Switches and Push buttons interfacing with 8051

There are no simpler devices than switches and push-buttons. This is the simplest way of detecting appearance of a voltage on the microcontroller input pin.

Nevertheless, it is not so simple in practice... It is about contact bounce- a common problem with m e c h a n i c a l switches. When the contacts strike together, their momentum and elasticity act together to cause bounce. The result is a rapidly pulsed electrical current instead of a clean transition from zero to full current. It mostly occurs due to vibrations, slight rough spots and dirt between contacts. This effect is usually unnoticeable when using these components in everyday life because the bounce happens too quickly. In other words, the whole this process does not last long (a few micro- or miliseconds), but it is long enough to be registered by the microcontroller. When using only a push-button as a pulse counter, errors occur in almost 100% of cases!

The simplest solution to this problem is to connect a simple RC circuit to suppress quick voltage changes. Since the bounce period is not defined, the values of components are not precisely determined. In most cases, it is recomended to use the values shown in figure below.

If complete stability is needed then radical measures should be taken. The output of the circuit, shown in figure (RS flip-flop), will change its logic state only after detecting the first pulse triggered by contact bounce. This solution is expensive (SPDT switch), but effecient, the problem is definitely solved. Since the capacitor is not used, very short pulses can also be registered in this way.

In addition to these hardware solutions, there is also a simple software solution. When a program tests the state of an input pin and detects a change, the check should be done one more time after a certain delay. If the change is confirmed, it means that a switch or push button has changed its position. The advantages of such solution are obvious: it is free of charge, effects of noises are eliminated and it can be applied to the poorer quality contacts as well. Disadvantage is the same as when using RC filter, i.e. pulses shorter than program delay cannot be registered.