Touchscreen linearity on STM32F469 issue

[Jerry Hancock]

Posted on December 11, 2016 at 01:29

After spending a lot of time with an STM32F469 discovery board I found that the touch screen doesn't have good linearity at all.  You can't draw a straight line across the screen.  If you for instance, start on the left drawing across even with a ruler, the resulting trace goes about half way across and then dips several millimeters and comes back up.  The resulting trace looks like the side view of a spoon.  I know these boards are cheap but I haven't seen anything like this with others.  Has anyone else had this experience?  I would hate to have to go thru the hassle of returning it as I doubt Mouser would take it back.

Thanks

Jerry

#stm32f469 #touchscreen

[Imen.D]

Posted on December 13, 2016 at 18:44

Hello

Hancock.Jerry

‌,

You can try an example within STM32CubeF4which describes how to configure LCD touch screen and attribute an action related to configured touch zone:

STM32Cube_FW_F4_V1.0\Projects\STM324x9I_EVAL\Applications\Display\

LTDC_Paint

Hope this help you to deduce what is going wrong in thiscase.

Best Regards

-Imen-

[Jerry Hancock]

Posted on December 22, 2016 at 18:19

Imen, Thank you for the reply.  I have since also purchased the STM32F769 Discovery which has the same display and touchscreen.  Unfortunately, though this board is better, it has the same non-linearity problem.  For instance, drawing across the board and placing a pixel at the touched locations, the resulting line is not straight.  I would be able to work with it being higher or lower on the board as long as the line was straight.  The line curves and the only way to resolve this would be to plot each point into an array with the resulting actual location.  I've looked at one of the paint programs, I believe the one you reference, and it uses a  calibration routine that only handles the up/down and left/right issues.  It can't possibly handle a wavy line calibration issue.

Other boards that use the smaller, lower resolution screens including the STM32F746 do not have this problem.  So the touch screen on these boards are basically unusable for any amount of detailed resolution.  If your application keeps the touch sensitive areas from stacking on top of each other, then it will work since the linearity issue is related to the vertical resolution (e.g. if you draw a straight line vertically when the display is in landscape mode then the line is straight).

Again, thank you for the reply.