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jean_prieur
jean_prieur
Associate III
September 11, 2013
Question

When I touch the SCL pin with my finger the SPI works

  • September 11, 2013
  • 12 replies
  • 2010 views
Posted on September 11, 2013 at 17:32

Hello,

I programmed a LCD screen with SPI on a prototype and everything was working fine.

But I mounted

 the screen on the final PCB and now I have a 

problem: the screen didn't understand the datas I send to it. The screen 

remains white. Datas are correctly send, I know it because I compare signals 

with my working prototype.

When I touch with my finger the clock pin, the screen is able to 

understand datas and works fine.

Do you ever had this problem ? I am sure of the screen connexions and 

I tested it with different SPI speeds and configurations...

Thanks !
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    12 replies

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    dthedens23
    dthedens23
    Associate
    September 13, 2013
    Posted on September 13, 2013 at 17:32

    The data sheet specifically mentions attention to line-end reflections and noise on SCL.

    Some Arduino users mention using internal pull ups.  Not sure that is a good idea.

    the finger does add R/C which can effect ringing/overshoot/rise time/....

    you just have to probe with a good scope.

    jean_prieur
    jean_prieurAuthor
    Associate III
    September 15, 2013
    Posted on September 15, 2013 at 13:07

    Hello everybody,

    I find the problem: it was another SPI device linked on 

    the same CLK and SDA pins who caused troubles. When I remove this device (this is a hard to remove device because of his package, this is why I have been slow to disconnect it), the 

    screen works perfectly. 

    I hope the screen is able to share the SPI with another devices.

    This not resolved the finger mystery, I try to put end line resistors or capacitors but no effects...

    Anyway thanks for your advices !

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