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.. _2.1.7_c_mcp3008:

2.1.7 Potentiometer(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
------------

The ADC function is used to convert analog signals into digital values. 
In this experiment, we use the MCP3008 ADC chip to perform this conversion. 
A potentiometer is used to generate a variable voltage, which changes the physical quantity. 
The MCP3008 then converts this analog voltage into a digital value that can be read and processed by the Raspberry Pi.

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

In this project, we need the following components. 

.. image:: ../img/list2_2.1.4_potentiometer.png

It's definitely convenient to buy a whole kit, here's the link: 

.. list-table::
    :widths: 20 20 20
    :header-rows: 1

    *   - Name	
        - ITEMS IN THIS KIT
        - LINK
    *   - Raphael Kit
        - 337
        - |link_Raphael_kit|

You can also buy them separately from the links below.

.. list-table::
    :widths: 30 20
    :header-rows: 1

    *   - COMPONENT INTRODUCTION
        - PURCHASE LINK

    *   - :ref:`cpn_gpio_board`
        - |link_gpio_board_buy|
    *   - :ref:`cpn_breadboard`
        - |link_breadboard_buy|
    *   - :ref:`cpn_wires`
        - |link_wires_buy|
    *   - :ref:`cpn_resistor`
        - |link_resistor_buy|
    *   - :ref:`cpn_led`
        - |link_led_buy|
    *   - :ref:`cpn_potentiometer`
        - |link_potentiometer_buy|
    *   - :ref:`cpn_mcp3008`
        - \-

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

.. list-table::
    :widths: 30 30 30 30
    :header-rows: 1

    *   - 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_2.1.7_potentiometer_mcp3008.png


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

**Step 1:** Build the circuit.

.. image:: ../img/july24_2.1.7_potentiometer_mcp3008.png


.. note::
    Please place the chip by referring to the corresponding position
    depicted in the picture. Note that the grooves on the chip should be on
    the left when it is placed.

**Step 2:** Open the code file.

.. raw:: html

   <run></run>

.. code-block::

    cd ~/raphael-kit/c/2.1.7-2/

**Step 3:** Compile the code.

.. raw:: html

   <run></run>

.. code-block::

    gcc 2.1.7_Potentiometer.c -lwiringPi

**Step 4:** Run.

.. raw:: html

   <run></run>

.. code-block::

    sudo ./a.out

After the code runs, rotate the knob on the potentiometer, the intensity
of LED will change accordingly.

.. 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`.

**Code**

.. code-block:: c

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

        #define SPI_CHANNEL 0  // CE0
        #define SPI_SPEED   1000000  // 1MHz
        #define LedPin      3

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

            unsigned char buffer[3];
            buffer[0] = 1;                             // Start bit
            buffer[1] = (8 + channel) << 4;            // Single-ended mode, channel
            buffer[2] = 0;

            wiringPiSPIDataRW(SPI_CHANNEL, buffer, 3);

            int value = ((buffer[1] & 3) << 8) | buffer[2];
            return value;
        }

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

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

            softPwmCreate(LedPin, 0, 100);

            while (1) {
                int analogVal = readADC(0);  // CH0
                printf("ADC Value: %d\n", analogVal);

                int pwmVal = analogVal * 100 / 1023;  // Normalize to 0–100
                softPwmWrite(LedPin, pwmVal);

                delay(100);
            }

            return 0;
        }


**Code Explanation**

.. code-block:: c

    #define SPI_CHANNEL 0  // CE0
    #define SPI_SPEED   1000000  // 1MHz
    #define LedPin      3

Define the SPI channel as CE0 (chip enable 0), set the SPI speed to 1MHz, and assign GPIO3 to the LED pin.

.. code-block:: c


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

        unsigned char buffer[3];
        buffer[0] = 1;                             // Start bit
        buffer[1] = (8 + channel) << 4;            // Single-ended mode, channel
        buffer[2] = 0;

        wiringPiSPIDataRW(SPI_CHANNEL, buffer, 3);

        int value = ((buffer[1] & 3) << 8) | buffer[2];
        return value;
    }


This function is used to read analog data from MCP3008.

* First, it checks if the channel number is within the valid range (0 to 7).

* It initializes a 3-byte array, where:

  * `buffer[0] = 1`: Start bit for MCP3008 communication.
  * `buffer[1] = (8 + channel) << 4`: Constructs the configuration byte for single-ended mode and selects the desired channel.
  * `buffer[2] = 0`: Placeholder byte to receive the result.

* `wiringPiSPIDataRW` sends and receives data through the SPI channel.

* The return value is extracted from the last two bytes using bitwise operations to obtain a 10-bit ADC result.

.. code-block:: c


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

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

        softPwmCreate(LedPin, 0, 100);

        while (1) {
            int analogVal = readADC(0);  // CH0
            printf("ADC Value: %d\n", analogVal);

            int pwmVal = analogVal * 100 / 1023;  // Normalize to 0–100
            softPwmWrite(LedPin, pwmVal);

            delay(100);
        }

        return 0;
    }


In the main function:

* `wiringPiSetup()` initializes the WiringPi library.
* `wiringPiSPISetup()` initializes SPI communication on channel 0 at 1MHz.
* `softPwmCreate()` sets up software PWM on GPIO3 with an initial duty cycle of 0 and a range of 0–100.

The program enters an infinite loop where:

* It reads the ADC value from channel 0 (connected to a potentiometer).
* Prints the ADC value to the terminal.
* Converts the 10-bit ADC value to a PWM duty cycle between 0 and 100.
* Writes the PWM value to the LED, so the brightness reflects the potentiometer's position.

`delay(100)` pauses for 100 milliseconds before the next read/write cycle.


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

.. .. image:: ../img/image181.jpeg