• State
  • Locked Locked
  • Replies 1 reply
  • Subscribers 98 subscribers
  • Views 1621 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

Resistive touch screen problems with ESD diodes, BBB AM335x

Cyril Moore
Prodigy 30 points

Hi Guys,

I have been working on an expansion board for the BeagleBone Black that includes a resistive LCD touchscreen. The problem that we are having is that our ESD protection diodes seem to break the break the touch input. 
We are using a 4 wire touch interface on AN0, AN1, AN2, and  AN3. The symptom we are seeing is that when the diode is connected to the touch interrupt seems to be triggered inconsistently. 
When checking the signals with a scope, it shows that the interrupt seems to work fine until an application that uses touch is run then it becomes inconsistent. This does not happen if the ESD diode is removed. The apps we are running are the Qt calibration and Qt touch test. Looking at the raw data from the touch device from within Linux on the BeagleBone Black, we see a steady stream of data before a touch app is run. We see inconsistent data streams once a touch app is running AND after the app closes.  Again this symptom does not happen with the diode removed.
My question is 2 parts:
1)What could be happening that would make the touch interrupt inconsistent only when that diode is present AND one of these applications is run?
2) Do we really need additional ESD protection? Does Anyone have any experience with a AM335x and a heavy usage resistive touch screen, that is not have additional ESD protection? 
Thanks 
Cyril
  • 0
    TI__Guru**** 393215 points
    Hi Cyril,
     
    None of our reference designs have any ESD protection on resistive TS lines. As I see it there could be two possible causes - capacitance added by the ESD device and leakage current. In my opinion the problem is caused by the device capacitance (60 - 80pF, compared to the AM335X input sampling capacitance of 5.5pF nominal).
     
    I can't say about software behaviour.