74hc595 with 16f877a interfacing problem

Thread Starter

varunme

Joined Sep 29, 2011
60
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
 

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Papabravo

Joined Feb 24, 2006
21,013
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.
 

Papabravo

Joined Feb 24, 2006
21,013
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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