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Let's first come to an easy beginner project - blink the onboard LED. The LED will be on and off per second. It is always the first try when you begin to program the microcontrollers, like a hello world project.

What you need

  • SwiftIO Feather (or SwiftIO board)

The projects in the section General will use SwiftIO Feather as an example. You can also use the SwiftIO board instead. And you may need to change your code accordingly.


For this project, you only need the board. There is a built-in RGB LED on the board as shown in the image above. You can control it using the methods in DigitalOut class.

circuit diagram

Just plug the board into your computer with a USB cable to download your code.

Example code

It's time for the code. Let's see how it works. You can find the example code at the bottom left corner of IDE: example / GettingStarted / Blink.

// Turn on and off the onboard LED continuously.

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

// Import the board library to use the Id of the specific board.
import MadBoard

// Initialize the onboard green LED with other parameters set to default.
let green = DigitalOut(Id.GREEN)

// Blink the LED over and over again.
while true {
// Apply a high votage and turn off the LED.
// Keep the light off for a minute.
sleep(ms: 1000)
// Apply a low voltage and turn on the LED.
// Keep the light on for a minute.
sleep(ms: 1000)


Digital signal

The digital signal usually has two states, whose value is either 1 or 0. In our cases, 1 represents 3.3V and 0 represents 0V. There are also other ways to express the same meaning: high or low, true or false. Now you will control the output voltage to turn on or off the LED.

digital signal


The LED, or light-emitting diode, is one kind of diode. It has a positive leg (anode) and a negative leg (cathode). The long leg is positive, and the short leg is negative. The current could only flow in one direction, from positive to negative, so you should connect the positive to the current source. Only when you connect it in the right direction, the current could flow.


There are two ways to connect the LED:

LED circuit
  1. Connect the LED to the digital pin and ground. If the pin outputs a high voltage, the current will flow from the pin to the ground, and thus the LED will be on. If it outputs a low voltage, the LED is off.

  2. Connect the LED to the power and a digital pin. Since the current always flows from high to low voltage, if the pin outputs a high voltage, there is no voltage difference between the two ends of the LED, so the LED is off. Only when the pin outputs a low voltage, the current could flow from the power to the pin so the LED will be on. And the onboard LEDs work in this way.

You can find an RGB LED on your board. It has three colors: red, green and blue. As you download the code, it serves as a status indicator. Besides, you could also control its color and state by setting the output voltage.


Since there are three colors, you could light any of them: if you turn on red and blue, you could notice it appears magenta. If all three are on, the LED seems to be white.

While the onboard LED is connected to 3.3V internally. If you set it to high voltage, there will be no current. So it will be lighted when you apply low voltage.

Code analysis

import SwiftIO
import MadBoard

SwiftIO consists of all the functionalities to control your board. All programs must first reference it to use everything in it, like classes and functions.

MadBoard defines the corresponding pin id of the different boards. You will not tell a specific board here since you have chosen a specified board for your project when creating it.

let green = DigitalOut(Id.GREEN)

Before you set a specific pin, you need to initialize it. let is a keyword for Swift language to declare constants. You will often use it to assign a name to the pin for easier reference later.

This statement is to create an instance for DigitalOut class and initialize that pin. So you need to indicate its id. All ids are in an enumeration, and the built-in RGB LEDs use the id RED, GREEN, or BLUE, thus the id of blue LED here is written as Id.GREEN using dot syntax.

while true {


In the dead loop while true, all code in the brackets will run over and over again unless you power off the code.


The method write(_:) is used to set the pin to output high or low voltage. Its parameter is a boolean type: true or false: true corresponds to a high level (3.3V) and false corresponds to a low level (0V). And as mentioned above, you need to set a low voltage to turn on the LED.

sleep(ms: 1000)

The function sleep(ms:) will stop the microcontroller's work for a specified period. It needs a period in milliseconds as its parameter. It is a global function in the library, so you can directly use it in your code.

In the loop, the pin outputs high voltage and then sleeps for 1 second. So in the first 1s, there is always a high voltage. Similarly, in the next 1s, the pin outputs low voltage.


DigitalOut - set whether the pin output a high or low voltage.

  • init(_:mode:value:) - initialize the digital output pin. The first parameter needs the id. You can refer to the corresponding Id enumeration. The parameters mode and value already have their default value.

  • write(_:) - set a specific pin to output high or low voltage. Its parameter is a boolean type. true corresponds to a high level, and false corresponds to a low level.

sleep(ms:) - suspend the microcontroller's work and thus make the current state last for a specified time, measured in milliseconds.

MadBoard - find the corresponding pin id of your board.