9.2.1.2 Input Considerations

Input signals must cross V_t-(min) to be considered a logic LOW, and V_t+(max) to be considered a logic HIGH. Do not exceed the maximum input voltage range found in the Absolute Maximum Ratings.

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 SN74HCS20-Q1, as specified in the Electrical Characteristics, and the desired input transition rate. A 10-kΩ resistor value is often used due to these factors.

The SN74HCS20-Q1 has no input signal transition rate requirements because it has Schmitt-trigger inputs.

Another benefit to having Schmitt-trigger inputs is the ability to reject noise. Noise with a large enough amplitude can still cause issues. To know how much noise is too much, please refer to the ΔV_T(min) in the Electrical Characteristics. This hysteresis value will provide the peak-to-peak limit.

Unlike what happens with standard CMOS inputs, Schmitt-trigger inputs can be held at any valid value without causing huge increases in power consumption. The typical additional current caused by holding an input at a value other than V_CC or ground is plotted in the Typical Characteristics.

Refer to the Feature Description for additional information regarding the inputs for this device.