TFT touch panel calibration

[georpo]

Hello!

I have a 4.3" TFT display with resistive touch screen and SSD1963 controller connected to a PIC24HJ256GP206.

Everything is working perfectly with graphics etc but I cannot think of a way to calibrate the touch screen.

The display resolution is 480x272 pixels.
The ADC readings is 0-580 for X and 0-360 for Y.

I need a formula to fit the ADC readings linearly in the display resolution.

I tried:

[John P]

I don't understand what "actual coordinates of the display" means. With your division calculations, you've already got inputs on a 0-480 and 0-272 scale. What else do you need?

[georpo]

I mean that the "X 100" reading on the ADC does not correspond to PIXEL X 100
_________________
George.

[Ttelmah]

It is impossible for us to help without knowing something about what is really happening.
Now, you say the ADC reading is 0-580, for X, but the first thing is where is 0 actually read?. Left edge of screen, right edge of screen?.
The same applies for Y.
Then is it really 0-580, or are you just guessing at the 0, with 580 being the max, but without data for the min?.

The normal way to calibrate a touch screen, is to display points at four known co-ordinates near the corners. Then take ADC readings as each of these points is touched. So (for example), display a cross at X=10, Y=10 (display coordinates) and read the ADC at this point. Then do the same at X=470, Y=10, then X=10, Y=370, and finally X=470, Y=370.

You then have four coordinate pairs giving the ADC readings for these points. Calling them in order, x1, y1, x2, y2 etc.. The maths is then:
Deltax=(x4+x2)-(x1+x3)/2
Deltay=(y4+y3)-(y1+y2)/2
offsetx=(x1+x3)/2
offsety=(y1+y2)/2

Position from a new adc reading x,y then becomes:

(((x-offsetx)*460)/Deltax)+10
Key thing to note, is that this can be done in signed int32 maths, rather than float maths - much faster, because the multiplication is performed first.
Note also that this will handle what I suspect is happening to you. Two things may apply:
1) '0', may not be the actual edge of the display. ADC readings may be something like 20 to 580, not 0 to 580.
2) '0', may not (likely) be the bottom left....


Best Wishes

[georpo]

Thanks for all this info!

X0,Y0 is left top edge.
The actual ADC readings are 98-685 for X and 159-520 for Y.

I just subtract 98 from the X reading and 159 from the Y reading
to make my readings start from 0.
_________________
George.

[Ttelmah]

So you need:

touchY=360-(touchY/1.32352);

To make the screen coordinates agree.

Best Wishes

[georpo]

[Ttelmah]

No, I mean 460.
Remember this was based on calibrating from points at +10, and +470 in X, not the edges of the screen.

Seriously, have you checked what the impedance specification of the touch screen output is?. I'd guess it is quite high. Hence would require a buffer amplifier....

Best Wishes

[georpo]

I found the problem!

I was using pull down resistors on the 2 analog inputs.
This made the reading non linear.

Your method works perfectly!

Thank you very much!
_________________
George.