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Meaning of simple set-up (LOGIC board)

  1. Feb 8, 2012 #1
    Hello all,
    I had a lab pertaining to LOGIC gates, and the instructor drew something on the board (without elaborating) and asked us if we knew the significance of what he had drawn on the board. No one knew, and as such we moved on. However, I am now wondering. Attached is what he had drawn. To me it looks rather pointless. Is it supposed to signify a one way path (input----->output), or am I missing something?
     

    Attached Files:

  2. jcsd
  3. Feb 8, 2012 #2

    berkeman

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    Staff: Mentor

    It looks like the figure is wrong. If the right-hand diode went to Vcc, then it would represent ESD protection diodes for the IO line.
     
  4. Feb 8, 2012 #3
    Does ESD come from outside of the circuit, or is it from within (or both)? If the cathode of D2 is exposed to a potential of Vcc, then essentially D2 is cut off unless the potential at the I/O exceeds ~(Vcc+0.7V)? The way I am seeing it is if a voltage is applied to D2, then I/O is not grounded.
     
  5. Feb 8, 2012 #4

    berkeman

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    Staff: Mentor

    ESD generally comes form external sources (like people touching your device). The diodes are both cut off normally, and only conduct when there is a negative (bottom diode conducts) or positive (upper diode conducts) ESD transient that hits the IO line. You either use a bypass cap on Vcc to help get the ESD transient current back to ground through the upper diode, or some folks prefer to use a fast Zener diode to clamp the upper diode to ground, as in the below figure.

    http://www.ce-mag.com/archive/03/ARG/Images/03ARGCE82c.jpg
     
  6. Feb 8, 2012 #5
    Interesting. Thanks!
     
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