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Button

Overview

A simple button demo showcasing the use of GPIO input with interrupts. The sample prints a message to the console each time a button is pressed.

Requirements

The board hardware must have a push button connected via a GPIO pin. These are called “User buttons” on many of Zephyr’s Supported Boards.

The button must be configured using the sw0 devicetree alias, usually in the BOARD.dts file. You will see this error if you try to build this sample for an unsupported board:

Unsupported board: sw0 devicetree alias is not defined

You may see additional build errors if the sw0 alias exists, but is not properly defined.

The sample additionally supports an optional led0 devicetree alias. This is the same alias used by the Blinky. If this is provided, the LED will be turned on when the button is pressed, and turned off off when it is released.

Devicetree details

This section provides more details on devicetree configuration.

Here is a minimal devicetree fragment which supports this sample. This only includes a sw0 alias; the optional led0 alias is left out for simplicity.

/ {
     aliases {
             sw0 = &button0;
     };

     soc {
             gpio0: gpio@... {
                     status = "okay";
                     gpio-controller;
                     #gpio-cells = <2>;
                     /* ... */
             };
     };

     buttons {
             compatible = "gpio-keys";
             button0: button_0 {
                     gpios = < &gpio0 PIN FLAGS >;
                     label = "User button";
             };
             /* ... other buttons ... */
     };
};

As shown, the sw0 devicetree alias must point to a child node of a node with a “gpio-keys” compatible.

The above situation is for the common case where:

  • gpio0 is an example node label referring to a GPIO controller

  • PIN should be a pin number, like 8 or 0

  • FLAGS should be a logical OR of GPIO configuration flags meant to apply to the button, such as (GPIO_PULL_UP | GPIO_ACTIVE_LOW)

This assumes the common case, where #gpio-cells = <2> in the gpio0 node, and that the GPIO controller’s devicetree binding names those two cells “pin” and “flags” like so:

gpio-cells:
  - pin
  - flags

This sample requires a pin cell in the gpios property. The flags cell is optional, however, and the sample still works if the GPIO cells do not contain flags.

Building and Running

This sample can be built for multiple boards, in this example we will build it for the nucleo_f103rb board:

west build -b nucleo_f103rb samples/basic/button

After startup, the program looks up a predefined GPIO device, and configures the pin in input mode, enabling interrupt generation on falling edge. During each iteration of the main loop, the state of GPIO line is monitored and printed to the serial console. When the input button gets pressed, the interrupt handler will print an information about this event along with its timestamp.