Xpt2046 four wire resistive touch screen

One 、 brief introduction

XPT2046 Four wire resistive touch screen , It is mainly plated with ITO Film composition of the coating . One layer has a vertical bus on the left and right edges of the screen , The other layer has a horizontal bus at the bottom and top of the screen , If voltage is applied to two buses of a thin film , stay ITO A uniform electric field will be formed on the coating . When the user touches the touch screen , At the point of contact, two layers of film will contact , The voltage at the contact point can be measured on another layer of film .

The following figure for XPT2046 Functional block diagram , so XPT2046 There is a multiplexer inside , Able to measure battery voltage 、AUX voltage 、 Chip temperature . One 12 Bit ADC Used for analog-to-digital conversion of the selected analog input channel , Get the digital quantity , Then it is sent to the control logic circuit , For master control CPU To read , meanwhile , Which channel to choose for conversion , It is also controlled by the master CPU Send commands to the control logic to set .

XPT Support stroke interruption , That is, when the touch screen detects that the touch is pressed , It can immediately produce stroke interruption , Inform the main controller that it can control the start of conversion and read data . In the process of transformation , adopt busy The signal indicates the current busy state , To avoid the master sending a new command to interrupt the previous command .

Two 、 Pins and their functions

XPT2046 adopt SPI Interface communicates with the main controller , Its interface with the main controller includes the following signals ：

PENIRQ_N： Stroke interrupt signal , When the stroke interrupt signal is set to be valid , Whenever the touch screen is pressed , This pin is pulled low . When the main control detects the signal , You can send a control signal to prohibit the interruption of strokes , So as to avoid triggering the controller interrupt by mistake in the conversion process . This pin is internally connected with a 50K The pull-up resistance of .

CS_N： Chip select signal , When CS_N When pulled down , It is used to control the conversion sequence and enable serial input / Output registers to move data out or in . When this pin is high , chip （ADC） Enter power down mode .

DCLK： External clock input , This clock is used to drive SARADC And drive numbers IO Serial data transmission on .

DIN： The data serial input pin of the chip , When CS Low power level , The data is on the serial clock DCLK The rising edge of is latched to the on-chip register .

DOUT： Serial data output , In the serial clock DCLK The falling edge data of is moved out of this pin , When CS_N Pin is high power level , This pin is in high resistance state .

BUSY： Busy output signal , When the chip receives the command and starts the conversion , This pin generates a DCLK High level of cycle . When the pin changes from high level to low level , The highest data of the conversion result is shown in DOUT On the pin , The master can read DOUT Value . When CS_N Pin is high power level ,BUSY Pin is in high resistance state .

3、 ... and 、 working principle

XPT2046 It is a typical successive approximation analog-to-digital converter （SARADC）, Including sampling / keep 、 A / D conversion 、 Serial port data output and other functions . At the same time, the chip integrates a 2.5V Internal reference voltage source 、 Temperature detection circuit , Use external clock when working .XPT2046 It can be powered by a single power supply , The power supply voltage range is 2.7V～5.5V. The reference voltage value is directly determined ADC Input range of , The reference voltage can use an internal reference voltage , You can also input directly from the outside 1V～VCC Reference voltage in the range （ Low output impedance of external reference voltage source is required ）.X、Y、Z、VBAT、Temp and AUX The analog signal enters after being selected by the on-chip control register ADC,ADC It can be configured as single ended or differential mode . choice VBAT、Temp and AUX It can be configured as single ended mode ; As a touch screen application , It can be configured as differential mode , This can effectively eliminate the measurement error caused by parasitic resistance of driving switch and external interference , Improve conversion accuracy .

The following figure for XPT2046 Typical working circuit ：

XPT2046 There are four pins , For connecting to a four wire resistive screen FPC On , Respectively XP、XN、YP、YN, Connected to the corresponding four wire resistance screen X The positive end of the electrode 、 Negative end and Y The positive end of the electrode 、 Negative end . Each of these four pins can work in two states , They are power supply /GND Output 、ADC Input . For example setting ADC Working in differential mode , When measuring X Coordinate of direction ,XP Output VCC、XN Connect to GND, here ,YP and YN As ADC The differential input pin of is connected to ADC On , By measuring YP and YN To get the current touch point X Location . Empathy , When measuring Y Coordinate of direction ,YP Output VCC、YN Connect to GND、 here ,XN and XP As ADC The differential input pin of is connected to ADC On , By measuring YP and YN To get the current touch point Y Location .

Four 、 internal structure

5、 ... and 、 Function and use

Typical applications ：

24 Clock cycle conversion

Read write timing

I understand XPT2046 The interface circuit , Then we can control MCU or FPGA To control the chip to read the coordinates . To read correctly X、Y coordinate , It is necessary to read and write data according to the control protocol specified by the chip .XPT2046 Make it happen once X、Y The reading of coordinates requires two transformations , A single conversion can only get a single X or Y Coordinates of , therefore , We have to control twice to get the result . The object of each conversion is X or Y coordinate , Determined by the control word sent by the controller .ADC It can be configured as single ended or differential mode during conversion , The specific control word is at the beginning of each transmission , Controlled by master MCU drive DIN Signal transmission . The following figure for XPT2046 Typical 24 Clock cycle conversion control timing ：

XPT2046 The data interface is a serial interface , Its typical working sequence is shown in the figure above , The signal shown in the figure comes from a single chip microcomputer or data signal processor with a basic serial interface . Communication between the processor and the converter requires 8 Clock cycles , May adopt SPI、SSI and Microwire Isochronous serial interface . A complete transformation requires 24 A serial synchronization clock （DCLK） To complete .

front 8 A clock is used to pass DIN Pin input control byte . When the converter gets enough information about the next conversion , Then, the input multiplexer and the reference source input are set according to the obtained information , And enter the sampling mode , if necessary , The touch panel drive will start .3 After several clock cycles , Control byte setting is complete , The converter enters the conversion state . At this time , Input sampling － The holder enters the hold state , The touch panel driver stops working （ Single ended operating mode ）. And then 12 A clock cycle will complete the real analog-to-digital conversion . If it is a measurement ratio conversion method （SER/DFR——＝0）, The drive will work all the time during the conversion , The first 13 A clock will output the last bit of the conversion result . The rest 3 Multiple clock cycles will be used to complete the last byte ignored by the converter （DOUT Buy low ）.

6、 ... and 、 Sample code

XPT2046 Read AD value

unsigned int XPT2046_ReadAD(unsigned char Command)
{
    
	unsigned char i;
	unsigned int Data=0;
	XPY2046_DCLK=0;
	XPY2046_CS=0;
	for(i=0;i<8;i++)
	{
    
		XPY2046_DIN=Command&(0x80>>i);
		XPY2046_DCLK=1;
		XPY2046_DCLK=0;
	}
	for(i=0;i<16;i++)
	{
    
		XPY2046_DCLK=1;
		XPY2046_DCLK=0;
		if(XPY2046_DOUT){
    Data|=(0x8000>>i);}
	}
	XPY2046_CS=1;
	return Data>>8;
}