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.. _1.1.4_py_pi5:

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:: ../python_pi5/img/1.1.4_7_segment_list.png

.. raw:: html

   <br/>

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 BCM
GPIO17       Pin 11   17
GPIO18       Pin 12   18
GPIO27       Pin 13   27
============ ======== ===

.. image:: ../python_pi5/img/1.1.4_7_segment_schematic.png


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

**Step 1:** Build the circuit.

.. image:: ../python_pi5/img/1.1.4_7-Segment_circuit.png

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

.. raw:: html

   <run></run>

.. code-block::

    cd ~/davinci-kit-for-raspberry-pi/python-pi5

**Step 3:** Run.

.. raw:: html

   <run></run>

.. code-block::

    sudo python3 1.1.4_7-Segment.py

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

.. warning::

    If there is an error prompt  ``RuntimeError: Cannot determine SOC peripheral base address``, please refer to :ref:`faq_soc` 

**Code**

.. note::
    You can **Modify/Reset/Copy/Run/Stop** the code below. But before that, you need to go to  source code path like ``davinci-kit-for-raspberry-pi/python-pi5``. After modifying the code, you can run it directly to see the effect. After confirming that there are no problems, you can use the Copy button to copy the modified code, then open the source code in Terminal via ``nano``  cammand and paste it.

.. raw:: html

    <run></run>

.. code-block:: python

   #!/usr/bin/env python3
   from gpiozero import OutputDevice
   from time import sleep

   # GPIO pins connected to 74HC595 shift register
   SDI = OutputDevice(17)   # Serial Data Input
   RCLK = OutputDevice(18)  # Memory Clock Input (Register Clock)
   SRCLK = OutputDevice(27) # Shift Register Clock

   # Hexadecimal digit codes for a common cathode 7-segment display
   segCode = [
       0x3f, 0x06, 0x5b, 0x4f, 0x66, 0x6d, 0x7d,
       0x07, 0x7f, 0x6f, 0x77, 0x7c, 0x39, 0x5e, 0x79, 0x71
   ]

   def hc595_shift(data):
       # Shift 8 bits of data into the 74HC595
       for bit in range(8):
           # Set SDI high or low based on data bit
           SDI.value = 0x80 & (data << bit)
           # Trigger shift register clock
           SRCLK.on()
           sleep(0.001)
           SRCLK.off()
       # Latch data to output by triggering memory clock
       RCLK.on()
       sleep(0.001)
       RCLK.off()

   def display_all_on():
       # Function to turn all segments on (for common cathode 7-segment display)
       all_on_code = 0x3f
       hc595_shift(all_on_code)
       print("Displaying all segments on")

   try:
       while True:
           # Display each hexadecimal digit on 7-segment display
           for code in segCode:
               hc595_shift(code)  # Shift the code into 74HC595
               # Print the displayed segment code
               print(f"Displaying segCode[{segCode.index(code)}]: 0x{code:02X}")
               sleep(0.5)  # Pause between displaying each digit

   except KeyboardInterrupt:
       # Gracefully handle script interruption (e.g., Ctrl+C)
       pass


**Code Explanation**

#. This snippet imports the necessary classes for the project. ``OutputDevice`` from ``gpiozero`` is used to control hardware components connected to GPIO pins, and ``sleep`` from ``time`` is for adding delays.

   .. code-block:: python

       #!/usr/bin/env python3
       from gpiozero import OutputDevice
       from time import sleep

#. SDI, RCLK, and SRCLK correspond to the Serial Data Input, Memory Clock Input (Register Clock), and Shift Register Clock pins of the 74HC595.

   .. code-block:: python

       # GPIO pins connected to 74HC595 shift register
       SDI = OutputDevice(17)   # Serial Data Input
       RCLK = OutputDevice(18)  # Memory Clock Input (Register Clock)
       SRCLK = OutputDevice(27) # Shift Register Clock


#. ``segCode`` is an array containing hexadecimal codes for each digit to be displayed on the 7-segment display.

   .. code-block:: python

       # Hexadecimal digit codes for a common cathode 7-segment display
       segCode = [
           0x3f, 0x06, 0x5b, 0x4f, 0x66, 0x6d, 0x7d,
           0x07, 0x7f, 0x6f, 0x77, 0x7c, 0x39, 0x5e, 0x79, 0x71
       ]

#. This function shifts 8 bits of data into the 74HC595. It serially inputs each bit into ``SDI``, toggles ``SRCLK`` to shift the bit, and uses ``RCLK`` to latch the data to the output.

   .. code-block:: python

       def hc595_shift(data):
           # Shift 8 bits of data into the 74HC595
           for bit in range(8):
               # Set SDI high or low based on data bit
               SDI.value = 0x80 & (data << bit)
               # Trigger shift register clock
               SRCLK.on()
               sleep(0.001)
               SRCLK.off()
           # Latch data to output by triggering memory clock
           RCLK.on()
           sleep(0.001)
           RCLK.off()

#. This function turns all segments of the display on by sending a specific code to ``hc595_shift``.

   .. code-block:: python

       def display_all_on():
           # Function to turn all segments on (for common cathode 7-segment display)
           all_on_code = 0x3f
           hc595_shift(all_on_code)
           print("Displaying all segments on")

#. In the main loop, each code in ``segCode`` is sent to the display in sequence, with a delay between each.

   .. code-block:: python

       try:
           while True:
               # Display each hexadecimal digit on 7-segment display
               for code in segCode:
                   hc595_shift(code)  # Shift the code into 74HC595
                   # Print the displayed segment code
                   print(f"Displaying segCode[{segCode.index(code)}]: 0x{code:02X}")
                   sleep(0.5)  # Pause between displaying each digit

#. This part of the code gracefully handles script interruption (like Ctrl+C).

   .. code-block:: python

       except KeyboardInterrupt:
           # Gracefully handle script interruption (e.g., Ctrl+C)
           pass