Embedded Systems – Assignment 02
Make a Melody
Introduction
The tasks of this laboratory assignment involve connecting a speaker to a microprocessor system and making simple
music with it. It requires writing software to make the speaker switch on/off at fixed frequencies, required by the
sounds of the music. The software to be written consists of two functions: SoundOn and TimeWait. A program,
melody1.cpp has been provided that will interpret simple melodies. These functions will be placed within it.
The expected outcomes of this assignment are (1) getting further practical experience with making hardware
connections to a microprocessor device and (2) having experience with time calibrating real-time software.
Task 1
Download melody1.cpp. Read through the code giving special attention to how melody1.cpp uses SoundOn and
TimeWait. These functions are unique in that they must operate with real timing constraints.
In the laboratory write-up include a paragraph or two about strategies for implementing the SoundOn and TimeWait
functions. Explain how to incorporate both frequency and timing information into the sound function. Be as
detailed as possible in explaining the algorithm.
Task 2
Write and calibrate SoundOn and TimeWait functions. Use the processor’s signal P10 (not the same as Pin 10 on
the 40 pin connector!) for the signal ouput that will drive the speaker.
The sound function must output a 50% duty cycle square wave at the correct frequency (plus or minus a few hertz is
ok, it just has to “sound” good). Outputting a square wave is not har. For instance, the blink program in the first lab
outputs a very slow square wave and it is extremely simple. The complication arises when the function must be
configurable for many different frequencies. It is probably not reasonable to write a mathematical function that
translates a frequency into square wave parameters. Because of time delays in the executing code and the
logarithmic nature of music, solving for such an equation is difficult. Instead, a simple method is to graph frequency
vs. square wave parameters for several values and to use a graph to approximate the rest of the values. [This is a
basic, easy-to-do solution. Better (and more complex) methods are surely available. Report on one if it becomes
apparent.]
In order to pass off task 2, verify that a call to SoundOn outputs a square wave at the appropriate frequencies and
durations.
Task 3
Put together the speaker circuit. Integrate the designed SoundOn and TimeWait functions into melody1.cpp and play
some tunes. Get the project working and pass it off.
Note: the speaker has about eight ohms of resistance. A reasonable current of about 20mA is adequate for sound
generation. The biasing resistors in the circuit are used to control the current. If the speaker plays too softly,
measure the voltage across it and compute the current. Make sure the current is around 20 mA.
In order to pass off, demonstrate your working system to the instructor/TA.
Task 4
Write a report detailing everything done to accomplish the lab. Include all source files, any pertinent information
about the hardware, any neat or interesting things learned and any problems encountered. As always, all program
listings are to include name, date and assignment number. Make sure the report includes an introduction and a
conclusion. Follow all the guidelines listed in the reporting format document.
