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Button

Introduction​

The button is the simplest and most common input device. Many products rely on buttons, including early iPhones. Today we'll learn how to write a button program using Python. With button input functionality, we can create many fun projects.

Experiment Objective​

Program to detect button input.

Experiment Explanation​

The Walnut Pi has an onboard button, located next to the Type-C power port:

key1

From the Walnut Pi schematic, we can see the button is connected to the main control pin PC12. When not pressed, it inputs a high level (1); when pressed, it connects to ground and outputs a low level (0):

key2

Since we are using a Python library, we only need to know the library pin name. The button's name in the Python library is board.KEY:

key3

digitalio Object​

In CircuitPython, you can directly use the digitalio module to program I/O input for detecting the button's input level. Details are as follows:

Constructor​

key=digitalio.DigitalInOut(pin)

Parameter description:

  • pin Development board pin number. Example: board.PC8

Usage​

key.direction = value

Defines pin as input/output. Value options:

  • digitalio.Direction.INPUT : Input.
  • digitalio.Direction.OUTPUT : Output.

key.pull = value

Set pull-up/pull-down resistor. Value options:

  • digitalio.Pull.UP : Pull-up.
  • digitalio.Pull.DOWN : Pull-down.

value = key.value

Key input return value. Value return values:

  • True or 1 : High level.
  • False or 0 : Low level.

For more usage, please refer to the official documentation:
https://docs.circuitpython.org/en/latest/shared-bindings/digitalio/index.html

KEY, like the LED in the previous section, also uses the digitalio object, but changed from output mode to input mode. We can write code to light up the blue LED when the button is pressed (input low level) and turn it off when released (input high level).

Reference Code​

'''
Experiment Name: Button
Experiment Platform: Walnut Pi
'''

# Import related modules
import board
from digitalio import DigitalInOut, Direction, Pull

# Build LED object and initialize
led = DigitalInOut(board.LED) # Define pin number
led.direction = Direction.OUTPUT  # IO as output

# Build key object and initialize
key = DigitalInOut(board.KEY) # Define pin number
key.direction = Direction.INPUT # IO as input
key.pull = Pull.UP # Enable pull-up resistor

while True:

    if key.value == 0: # Button pressed
        led.value = 1 # Light up LED

    else: # Released
        led.value = 0 # Turn off LED

Experiment Results​

Here we use Thonny to remotely run the above Python code on the Walnut Pi. For instructions on running Python code on the Walnut Pi, please refer to: Running Python Code

key4

Press the button, LED lights up.

key5

Release, LED turns off

key6

Besides using the onboard button and LED, you can also build your own circuit. Just modify the GPIO pin number in the code accordingly.

key7

The essence of the button is also GPIO operation. Besides the onboard LED and button, you can also connect to other GPIO pins (on the 40-pin header) using Dupont wires to use related functions.