[How to fix irresponsive touchscreen] [with solution]
Hi,
StoryI received a broken unit: the touchscreen was very irresponsive and would jump all over the place
. I think the unit Dave tested had a similar issue at the beginning of his video, but mine was WAY worse. Not usable at all.
I tried to send it back, but aliexpress decided I should pay for the postage and since I rejected that, they stopped asking me and refunded 30€ back. GREAT! I have a 100€ paperweight!
I thought it may had a bad solder joint or some solder bridge so I took it apart a month ago, cleaned some solder droplets and cleaned and reconnected the display flat cables. That didn't do s*t.
Yesterday I decided to give it a second try before I throw it away and
since it is trash I was very careless and started touching everything with bare hands. Maybe some static fixes it hehe. And then something happened
:
ConfusionI connected the clock line of the I2C to another oscilloscope and realised the clock was not a nice square wave all the time, so I looked a bit closer and realised there is what seems to be a cap there (see question 1). The other side of the cap is neither ground nor vcc, it is another different square signal even of a different frequency (Question 2).
-> see img2.jpeg
Lucky FixAnyway, I realised that the touchscreen worked perfectly while I was touching the ends of the cap with my bare hands
, and since both sides are connected through resistors, I
decided the added capacitance was what was fixing it.
So I tried a couple of low value caps (1pF, 5pF, 10pF and 100pF) and they all made things a bit better, but the 100pF one made it just PERFECT
!
-> see img1.jpeg
Questions1. Why is there a cap connected to SCL?
1.1 That's a capacitor, right?
2. Why is the cap connected across two signal lines?
2.1 I never saw anything like that in I2C lines, is this some advance hack?
2.2 What could the other signal be?
3. Why do you think the touchscreen was defective in the first place?
Last wordsThis neat toy is now fully functional as far as I can tell. I'm obviously a hobbyist so although I hope this may help somebody else, this opened a bunch of questions for me. I'd be very thankful if some of the old wolves here could clarify wtf is a cap doing in an I2C line and why it is connected across two signal lines.
Sorry if this appears more than once, last two posts didn't work, probably imgs too big