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.. _2.1.6_c_pi5_mcp3008:

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

.. 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.

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

In this project, we need the following components. 

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

Principle
---------

**Joystick**

The basic idea of a joystick is to translate the movement of a stick
into electronic information that a computer can process.

In order to communicate a full range of motion to the computer, a
joystick needs to measure the stick's position on two axes -- the X-axis
(left to right) and the Y-axis (up and down). Just as in basic geometry,
the X-Y coordinates pinpoint the stick's position exactly.

To determine the location of the stick, the joystick control system
simply monitors the position of each shaft. The conventional analog
joystick design does this with two potentiometers, or variable
resistors.

The joystick also has a digital input that is actuated when the joystick
is pressed down.

.. image:: img/image318.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.png


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

**Step 1:** Build the circuit.

.. image:: ../img/july24_2.1.9_joystick_mcp3008.png

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

.. raw:: html

   <run></run>

.. code-block::

    cd ~/davinci-kit-for-raspberry-pi/c/2.1.6-2/

**Step 3:** Compile the code.

.. raw:: html

   <run></run>

.. code-block::

    gcc 2.1.6_Joystick.c -o joystick -lwiringPi

**Step 4:** Run the executable file.

.. raw:: html

   <run></run>

.. code-block::

    ./joystick

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


.. note::

    If it does not work after running, or there is an error prompt: \"wiringPi.h: No such file or directory\", please refer to :ref:`install_wiringpi_pi5`.

**Code**

.. code-block:: c

    #include <wiringPi.h>
    #include <wiringPiSPI.h>
    #include <stdio.h>

    #define SPI_CHANNEL 0
    #define SPI_SPEED   1000000  // 1 MHz
    #define BtnPin      3        // WiringPi 3 = BCM GPIO22

    // Read from MCP3008 channel (0–7)
    int read_ADC(int channel) {
        if (channel < 0 || channel > 7) return -1;

        unsigned char buffer[3];
        buffer[0] = 1;                            // Start bit
        buffer[1] = (8 + channel) << 4;           // Channel config
        buffer[2] = 0;

        wiringPiSPIDataRW(SPI_CHANNEL, buffer, 3);

        int result = ((buffer[1] & 0x03) << 8) | buffer[2];
        return result;
    }

    int main(void) {
        if (wiringPiSetup() == -1) {
            printf("WiringPi setup failed!\n");
            return 1;
        }

        if (wiringPiSPISetup(SPI_CHANNEL, SPI_SPEED) == -1) {
            printf("SPI setup failed!\n");
            return 1;
        }

        pinMode(BtnPin, INPUT);
        pullUpDnControl(BtnPin, PUD_UP);

        while (1) {
            int x_val = read_ADC(0);     // VRX on CH0
            int y_val = read_ADC(1);     // VRY on CH1
            int btn_val = digitalRead(BtnPin);  // SW button

            printf("x = %d, y = %d, btn = %d\n", x_val, y_val, btn_val);
            delay(100);
        }

        return 0;
    }

**Code Explanation**

#. This section initializes the libraries needed for GPIO and SPI communication.

   .. code-block:: c

       #include <wiringPi.h>
       #include <wiringPiSPI.h>
       #include <stdio.h>

       #define SPI_CHANNEL 0
       #define SPI_SPEED   1000000  // 1 MHz
       #define BtnPin      3        // WiringPi 3 = BCM GPIO22

#. Defines a function ``read_ADC()`` to read analog data from MCP3008. It communicates over SPI to request data from a given channel (0–7), then parses the response to get a 10-bit ADC result.

   .. code-block:: c

       int read_ADC(int channel) {
           if (channel < 0 || channel > 7) return -1;

           unsigned char buffer[3];
           buffer[0] = 1;
           buffer[1] = (8 + channel) << 4;
           buffer[2] = 0;

           wiringPiSPIDataRW(SPI_CHANNEL, buffer, 3);

           int result = ((buffer[1] & 0x03) << 8) | buffer[2];
           return result;
       }

#. The main function initializes the WiringPi and SPI interfaces, configures the joystick's button pin, and continuously reads joystick values and prints them to the console.

   .. code-block:: c

       int main(void) {
           if (wiringPiSetup() == -1) {
               printf("WiringPi setup failed!\n");
               return 1;
           }

           if (wiringPiSPISetup(SPI_CHANNEL, SPI_SPEED) == -1) {
               printf("SPI setup failed!\n");
               return 1;
           }

           pinMode(BtnPin, INPUT);
           pullUpDnControl(BtnPin, PUD_UP);

           while (1) {
               int x_val = read_ADC(0);     // VRX to CH0
               int y_val = read_ADC(1);     // VRY to CH1
               int btn_val = digitalRead(BtnPin);  // SW to GPIO22

               printf("x = %d, y = %d, btn = %d\n", x_val, y_val, btn_val);
               delay(100);
           }

           return 0;
       }

#. In this program:

   - The joystick's VRX is connected to CH0 of MCP3008.
   - The VRY is connected to CH1.
   - The button (SW) is connected to GPIO22 (WiringPi pin 3).
   - The ``read_ADC()`` function is used to read analog values from VRX and VRY.
   - The digital value of the joystick button is read using ``digitalRead()``.
   - All values are printed continuously every 100 milliseconds.

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

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