Unused inputs must be terminated to either V_CC or ground. These can be directly terminated if the input is completely unused, or they can be connected with a pull-up or pull-down resistor if the input is to be used sometimes, but not always. A pull-up resistor is used for a default state of HIGH, and a pull-down resistor is used for a default state of LOW. The resistor size is limited by drive current of the controller, leakage current into the SN74HC21, as specified in the Section 6.4, and the desired input transition rate. A 10-kΩ resistor value is often used due to these factors.

The SN74HC21 has standard CMOS inputs, so input signal edge rates cannot be slow. Slow input edge rates can cause oscillations and damaging shoot-through current. The recommended rates are defined in the Section 6.2.

Refer to the Section 8.3 for additional information regarding the inputs for this device.