Application and operating principle of the touch-sensitive screen
1 touch-sensitive screen of basic principles
Made up of three parts job part on it is typical for touch-sensitive screen generally: Seals, electrodes between two layers of diaphanous resistance conductor layer, two storeys of conductors. The conductor layer of the resistance chooses the resistance material, for instance, indium tin oxidizes things(ITO) Scribble and form on the substrate, upper substrate uses the plastics, the substrate of the lower floor uses the glass. The seal is a viscous insulating liquide material, such as polyester membrane. The electrode chooses the material with fairly good conductivity (such as aluminum paint Chinese ink) Form, its conductivity is probably 1000 times of ITO.
When the touch-sensitive screen works, the upper and lower conductor layers are equivalent to the resistor network. When a floor of electrodes are added to the voltage, will form the voltage gradient in this network. If there is external force that makes two stories keep in touch at a certain point, the voltage at the place where another layer of the electrode without voltage can measure the contact point, thus know the coordinates of the contact point. For example, +, X- for the electrode (X on the top level) On add on voltage,form by voltage gradient at top level at conductor layer, as make two stories not kept in touch external force at a certain point, the voltage at the place where the bottom layer can measure the contact point, and then according to this voltage and electrode (X +) Between distance relation,know this the X-coordinates of the places. Then switch to voltage the intersection of bottom layer and electrode (Y +, Y-) Have, and the voltage at the place where the top level measures the contact point, thus know the Y-corrdinate.
The control of 2 touch-sensitive screens is realized
In a lot of PDA employs now, regards touchscreen as an input equipment, there is a specialized chip too in the control on touch-sensitive screen. Obviously, the control chip of the touch-sensitive screen should finish two things: First, finish the switch of electrode voltage; Second, it is the magnitude of voltage (namely A/D) of gathering contact point . This text is with BB (Burr-Brown) The chip ADS7843 that the company produces is the example, introduce the realization that the touch-sensitive screen controlled.
2.1 Basic characteristic and typical application of ADS7843
ADS7843 one built-in 12 analog to digital converter, low the intersection of on resistance and serial interface chip of analog switch. Service voltage 2.7~5 V, the reference voltage VREF is 1 V +VCC, the input span which changes the voltage is 0 VREF, the highest conversion rate is 125 kHz. Pin disposition of ADS7843. Table 1 is a pin functional specification, Fig. 4 is the typical application.
2.2 The internal structure of ADS7843 and mode selection of the reference voltage
The reason why ADS7843 can realize the control on touch-sensitive screen, because its internal structure is very apt to realize the switch of the electrode voltage, and can change fast A/D. 5 Fig. show for his internal structure, A2 A0 and SER / control bits in control registers, choice used for carrying on switch switching over and reference voltage.
ADS7843 supports two kinds of reference voltage import schemas: One kind is that fixing of reference voltage is VREF, another kind of adopts the differential mode, the reference voltage comes from driving electrodes. These two kinds of modes part. Adopt Fig. 6(b) Can last switch to last influence that pressure drop bringing. Table 2 and Table 3 are the correspondent internal switch state of import schema of two kinds of reference voltage.
2.3 Control word and data transmission format of ADS7843
The control word of ADS7843 is set out in Table 4, among them S is the initial mark bit of data transmission, this one must be " 1" . A2 A0 chooses (shown in Table 2 and 3) the passway .
The precision that MODE is used for choosing A/D to change, " 1" Choose 8, " 0" Choose 12.
SER / the import schema of choosing the reference voltage (shown in Table 2 and 3) . PD1, PD0 are chosen the power saving mode:
" 00" The power saving mode is allowed, the power down in A/D two times changes, and interrupt enabling;
" 01" Together " 00" ,Just disallowing of interruption;
" 10" Keep;
" 11" Power saving mode that it is forbidden.
Change in order to finish electrode VS and A/D, need to send the control word to ADS7843 through the serial port first, change and finish, and then read out the voltage conversion value through the serial port. Canonial primary switch needs 24 clock cycles. Because serial port support, go on, convey at the same time while being two-way, in one reading with can overlap send control word next time, so conversion rate can be improved by 16 clock cycles each time. If conditions permit, CPU can be produced of 15 CLKs (such as FPGAs and ASICs) ,Conversion rate can also be improved by 15 clock cycles each time.
