11.1 Layout Guidelines

When using multiple-bit logic devices, inputs should never float.

In many cases, functions or parts of functions of digital logic devices are unused. For example, when two inputs of a triple-input AND gate are used or only 3 of the 4 buffer gates are used. Such input pins should not be left unconnected because the undefined voltages at the outside connections result in undefined operational states. Figure 14 specifies the rules that must be observed under all circumstances. All unused inputs of digital logic devices must be connected to a high or low bias to prevent them from floating. The logic level that should be applied to any particular unused input depends on the function of the device. Generally, they will be tied to GND or V_CC, the deciding factor is based on whichever makes more sense or is more convenient at the time. It is generally acceptable to float outputs, unless the part is a transceiver. If the transceiver has an output enable pin, then asserting the output enable pin will disable the output section of the part. This will not disable the input section of the I/Os, so they cannot float when disabled.