Debounce

When we use a push button as a toggle switch, it often generates some flickering effects, due to mechanical and physical limitations. I.e, the processor may recognize the button being pushed several times in a short period of time. This example demonstrates how to debounce an input, which means checking twice in a short period of time to make sure the push-button is definitely being pressed, and only being pressed once.

What you need

• SwiftIO board

• Button

• Jumper wires

Circuit

Code

// Check if the button is definitely pressed.

// Import the library to enable the relevant classes and functions.
import SwiftIO

// Initialize the red onboard LED.
let red = DigitalOut(Id.RED)

// Initialize a digital input pin D10 the button is connected to.
let button = DigitalIn(Id.D10, mode: .pullUp)

// Declare the values in order to record and judge the button state.
var count = 0
var triggered = false


while true {
    // Read from the input pin.
    let value = button.read()

    // Ignore the change due to the noise.
    if value == false {
        count += 1
    } else {
        count = 0
        triggered = false
    }

    // Wait a certain period to check if the button is definitely pressed. 
    // Toggle the LED and then reset the value for next press.
    if count > 50 && !triggered {
        red.toggle()
        triggered = true
        count = 0
    }

    // Wait a millisecond and then read to ensure the current state last for enough time. 
    sleep(ms: 1)

}

Instruction

Swift has a basic Boolean type, called Bool. Boolean values are referred to as logical, because they can be either true or false. Swift provides two Boolean constant values, true and false. var triggered is define as Bool to store whether the DigitalInput is change.

Due to mechanical and physical issues, push-buttons often generate spurious open/close transitions when pressed, these transitions may be read as multiple presses in a very short time fooling the program. This example demonstrates how to debounce an input, which means checking twice in a short period of time to make sure the push-button is definitely pressed. Without debouncing, pressing the button once may cause unpredictable results. This code uses the sleep(ms: 1) 50 times to keep track of the time passed since the button was pressed.

See Also

• DigitalOut - The DigitalOut class is used to set a High or Low voltage output to a digital output pin.
• DigitalIn Mode - The Mode enumerate includes the available input modes.

References

• swift: Boolean type
• wiki: Push–pull output