Can I leave unused inputs of general-purpose CMOS logic open?
As shown in the figure below, CMOS input must always be connected to VCC or GND. Because the input impedance of CMOS is very high, output will be unstable if input is left open. Moreover, it may lead to increased consumption current due to the through current or malfunction.
Generally, consumption current depending on VCC or GND or noise do not make any difference.
[Figure: Example of open input]
* Open Input is one cause of malfunction, fault, or destruction.
[Figure: Example of connecting CMOS input to VCC or GND]
* Always connect unused input terminals to power source VCC or GND.