Visual glitches upon touch input

[Volcano339]

I'm using a Teensy 4.1 with a display using the default pins.
Whenever I touch the center of the display, the screen starts having weird artifacts (colors inverted, image misaligned and repeated, flickering, and usually a white screen after the others).

Sometimes, I don't even need to touch the display (this usually happens if any pin other than PIN_BACKLIGHT is set to 255).
When this is the case, the display usually alternates between a white screen and another white screen with gray horizontal stripes.

This happens with every demo except for touch calibration.

The visual problems go away after rebooting (as long as you don't use touch again).

This is the sketch where the screen goes white if you touch the center:
https://gist.github.com/Volcano339/621a4a85d2d7d4b0491ddec014bf92fb

I don't know if this is a hardware issue or not, but the fact that the glitches go away after rebooting suggests to me that it is more likely a software issue.

If you think it is a hardware issue, feel free to ask questions about the hardware.
This might just be a simple hardware mistake.

Any help is appreciated!

[vindar]

Hi,

I just tried your sketch but it works without glitches for me. The only difference with my hardwired test setup is that the backlight pin of the display is directly connected to +3.3v and not to PIN 2. All the other connections are the same.

I suspect it is a wiring problem... Maybe a loose connection ? How long are your wires, breadboard wires longer than 20/30cm do not work well with high SPI speed. You can try setting the SPI speed to 10Mhz just to see if it helps.

It also looks like the touchscreen may be interfering with the display. Are you sure that T_CS is routed to PIN 4 on the Teensy ?

With the sketch provided, the complete wiring should be:

• display CS <-> Teensy pin 9
• display DC <-> Teensy pin 10
• display SDI <-> Teensy pin 11
• display SDO <-> Teensy pin 12
• display SCK <-> Teensy pin 13
• display RST <-> Teensy pin 6
• display LED <-> Teensy pin 2
• display T_CS <-> Teensy pin 4
• display T_IRQ <-> Teensy pin 3
• display T_DIN <-> Teensy pin 11 (same pin as SDI)
• display T_DO <-> Teensy pin 12 (same pin as SDO)
• display T_CLK <-> Teensy pin 13 (same pin as CLK)

What happens if you physically disconnect all the touchscreen pins (ie all the T_* ones) from the teensy and set PIN_TOUCH_IRQ and PIN_TOUCH_CS to 255 in the sketch ? Are there still glitches ?

Finally, do you use the usual cheap red ILI9341 breakout board ? Some of those boards do not even have a touchscreen present even though the touch pins are still present on the PCB. Can you post a picture of your setup ?

[Volcano339]

I tried it without the T_* pins and it seemed to not have any problems.

I made sure that all connections were correct and I set the SPI speed to 10Mhz.
I still get issues if the T_* pins are connected.

The touch chip is soldered to the display breakout and the touch calibration wizard works just fine.

Maybe it has something to do with how I'm powering it.
The power supply outputs 5V.

The LED is being powered by pin 2 with a 110Ohm resistor.

Maybe my unit is just defective.

Here is a picture of it:

[vindar]

That is weird indeed.. I do not understand why it works with the touchscreen calibration demo and not the others sketches...

I will try to replicate your exact setup on a breadboard this evening when I have some time.

Two remarks:

• These breadboard voltage regulators are notoriously very bad quality... You may try removing it and powering the Teensy and screen directly from USB. When connected to USB, the +5V pin of the Teensy will deliver more than enough current to power the ILI9341 screen. This is how I usually do it.

• On the other hand, I think to recall that some red ILI9341 screen do NOT have a transistor attached to the backlight pin in which case it can draw a lot (too much!) current from the digital pin 2 of the teensy. Theoretically up to 30ma at 3.3V with the 110Ohm resistor... This is really too much for a digital pin who is rated for something like 10ma maximum (or even less, I do not recall the exact value)... You should instead connect the backlight pin from the screen to the +3.3V output of the Teensy (via the 110Ohm resistor just to be safe).

[Volcano339]

I tried it with USB power and connected the backlight to a 3.3V pin (with the 110Ohm resistor) and it made no difference.
I might just have to get a replacement display.

Let me know if you have any more questions about my wiring.
I think your tests are my last hope before I return the display.

[vindar]

Hi,

I replicated your setup on a breadboard (with the LED pin connected to +3.3V through a 100Ohm resistor).

It works ok. No bug/freeze when I touch the screen. I just let it run for 30 minutes without any error. I guess it really is an hardware problem on your side...

[Volcano339]

Thanks for the help!
I guess I'll just get a replacement.

There is something that is still bothering me though.

If the issue really is on the hardware side, why does the touch calibration demo have no issues?

[vindar]

Yes, I do not really understand why the touch calibration demo works.

However, it is worth noting that while doing performing calibration the driver is not using all the fancy features like DMA transfers, double buffering, vsync... This may make this mode of operation more resilient to failure/noise/temporary disconnection between the MCU and the display (e.g. if the signal on some lines SCK/MISO/MISO is shortly interrupted) and may enable it to recover... But that is just a guess.

[Volcano339]

Now I have to figure out if the problem lies with the display itself or if my wiring is just not good.
Unfortunately, results from testing are horribly inconsistent.
One last observation I made is that the problem can be triggered if the T_* wires are at all moved (sometimes this has no effect). I think this might also happen with the backlight wire, but the tests are too inconsistent to really tell.
The only thing that breaks the display every time is applying pressure to the center of the screen.

Do you think I should just try it with a different display, or are there other things that I should look out for with my wiring?

[vindar]

I do not know the answer really :) My feeling is that, even if the wiring is inconsistent, your display is faulty anyway since pressing the screen always freeze/crashes the program... These display are very cheap (I usually buy them on aliexpress for a few euros) so I would simply get another one...

[Volcano339]

Alright, I got a replacement display and I still have the exact same issues.
I tried using the alternate pins, but the problem persists (with USB power).
The only thing I can do that will stop the problem is not connect the T_* pins.

Now I'm really confused :/

I wonder why it's the T_* pins specifically that cause problems.
Could something be wrong with my wiring?
Could something be wrong with my Teensy?
Maybe the breadboards have issues?

Any ideas?

[vindar]

That is really weird...

I have never had any problem with a Teensy 4 (I did burn a couple of them inadvertently but it was my fault and they just stopped working completely). Just in case, you can try resetting the teensy to it's default state by pressing the button on the board for about 15 second and then releasing it when the led starts to blink. I do not expect this will really help but it won't hurt and does not cost anything to try...

As for the breadboard, I did get a faulty one once where some connections did not work. It took me some time to figure it out... Maybe yours also has some defects ? Can you use another one or at least change the position of the display on the breadboard?

Finally, you may also try shorter breadboard wire. Long ones cause problem with high SPI rates. You should also lower the SPI rate to 10Mhz during testing...

Apart from those generic advices, I cannot help much because I have never experienced these particular problems, sorry! Hopefully you will find what is wrong.

[Volcano339]

I moved the display to the other half of the breadboard and it still has problems.

The SPI speed is set to 10Mhz and all wires are <30cm.

I wrote a test program for the Teensy that makes every pin alternate between high and low every 0.5s, and every pin seemed to work just fine.
Is it safe to assume that the Teensy isn't the problem if the test program worked?

The two weirdest behaviors from this have been the fact that this only happens if the touch pins are connected, and that the touch calibration demo has no issues.

I'm running out of ideas

[vindar]

:(

I am also running out of ideas...

Do the problem still appears if you disconnect the touch_irq wire and set it to 255 in the code ? The IRQ pin is not mandatory to use the touchscreen.

Also, what happens if you leave everything connected but set all touch related pin to 255 in the driver and add:

pinmode(4, OUTPUT)
digitalWrite(4, HIGH)

before the begin() call to the driver to disable SPI communication with the touch device. More generally, you may test which touch pin/ wire causes problem.

Also, when the screen freeze, what is the output on the serial monitor. Does the driver still print the stats periodically or does the program completely hang ?

[Volcano339]

I don't think the program itself has ever frozen.
It seems to work now without touch_irq connected.
I tried connecting touch_irq to pin 28, but it has the same issues.

What functionality am I missing out on if I don't use touch_irq, and do you have any idea why touch_irq specifically would cause problems?

[vindar]

Good, seems like we are narrowing on the issue !

The IRQ pin is used by the display to indicate when the screen is being touched. When connected to the Teensy and registered within the ILI9341_T4 driver, it will trigger an interrupt which register the current time. In this way, when readTouch() is called, the method can check whether the screen was touched since the previous call to readTouch(). If not, then it is not necessary update the touch status which saves a SPI transaction. On the other hand, when the IRQ pin is not set, any call to readTouch() will require a SPI transaction and will therefore need to block/wait until the current screen update completes which may be inefficient when called often.... Also, if the IRQ pin is not set within the driver, the lastTouched() method will always return -1.

The IRQ pin can be connected to any (unused) pin on the Teensy. I find it surprising that the IRQ pin is the one causing troubles. Can you try whether the bug occurs in any of the following two cases ?

1. The IRQ is connected to the Teensy but the corresponding TOUCH_IRQ is set to 255 in the sketch (so we pretend is it not connected).
2. The IRQ pin is disconnected but TOUCH_IRQ is set to a pin pull up to +3.3V (so we pretend it is connected but we make sure the interrupt never fires since it is active low).

As you mention that the program itself has never frozen, what does serial output when the display stops working ? Does it keep outputting the periodic printStats() in the sketch as usual ?