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.. _1.1.4_c:

1.1.4 7-segment Display
=============================

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

Let's try to drive a 7-segment display to show a figure from 0 to 9 and
A to F.

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

In this project, we need the following components. 

.. image:: ../img/list_7_segment.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_7_segment`
        - |link_7segment_buy|
    *   - :ref:`cpn_74hc595`
        - |link_74hc595_buy|

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

Connect pin ST_CP of 74HC595 to Raspberry Pi GPIO18, SH_CP to GPIO27, DS
to GPIO17, parallel output ports to 8 segments of the LED segment
display. Input data in DS pin to shift register when SH_CP (the clock
input of the shift register) is at the rising edge, and to the memory
register when ST_CP (the clock input of the memory) is at the rising
edge. Then you can control the states of SH_CP and ST_CP via the
Raspberry Pi GPIOs to transform serial data input into parallel data
output so as to save Raspberry Pi GPIOs and drive the display.

============ ======== ======== ===
T-Board Name physical wiringPi BCM
GPIO17       Pin 11   0        17
GPIO18       Pin 12   1        18
GPIO27       Pin 13   2        27
============ ======== ======== ===

.. image:: ../img/schematic_7_segment.png


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

**Step 1:** Build the circuit.

.. image:: ../img/image73.png

**Step 2:** Get into the folder of the code.

.. raw:: html

   <run></run>

.. code-block::

    cd ~/raphael-kit/c/1.1.4/

**Step 3:** Compile.

.. raw:: html

   <run></run>

.. code-block::

    gcc 1.1.4_7-Segment.c -lwiringPi

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

.. raw:: html

   <run></run>

.. code-block::

    sudo ./a.out

After the code runs, you'll see the 7-segment display display 0-9, A-F.

.. 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 <stdio.h>
    #define   SDI   0   //serial data input
    #define   RCLK  1   //memory clock input(STCP)
    #define   SRCLK 2   //shift register clock input(SHCP)
    unsigned char SegCode[16] = {0x3f,0x06,0x5b,0x4f,0x66,0x6d,0x7d,0x07,0x7f,0x6f,0x77,0x7c,0x39,0x5e,0x79,0x71};

    void init(void){
        pinMode(SDI, OUTPUT); 
        pinMode(RCLK, OUTPUT);
        pinMode(SRCLK, OUTPUT); 
        digitalWrite(SDI, 0);
        digitalWrite(RCLK, 0);
        digitalWrite(SRCLK, 0);
    }

    void hc595_shift(unsigned char dat){
        int i;
        for(i=0;i<8;i++){
            digitalWrite(SDI, 0x80 & (dat << i));
            digitalWrite(SRCLK, 1);
            delay(1);
            digitalWrite(SRCLK, 0);
        }
            digitalWrite(RCLK, 1);
            delay(1);
            digitalWrite(RCLK, 0);
    }

    int main(void){
        int i;
        if(wiringPiSetup() == -1){ //when initialize wiring failed, print messageto screen
            printf("setup wiringPi failed !");
            return 1;
        }
        init();
        while(1){
            for(i=0;i<16;i++){
                printf("Print %1X on Segment\n", i); // %X means hex output
                hc595_shift(SegCode[i]);
                delay(500);
            }
        }
        return 0;
    }

**Code Explanation**


.. code-block:: c

    unsigned char SegCode[16] = {0x3f,0x06,0x5b,0x4f,0x66,0x6d,0x7d,0x07,0x7f,0x6f,0x77,0x7c,0x39,0x5e,0x79,0x71};

A segment code array from 0 to F in Hexadecimal (Common cathode).

.. code-block:: c

    void init(void){
        pinMode(SDI, OUTPUT); 
        pinMode(RCLK, OUTPUT); 
        pinMode(SRCLK, OUTPUT); 
        digitalWrite(SDI, 0);
        digitalWrite(RCLK, 0);
        digitalWrite(SRCLK, 0);
    }

Set ds, st_cp, sh_cp three pins to OUTPUT, and the initial state as 0.

.. code-block:: c

    void hc595_shift(unsigned char dat){}

To assign 8 bit value to 74HC595’s shift register.

.. code-block:: c

    digitalWrite(SDI, 0x80 & (dat << i));

Assign the dat data to SDI(DS) by bits. Here we assume dat=0x3f(0011 1111, when i=2, 0x3f will shift left(<<) 2 bits. 1111 1100 (0x3f << 2) & 1000 0000 (0x80) = 1000 0000, is true.

.. code-block:: c

    digitalWrite(SRCLK, 1);

SRCLK's initial value was set to 0, and here it's set to 1, which is to generate a rising edge pulse, then shift the DS date to shift register.

.. code-block:: c

    digitalWrite(RCLK, 1);

RCLK's initial value was set to 0, and here it's set to 1, which is to generate a rising edge, then shift data from shift register to storage register.

.. code-block:: c

    while(1){
            for(i=0;i<16;i++){
                printf("Print %1X on Segment\n", i); // %X means hex output
                hc595_shift(SegCode[i]);
                delay(500);
            }
        }

In this for loop, we use ``%1X`` to output i as a hexadecimal number. Apply i to find the corresponding segment code in the ``SegCode[]`` array, and employ ``hc595_shift()`` to pass the SegCode into 74HC595's shift register.

.. note::
    The hexadecimal format of number 0~15 are (0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F)

**Phenomenon Picture**

.. image:: ../img/image74.jpeg