ATtiny85: Debounce Your Pushbuttons!

The image above exemplifies why you need to debounce your pushbuttons.

When people push a button, they expect one reaction per push. Due to the springy nature that pushes back at you when pressing them, buttons tend to bounce around when pressed and released, which will mess up the signal from them.

For example, let’s say we have a button that we intend to output 0V (logic 0) when pressed, and 5V (logic 1) when unpressed. If we probed the signal coming from the button during the transition from pushing it down to letting go, we would expect an immediate and clean transition from logic 0 to logic 1. What we end up seeing instead is the capture above. Before the signal settles to a flat 5V, it bounces between the two logic states many times.

Imagine if this was the signal your TV received when you pressed a button on your remote. If the signal was taken as is, and a transition from 0 to 1 meant increment the channel, you would probably have an aneurysm trying to navigate to a specific channel. This is why we need to debounce our buttons!

Debouncing attempts to ignore any intermittent jumping between logic states during an actual intended transition from 0 to 1 or 1 to 0. This can be done in hardware, with RC circuits and Schmitt trigger inverters, or in software with just the microcontroller. Let’s focus on software debouncing for now.

In this post we will first consider how to read a pushbutton input and turn on some LEDs in response. Next we will use the same hexadecimal counter circuit used in the previous post, with a pushbutton used to increment the counter, first with an undebounced implementation and then with a software debounced implementation

Continue reading