What is Switch Debouncing?

All mechanical switches will ‘bounce’ when closed which means the contacts do not make immediate firm contact. The consequence of this is that the actual voltage from the switch oscillates when contact is made. If this signal is to act the input to a digital circuit then instead of the circuit seeing only a single transition it will actually see many. A common use of an SR flip-flop is to dean up this signal to ensure only a single transition is produced. The SR can do this because once set or reset it will remain in this state if both inputs are held at 0 (and so be immune to the oscillations).

Example 6.1                                 

Draw the outputs that will be obtained from the circuit in Fig. 6.2 and determine how an SR flip-flop can be used to clean up these signals.

Solution

Fig. 6.3 shows the signals that will be obtained from points A and B. We note that the contact bounce gives conditions of A and B both 1 during which time we require the outputs to remain constant. We cannot use inputs of  1 for an SR flip-flop but note that if both inputs to the flip-flop arc inverted (to give an SR flip-flop) then we will obtain the required clean waveform shown in Fig. 6.4.

Input A going LOW, and so 5 LOW, will set Q=l. Then as transition occurs the contact bounce giving A and B both HIGH will cause both 5 and R to be HIGH (i.e. S=R=O) and so the flip-flop will remain in the same (Q=1) state as required. Similarly, input B going LOW will reset Q to 0. Contact bounce giving A 1 will not affect Q which remains LOW.