74hc595 with 16f877a interfacing problem

Discussion in 'General Electronics Chat' started by varunme, Mar 8, 2015.

  1. varunme

    Thread Starter Member

    Sep 29, 2011
    59
    0
    74hc595 without connecting the pic also, the output leds to the 595 is always on, the connection diagram is as below
    when i touch the pins of 595, the output pins on and off irregularly

    what canbe the problem?

    connection diagram as below
     
    • 595.jpg
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  2. Papabravo

    Expert

    Feb 24, 2006
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    Unconnected inputs can cause erratic behavior. Lack of bypass can also cause it.
     
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  3. varunme

    Thread Starter Member

    Sep 29, 2011
    59
    0
    which all input to be connected still?
    Bypass means Bypass capacitor??
     
    Last edited: Mar 8, 2015
  4. Papabravo

    Expert

    Feb 24, 2006
    10,180
    1,800
    You said that you were testing the 74HC595 without connecting the PIC. You tell me which pins on the 74HC595 are not connected to any driving source. I meant to say "bypass capacitors", you are correct.
     
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  5. varunme

    Thread Starter Member

    Sep 29, 2011
    59
    0
    yes, only VCC and gnd were connected, all others unconnected
     
  6. Papabravo

    Expert

    Feb 24, 2006
    10,180
    1,800
    And those floating inputs are high impedance which means they can assume any value without drawing or supplying any significant current. Floating pins on a CMOS devices implies erratic operation. Why does this surprise you?
     
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