PC/CP320 Physical Computing

Raspberry Pi Python GPIO and Serial Console Introduction

Objectives

1. To introduce Python programming on the Raspberry Pi
2. To become familiar with Raspberry Pi GPIO pins and BOARD and BCM numbering schemes
3. To learn nano editor commands
4. To introduce using the serial console on the Raspberry Pi

Preparation

Equipment

• Raspberry Pi
• TTL-serial to USB adapter
• LED and 1k resistor. (Depending on brightness, you may need a smaller resistor.)

Procedure

The Rapsberry Pi GPIO pins can easily be damaged with improper handling.

1. Testing the Serial adapter driver installation

   Connect the TTL-serial cable via the USB cable to the computer,
   Open the Device Manager and check to see what COM port the adapter appeared on.
   
   
2. Open putty, and set it for serial on the COM port just noted and set the baud rate to 115200.
   
   
   
3. Any time you connect anything to the GPIO pins, make sure you know the correct orientation!!!
   
   Connect the TTL-serial cable to the Raspberry Pi as follows:
   
   • Don't connect the red wire.
   • Connect the black wire to GROUND.
   • Connect the white wire to TxD. Use a white wire for the connection!
   • Connect the green wire to RxD. Use a green wire for the connection!
   
   
4. Plug in the Pi, and you should see the boot process in the serial console and get a login prompt as expected.
   
   Show this to the lab instructor.
   
   
   
5. Here are some common commands for linux. Note that the cursor up and down keys take you through a history of previously used commands.

   Common linux commands
   Command	Description	Example
   ls	list files in directory	ls *.py
   cp	copy a file	cp test.py newtest.py
   mv	move (or rename) a file	mv oldtest.py newtest.py
   sudo	execute program as superuser	sudo python3 test.py
   mount	mount external (flash) drive	mount /mnt/usb
   umount	unmount external (flash) drive	umount /mnt/usb
   shutdown	shutdown computer properly	sudo shutdown -h now

   
   
   Here are some common commands for nano.

   Common nano commands
   Command	Description
   CTRL-K	cut line(s)
   CTRL-U	uncut line(s) (i.e. paste)
   CTRL-X	save and quit
   CTRL-O	save without quitting

6. Running the first python test program

   Since there may be code for some other purpose already on the Pi, delete it by typing:
   rm gpio_test_basic.py
   
   
   • Download gpio_test_basic.py according to the serial console wifi instructions.
     Note: An alternative way to copy a program is to
     1. Open the file on the PC
     2. Start the nano editor on the Raspberry Pi
     3. Copy the program from the window on the PC and paste it into the terminal window. (On the PC, copy with CTRL-C and paste in the terminal window by clicking the right mouse button.)
     4. Exit nano giving the file whatever name you want. (A good choice would be gpio_test_basic.py. )
     
     
   • Connect the GPIO pin to the LED in series with the resistor to ground to observe the output.
     
     
   • To run a program, type
     sudo python3 gpio_test_basic.py
     
     (Note: typing "python" instead of "python3" will use python 2 instead.)
     
     Note: If the program doesn't access GPIO pins you don't need sudo.
   • To exit a program type CTRL-C.
   • Now try to re-run the program. What happens?
     GPIO pins are not automatically released when a program stops. In order for this to happen you must release them before the program exits.
   • Download gpio_test_2.py
   • Run the program, as before, and then exit out. Rerun it and see what happens.
     This program makes sure that GPIO pins are released at the end. This is a template you should always follow.
7. Make a copy of the program so that you can edit the copy.
   To do this, type
   cp gpio_test_2.py test.py
   Now you can edit test.py using nano. (Save and exit nano with CTRL-X.)
   
   
8. Modify the program to use the same pin GPIO18 but using the Broadcom numbering scheme. Show the modified working program to the lab instructor.
   
   

9. Using Pulse Width Modulation on GPIO18

   
   Broadcom pin GPIO18 has the ability to produce asquare wave output using Pulse Width Modulation or PWM. Once started, a square wave will be produced without the program having to toggle the pin explicitly. To set it up, after the pin is set to output
   p = GPIO.PWM(12, 60)
   will set up the pin for a 60Hz waveform.
   p.start(50)
   will turn it ON with a 50% duty cycle and
   p.stop()
   will turn it OFF.
   
   p.ChangeDutyCycle(20)
   will change the duty cycle to 20% without stopping it and
   p.ChangeFrequency(100)
   will change the frequency to 100Hz without stopping.
   
   Download pwm_test_2018.py
   
   
10. Now connect the LED and resistor to the pin, and run the program to see how changing the duty cycle changes the brightness.
    
    
11. Change the frequency to see how low it can go before you see the LED flickering.
    
    
12. Change the duty cycles to see how small a difference in brightness you can detect.
    
    
13. Remove (or comment out) the command to stop the pwm at the end of the program. Run the program and see what happens when the program stops. (You may need to remove the cleanup line as well.)
    
    
14. As before, modify the program to use the same pin GPIO18 but using the Broadcom numbering scheme.

    
    Demonstrate the output using the LED and the effect of changing the duty cycle to the lab instructor.
    

15. Backing up test programs to flash drive

    If you have a flash drive, in sert in into one of the USB ports and type
    mount /mnt/usb
    You should be able to use the cp command to copy files to the flash drive.
    You should be able to use the ls command to see that the files were copied to the flash drive.
    Before you remove the drive, be sure to unmount it using umount /mnt/usb
    Show this to the lab instructor.
    
    

16. Proper shutdown

    Shut down the pi using
    sudo shutdown -h now
    When shutdown is complete, you can disconnect power and put everything away.