We use switches a lot in our projects. Pushbutton switches, slide switches, micro switches. We also have touch sensors, light sensors, sound sensors, and many others.
These physical devices are messy. By that I mean that the change from one state to the other (e.g. pressing or releasing a button) bounces back and forth before settling into the new state. You can see this in the oscilloscope trace below.
These bounces are fast. The bouncy section of the above trace lasts for a couple milliseconds. If you hooked a switch to an LED as below, you'd never notice the bounce. They're far to fast for that.
However code running on a microcontroller is a lot faster that our eyes and brains. It can see those bounces. If you have code that's looking for the push and release of a switch, it might be something like this:
import board
import digitalio
pin = digitalio.DigitalInOut(board.D12)
pin.direction = digitalio.Direction.INPUT
pin.pull = digitalio.Pull.UP
button_state = False
while True:
pressed = pin.value
if pressed != button_state:
print(pressed)
button_state = pressed
Some switches bounce more than others and with some CircuitPython code won't see the bounces whereas C++ code would. Python's slower performance can actually do you a favor sometimes. But not always. And as the microcontrollers CircuitPython runs on get faster, bouncing will be more of a problem.
There are two straightforward ways of doing debouncing in CircuitPython.
- The
keypadmodule returns debounced key or button press and release events.keypadis the preferred way to do debouncing, but it is not available on SAMD21 builds for space reasons. For more information on keypad, see the Keypad and Matrix Scanning in CircuitPython guide. - This guide introduces the
adafruit_debouncerlibrary and itsDebouncerclass. It starts with the basic usage and proceeds to more advanced capabilities.
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