CP316: Microprocessor Systems and Interfacing

Testing Hardware Timing

Objectives

The speed with which a microprocessor or microcontroller communicates with hardware is dependent on many factors. One obvious factor is the clock speed, however this is often not the most important factor.

Equipment

• Raspberry Pi
• Arduino Uno
• AVR Data Sheet [pdf,660pp; ©2015]

Procedure

Raspberry Pi Manual Pin Toggling

1. Connect the Raspberry Pi with the serial monitor.
2. Download gpio_test_basic.py.
3. Connect the oscilloscope to the output pin.
4. Run the program to see that the square wave appears on the scope.
5. Comment out the delay instructions in the program and rerun it so you can see the maximum frequency that can be produced.
   Sketch the output, and note the oscilloscope settings.
   
   
   Is the output sharp or fuzzy? What does this tell you about the stability of the timing?
   
   
6. Press the Run/Stop button on the oscilloscope to freeze the image, and note how consistent the pulses are. Repeat this 5 or 6 times to find the maximum variation between pulses that you observe.
   Sketch the output, and note the oscilloscope settings.
   
   
   Demonstration - demonstrate the screen showing the variation.
   Because the operating system is scheduling several tasks, your program will be suspended periodically, which will result in occasional long pulses.

Raspberry Pi PWM Operation

1. Download pwm_test_continuous.py .
2. Change the frequency to the maximum frequency observed previously, run the program and observe the output.
3. As before, press the Run/Stop button on the oscilloscope to freeze the image, and note how consistent the pulses are. Repeat this 5 or 6 times to find the maximum variation between pulses that you observe.
   Sketch the output, and note the oscilloscope settings.
   Demonstration - demonstrate the screen showing the variation.
   Does the PWM capability avoid the problem of manual toggling of pins? What does that tell you about whether PWM is a feature of the hardware or the software?
4. Increase the frequency, and see what maximum frequency you can achieve using the PWM feature.
5. When you're done, shutdown and put away the Pi.

Arduino blink program

1. Connect the Arduino board.
2. From the Examples, load the Blink sketch.
3. Connect the oscilloscope to output pin 13.
4. Run the program to see that the square wave appears on the scope.
5. Comment out the delay instructions in the program and rerun it so you can see the maximum frequency that can be produced.
   Sketch the output, and note the oscilloscope settings.
   
   
   
6. Press the Run/Stop button on the oscilloscope to freeze the image, and note how consistent the pulses are. Repeat this 5 or 6 times to find the maximum variation between pulses that you observe.
   Sketch the output, and note the oscilloscope settings.
   Demonstration - demonstrate the screen showing the variation.
   Because there is no operating system, your program is the only task running.

Arduino blinkfast program - using low level features

1. Create a new sketch, and download the Blinkfast sketch.
2. Run the program to see that the square wave appears on the scope.
3. Comment out the delay instructions in the program and rerun it so you can see the maximum frequency that can be produced.
   Sketch the output, and note the oscilloscope settings.
   
   Is the duty cycle 50%? If not, what does that mean about the assembly language code?
   
   
4. Press the Run/Stop button on the oscilloscope to freeze the image, and note how consistent the pulses are. Repeat this 5 or 6 times to find the maximum variation between pulses that you observe.
   Sketch the output, and note the oscilloscope settings.
   Demonstration - demonstrate the screen showing the variation.
   Because there is no operating system, your program is the only task running.

Inline assembly language

1. Insert NOP instructions into the blinkfast code to make the output have a 50% duty cycle.
   Demonstration - demonstrate the revised code running.

Arduino diasassembly

1. Open the File menu and choose Preferences.
   
   
   
2. Choose the verbose output option.
   
   
   
3. File the location of the elf file when you compile the program.
   
   
   
4. Change to the directory containing the elf file. (Note how your username may not show up as expected when it shows the path.)
   
   
   
5. Run the command to dissasemble the elf file.
   
   
   
6. Note the dissasembled (txt) file is much bigger than the elf file.
   
   
   
7. Open the dissasembled (txt) file in any text editor.
   
   
   
8. It should look like this:
   Blink.ino.txt
   Search for void loop() to see where your code starts.