Controlling an LED through a button (polling based)

In this exercise, we will modify the blinky example so that LED1 is turned on only when button 1 is being pressed, as shown in the illustration below.

To do this, we will use the polling method, as discussed in GPIO Generic API. This is done by continuously polling the CPU to check if the button was pressed and then update the LED accordingly. In Exercise 2 that follows, we will learn how to use GPIO interrupts, which is more power-efficient than polling.

If we recall the schematic of the nRF52833 DK, there are four push-buttons connected to the same GPIO peripheral as the LEDs, GPIO0 (node label: gpio0). This can be seen in the GPIO pin mapping, for instance, notice that button 1 is connected to P0.11. The 0 in P0 symbolizes it is &gpio0.

This is why the same driver is needed for both the LEDs and buttons. This is also evident from the DK’s devicetree file nrf52833dk_nrf52833.dts.

	buttons {
		compatible = "gpio-keys";
		button0: button_0 { 
			gpios = <&gpio0 11 (GPIO_PULL_UP | GPIO_ACTIVE_LOW)>;
			label = "Push button switch 0";
		};
		button1: button_1 {
			gpios = <&gpio0 12 (GPIO_PULL_UP | GPIO_ACTIVE_LOW)>;
			label = "Push button switch 1";
		};
/* ... */
		};
	};

/* ... */
	/* These aliases are provided for compatibility with samples */
	aliases {
/* ... */
		sw0 = &button0;
		sw1 = &button1;
		sw2 = &button2;
		sw3 = &button3;
		bootloader-led0 = &led0;
	};
};

Button 1 on the nRF52833 DK is given the node name button_0, which has the alias (sw0) and node label (button0) and is connected to &gpio0, pin 11. Remember, indexing in the devicetree files always starts from 0, which is why button 1 on the board is called button0.

gpios = <&gpio0 11 (GPIO_PULL_UP | GPIO_ACTIVE_LOW)>;

Exercise steps

1. Clone the nRF Connect SDK Fundamentals GitHub repository. We have two options here. We can clone the GitHub repository using the graphical user interface in VS Code (described in step 1.1) or the command line (described in step 1.2). Pick either 1.1 or 1.2.

1.1 Copy the link to the repository and use VS Code’s Command Palette to clone the repository.

Go to View -> Command Palette -> type Git Clone and paste in the repository link. Save the repo somewhere close to the root directory.

1.2 Or, create a folder close to your root directory and clone the repository by running the following command in nRF Connect Terminal.

git clone https://github.com/NordicDeveloperAcademy/ncs-fund.git

Once you issue the command, the course code base will be cloned in the current working directory.

2. Open the exercise code base for this exercise in VS Code.

2.1 From the Welcome View in the nRF Connect extension, click on Open an existing application.

Select the base code for this exercise, found in ncs-fund/v2.8.x-v2.7.0/l2/l2_e1, and click on Select Folder.

2.2 Open main.c from the Explorer in VS Code as shown in the screenshot below:

3. Initialize the button on the hardware.

3.1 Get the node identifier for button 1 through its alias sw0.

Recall from the Devicetree section, that the /aliases node in the DK devicetree defined the alias sw0 for node &button0. The line below uses the macro call DT_ALIAS() to access the node identifier for the node through its alias.

In main.c , search for STEP 3.1 and add (copy and paste) the following lines:

#define SW0_NODE	DT_ALIAS(sw0) 

This is done to ensure compatibility across different hardware.

3.2 Get the device pointer, pin number and pin’s configuration flags through gpio_dt_spec.

The GPIO API has the struct called gpio_dt_spec that encapsulates all this information, and can be retrieved using GPIO_DT_SPEC_GET().

In main.c , search for STEP 3.2 and add (copy and paste) the following lines:

	static const struct gpio_dt_spec button = GPIO_DT_SPEC_GET(SW0_NODE, gpios);

This line uses the macro GPIO_DT_SPEC_GET( ) to populate the members of the gpio_dt_spec variable with:

4. It is important to verify that the device is ready to use by calling device_is_ready() before using it.

In main.c , search for STEP 4 and add (copy and paste) the following lines:

if (!device_is_ready(button.port)) {
	return -1;
}

5. Configure the pin connected to the button to be an input pin and set its hardware specifications.

Inside main(), search for STEP 5 and add the following lines:

ret = gpio_pin_configure_dt(&button, GPIO_INPUT);
if (ret < 0) {
	return -1;
}

6. In the main loop of our exercise, we will indefinitely poll the CPU to read the status of the button (pressed = 1, unpressed = 0) and update the LED to the status of the button.

6.1 Read the status of the button and store it.

Using gpio_pin_get_dt(), read the current status of button.pin and save it in the variable val.

Search for STEP 6.1 and add the following code inside the infinite loop:

bool val = gpio_pin_get_dt(&button);

6.2 Update the LED to the status of the button

Update the LED to reflect the current status of the button that was saved in the variable val in the previous step, using gpio_pin_set_dt().

Search for STEP 6.2 and add the following code inside the infinite loop:

 gpio_pin_set_dt(&led,val);

7. Change the sleep time from 1000 ms to 100 ms. 100 ms is about the right time to put the main thread to sleep and still be able to respond in time when the button is pressed.

8. Add a build configuration, like we did in Lesson 1 Exercise 2.

9. Build the exercise and flash it to the board as we have done in the previous lesson. Observe that when button 1 is pressed, LED1 is turned ON. (Remember on the nRF54L15 DK that would be button0 and LED0)

The solution for this exercise can be found in the GitHub repository, in l2/l2_e1_sol of the directory v2.8.x-v2.7.0