Touchscreen
MBED Lib ERROR
At the moment you have to use a old version of the mbed lib to use this library, because of a error in using the analog pins. The actual mbed lib is not switching the analog pins to digital mode !
The latest working mbed version is 43 from 26. oct 12.
To connect a resistiv touchscreen to the mbed we need four pins. Two analog inputs and two digital IOs. The connections on the touchscreen are normaly marked as x+,x-,y+ and y-.
The touchscreen is working like two potenciometers. First we have to read the X direction. To get a voltage across the X direction we connect the Y lines one to 0V and one to 3,3V. If the screen is hit we get a voltage between 0 to 3,3V - depending on the position. After that we connect the X lines to 0V and 3,3V and we detect the voltage on one of the Y lines. If there is no hit the line is not connected and we get a low reading.
Library
http://mbed.org/users/dreschpe/libraries/Touch_tft/lx1972
The commands :
- calibrate() ask the user to point onto two point to calibrate the screen
- point get_touch() return analog x and y
- bool is_touched(point) test if the screen is touched
- point to_pixel(point) use the calibration data to calculate the pixel position
The demo code :
#include "mbed.h"
#include "SPI_TFT.h"
#include "Arial12x12.h"
#include "touch_tft.h"
// the TFT is connected to SPI pin 5-7
// the touch is connected to 19,20,16,17
touch_tft tt(p19,p20,p16,p17,p5, p6, p7, p8, p15,"TFT"); // x+,x-,y+,y-,mosi, miso, sclk, cs, reset
int main() {
point p;
tt.claim(stdout); // send stdout to the TFT display
tt.background(Black); // set background to black
tt.foreground(White); // set chars to white
tt.cls(); // clear the screen
tt.set_font((unsigned char*) Arial12x12); // select the font
tt.set_orientation(1);
tt.calibrate(); // calibrate the touch
while (1) {
p = tt.get_touch(); // read analog pos.
if (tt.is_touched(p)) { // test if touched
p = tt.to_pixel(p); // convert to pixel pos
tt.fillcircle(p.x,p.y,3,Blue); // print a blue dot on the screen
}
}
will print a blue dot if the screen is hit.
Also see the demo :
32 comments on Touchscreen:
maryam wasim wrote:
I have a problem my touchscreen LCD HY32D does not have x,y pins.however it does have XPT2046.
http://www.hotmcu.com/32-touch-screen-tft-lcd-with-16-bit-parallel-interface-p-36.html
How can i use this LCD to detect touch ?
The XPT2046 does all the analog to digital conversion stuff. It has a serial SPI interface. You need a library that controls the device and reads out the internal registers to get the X,Y coordinates. There are some libs on the mbed site that should help you. Here is one.
Maxime Nounckele wrote:
Hi, I try this code but I have a grey screen. Anybody have an idea ? I note that "SPI_TFT library" works great.
It's OK I solved my problem. I used the new version of "SPI_TFT library"
But I have a new problem : When I touched the screen the position is always X=0 and Y=0.
The Pencil is mandatory for this ? Or I can do it with other thing ?
Hi Peter, I encountered the same problem as maxime and did the same, updated with the new SPI_TFT_ILI9341 file. It worked but now i have another problem. I'm stuck in the calibration part, the 1st cross on the top left corner of the screen. I keep pressing the cross and the screen but nothing happens.
I tried to display the values of p.x and p.y using the code below (i've added display to the calibrate function):
for (i=0; i<5; i++) { do { p = get_touch(); float x,y; x = p.x; y = p.y; printf("%f \n",x); printf("%f \n",y);
} while (p.x < 0x2000 | p.y < 0x2000); wait for touch a += p.x; b += p.y; }
if i dont touch the screen, values range from x(0-150) y(0). if i touch the screen, values range from x always 0, y around 64000+
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Touchscreen
Add a resistiv touchscreen to 4 mbed pins. We need two analog pins and two IOs.
Page owner: 
Peter Drescher
Created 14 Jul 2011.
Last updated 09 Mar 2013
Hello
i have some questions if u may... in the data sheet at mikroelectonika they mention its important to use pull-up resistor .. so my question is do i have to connect resistor or do i connect directly .. and what are the values of the resistors if i have to use any ???
Thanks in advance