The cross-coupled nand gate method is ideal for projects where two processor pins are not available for use with one switch.
Output Q is high when the switch is idle, and low when it is activated. The /Q output is the opposite: low when idle and high when activated. Note that only Methods 1, Two-Pin Debounce, Method 2, Cross-coupled Nand Debounce or Method 3, D-Type Flip Flop Debounce provide immediate outputs with no delay on activation and release (see Waveform 1).
For some industrial automation and robotics applications, the immediate attention of signaling from these debounce methods is needed.
Note that the LogiSwitch integral debounce switches or LS100 series debouncer ICs incorporate a circuit similar to the circuitry of Method 2.