ADS7845: touch screen controller: wrong readings

Luca Ellero

Part Number: ADS7845
Other Parts Discussed in Thread: ADS7846

Hi,
we are using an ADS7845 touch screen controller in our NXP iMX8MP board.
We are using Linux kernel version 5.15 with the in-tree ads7846 driver (which supports ADS7845 as well).
Driver source code is located in drivers/input/touchscreen/ads7846.c.

With some efforts we managed to get it working.
The problem is that, sometimes, the touch screen controller detects a "spurious" touch that is very far from the coordinates been actually touched.
This can produce very bad results (imagine that, for example, you have a push button at that point).
The problem is particularly visible when the touch pressure is low. If the surface is touched using heavy pressure there aren't any problems.

Using the software debounce filter is not helping here because (analyzing the touch coordinates produced by the controller) the spurious touch is not produced by a single touch but by a long series of wrong readings.

Are there any workarounds to avoid this problem?
Thanks

Best regards
Luca Ellero

over 2 years ago

0 Sanjay Dixit over 2 years ago

TI__Mastermind 23030 points

shall respond tomorrow

0 Sanjay Dixit over 2 years ago in reply to Sanjay Dixit

TI__Mastermind 23030 points

Can you please send me your schematic?

0 Luca Ellero over 2 years ago in reply to Sanjay Dixit

Prodigy 10 points

Hi,

thank you for your reply.

In attachment you can find the schematics.

As as side note:
- all SPI signals go directly to CPU pins through a voltage converter 1.8V/3.3V
- PENIRQ goes directly to CPU pin
- we tried also a 100k pull-up resistor on PENIRQ, no benefits.

Regards

0 Sanjay Dixit over 2 years ago in reply to Luca Ellero

TI__Mastermind 23030 points

There are 2 things that can be tried.

1. I see 10nf noise filter capacitors on the the lines going to the touchscreen. I would suggest to reduce these to 1n to see if the sensitivity on a soft touch is improved.

2. There is a mechanical bounce that happens on Resistive touch screens on touching and data is stable after a certain time has elapsed. Therefore you should try waiting for some time before taking a set of readings and average these readings.

You could even try a DC Check on the touch panel. A fixed DC Voltage of 3.3v can be given to the terminals of the touchscreen and the waveform on the wiper can be examined.

A soft touch can be made and the voltage on the wiper can be measured with an oscilloscope. This shall show the effect of the mechanical bounce .The waveform you see with a soft touch can be used to evolve the algorithm.

0 Luca Ellero over 2 years ago in reply to Sanjay Dixit

Prodigy 10 points

Hi,

thank you for your suggestions.

We are working on this and report results as soon as possible.

Anyway, regarding point 2, we are already waiting some time before acquiring, we have tried times from 1 ms to 20 ms, but this doesn't solve the problem.

Regards

0 Sanjay Dixit over 2 years ago in reply to Luca Ellero

TI__Mastermind 23030 points

I guess its best ti take a look with an oscilloscope.

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ADS7845: touch screen controller: wrong readings