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.. _2.1.6_js_pi5_mcp3008:

2.1.6 Joystick(MCP3008)
==========================

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

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

Required Components
-------------------------------

In this project, we need the following components. 

.. image:: ../img/image317-copy.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 MCP3008 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

    *   - T-Board Name
        - physical
        - WiringPi
        - BCM

    *   - SPICE0
        - pin24
        - 10
        - 8
    *   - SPIMOSI
        - pin19
        - 12
        - 10
    *   - SPIMISO
        - pin21
        - 13
        - 9
    *   - SPISCLK
        - pin23
        - 14
        - 11
    *   - GPIO22
        - pin15
        - 3
        - 22

.. image:: ../img/schematic_2.1.9_joystick_mcp3008_js.png


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

**Step 1:** Build the circuit.

.. image:: ../img/2.1.9_Joystick_js.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-2.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 mcpadc = require('mcp-spi-adc');

    // Open channel 1 (X-axis)
    const xChannel = mcpadc.openMcp3008(1, { speedHz: 1350000 }, (err) => {
    if (err) {
        console.error('Failed to open X channel:', err);
        process.exit(1);
    }
    });

    // Open channel 2 (Y-axis)
    const yChannel = mcpadc.openMcp3008(2, { speedHz: 1350000 }, (err) => {
    if (err) {
        console.error('Failed to open Y channel:', err);
        process.exit(1);
    }
    });

    // Button input on GPIO22 with pull-up
    const btn = new Gpio(22, {
    mode: Gpio.INPUT,
    pullUpDown: Gpio.PUD_UP,
    });

    // Read loop
    setInterval(() => {
    xChannel.read((errX, xReading) => {
        if (errX) {
        console.error('X channel read error:', errX);
        return;
        }

        yChannel.read((errY, yReading) => {
        if (errY) {
            console.error('Y channel read error:', errY);
            return;
        }

        const x_val = Math.round(xReading.value * 1023);
        const y_val = Math.round(yReading.value * 1023);
        const btn_val = btn.digitalRead();

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


**Code Explanation**

.. code-block:: js

    const mcpadc = require('mcp-spi-adc');

This line imports the ``mcp-spi-adc`` module, which allows communication with the MCP3008 ADC using the Raspberry Pi's hardware SPI interface.

.. code-block:: js

    const xChannel = mcpadc.openMcp3008(1, { speedHz: 1350000 }, ...);
    const yChannel = mcpadc.openMcp3008(2, { speedHz: 1350000 }, ...);

These lines open MCP3008 analog input channels 1 and 2 for reading joystick X and Y axis signals, respectively. The SPI communication speed is set to 1.35 MHz.

.. code-block:: js

    const btn = new Gpio(22, {
      mode: Gpio.INPUT,
      pullUpDown: Gpio.PUD_UP,
    });

Initializes GPIO pin 22 as a digital input with an internal pull-up resistor enabled. This pin is used to read the state of a push button.

.. code-block:: js

    setInterval(() => {
      xChannel.read(...);
      yChannel.read(...);
    }, 100);

This function runs every 100 milliseconds. It reads the joystick's X and Y axis values via MCP3008 channels 1 and 2 using SPI. The floating-point values (range 0.0–1.0) are converted to 10-bit integers (0–1023). It also reads the button state using ``digitalRead()`` on GPIO22, returning 0 when pressed and 1 when released. All values are printed to the console.

.. Phenomenon Picture
.. ------------------

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