+ block (CHYAD)

CHYAD: MOV AD_CH, #CHY

LCALL AD_RUN

MOV CHYAdResultH, AD_RESULTH

MOV CHYAdResultL, AD_RESULTL

RET

; A / D conversion subroutine (AD_RUN)

; Input: AD_CH-mode and channel selection command word

; Output: AD_RESULTH, L; 12 bit A/D conversion value

; Use: R2 ; Auxiliary working register

AD_RUN:

CLR CS ; chip allowed

CLR DCLK

MOV R2, #8; write 8 bit command word first

MOV A, AD_CH

AD_LOOP:

MOV C, ACC.7

MOV DIN, C; clock rising edge latch DIN

SETB DCLK ; start sending command words

CLR DCLK; clock pulse, a total of 24

RL A

DJNZ R2, AD_LOOP

NOP

NOP

NOP

NOP

ADW0: JNB BUSY, AD_WAIT; waiting for conversion to complete

SJMP ADW1

AD_WAIT:

LCALL WATCHDOG

NOP

SJMP ADW0

CLR DIN

ADW1: MOV R2, #12 ; start reading 12bit results

SETB DCLK

CLR DCLK

AD_READ:

SETB DCLK

CLR DCLK ; read with the falling edge of the clock

MOV A, AD_RESULTL

MOV C, DOUT

RLC A

MOV AD_RESULTL, A

MOV A, AD_RESULTH

RLC A

MOV AD_RESULTH, A

DJNZ R2, AD_READ

MOV R2, #4; Finally, 4 clocks that are useless

IGNORE:

SETB DCLK

CLR DCLK

DJNZ R2, IGNORE

SETB CS; banned chip

ANL AD_RESULTH, #0FH ; shield high 4 bit

RET

2.5 A/D conversion result data format

The ADS7843 conversion result is in binary format. It should be noted that the reference voltage is different in the two input modes when formula calculation is performed. Moreover, if the 8-bit conversion precision is selected, 1LSB=VREF/256, the conversion completion time can be advanced by 4 clock cycles, and the serial clock rate can be doubled.

Conclusion

In many embedded systems, the CPU provides specialized modules to support the input of the LCD and touch screen, making the interface very simple. For example, MOTOROLA's MC68VZ328 (called Dragon Ball) provides dedicated pins to support 8-bit and 4-bit LCD displays. Support for touch screens is also easily accomplished with SPI2 via the ADS7843.

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