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2.1.6 Joystick
==============


.. note::

   .. image:: ../img/mcp3008_and_adc0834.jpg
      :width: 25%
      :align: left
    

   Depending on your kit version, please identify whether you have **ADC0834** or **MCP3008** and proceed with the matching section.



Introduction
------------

In this project, We're going to learn how joystick works. We manipulate
the Joystick and display the results on the screen.

Components
----------

.. image:: ../img/image317.png



Schematic Diagram
-----------------

When the data of joystick is read, there are some differents between
axis: data of X and Y axis is analog, which need to use ADC0834 to
convert the analog value to digital value. Data of Z axis is digital, so
you can directly use the GPIO to read, or you can also use ADC to read.

.. image:: ../img/image319.png


.. image:: ../img/image320.png


Experimental Procedures
-----------------------

**Step 1:** Build the circuit.

.. image:: ../img/image193.png

**Step 2:** Go to the folder of the code.

.. raw:: html

   <run></run>

.. code-block::

    cd ~/davinci-kit-for-raspberry-pi/nodejs/

**Step 3:** Run the code.

.. raw:: html

   <run></run>

.. code-block::

    sudo node joystick.js

After the code runs, turn the Joystick, then the corresponding values of
x, y, Btn are displayed on screen.

**Code**

.. code-block:: js

    const Gpio = require('pigpio').Gpio;
    const ADC0834 = require('./adc0834.js').ADC0834;

    const adc = new ADC0834(17, 18, 22);
    const btn = new Gpio(25, {
        mode: Gpio.INPUT,
        pullUpDown: Gpio.PUD_UP,
    });

    setInterval(async() => {

        x_val = await adc.read(0);
        y_val = await adc.read(1);

        btn_val = btn.digitalRead();
        console.log(`x = ${x_val}, y = ${y_val}, btn = ${btn_val}\n`);
    }, 100);



**Code Explanation**

.. code-block:: js

    const ADC0834 = require('./adc0834.js').ADC0834;

We import an ``ADC0834`` constructor to use the adc0834 module.

.. code-block:: js

    setInterval(async() => {

        x_val = await adc.read(0);
        y_val = await adc.read(1);

        btn_val = btn.digitalRead();
        console.log(`x = ${x_val}, y = ${y_val}, btn = ${btn_val}\n`);
    }, 100);


When reading the values of multiple channels of ADC0834 at the same time, asynchronous programming is required.
We build a promise function here,
And use the await instruction of async function to elegantly write this complex asynchronous task.

* `Promise <https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise>`_
* `Async Function <https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/async_function>`_



Phenomenon Picture
------------------

.. image:: ../img/image194.jpeg