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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.

Components
----------------

.. image:: img/list_7_segment.png


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.

.. image:: img/schematic_7_segment.png

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

**Step 1:** Build the circuit.

.. image:: img/image73.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 7-segment_display.js

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

**Code**

.. code-block:: js

    const Gpio = require('pigpio').Gpio;

    const segCode = [0x3f, 0x06, 0x5b, 0x4f, 0x66, 0x6d, 0x7d, 0x07, 0x7f, 0x6f, 0x77, 0x7c, 0x39, 0x5e, 0x79, 0x71];

    const SDI = new Gpio(17, { mode: Gpio.OUTPUT });
    const RCLK = new Gpio(18, { mode: Gpio.OUTPUT });
    const SRCLK = new Gpio(27, { mode: Gpio.OUTPUT });

    function hc595_shift(dat) {
        for (let j = 0; j < 8; j++) {
            let code = 0x80 & (dat << j);
            if (code != 0) {
                code = 1;
            }
            SDI.digitalWrite(code);
            SRCLK.trigger(1,1);
        }
        RCLK.trigger(1,1);
    }

    let index = -1;
    setInterval(() => {
        index = (index+1)%16;
        hc595_shift(segCode[index]);
    }, 1000);


**Code Explanation**

.. code-block:: js

    const segCode = [0x3f,0x06,0x5b,0x4f,0x66,0x6d,0x7d,0x07,0x7f,0x6f,0x77,0x7c,0x39,0x5e,0x79,0x71];

Define a hexadecimal (common cathode) segment code array from 0 to F.   

.. code-block:: js

    const SDI = new Gpio(17, { mode: Gpio.OUTPUT });
    const RCLK = new Gpio(18, { mode: Gpio.OUTPUT });
    const SRCLK = new Gpio(27, { mode: Gpio.OUTPUT });

Initialize pins 17, 18, and 27 as output mode, and copy them to ``SDI``, ``RCLK``, and ``SRCLK`` respectively.

.. code-block:: js

    function hc595_shift(dat) {
        for (let j = 0; j < 8; j++) {
            let code = 0x80 & (dat << j);
            if (code != 0) {
                code = 1;
            }
            SDI.digitalWrite(code);
            SRCLK.trigger(1,1);
        }
        RCLK.trigger(1,1);
    }

Implement a ``hc595_shift`` function to convert the fields in the array ``segCode`` into numbers 
and display them on the digital tube.

.. code-block:: js

    let code = 0x80 & (dat << j);
    if (code != 0) {
        code = 1;
    }
    SDI.digitalWrite(code); 

Assign the dat data to SDI(DS) by bits. 
Here we assume dat=0x3f(0011 1111, when j=2, 0x3f will shift right(<<) 2 bits. 
1111 1100 (0x3f << 2) & 1000 0000 (0x80) = 1000 0000, is true.
At this time, 1 is written to SDI.

.. code-block:: js

    SRCLK.trigger(1,1);

Generate a rising edge pulse and move the DS data to the shift register.


``trigger(pulseLen, level)``
    * pulseLen - pulse length in microseconds (1 - 100)
    * level - 0 or 1

Sends a trigger pulse to the GPIO. 
The GPIO is set to level for pulseLen microseconds and then reset to not level.

.. code-block:: js

    RCLK.trigger(1,1);

Generate a rising edge pulse and move the data from the shift register to the storage register.


.. code-block:: js

    let index = -1;
    setInterval(() => {
        index = (index+1)%16;
        hc595_shift(segCode[index]);
    }, 1000);

Finally, use the function ``hc595_shift()`` to convert the fields in ``segCode`` 
and display them through the digital tube.   

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

.. image:: img/image74.jpeg