GPIO Demo
by Florian Wesch <fw@info-beamer.com>

GPIO demo package

info-beamer OS (from version 10) has support for GPIO access. The package sdk makes it easy to read GPIO values from your Python package service. This package demonstrates this in a minimal way: Pressing a physical button toggles the screen color from red to green. Here's the wiring you'll need and how it works:

The button connects the 3.3V output PIN to GPIO18 of your Pi via a 1KΩ resistor.

How this package works

All info-beamer packages contain multiple files that together control how to configure a package, how it runs background tasks on a device and what is shown on the screen. Here's all files needed for the GPIO demo.

node.json file

This package only has a single configuration option named pin. The follow snippet shows the complete node.json file (reference documentation).

{
    "name": "GPIO Demo",
    "permissions": {
        "gpio": "Access GPIO"
    },
    "options": [{
        "title": "PIN",
        "ui_width": 4,
        "name": "pin",
        "type": "integer",
        "default": 18
    }]
}

It specifies that this node wants access to gpio and defines a configuration option named pin as an integer with a default value of 18.

service file

The package service, bundled as the file service in this package, looks like this:

#!/usr/bin/python
from hosted import device, node, config
config.restart_on_update()

device.gpio.monitor(config.pin)
for pin, state in device.gpio.poll_forever():
    node.send('/state:%d' % state)

This file is executed on any device you install this package on. It runs forever and is automatically restarted by the info-beamer OS if it terminates for any reason. The code imports the info-beamer package sdk from hosted.py. The config class will automatically be populated with the configuration values set by the user. Right now only pin can be configured and the value set by the user is available in config.pin.

The config.restart_on_update() line tells the package sdk to automatically restart the service every time the configuration changes. So if the user changes the PIN value from 18 to (for example) 17, the Python process will exit and is automatically restarted by the info-beamer OS.

Next we set up a GPIO monitor for the configured PIN using the device.gpio.monitor call. The for loop then forever loops and returns a new observation in every loop.

Inside the loop we get a state value each time. True if GPIO is high, False if it is low. We immediately send this value to the running info-beamer code in node.lua (see below). Since we convert the boolean state value to a numeric value with %d it is converted to either 1 or 0. The node.send call internally sends a UDP packet to the info-beamer process with the following content assuming this package is the top-level node (/root) in your setup:

/root/state:0

or

/root/state:1

node.lua

The node.lua file is responsible for controlling the content of the screen. Here's the complete file:

gl.setup(NATIVE_WIDTH, NATIVE_HEIGHT)

local on = false

util.data_mapper{
    state = function(state)
        on = state == '1'
    end,
}

function node.render()
    if on then
        gl.clear(0, 1, 0, 1) -- green
    else
        gl.clear(1, 0, 0, 1) -- red
    end
end

The variable on decides if we clear the screen in green or red: You can see how this decision is made in the node.render function at the bottom. The node.render function is usually called 60 times per second and is responsible to creating the screen outout.

The util.data_mapper receives incoming UDP packets (remember we send them in the package service above) and decides how to react the them. The value state matches to the node.send call from above. Its callback function receives the value sent: In our case we sent either 0 or 1 (see above). So we test for that and update the on variable accordingly.

That's all. You can install this package on your info-beamer devices and can see how the screen turns green while the button is pressed.