Debounced Counter Made From Off-The-Shelf Chips

Posted on Feb 5, 2014

The real usefulness of the SR latch comes during input switch transitions. Let`s assume the input switch is a break-before-make and the switch starts in the up position. From the prior analysis, the inputs values when the switch is up are low, low, high, high, and the output is high. As the switch is toggled, it breaks the first input, changing it

Debounced Counter Made From Off-The-Shelf Chips
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to high, low, high, high. Those values can be used to determine the output value. This means that even if the input switch signal bounces multiple times between low and high signals as it breaks the connection, the debounce output remains steady. Because the debounce output is steady, the counter doesn`t increment spuriously. Not surprisingly, the same steadiness occurs when the input switch starts in the down position and starts to break contact as it transitions up. The output remains low. While this is exactly what we want, this also explains why a single-throw switch or pushbutton won`t work. This circuit keeps the output in a steady state when only one input is changing back and forth. Pushbuttons and single-throw switches only change one input. Double-throw switches provide more information. By requiring two inputs to change states, not just one, you can ignore all errors generated by either input at a given time. You don`t care how long it takes for the errors (bouncing) to go away, but you`re ready to change states again right away if necessary. Software debouncing of pushbuttons relies on time as the second input. That is, the software assumes it takes a certain amount of time for the switch to settle down before more input can be reliably accepted. This slows down the maximum switching rate and the software method is still vulnerable to long bounces. In the previous two examples, it was assumed that the switch is...

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