Hardware momentary push-buttons the Linux way
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README.md

gpio-button

A JavaScript package for interfacing with hardware momentary push-buttons connected to GPIO pins on Linux systems. Many Linux systems like the Raspberry Pi have a driver called gpio-keys which can be used to emit events when momentary push-buttons are pressed and released. This package conforms to the conventions of the gpio-keys driver and enables very efficient interfacing with momentary push-buttons. The technique may appear a little complex at first, but when used, it results in an efficient pure JavaScript solution for momentary push-buttons.

Installation

npm install gpio-button

Usage

Assume that there's a momentary push button connected to GPIO4 on a Raspberry Pi:

Let's start with the complex bit, device tree overlays, after that everything is easy. A device tree overlay can be used to tell the Linux kernel about hardware that is connected to the system, in this case, a momentary push-button. Once the Linux kernel knows about the device tree overlay, it arranges everything so that the momentary push-button can be used with ease.

The device tree overlay for a hardware device is described in source code and compiled into a binary format understood by the Linux kernel using the device tree compiler.

The source code for the momentary push-button overlay connected to GPIO4 in the circuit diagram above is:

/dts-v1/;
/plugin/;

/ {
    compatible = "brcm,bcm2835", "brcm,bcm2708", "brcm,bcm2709";

    fragment@0 {
        target = <&gpio>;
        __overlay__ {
            button4_pin: button4_pin {
                brcm,pins = <4>;     /* gpio4 */
                brcm,function = <0>; /* input */
                brcm,pull = <1>;     /* pull-down */
            };
        };
    };

    fragment@1 {
    target-path = "/soc";
        __overlay__ {
            button4: button4 {
                compatible = "gpio-keys";
                #address-cells = <1>;
                #size-cells = <0>;
                pinctrl-names = "default";
                pinctrl-0 = <&button4_pin>;
                status = "okay";

                button@4 {
                    label = "button gpio4";
                    linux,code = <4>;
                    gpios = <&gpio 4 0>;
                };
            };
        };
    };
};

Beautiful, isn't it :). The source code can also be found in button4-overlay.dts in the example directory.

On Raspbian, if the device tree compiler is not already installed it can be isntalled with the following command:

sudo apt-get install device-tree-compiler

And the overlay is compiled with the following command:

dtc -@ -I dts -O dtb -o button4-overlay.dtb button4-overlay.dts

The device tree compiler emits three warnings that can be safely ignored. For further information related to the warnings see here.

The device tree blob button4-overlay.dtb produced by the compiler is the binary format understood by the Linux kernel and should be copied to directory /boot/overlays:

sudo cp button4-overlay.dtb /boot/overlays

The last piece of the puzzle is adding the following line at the end of /boot/config.txt so that the overlay gets loaded at boot time:

device_tree_overlay=overlays/button4-overlay.dtb

After the Pi has been rebooted, the following JavaScript program can be used to print information when the momentary push-button is pressed, held, or released:

var Button = require('gpio-button'),
  button4 = new Button('button4');

button4.on('press', function () {
  console.log('press');
});

button4.on('hold', function () {
  console.log('hold');
});

button4.on('release', function () {
  console.log('release');
});

API

Button(name)

Returns a new Button object which inherits from EventEmitter. A 'ready' event will be emitted when the hardware button itself is ready for user interaction. The specified name is a string and and corresponds to the name of the node for the button in the device tree overlay.

pressed()

Returns true if the button is pressed, else false.

held()

Returns true if the button is held, else false.

released()

Returns true if the button is released, else false.

Event: press

Emitted when the button is pressed.

Event: hold

Emitted continuously when the button is held.

Event: release

Emitted when the button is released.