Hot!Touchscreen not responding:

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TomWr
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2018/01/13 06:01:15 (permalink)
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Touchscreen not responding:

Problem:  Touchscreen is not working.
 
Components:
PIC32MZ2048EFG064
TST350MLQZ-06P LCD with touchscreen  (I tried two different units)
SSDD2219 SPI interface for display driver (SPI)
STMPE811 driver for touchscreen (I2C)
 
On a previous prototype board using the PIC32MX, we had the LCD and touch fully working.  We needed more capabilities on the PIC so we changed processors.
 
The LCD display works fine.  the touchscreen does not.  I can write/read to both the SSD2219 and the STMPE811.  I can read back, thus confirming that I can configure the driver chips.  The STMPE811 handles the touchscreen.  There are a series of registers that it uses to configure and capture touch activity.  There is an interrupt, which is configurable.  We use it when the FIFO contains a touch response.
 
I do not receive the interrupt on the PIC.  When I scope the pin, I do not see any activity when I touch the display.  So, it isn't the PIC.  the STMPE811 is not generating the interrupt.
 
From the PIC, I also read the STMPE811's FIFO and status.  There is nothing.
 
I checked the control related pins on the LCD display and they are seemingly fine.  The only thing I found a little od was the Backplane voltage was only 5.4volts.  
 
I then checked the 4 wire touchscreen interface.  When I touch the display, I see no change in state for any of the pins.  X+ is 3.3 volts.  Y+ and X- exhibit similar signals.  They are roughly zero volts, but you can see tiny periodic spikes down to -0.1 volts and +0.17volts.  Y- is -0.02 to -0.1 volts.
 
I don't know if those are normal readings, but I would naturally and unknowingly think Y+ would also be 3.3volts.
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Any help in debugging would be greatly appreciated.  What readings should I expect on the 4 wire interface pins?  I am guessing the power supply for the analog portion of the display provides the power.  But, heck, I don't know.
 
Is it possibly a configuration issue with the STMPE811 touch driver.  I am the software guy and I think it is a configuration issue.  But, this configuration worked on the previous board.
 
I am stumped.
 
Thanks for any help I received,
Tom
#1

3 Replies Related Threads

    laurie21
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    Re: Touchscreen not responding: 2018/01/13 10:17:24 (permalink)
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    Have you confirmed all the pins on the STMPE811 are soldered properly? I just went threw a
    nightmare due to that reason, some of the pins on my FT232 using the same package did
    not take the solder as they should have. Your testing the (irq-out) and finding no pulse
    makes me think it could be a hardware problem.
    #2
    jcandle
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    Re: Touchscreen not responding: 2018/01/13 20:38:47 (permalink)
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    I am trying to remember but first I would think one plane (the X resistance or the Y) is driven high and the other is weakly pulled low, or vice versa.  A touch would make contact between x and y resistive planes. 
     
    For the case where X+ is pulled high and X- floated (should be close to 3.3V!. Y+ would be weakly pulled low and Y- floated (so weakly pulled low.  A push should pull Y+ up to X+ as long as the RTP sheet's resistance is low compared to the pull down resistor.   
     
    I agree - check the solder...  
    #3
    TomWr
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    Re: Touchscreen not responding: 2018/01/15 09:16:33 (permalink)
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    I appreciate the responses.  Thank you.  Interestingly, We have two units and both of them have this problem.
    #4
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