1 VDC always on data pin

Discussion in 'General Electronics Chat' started by spinnaker, Feb 1, 2015.

  1. spinnaker

    Thread Starter AAC Fanatic!

    Oct 29, 2009
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    I was not sure if I should post this in the micro controller forum. It does seem to be a more general issue. If a mod thinks it belongs elsewhere go ahaed and move it,

    I bought one of these TFT displays a long time ago.

    http://www.ebay.com/itm/251502637209?_trksid=p2060778.m2749.l2649&ssPageName=STRK:MEBIDX:IT

    It is based on the HX8347 for the display portion.

    I decieded to pull it out of my inventory and start working with it. I was having issues and started my trouble shooting. I noticed that when I pulled a data pin low on my mcu it really never went to zero, the lowest I can get a low is 1VDC. . So I started pulling wires. When I disconnected the anode connection for the backlight, my data pins dropped to zero.

    Reconnected and the 1VDC appeared back on the data pins.

    Figuring the display has been in my stash for a long time and never tested it might just be a bad display. So I ordered 2 more about a week ago. Wired everything up and both displays are doing the same thing.

    Does this sound normal?

    What is strange is I have 3 displays, purchased at 2 different periods, all doing the same thing wrong. Makes me think I am doing something wrong but can't imagine what.


    I can post a schematic if needed but on two of the displays I did not wire much up. Just ground, VCC and the anode backlight pin. I tied the anode to %VDC through a 22ohm resistor. All per their very limited "datasheet".
     
  2. spinnaker

    Thread Starter AAC Fanatic!

    Oct 29, 2009
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    Here are their "datasheets" as far as it goes for wiring for what it is worth.
     
  3. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    So does the backlight come on when you connect it to your %VDC supply? If you have the correct pins it should.

    As far as the 1V on the output pins go, well, no schematic leads directly to no opinion.
     
  4. spinnaker

    Thread Starter AAC Fanatic!

    Oct 29, 2009
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    Yes back light comes on when connected to VDC and goes off when disconnected.

    My mcu is wired similar as in the schematic posted with the exception it is a PIC18F46K22 using internal RC OSC instead of the the 89C5X with crystal OSC.

    RD is tied high through a 1ok ohm resistor.

    But I don't need to wire everything up to see the problem. All I need to do is wire the anode and ground and the data pins float up 1VDC.

    I'll post a schematic in a bit showing those pins but it really is that simple. Just 2 pins wired and it floats.
     
  5. spinnaker

    Thread Starter AAC Fanatic!

    Oct 29, 2009
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    Here is pretty much all I have to wire up to see the issue.

    ebaydisplay.PNG

    I don't have a schematic to the display PCB.
     
  6. RamaD

    Active Member

    Dec 4, 2009
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    The microcontroller output, even when zero is output, can go higher when sinking current. It is specified in the datasheet. Checked some PICs, where the output is (Vol) is specified at a max of 0.6V for 7mA. Just the I2R drop of the output mosfet's Rdson. Even higher current will not damage the chip, provided total power dissipated does not exceed, blah, blah. So it is likely that when the Anode is connected, the uC output is sinking some current, in excess of 7mA.
     
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