EENG 383

EENG 383

Ultrasonic Range Finder

External Hardware

To answer the following questions you will need to consult the HCSR04 - Ultra-Sonic Ranging Module HC-SR04 Technical Specification. The speed of sound has some relationship to the elevation at which it is measured. Within a reasonable degree of accuracy, the speed of sound in Golden Colrado is 334 m/s. Use this value in your calculations.
  1. Given a range of x centimeters from the ultrasonic sensor to an object, determine the duration of the echo pulse generated by the ultrasonic sensor in milliseconds. Remember that the acoustic pulse emitted by the ultrasonic sensor must travel out to the object and then reflect back to the sensor. The echo pulse is held high during the flight time (out and back) of this acoustic pulse. Use dimensional analysis and show your work for full credit.
  2. The working range of the ultrasonic sensor is 10cm to 500cm. Determine the duration of the echo pulse (in milliseconds) for an object placed at these two distances away from the ultrasonic sensor. Use dimensional anslysis and show your work for full credit.
  3. Assuming that we are measuring the duration of the echo pulse using a 16-bit timer, what is the smallest prescaler that can be used to measure the duration of the echo pulse.
  4. Given an echo pulse of x milliseconds generated by the ultrasonic sensor, determine the range (in centimeters) from the object to the ultrasonic sensor. Use dimensional analysis and show your work for full credit.
  5. Find the HC-SR04 - Ultra-Sonic Ranger technical documents found on the Digikey web site. Hint, search for "HC-SR04" on the Digikey web site. What is the minimum duration of the pulse on the trigger input?

Internal Subsystem

Discuss capture subsystem, rising and falling edge. Interrupt subsystem.
  1. Given an echo pulse of duration x timer counts with the timer configured with a 1:8 prescaler, find the distance in centimeters to the object. Use dimensional analysis for full credit. Note, the constant relating timer counts to centimeters will be called the conversion factor.
    2. Let's say that you measured the duration of the echo pulse using a 1:8 prescaled timer and stored the number of timer counts in a variable called timerCounts. I want you to convert the timer counts into a distance (in centimeters) by multiplyng by the conversion factor determined in the previous problem. We will use 0.008 as the approximation in the following discussion, but you should use the true value determined in the previous problem. To summarize I would the equivlent of the following line of code in your program:
    distanceInCm = 0.008 * timerCounts;
    However since we are not allow to use floating point math on the PIC, we will need to find an alternative.

    The idea will be to multiply the conversion factor by 216, mutiple the result with timerCounts and then divide by 216. In other words:
    distanceInCm = ((216*0.008) * timerCounts)/216;
    Since we are multiplying and then dividing by the same factor (216), the factors cancel leaving us with the same answer as if we had just multiplied by 0.008. However, the advantage is that the product 216*0.008 = 524 (rounded to nearest integer) which is easy to multiply by timerCounts using regular integer math. Then the division by 216 can be accomplished by shifting right by 16-bits using the C shift operation.
  2. Write a C-code snippet to convert timerCounts into distance in centimeters.

Firmware Organization

Build a project using the following MCC configuration:
  • Click on the "Generate" button in the Project Resources area of the project manager window. In the MCC Save Configuration File, keep the defaults and Save. Anytime that you make a change to the configuration you must re-generate the supporting files by clicking on the generate button,
  • Click on the Project tab in the project manager window, expand the Source Files folder and double click main.c to open it in the editor window.
  • Replace the contents of main.c with inlab05.c,
  • Click on the Project tab in the project manager window, expand the Source Files folder, expand the MCC Generated Files folder and double click tmr0.c to open it in the editor window.
  • Scroll down to the void TMR0_DefaultInterruptHandler(void) function (on line 158), and delete the entire function (remove lines 158 - 161). We are doing this because we have defined this function, TMR0_DefaultInterruptHandler, in main.
  • Compile and download the code to the PIC,
  • Move the slide switch from PROG to VCOM, this is easy to forget!
  • Wire up the ultrasonic range finder to the PIC using 4 jumper wires according to the splash screen instructions guide. In order to provide power (Vcc) to the range finder you will need to remove the power jumper from JP1 and connect the jumper wire to the pin closest to the USB mini connector. It should look something like the following; this setup will enable you to move the range finder while keeping the Dev'17 board still. Experiment with the range finder and terminal application by pointing the range finder at nearby vertical surfaces and observing the values returned by the program. You may experience interference if your neighbors at the lab bench are also experimenting.


    Let's examine the structure of the firmware before moving on with this lab. In the Project area of the project window, expand the Source File folder then expand the MCC Generated Files folder.
    1. Open the MCC generated file interrupt_manager.c, what function is called when timer 0 is enabled and when timer 0 has rolled over?
    2. At the start of main the function "SYSTEM_Initialize" is called. Look through the MCC generated files to find this function. What file is this function call defined in and list the initialization functions that SYSTEM_Initialize function calls.
    3. Look through the MCC generated files to find the TMR0_SetInterruptHandler function. The argument to TMR0_SetInterruptHandler is a function pointer; that is the name of a function! In the body of TMR0_SetInterruptHandler the function passed into the TMR0_SetInterruptHandler function is aliased to "TMR0_InterruptHandler." So if you called "TMR0_SetInterruptHandler(pugs);" where pugs was a subroutine that you defined (say in your main.c file), then a call to TMR0_InterruptHandler(); would call the pugs function.
      Find the TMR0_Initialize function, what is the name of the function that is passed into the TMR0_SetInterruptHandler function? In what source file is this function defined? What does this function do?
    After setting up the subsystems using MCC, you may find that you need to modify a setting. This may require MCC to modify an existing generate file. When this happens you will have to work with the MCC Diff Checking Tool. The diff checking tool presents you with a new version of the file and the old version of the file, allowing you to make decisions about weather to incorperate changes or not. To understand this better, let's look at a practical example of making changes to the timer 0 configuration. You can find more detailed information starting on page 22 of the MPLAB® Code Configurator User’s Guide

    Firmware Experiments

    Lay a 30 centimeter-long piece of tape down on the lab bench. Mark off 2 cm intervals along the length of the tape. Place the ultrasonic range finder transducer along the 0 cm mark. You will be placing a solid hard surface along the length of the tape so that the ultrasonic acoustic pulse better reflect.




    1. Include the screenshot from the waveform save on your oscilloscope when an object is placed about 1 meter away.
    2. Using the time per division information on the oscilloscope screen and the duration the waveform (described in divisions), show your calculation for the period of the waveform on the echo line by multiplying the time division setting by the number of division of the echo pulse. For example, if your oscilloscope was set to 20ms/division and the waveform had a duration of 4.6 divisions, then your answer would look like: 
      Oscilloscope set to 20ms/division
The duration of the waveform is 4.6 divisions

	  20ms        
	-------- *  4.6 divisions = 92ms
	division
    3. Modify the convertEchoToCm function using your conversion math from a previous answer. Show the complete function as your answer.
    4. Complete the table below by placing a hard reflective object at the distance given in the "Actual distance" column. Follow the following instructions to complete the remaining columns in the table.
      • Measure the duration of the echo pulse (in timer counts using the "s") and enter it into the "Echo duration" column below.
      • Use your answer to questions above to convert the Echo counts into distances (in cm). For each echo count, enter the corresponding distance (to 3 significnt figures) into the "Calculated distance" column below.
      • For each row in the table below, compute the % error between the actual distance of the object and calculated distance in the "Calculated distance (cm)" column and enter it in the "% error" column.
      • Finally record the distance reported by the convertEchoToCm function statement in the column "convertEchoToCm".
      Actual distance Echo duration (counts) Calculated distance (cm)% error convertEchoToCm distance
      10 cm        
      20 cm        
      30 cm        
      40 cm        
      50 cm