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Lighting the First LED

Introduction​

It's believed that most people start learning embedded MCU programming by lighting an LED, and learning on the Walnut Pi is no exception. Lighting the first LED will give you a certain understanding of Walnut Pi GPIO applications and programming methods, laying the foundation for future learning and larger programs while boosting confidence.

Experiment Objective​

Light up the onboard blue LED.

Experiment Explanation​

The Walnut Pi has an onboard programmable LED, located next to the button: led1

From the Walnut Pi schematic, we can see that the LED is connected to the main control pin PC13, which lights up when outputting a high level: led2

Since we are using a Python library, we only need to know the library pin name.

We can check the pin number through Python commands in the terminal.

Enter python in the terminal to start Python interactive mode:

python

Then enter:

import board

Then enter:

board.

Press the Tab key to autocomplete and see all Walnut Pi Python library pin names.

The LED's name in the Python library is board.LED:

led3

digitalio Object​

In CircuitPython, you can directly use the digitalio module to program I/O output to light up the LED. Details are as follows:

Constructor​

led=digitalio.DigitalInOut(pin)

Parameter description:

  • pin Development board pin number. Example: board.LED, board.PB6

Usage​

led.direction = value

Defines pin as input/output. Value options:

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

led.pull = value

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

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

led.value = value

GPIO output value. Value options:

  • 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

The above describes CircuitPython's DigitalInOut object in detail. digitalio is the main module, and DigitalInOut, Direction, Pull, DriveMode are submodules under digitalio. In Python programming, there are two ways to reference related modules:

  • Method 1: import digitalio, then operate via digitalio.DigitalInOut;
  • Method 2: from digitalio import DigitalInOut, meaning directly import the DigitalInOut module from digitalio and then operate directly via DigitalInOut. Method 2 is clearly more intuitive and convenient. This experiment also uses Method 2 for programming. The code flow is as follows:

Reference Code​

'''
Experiment Name: Lighting the First LED
Experiment Platform: Walnut Pi
'''

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

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

led.value = 1 # Output high level, light up onboard blue LED

#led.value = 0 # Output low level, turn off onboard blue 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

led4

The LED is lit. However, the Walnut Pi's blue LED is used as a system startup indicator by default (blue LED lit when system starts successfully), so the effect may not be noticeable.

led5

You can modify the code to turn off the LED:

led.value = 0 # Output low level, turn off onboard blue LED

led6

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

led7

From this first experiment, we can see that using Python to develop Walnut Pi hardware requires learning the constructor and usage of Python libraries, enabling programming operations on related objects. With the support of powerful Python libraries, the experiment only needed a few lines of code to light up the LED.