What are these little blips?

Discussion in 'Embedded Systems and Microcontrollers' started by spinnaker, Nov 10, 2012.

  1. spinnaker

    Thread Starter AAC Fanatic!

    Oct 29, 2009
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    I am seeing these little blips right in the middle of the square wave output form a Pic EUSART outpin pin.

    I am seeing them on my Tek and Rigol scope. They are probably much more pronounced on the Tek, Any idea what they are?

    [​IMG]
     
  2. spinnaker

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    I got a better shot of them here. I have flipped the channels around in this shot.


    [​IMG]
     
  3. MrChips

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    Oct 2, 2009
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    What you are seeing is cross-talk between the two input channels.
    You will see that the blips align with the rising and falling edges of the faster clock.
    This is normal and not anything to be concerned about.
     
  4. spinnaker

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    But if I disconnect one channel, I am still seeing them.
     
  5. MrChips

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    It is also present on the logic board as power supply and ground noise,
    one of the reasons we stress good power supply decoupling capacitors.
     
  6. spinnaker

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    How do you know this? I have not put a scope to my VDD pin.

    Funny though I just measured the output again and they are gone. :confused:
     
  7. MrChips

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    Are you looking at the lower or higher frequency clock? You will not see the blips on the higher frequency.
     
  8. MrChips

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    Because I have seen this too many times.
     
  9. nigelwright7557

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    Classic poor decoupling.
     
  10. takao21203

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    Apr 28, 2012
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    If you look close you see the spikes on the data line occurs exactly at the time the clock line floats to zero.

    This might be due to the internal structure of the PIC.

    Isn't so much a matter of concern as long as it is just a little as seen here.

    I had serious issues when I used synchronous serial port over long distance- 30 cm or so, at MHz frequencies. Had to add small 22uH coils at the endpoint...problem solved.
     
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