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Switch Debouncing. Switch Debouncing. Switches connected to sources of constant logic 0 and 1 are often used in digital systems to supply “user inputs”. In high speed digital logic applications we must consider another aspect of switch operation, the time dimension.
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Switch Debouncing • Switches connected to sources of constant logic 0 and 1 are often used in digital systems to supply “user inputs”. • In high speed digital logic applications we must consider another aspect of switch operation, the time dimension. • A simple make or brake operation, which occurs instantly as far as we slow-moving humans are concerned, actually has several phases that are distinguishable by high-speed digital logic.
Switch Debouncing • Figure (a) shows how a single-pole-single-through (SPST) switch might be used to generate a single logic input. A pull-up resistor provides a logic-1 when the switch is opened, and the switch contact is tied to ground to provide a logic-0 when the switch is pushed. • As shown in (b), it takes a while after a push for the wiper to hit the bottom contact. Once it hits, it does not stay there for long; it bounces a few times before finally settling. The result is that several transitions are seen on the SW_L and DSW logic signals for each single switch push. • This behaviour is called contact bounce. • Typical switches bounce for 10-20 ms, a very long time compared to the switching speeds of logic gates.
Switch Debouncing • This contact bounce is a problem if a switch is being used to count or signal the clock events for our lab exercise. • Then we must provide a circuit to debounce the switch - to provide just one signal change or pulse for each switch push.
Simplest Switch Debouncer • The simplest sequential circuit can be used to make the simplest switch debouncer as shown in figure (a). • This circuit uses a single-pole-double-through (SPDT) switch. • Feedback in the bistable holds SW at VOL, a valid logic 0. • Feedback in the bistable maintains the logic 0 on SW_L even if the wiper bounces off the bottom contact. It does not bounce far enough to touch the top contact again.
Switch Debouncing using S’R’ latch • In situations where momentarily shorting gate outputs must be avoided, a similar circuit can be designed using S’R’ latch and pull-up resistors as shown in figure.
