JAJSOB3E November 1998 – May 2024 CD54HC164 , CD54HCT164 , CD74HC164 , CD74HCT164
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When using multiple-input and multiple-channel logic devices, inputs must not be left floating. In many cases, functions of digital logic devices, or parts of functions, are unused. For example, when a triple-input AND gate only uses two inputs or the buffer gates only use three of the four buffers. Such unused input pins must not be left unconnected because the undefined voltages at the outside connections result in undefined operational states. All unused inputs of digital logic devices must be connected to a logic high or logic low voltage, as defined by the input voltage specifications, to prevent them from floating. The logic level that must be applied to any particular unused input depends on the function of the device. Generally, the inputs are tied to GND or VCC, whichever makes more sense for the logic function or is more convenient.