4 Wire resistive touch screen, Need help

[rsocor01]

1) Can you please explain how is "Start reading average pressure value" routine is working. Upto now I have only encountered putting 5v on y+ or x+ and corresponding y- or x- to be 0v. What is 5v @ y+ and 0v and x- doing?

This application note will help you understand better what is going on.
http://www.eetasia.com/ARTICLES/2004...URCES=DOWNLOAD

Before you start reading the touchscreen coordinates, you need to know if the screen has been touched. By having 5V at Y+ and 0V at X- you monitor if the screen was touched. The setup is as follows



And the code is

START:

    '################ START READING AVERAGE PRESSURE VALUE ################    

    TRISA = %00001001           ' Set Y+ (portA.2, AN2) and X- (portA.1, AN1) as outputs 
                                ' and Y- (portA.3, AN3) and X+ (portA.0, AN0) as inputs 
    
    low PORTA.1                 ' Set X- = 0V    
    high PORTA.2                ' Set Y+ = 5V     
    ADCON0 = %00000001          ' Bits(5-2): 0000=AN0. Start sampling at X+   
    pauseus 10        
    ADCON0.1 = 1                ' Stops or continues sampling depending on ADCON2 bits(5-3), 
                                ' then begins conversion         
    
    GOSUB CONVERSION_ROUTINE    ' Obtain value for A/D conversion resolution
    PRESSURE_XP = RESOLUTION    ' Obtain PRESSURE_XP from X+ sampling, X- = 0V, and Y+ = 5V  
    
    ADCON0 = %00001101          ' Bits(5-2): 0011=AN3. Start sampling at Y-   
    pauseus 10        
    ADCON0.1 = 1                ' Stops or continues sampling depending on ADCON2 bits(5-3), 
                                ' then begins conversion         

    GOSUB CONVERSION_ROUTINE    ' Obtain value for A/D conversion resolution
    PRESSURE_YM = RESOLUTION    ' Obtain PRESSURE_YM from Y- sampling, X- = 0V, and Y+ = 5V 

    IF (PRESSURE_XP > 24) AND (PRESSURE_YM < 1000) THEN         ' THE TOUCHSCREEN HAVE BEEN TOUCHED         
        GOSUB READ_COORDINATES
        gosub CONVERT_TO_PIXELS 
    ENDIF      

    LCDOUT $FE, 1                                                       ' Clear LCD                               
    LCDOUT $FE,2,"X=",DEC5 X_VALUE,", Y=",DEC5 Y_VALUE                  ' DISPLAYS A/D CONVERSION VALUES            
    LCDOUT $FE,$C0,"P1=",DEC4 PRESSURE_XP,", P2=",DEC4 PRESSURE_YM   
    
    GOTO START

If the screen is not touched then the reading for PRESSURE_XP is going to be very close to zero and the reading for PRESSURE_YM is going to be very close to 1024, remember it is a 10-bit ADC. The screen has been touched when the following condition has been met

    IF (PRESSURE_XP > 24) AND (PRESSURE_YM < 1000) THEN         ' THE TOUCHSCREEN HAVE BEEN TOUCHED

To read the X,Y coordinates you need the following setup.



And the code is

'****************************************************************
'*         READ X AND Y VALUES SUBROUTINE                       *
'**************************************************************** 
READ_COORDINATES:
    ' PORTA.0 (AN0) -> X+
    ' PORTA.1 (AN1) -> X-
    ' PORTA.2 (AN2) -> Y+
    ' PORTA.3 (AN3) -> Y-

    '################ READ AVERAGE X-VALUE ################

    TRISA = %00000011                   ' Set X+ (portA.0, AN0) and X- (portA.1, AN1) as inputs 
                                        ' and Y+ (portA.2, AN2) and Y- (portA.3, AN3) as outputs 

    low PORTA.3                         ' Set Y- = 0V    
    high PORTA.2                        ' Set Y+ = 5V      
    ADCON0 = %00000001                  ' Bits(5-2): 0000=AN0. Start sampling at X+    
    pauseus 10        
    ADCON0.1 = 1                        ' Stops or continues sampling depending on ADCON2 bits(5-3), 
                                        ' then begins conversion         
    GOSUB CONVERSION_ROUTINE            ' Obtain value for A/D conversion resolution  
    X_VALUE = RESOLUTION                ' Obtain X_VALUE from X+ sampling, Y- = 0V, and Y+ = 5V  
  
    ADCON0 = %00000101                  ' Bits(5-2): 0001=AN1. Start sampling at X-    
    pauseus 10        
    ADCON0.1 = 1                        ' Stops or continues sampling depending on ADCON2 bits(5-3), 
                                        ' then begins conversion          
    GOSUB CONVERSION_ROUTINE            ' Obtain value for A/D conversion resolution   
    X_VALUE = (X_VALUE + RESOLUTION)      ' Obtain X_VALUE from X- sampling, Y- = 0V, and Y+ = 5V  
    
    '################ READ AVERAGE Y-VALUE ################    

    TRISA = %00001100                   ' Set X+ (portA.0, AN0) and X- (portA.1, AN1) as outputs 
                                        ' and Y+ (portA.2, AN2) and Y- (portA.3, AN3) as inputs 

    low PORTA.1                         ' Set X- = 0V    
    high PORTA.0                        ' Set X+ = 5V      
    ADCON0 = %00001001                  ' Bits(5-2): 0010=AN2. Start sampling at Y+    
    pauseus 10        
    ADCON0.1 = 1                        ' Stops or continues sampling depending on ADCON2 bits(5-3), 
                                        ' then begins conversion         
    GOSUB CONVERSION_ROUTINE            ' Obtain value for A/D conversion resolution
    Y_VALUE = RESOLUTION                ' Obtain Y_VALUE from Y+ sampling, X- = 0V, and X+ = 5V    
  
    ADCON0 = %00001101                  ' Bits(5-2): 0011=AN3. Start sampling at Y-    
    pauseus 10        
    ADCON0.1 = 1                        ' Stops or continues sampling depending on ADCON2 bits(5-3), 
                                        ' then begins conversion         
    GOSUB CONVERSION_ROUTINE            ' Obtain value for A/D conversion resolution
    Y_VALUE = (Y_VALUE + RESOLUTION)      ' Obtain Y_VALUE from Y- sampling, X- = 0V, and X+ = 5V 
   

RETURN

'****************************************************************
'*         CONVERSION SUBROUTINE                                *
'**************************************************************** 
CONVERSION_ROUTINE:
    REPEAT          ' Wait until conversion is finished. When finished ADCON0.1 will be =0        
        PAUSEUS 10
        CONV_STATUS = ADCON0.1    
    UNTIL CONV_STATUS = 0
    
    RESOLUTION.BYTE0 = ADRESL : RESOLUTION.BYTE1 = ADRESH       ' READ A/D CONVERSION VALUE

RETURN

For the final routine "CONVERT_TO_PIXELS" the constants Ax, Bx, Ay, and By should be found experimentally. They will vary depending on the resolution of the LCD screen. One thing to remember is that the relation of the 4-wire touchscreen readings and the corresponding LCD pixels behaves almost linearly.

'****************************************************************
'*         CONVERT_TO_PIXELS ROUTINE                            *
'**************************************************************** 
CONVERT_TO_PIXELS:

     'Ax, Bx, Ay, and By should be found experimentally
    X_VALUE = Ax * X_VALUE + Bx    
    Y_VALUE = Ay * Y_VALUE + By
RETURN

[Demon]

nevermind, read code few more times...

Robert

[Megahertz]

Makes much more sense now. Thanks a lot for all the info.

[Art]

I can't see why you'd use an interrupt for a touch screen,
and if there is a valid reason I'd like to know what it is.

Why not follow the way 2D games work?

1) Draw current frame to screen,

2) Check for user input,

3) act on user input, and apply any movement/calculations for next frame,

4) goto step one.

It would seem if you're using an interrupt, and your program is in the middle of step 3
while receiving new input you are complicating things.

[rsocor01]

I can't see why you'd use an interrupt for a touch screen,
and if there is a valid reason I'd like to know what it is.

Why not follow the way 2D games work?

1) Draw current frame to screen,

2) Check for user input,

3) act on user input, and apply any movement/calculations for next frame,

4) goto step one.

It would seem if you're using an interrupt, and your program is in the middle of step 3
while receiving new input you are complicating things.

No, there is no interrupt in the program I posted.

Robert