Electronic – TFT Resistive screen made multitouch

touchscreen

Can a TFT Resistive screen be made/used as a multitouch screen?

I have a Android 2.2 SuperPad III tablet which comes with a resistive screen and I am developing an app that needs a few features of a multitouch screen. When testing the tablet for multitouch it fails: it doesn't register 2 different points simultaneously. I know as a fact that Android OS has support for multitouch since I have performed the same tests on a capacitive screen and all tests were successful.

Would it be possible that Android OS disables the multitouch functionality on an resistive screen tablet or is it a hardware limitation? If needed, I don't mind modifying the Android kernel to reflect my needs however I would like to know that the hardware can handle it.

Best Answer

It's a hardware limitation. Resistive screens measure the resistance from the top and the left, and bottom and right of the pressed point, and derive X and Y coordinates from these.

Image portraying physical touchscreen and how the resistances are created

Pressing two points simultaneously can't give you multiple coordinates. It can confuse the controller, however, resulting in a complete irrelevant set of coordinates.
No matter what technology, to get multiple sets of coordinates you have to scan the grid, looking at one point at a time, which resistive doesn't.

The application schematic for the MXB7843 touchscreen controller shows the electrical model of the touchscreen as the 4 resistors on the left.

example resistive screen reader application schematic

When you are looking at the image you can see that it only has physical inputs for one point so asking for two is just not there.

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Electronic – Resistive Touch Screens

I think the reading is taken from the other axis. So if you want to read the X axis you apply power/ground to the X wires and read the ADC connected to Y.
If you notice in the code snippet it says power is applied to X axis but reading is taken from CH0 (RA0) and named result_y (I think the diagram may have RC0 and RC3 connections mixed up though):

//Set PORTA To Inputs/High Impedance
TRISAbits.TRISA0 = 1;
TRISAbits.TRISA1 = 1;

//Set Lower 2 Bits to High Impedance
TRISCbits.TRISC0 = 1;
TRISCbits.TRISC1 = 1;
//Set Higher 2 Bits to Output
TRISCbits.TRISC2 = 0;
TRISCbits.TRISC3 = 0;

PORTCbits.RC0 = 0;
PORTCbits.RC1 = 0;
//Provide Ground To X-axis Of Touch Screen
PORTCbits.RC2 = 0;
//Provide Power To X-axis Of Touch Screen
PORTCbits.RC3 = 1;

// configure A/D convertor
OpenADC( ADC_FOSC_32 & ADC_RIGHT_JUST & 
ADC_8ANA_0REF,ADC_CH0 & ADC_INT_OFF );

Delay10TCYx( 5 ); // Delay for 50TCY
ConvertADC(); // Start conversion
while( BusyADC() ); // Wait for completion
result_y = ReadADC(); // Read result
CloseADC();

Electronic – Resistive touchscreen to ADC

You don't need such a complicated circuit. You need 4 ports and possibly two resistors.

There are 4 pins for the touch Screen (Page 12 of the PDF):

Pin1 = Y-

Pin2 = X+

Pin3 = Y+

Pin4 = X-

To get the “X” touch position, the microprocessor sets Pin3 to +3.3V and Pin1 to GND (0V). Pin4 is made unconnected (high impedance) by setting it to an input or an open collector output set to high. The microprocessor uses Pin2 to read the voltage through the ADC.

To get the “Y” touch position do the opposite. The microprocessor sets Pin2 to +3.3V and Pin4 to GND (0V). Pin1 is made unconnected (high impedance). The microprocessor uses Pin3 to read the voltage.

Pin2 and Pin3 need to go to ADC, through a resistor if you want the control current down. Pin1 and Pin3 go to an open collector output or a regular I/O.

The resistor used on Pin2 and Pin3 will not really affect the ADC if you put a 100pF cap to ground right beside the microprocessor.

To summarize:

“X” touch position:

Pin1 = 0V output

Pin2 = ADC input

Pin3 = 3.3V output (with inline resistor if needed)

Pin4 = High impedance (open collector output or just an input)

“Y” touch position:

Pin1 = High impedance (open collector output or just an input)

Pin2 = 3.3V output (with inline resistor if needed)

Pin3 = ADC input

Pin4 = 0V output

Please see the link below if only for the pictures:

https://www.sparkfun.com/datasheets/LCD/HOW%20DOES%20IT%20WORK.pdf

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